summaryrefslogtreecommitdiffstats
path: root/documentation/dev-manual/dev-manual-common-tasks.xml
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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<chapter id='extendpoky'>

<title>Common Tasks</title>
    <para>
        This chapter describes fundamental procedures such as creating layers,
        adding new software packages, extending or customizing images,
        porting work to new hardware (adding a new machine), and so forth.
        You will find that the procedures documented here occur often in the
        development cycle using the Yocto Project.
    </para>

    <section id="understanding-and-creating-layers">
        <title>Understanding and Creating Layers</title>

        <para>
            The OpenEmbedded build system supports organizing
            <ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink> into
            multiple layers.
            Layers allow you to isolate different types of customizations from
            each other.
            For introductory information on the Yocto Project Layer Model,
            see the
            "<ulink url='&YOCTO_DOCS_OM_URL;#the-yocto-project-layer-model'>The Yocto Project Layer Model</ulink>"
            section in the Yocto Project Overview and Concepts Manual.
        </para>

        <section id='creating-your-own-layer'>
            <title>Creating Your Own Layer</title>

            <para>
                It is very easy to create your own layers to use with the
                OpenEmbedded build system.
                The Yocto Project ships with tools that speed up creating
                layers.
                This section describes the steps you perform by hand to create
                layers so that you can better understand them.
                For information about the layer-creation tools, see the
                "<ulink url='&YOCTO_DOCS_BSP_URL;#creating-a-new-bsp-layer-using-the-bitbake-layers-script'>Creating a New BSP Layer Using the <filename>bitbake-layers</filename> Script</ulink>"
                section in the Yocto Project Board Support Package (BSP)
                Developer's Guide and the
                "<link linkend='creating-a-general-layer-using-the-bitbake-layers-script'>Creating a General Layer Using the <filename>bitbake-layers</filename> Script</link>"
                section further down in this manual.
            </para>

            <para>
                Follow these general steps to create your layer without using
                tools:
                <orderedlist>
                    <listitem><para>
                        <emphasis>Check Existing Layers:</emphasis>
                        Before creating a new layer, you should be sure someone
                        has not already created a layer containing the Metadata
                        you need.
                        You can see the
                        <ulink url='http://layers.openembedded.org/layerindex/layers/'>OpenEmbedded Metadata Index</ulink>
                        for a list of layers from the OpenEmbedded community
                        that can be used in the Yocto Project.
                        You could find a layer that is identical or close to
                        what you need.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Create a Directory:</emphasis>
                        Create the directory for your layer.
                        When you create the layer, be sure to create the
                        directory in an area not associated with the
                        Yocto Project
                        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
                        (e.g. the cloned <filename>poky</filename> repository).
                        </para>

                        <para>While not strictly required, prepend the name of
                        the directory with the string "meta-".
                        For example:
                        <literallayout class='monospaced'>
     meta-mylayer
     meta-GUI_xyz
     meta-mymachine
                        </literallayout>
                        With rare exceptions, a layer's name follows this
                        form:
                        <literallayout class='monospaced'>
     meta-<replaceable>root_name</replaceable>
                        </literallayout>
                        Following this layer naming convention can
                        save you trouble later when tools, components, or
                        variables "assume" your layer name begins with "meta-".
                        A notable example is in configuration files as
                        shown in the following step where layer names without
                        the "meta-" string are appended
                        to several variables used in the configuration.
                        </para></listitem>
                    <listitem><para id='dev-layer-config-file-description'>
                        <emphasis>Create a Layer Configuration File:</emphasis>
                        Inside your new layer folder, you need to create a
                        <filename>conf/layer.conf</filename> file.
                        It is easiest to take an existing layer configuration
                        file and copy that to your layer's
                        <filename>conf</filename> directory and then modify the
                        file as needed.</para>

                        <para>The
                        <filename>meta-yocto-bsp/conf/layer.conf</filename> file
                        in the Yocto Project
                        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta-yocto-bsp/conf'>Source Repositories</ulink>
                        demonstrates the required syntax.
                        For your layer, you need to replace "yoctobsp" with
                        a unique identifier for your layer (e.g. "machinexyz"
                        for a layer named "meta-machinexyz"):
                        <literallayout class='monospaced'>
     # We have a conf and classes directory, add to BBPATH
     BBPATH .= ":${LAYERDIR}"

     # We have recipes-* directories, add to BBFILES
     BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
                 ${LAYERDIR}/recipes-*/*/*.bbappend"

     BBFILE_COLLECTIONS += "yoctobsp"
     BBFILE_PATTERN_yoctobsp = "^${LAYERDIR}/"
     BBFILE_PRIORITY_yoctobsp = "5"
     LAYERVERSION_yoctobsp = "4"
     LAYERSERIES_COMPAT_yoctobsp = "&DISTRO_NAME_NO_CAP;"
                        </literallayout>
                        Following is an explanation of the layer configuration
                        file:
                        <itemizedlist>
                            <listitem><para>
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBPATH'><filename>BBPATH</filename></ulink>:
                                Adds the layer's root directory to BitBake's
                                search path.
                                Through the use of the
                                <filename>BBPATH</filename> variable, BitBake
                                locates class files
                                (<filename>.bbclass</filename>),
                                configuration files, and files that are
                                included with <filename>include</filename> and
                                <filename>require</filename> statements.
                                For these cases, BitBake uses the first file
                                that matches the name found in
                                <filename>BBPATH</filename>.
                                This is similar to the way the
                                <filename>PATH</filename> variable is used for
                                binaries.
                                It is recommended, therefore, that you use
                                unique class and configuration filenames in
                                your custom layer.
                                </para></listitem>
                            <listitem><para>
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILES'><filename>BBFILES</filename></ulink>:
                                Defines the location for all recipes in the
                                layer.
                                </para></listitem>
                            <listitem><para>
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_COLLECTIONS'><filename>BBFILE_COLLECTIONS</filename></ulink>:
                                Establishes the current layer through a
                                unique identifier that is used throughout the
                                OpenEmbedded build system to refer to the layer.
                                In this example, the identifier "yoctobsp" is
                                the representation for the container layer
                                named "meta-yocto-bsp".
                                </para></listitem>
                            <listitem><para>
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_PATTERN'><filename>BBFILE_PATTERN</filename></ulink>:
                                Expands immediately during parsing to
                                provide the directory of the layer.
                                </para></listitem>
                            <listitem><para>
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_PRIORITY'><filename>BBFILE_PRIORITY</filename></ulink>:
                                Establishes a priority to use for
                                recipes in the layer when the OpenEmbedded build
                                finds recipes of the same name in different
                                layers.
                                </para></listitem>
                            <listitem><para>
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-LAYERVERSION'><filename>LAYERVERSION</filename></ulink>:
                                Establishes a version number for the layer.
                                You can use this version number to specify this
                                exact version of the layer as a dependency when
                                using the
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-LAYERDEPENDS'><filename>LAYERDEPENDS</filename></ulink>
                                variable.
                                </para></listitem>
                            <listitem><para>
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-LAYERSERIES_COMPAT'><filename>LAYERSERIES_COMPAT</filename></ulink>:
                                Lists the
                                <ulink url='&YOCTO_WIKI_URL;/wiki/Releases'>Yocto Project</ulink>
                                releases for which the current version is
                                compatible.
                                This variable is a good way to indicate if
                                your particular layer is current.
                                </para></listitem>
                        </itemizedlist>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Add Content:</emphasis>
                        Depending on the type of layer, add the content.
                        If the layer adds support for a machine, add the machine
                        configuration in a <filename>conf/machine/</filename>
                        file within the layer.
                        If the layer adds distro policy, add the distro
                        configuration in a <filename>conf/distro/</filename>
                        file within the layer.
                        If the layer introduces new recipes, put the recipes
                        you need in <filename>recipes-*</filename>
                        subdirectories within the layer.
                        <note>
                            For an explanation of layer hierarchy that
                            is compliant with the Yocto Project, see
                            the
                            "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-filelayout'>Example Filesystem Layout</ulink>"
                            section in the Yocto Project Board
                            Support Package (BSP) Developer's Guide.
                        </note>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Optionally Test for Compatibility:</emphasis>
                        If you want permission to use the Yocto Project
                        Compatibility logo with your layer or application that
                        uses your layer, perform the steps to apply for
                        compatibility.
                        See the
                        "<link linkend='making-sure-your-layer-is-compatible-with-yocto-project'>Making Sure Your Layer is Compatible With Yocto Project</link>"
                        section for more information.
                        </para></listitem>
                </orderedlist>
            </para>
        </section>

        <section id='best-practices-to-follow-when-creating-layers'>
            <title>Following Best Practices When Creating Layers</title>

            <para>
                To create layers that are easier to maintain and that will
                not impact builds for other machines, you should consider the
                information in the following list:
                <itemizedlist>
                    <listitem><para>
                        <emphasis>Avoid "Overlaying" Entire Recipes from Other Layers in Your Configuration:</emphasis>
                        In other words, do not copy an entire recipe into your
                        layer and then modify it.
                        Rather, use an append file
                        (<filename>.bbappend</filename>) to override only those
                        parts of the original recipe you need to modify.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Avoid Duplicating Include Files:</emphasis>
                        Use append files (<filename>.bbappend</filename>)
                        for each recipe that uses an include file.
                        Or, if you are introducing a new recipe that requires
                        the included file, use the path relative to the
                        original layer directory to refer to the file.
                        For example, use
                        <filename>require recipes-core/</filename><replaceable>package</replaceable><filename>/</filename><replaceable>file</replaceable><filename>.inc</filename>
                        instead of
                        <filename>require </filename><replaceable>file</replaceable><filename>.inc</filename>.
                        If you're finding you have to overlay the include file,
                        it could indicate a deficiency in the include file in
                        the layer to which it originally belongs.
                        If this is the case, you should try to address that
                        deficiency instead of overlaying the include file.
                        For example, you could address this by getting the
                        maintainer of the include file to add a variable or
                        variables to make it easy to override the parts needing
                        to be overridden.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Structure Your Layers:</emphasis>
                        Proper use of overrides within append files and
                        placement of machine-specific files within your layer
                        can ensure that a build is not using the wrong Metadata
                        and negatively impacting a build for a different
                        machine.
                        Following are some examples:
                        <itemizedlist>
                            <listitem><para>
                                <emphasis>Modify Variables to Support a
                                Different Machine:</emphasis>
                                Suppose you have a layer named
                                <filename>meta-one</filename> that adds support
                                for building machine "one".
                                To do so, you use an append file named
                                <filename>base-files.bbappend</filename> and
                                create a dependency on "foo" by altering the
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>
                                variable:
                                <literallayout class='monospaced'>
     DEPENDS = "foo"
                                </literallayout>
                                The dependency is created during any build that
                                includes the layer
                                <filename>meta-one</filename>.
                                However, you might not want this dependency
                                for all machines.
                                For example, suppose you are building for
                                machine "two" but your
                                <filename>bblayers.conf</filename> file has the
                                <filename>meta-one</filename> layer included.
                                During the build, the
                                <filename>base-files</filename> for machine
                                "two" will also have the dependency on
                                <filename>foo</filename>.</para>
                                <para>To make sure your changes apply only when
                                building machine "one", use a machine override
                                with the <filename>DEPENDS</filename> statement:
                                <literallayout class='monospaced'>
     DEPENDS_one = "foo"
                                </literallayout>
                                You should follow the same strategy when using
                                <filename>_append</filename> and
                                <filename>_prepend</filename> operations:
                                <literallayout class='monospaced'>
     DEPENDS_append_one = " foo"
     DEPENDS_prepend_one = "foo "
                                </literallayout>
                                As an actual example, here's a line from the recipe
                                for gnutls, which adds dependencies on
                                "argp-standalone" when building with the musl C
                                library:
                                <literallayout class='monospaced'>
     DEPENDS_append_libc-musl = " argp-standalone"
                                </literallayout>
                                <note>
                                    Avoiding "+=" and "=+" and using
                                    machine-specific
                                    <filename>_append</filename>
                                    and <filename>_prepend</filename> operations
                                    is recommended as well.
                                </note>
                                </para></listitem>
                            <listitem><para>
                                <emphasis>Place Machine-Specific Files in
                                Machine-Specific Locations:</emphasis>
                                When you have a base recipe, such as
                                <filename>base-files.bb</filename>, that
                                contains a
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                                statement to a file, you can use an append file
                                to cause the build to use your own version of
                                the file.
                                For example, an append file in your layer at
                                <filename>meta-one/recipes-core/base-files/base-files.bbappend</filename>
                                could extend
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
                                using
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
                                as follows:
                                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${BPN}:"
                                </literallayout>
                                The build for machine "one" will pick up your
                                machine-specific file as long as you have the
                                file in
                                <filename>meta-one/recipes-core/base-files/base-files/</filename>.
                                However, if you are building for a different
                                machine and the
                                <filename>bblayers.conf</filename> file includes
                                the <filename>meta-one</filename> layer and
                                the location of your machine-specific file is
                                the first location where that file is found
                                according to <filename>FILESPATH</filename>,
                                builds for all machines will also use that
                                machine-specific file.</para>
                                <para>You can make sure that a machine-specific
                                file is used for a particular machine by putting
                                the file in a subdirectory specific to the
                                machine.
                                For example, rather than placing the file in
                                <filename>meta-one/recipes-core/base-files/base-files/</filename>
                                as shown above, put it in
                                <filename>meta-one/recipes-core/base-files/base-files/one/</filename>.
                                Not only does this make sure the file is used
                                only when building for machine "one", but the
                                build process locates the file more quickly.</para>
                                <para>In summary, you need to place all files
                                referenced from <filename>SRC_URI</filename>
                                in a machine-specific subdirectory within the
                                layer in order to restrict those files to
                                machine-specific builds.
                                </para></listitem>
                        </itemizedlist>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Perform Steps to Apply for Yocto Project Compatibility:</emphasis>
                        If you want permission to use the
                        Yocto Project Compatibility logo with your layer
                        or application that uses your layer, perform the
                        steps to apply for compatibility.
                        See the
                        "<link linkend='making-sure-your-layer-is-compatible-with-yocto-project'>Making Sure Your Layer is Compatible With Yocto Project</link>"
                        section for more information.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Follow the Layer Naming Convention:</emphasis>
                        Store custom layers in a Git repository that use the
                        <filename>meta-<replaceable>layer_name</replaceable></filename>
                        format.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Group Your Layers Locally:</emphasis>
                        Clone your repository alongside other cloned
                        <filename>meta</filename> directories from the
                        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='making-sure-your-layer-is-compatible-with-yocto-project'>
            <title>Making Sure Your Layer is Compatible With Yocto Project</title>

            <para>
                When you create a layer used with the Yocto Project, it is
                advantageous to make sure that the layer interacts well with
                existing Yocto Project layers (i.e. the layer is compatible
                with the Yocto Project).
                Ensuring compatibility makes the layer easy to be consumed
                by others in the Yocto Project community and could allow you
                permission to use the Yocto Project Compatible Logo.
                <note>
                    Only Yocto Project member organizations are permitted to
                    use the Yocto Project Compatible Logo.
                    The logo is not available for general use.
                    For information on how to become a Yocto Project member
                    organization, see the
                    <ulink url='&YOCTO_HOME_URL;'>Yocto Project Website</ulink>.
                </note>
            </para>

            <para>
                The Yocto Project Compatibility Program consists of a layer
                application process that requests permission to use the Yocto
                Project Compatibility Logo for your layer and application.
                The process consists of two parts:
                <orderedlist>
                    <listitem><para>
                        Successfully passing a script
                        (<filename>yocto-check-layer</filename>) that
                        when run against your layer, tests it against
                        constraints based on experiences of how layers have
                        worked in the real world and where pitfalls have been
                        found.
                        Getting a "PASS" result from the script is required for
                        successful compatibility registration.
                        </para></listitem>
                    <listitem><para>
                        Completion of an application acceptance form, which
                        you can find at
                        <ulink url='https://www.yoctoproject.org/webform/yocto-project-compatible-registration'></ulink>.
                        </para></listitem>
                </orderedlist>
            </para>

            <para>
                To be granted permission to use the logo, you need to satisfy
                the following:
                <itemizedlist>
                    <listitem><para>
                        Be able to check the box indicating that you
                        got a "PASS" when running the script against your
                        layer.
                        </para></listitem>
                    <listitem><para>
                        Answer "Yes" to the questions on the form or have an
                        acceptable explanation for any questions answered "No".
                        </para></listitem>
                    <listitem><para>
                        Be a Yocto Project Member Organization.
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                The remainder of this section presents information on the
                registration form and on the
                <filename>yocto-check-layer</filename> script.
            </para>

            <section id='yocto-project-compatible-program-application'>
                <title>Yocto Project Compatible Program Application</title>

                <para>
                    Use the form to apply for your layer's approval.
                    Upon successful application, you can use the Yocto
                    Project Compatibility Logo with your layer and the
                    application that uses your layer.
                </para>

                <para>
                    To access the form, use this link:
                    <ulink url='https://www.yoctoproject.org/webform/yocto-project-compatible-registration'></ulink>.
                    Follow the instructions on the form to complete your
                    application.
                </para>

                <para>
                    The application consists of the following sections:
                    <itemizedlist>
                        <listitem><para>
                            <emphasis>Contact Information:</emphasis>
                            Provide your contact information as the fields
                            require.
                            Along with your information, provide the
                            released versions of the Yocto Project for which
                            your layer is compatible.
                            </para></listitem>
                        <listitem><para>
                            <emphasis>Acceptance Criteria:</emphasis>
                            Provide "Yes" or "No" answers for each of the
                            items in the checklist.
                            Space exists at the bottom of the form for any
                            explanations for items for which you answered "No".
                            </para></listitem>
                        <listitem><para>
                            <emphasis>Recommendations:</emphasis>
                            Provide answers for the questions regarding Linux
                            kernel use and build success.
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>

            <section id='yocto-check-layer-script'>
                <title><filename>yocto-check-layer</filename> Script</title>

                <para>
                    The <filename>yocto-check-layer</filename> script
                    provides you a way to assess how compatible your layer is
                    with the Yocto Project.
                    You should run this script prior to using the form to
                    apply for compatibility as described in the previous
                    section.
                    You need to achieve a "PASS" result in order to have
                    your application form successfully processed.
                </para>

                <para>
                    The script divides tests into three areas: COMMON, BSP,
                    and DISTRO.
                    For example, given a distribution layer (DISTRO), the
                    layer must pass both the COMMON and DISTRO related tests.
                    Furthermore, if your layer is a BSP layer, the layer must
                    pass the COMMON and BSP set of tests.
                </para>

                <para>
                    To execute the script, enter the following commands from
                    your build directory:
                    <literallayout class='monospaced'>
     $ source oe-init-build-env
     $ yocto-check-layer <replaceable>your_layer_directory</replaceable>
                    </literallayout>
                    Be sure to provide the actual directory for your layer
                    as part of the command.
                </para>

                <para>
                    Entering the command causes the script to determine the
                    type of layer and then to execute a set of specific
                    tests against the layer.
                    The following list overviews the test:
                    <itemizedlist>
                        <listitem><para>
                            <filename>common.test_readme</filename>:
                            Tests if a <filename>README</filename> file
                            exists in the layer and the file is not empty.
                            </para></listitem>
                        <listitem><para>
                            <filename>common.test_parse</filename>:
                            Tests to make sure that BitBake can parse the
                            files without error (i.e.
                            <filename>bitbake -p</filename>).
                            </para></listitem>
                        <listitem><para>
                            <filename>common.test_show_environment</filename>:
                            Tests that the global or per-recipe environment
                            is in order without errors (i.e.
                            <filename>bitbake -e</filename>).
                            </para></listitem>
                        <listitem><para>
                            <filename>common.test_signatures</filename>:
                            Tests to be sure that BSP and DISTRO layers do not
                            come with recipes that change signatures.
                            </para></listitem>
                        <listitem><para>
                            <filename>bsp.test_bsp_defines_machines</filename>:
                            Tests if a BSP layer has machine configurations.
                            </para></listitem>
                        <listitem><para>
                            <filename>bsp.test_bsp_no_set_machine</filename>:
                            Tests to ensure a BSP layer does not set the
                            machine when the layer is added.
                            </para></listitem>
                        <listitem><para>
                            <filename>distro.test_distro_defines_distros</filename>:
                            Tests if a DISTRO layer has distro configurations.
                            </para></listitem>
                        <listitem><para>
                            <filename>distro.test_distro_no_set_distro</filename>:
                            Tests to ensure a DISTRO layer does not set the
                            distribution when the layer is added.
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>
        </section>

        <section id='enabling-your-layer'>
            <title>Enabling Your Layer</title>

            <para>
                Before the OpenEmbedded build system can use your new layer,
                you need to enable it.
                To enable your layer, simply add your layer's path to the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BBLAYERS'>BBLAYERS</ulink></filename>
                variable in your <filename>conf/bblayers.conf</filename> file,
                which is found in the
                <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
                The following example shows how to enable a layer named
                <filename>meta-mylayer</filename>:
                <literallayout class='monospaced'>
     # POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
     # changes incompatibly
     POKY_BBLAYERS_CONF_VERSION = "2"

     BBPATH = "${TOPDIR}"
     BBFILES ?= ""

     BBLAYERS ?= " \
       /home/<replaceable>user</replaceable>/poky/meta \
       /home/<replaceable>user</replaceable>/poky/meta-poky \
       /home/<replaceable>user</replaceable>/poky/meta-yocto-bsp \
       /home/<replaceable>user</replaceable>/poky/meta-mylayer \
       "
                </literallayout>
            </para>

            <para>
                BitBake parses each <filename>conf/layer.conf</filename> file
                from the top down as specified in the
                <filename>BBLAYERS</filename> variable
                within the <filename>conf/bblayers.conf</filename> file.
                During the processing of each
                <filename>conf/layer.conf</filename> file, BitBake adds the
                recipes, classes and configurations contained within the
                particular layer to the source directory.
            </para>
        </section>

        <section id='using-bbappend-files'>
            <title>Using .bbappend Files in Your Layer</title>

            <para>
                A recipe that appends Metadata to another recipe is called a
                BitBake append file.
                A BitBake append file uses the <filename>.bbappend</filename>
                file type suffix, while the corresponding recipe to which
                Metadata is being appended uses the <filename>.bb</filename>
                file type suffix.
            </para>

            <para>
                You can use a <filename>.bbappend</filename> file in your
                layer to make additions or changes to the content of another
                layer's recipe without having to copy the other layer's
                recipe into your layer.
                Your <filename>.bbappend</filename> file resides in your layer,
                while the main <filename>.bb</filename> recipe file to
                which you are appending Metadata resides in a different layer.
            </para>

            <para>
                Being able to append information to an existing recipe not only
                avoids duplication, but also automatically applies recipe
                changes from a different layer into your layer.
                If you were copying recipes, you would have to manually merge
                changes as they occur.
            </para>

            <para>
                When you create an append file, you must use the same root
                name as the corresponding recipe file.
                For example, the append file
                <filename>someapp_&DISTRO;.bbappend</filename> must apply to
                <filename>someapp_&DISTRO;.bb</filename>.
                This means the original recipe and append file names are
                version number-specific.
                If the corresponding recipe is renamed to update to a newer
                version, you must also rename and possibly update
                the corresponding <filename>.bbappend</filename> as well.
                During the build process, BitBake displays an error on starting
                if it detects a <filename>.bbappend</filename> file that does
                not have a corresponding recipe with a matching name.
                See the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BB_DANGLINGAPPENDS_WARNONLY'><filename>BB_DANGLINGAPPENDS_WARNONLY</filename></ulink>
                variable for information on how to handle this error.
            </para>

            <para>
                As an example, consider the main formfactor recipe and a
                corresponding formfactor append file both from the
                <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>.
                Here is the main formfactor recipe, which is named
                <filename>formfactor_0.0.bb</filename> and located in the
                "meta" layer at
                <filename>meta/recipes-bsp/formfactor</filename>:
                <literallayout class='monospaced'>
     SUMMARY = "Device formfactor information"
     SECTION = "base"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
     PR = "r45"

     SRC_URI = "file://config file://machconfig"
     S = "${WORKDIR}"

     PACKAGE_ARCH = "${MACHINE_ARCH}"
     INHIBIT_DEFAULT_DEPS = "1"

     do_install() {
	     # Install file only if it has contents
             install -d ${D}${sysconfdir}/formfactor/
             install -m 0644 ${S}/config ${D}${sysconfdir}/formfactor/
	     if [ -s "${S}/machconfig" ]; then
	             install -m 0644 ${S}/machconfig ${D}${sysconfdir}/formfactor/
	     fi
     }                </literallayout>
                In the main recipe, note the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                variable, which tells the OpenEmbedded build system where to
                find files during the build.
            </para>

            <para>
                Following is the append file, which is named
                <filename>formfactor_0.0.bbappend</filename> and is from the
                Raspberry Pi BSP Layer named
                <filename>meta-raspberrypi</filename>.
                The file is in the layer at
                <filename>recipes-bsp/formfactor</filename>:
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
                </literallayout>
            </para>

            <para>
                By default, the build system uses the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
                variable to locate files.
                This append file extends the locations by setting the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS'><filename>FILESEXTRAPATHS</filename></ulink>
                variable.
                Setting this variable in the <filename>.bbappend</filename>
                file is the most reliable and recommended method for adding
                directories to the search path used by the build system
                to find files.
            </para>

            <para>
                The statement in this example extends the directories to
                include
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-THISDIR'><filename>THISDIR</filename></ulink><filename>}/${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink><filename>}</filename>,
                which resolves to a directory named
                <filename>formfactor</filename> in the same directory
                in which the append file resides (i.e.
                <filename>meta-raspberrypi/recipes-bsp/formfactor</filename>.
                This implies that you must have the supporting directory
                structure set up that will contain any files or patches you
                will be including from the layer.
            </para>

            <para>
                Using the immediate expansion assignment operator
                <filename>:=</filename> is important because of the reference
                to <filename>THISDIR</filename>.
                The trailing colon character is important as it ensures that
                items in the list remain colon-separated.
                <note>
                    <para>
                        BitBake automatically defines the
                        <filename>THISDIR</filename> variable.
                        You should never set this variable yourself.
                        Using "_prepend" as part of the
                        <filename>FILESEXTRAPATHS</filename> ensures your path
                        will be searched prior to other paths in the final
                        list.
                    </para>

                    <para>
                        Also, not all append files add extra files.
                        Many append files simply exist to add build options
                        (e.g. <filename>systemd</filename>).
                        For these cases, your append file would not even
                        use the <filename>FILESEXTRAPATHS</filename> statement.
                    </para>
                </note>
            </para>
        </section>

        <section id='prioritizing-your-layer'>
            <title>Prioritizing Your Layer</title>

            <para>
                Each layer is assigned a priority value.
                Priority values control which layer takes precedence if there
                are recipe files with the same name in multiple layers.
                For these cases, the recipe file from the layer with a higher
                priority number takes precedence.
                Priority values also affect the order in which multiple
                <filename>.bbappend</filename> files for the same recipe are
                applied.
                You can either specify the priority manually, or allow the
                build system to calculate it based on the layer's dependencies.
            </para>

            <para>
                To specify the layer's priority manually, use the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_PRIORITY'><filename>BBFILE_PRIORITY</filename></ulink>
                variable and append the layer's root name:
                <literallayout class='monospaced'>
     BBFILE_PRIORITY_mylayer = "1"
                </literallayout>
            </para>

            <note>
                <para>It is possible for a recipe with a lower version number
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
                in a layer that has a higher priority to take precedence.</para>
                <para>Also, the layer priority does not currently affect the
                precedence order of <filename>.conf</filename>
                or <filename>.bbclass</filename> files.
                Future versions of BitBake might address this.</para>
            </note>
        </section>

        <section id='managing-layers'>
            <title>Managing Layers</title>

            <para>
                You can use the BitBake layer management tool
                <filename>bitbake-layers</filename> to provide a view
                into the structure of recipes across a multi-layer project.
                Being able to generate output that reports on configured layers
                with their paths and priorities and on
                <filename>.bbappend</filename> files and their applicable
                recipes can help to reveal potential problems.
            </para>

            <para>
                For help on the BitBake layer management tool, use the
                following command:
                <literallayout class='monospaced'>
     $ bitbake-layers --help
     NOTE: Starting bitbake server...
     usage: bitbake-layers [-d] [-q] [-F] [--color COLOR] [-h] &lt;subcommand&gt; ...

     BitBake layers utility

     optional arguments:
       -d, --debug           Enable debug output
       -q, --quiet           Print only errors
       -F, --force           Force add without recipe parse verification
       --color COLOR         Colorize output (where COLOR is auto, always, never)
       -h, --help            show this help message and exit

     subcommands:
       &lt;subcommand&gt;
         show-layers         show current configured layers.
         show-overlayed      list overlayed recipes (where the same recipe exists
                             in another layer)
         show-recipes        list available recipes, showing the layer they are
                             provided by
         show-appends        list bbappend files and recipe files they apply to
         show-cross-depends  Show dependencies between recipes that cross layer
                             boundaries.
         add-layer           Add one or more layers to bblayers.conf.
         remove-layer        Remove one or more layers from bblayers.conf.
         flatten             flatten layer configuration into a separate output
                             directory.
         layerindex-fetch    Fetches a layer from a layer index along with its
                             dependent layers, and adds them to conf/bblayers.conf.
         layerindex-show-depends
                             Find layer dependencies from layer index.
         create-layer        Create a basic layer

     Use bitbake-layers &lt;subcommand&gt; --help to get help on a specific command
                </literallayout>
            </para>

            <para>
                The following list describes the available commands:
                <itemizedlist>
                    <listitem><para>
                        <emphasis><filename>help:</filename></emphasis>
                        Displays general help or help on a specified command.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>show-layers:</filename></emphasis>
                        Shows the current configured layers.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>show-overlayed:</filename></emphasis>
                        Lists overlayed recipes.
                        A recipe is overlayed when a recipe with the same name
                        exists in another layer that has a higher layer
                        priority.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>show-recipes:</filename></emphasis>
                        Lists available recipes and the layers that provide them.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>show-appends:</filename></emphasis>
                        Lists <filename>.bbappend</filename> files and the
                        recipe files to which they apply.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>show-cross-depends:</filename></emphasis>
                        Lists dependency relationships between recipes that
                        cross layer boundaries.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>add-layer:</filename></emphasis>
                        Adds a layer to <filename>bblayers.conf</filename>.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>remove-layer:</filename></emphasis>
                        Removes a layer from <filename>bblayers.conf</filename>
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>flatten:</filename></emphasis>
                        Flattens the layer configuration into a separate output
                        directory.
                        Flattening your layer configuration builds a "flattened"
                        directory that contains the contents of all layers,
                        with any overlayed recipes removed and any
                        <filename>.bbappend</filename> files appended to the
                        corresponding recipes.
                        You might have to perform some manual cleanup of the
                        flattened layer as follows:
                        <itemizedlist>
                            <listitem><para>
                                Non-recipe files (such as patches)
                                are overwritten.
                                The flatten command shows a warning for these
                                files.
                                </para></listitem>
                            <listitem><para>
                                Anything beyond the normal layer
                                setup has been added to the
                                <filename>layer.conf</filename> file.
                                Only the lowest priority layer's
                                <filename>layer.conf</filename> is used.
                                </para></listitem>
                            <listitem><para>
                                Overridden and appended items from
                                <filename>.bbappend</filename> files need to be
                                cleaned up.
                                The contents of each
                                <filename>.bbappend</filename> end up in the
                                flattened recipe.
                                However, if there are appended or changed
                                variable values, you need to tidy these up
                                yourself.
                                Consider the following example.
                                Here, the <filename>bitbake-layers</filename>
                                command adds the line
                                <filename>#### bbappended ...</filename> so that
                                you know where the following lines originate:
                                <literallayout class='monospaced'>
     ...
     DESCRIPTION = "A useful utility"
     ...
     EXTRA_OECONF = "--enable-something"
     ...

     #### bbappended from meta-anotherlayer ####

     DESCRIPTION = "Customized utility"
     EXTRA_OECONF += "--enable-somethingelse"
                                </literallayout>
                                Ideally, you would tidy up these utilities as
                                follows:
                                <literallayout class='monospaced'>
     ...
     DESCRIPTION = "Customized utility"
     ...
     EXTRA_OECONF = "--enable-something --enable-somethingelse"
     ...
                                </literallayout>
                                </para></listitem>
                        </itemizedlist>
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>layerindex-fetch</filename>:</emphasis>
                        Fetches a layer from a layer index, along with its
                        dependent layers, and adds the layers to the
                        <filename>conf/bblayers.conf</filename> file.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>layerindex-show-depends</filename>:</emphasis>
                        Finds layer dependencies from the layer index.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>create-layer</filename>:</emphasis>
                        Creates a basic layer.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='creating-a-general-layer-using-the-bitbake-layers-script'>
            <title>Creating a General Layer Using the <filename>bitbake-layers</filename> Script</title>

            <para>
                The <filename>bitbake-layers</filename> script with the
                <filename>create-layer</filename> subcommand simplifies
                creating a new general layer.
                <note><title>Notes</title>
                    <itemizedlist>
                        <listitem><para>
                            For information on BSP layers, see the
                            "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-layers'>BSP Layers</ulink>"
                            section in the Yocto Project Board Specific (BSP)
                            Developer's Guide.
                            </para></listitem>
                        <listitem><para>
                            In order to use a layer with the OpenEmbedded
                            build system, you need to add the layer to your
                            <filename>bblayers.conf</filename> configuration
                            file.
                            See the
                            "<link linkend='adding-a-layer-using-the-bitbake-layers-script'>Adding a Layer Using the <filename>bitbake-layers</filename> Script</link>"
                            section for more information.
                            </para></listitem>
                    </itemizedlist>
                </note>
                The default mode of the script's operation with this
                subcommand is to create a layer with the following:
                <itemizedlist>
                    <listitem><para>A layer priority of 6.
                        </para></listitem>
                    <listitem><para>A <filename>conf</filename>
                        subdirectory that contains a
                        <filename>layer.conf</filename> file.
                        </para></listitem>
                    <listitem><para>
                        A <filename>recipes-example</filename> subdirectory
                        that contains a further subdirectory named
                        <filename>example</filename>, which contains
                        an <filename>example.bb</filename> recipe file.
                        </para></listitem>
                    <listitem><para>A <filename >COPYING.MIT</filename>,
                        which is the license statement for the layer.
                        The script assumes you want to use the MIT license,
                        which is typical for most layers, for the contents of
                        the layer itself.
                        </para></listitem>
                    <listitem><para>
                        A <filename>README</filename> file, which is a file
                        describing the contents of your new layer.
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                In its simplest form, you can use the following command form
                to create a layer.
                The command creates a layer whose name corresponds to
                <replaceable>your_layer_name</replaceable> in the current
                directory:
                <literallayout class='monospaced'>
     $ bitbake-layers create-layer <replaceable>your_layer_name</replaceable>
                </literallayout>
                As an example, the following command creates a layer named
                <filename>meta-scottrif</filename> in your home directory:
                <literallayout class='monospaced'>
     $ cd /usr/home
     $ bitbake-layers create-layer meta-scottrif
     NOTE: Starting bitbake server...
     Add your new layer with 'bitbake-layers add-layer meta-scottrif'
                </literallayout>
            </para>

            <para>
                If you want to set the priority of the layer to other than the
                default value of "6", you can either use the
                <filename>&dash;&dash;priority</filename> option or you can
                edit the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILE_PRIORITY'><filename>BBFILE_PRIORITY</filename></ulink>
                value in the <filename>conf/layer.conf</filename> after the
                script creates it.
                Furthermore, if you want to give the example recipe file
                some name other than the default, you can
                use the
                <filename>&dash;&dash;example-recipe-name</filename> option.
            </para>

            <para>
                The easiest way to see how the
                <filename>bitbake-layers create-layer</filename> command
                works is to experiment with the script.
                You can also read the usage information by entering the
                following:
                <literallayout class='monospaced'>
     $ bitbake-layers create-layer --help
     NOTE: Starting bitbake server...
     usage: bitbake-layers create-layer [-h] [--priority PRIORITY]
                                        [--example-recipe-name EXAMPLERECIPE]
                                        layerdir

     Create a basic layer

     positional arguments:
       layerdir              Layer directory to create

     optional arguments:
       -h, --help            show this help message and exit
       --priority PRIORITY, -p PRIORITY
                             Layer directory to create
       --example-recipe-name EXAMPLERECIPE, -e EXAMPLERECIPE
                             Filename of the example recipe
                </literallayout>
            </para>
        </section>

        <section id='adding-a-layer-using-the-bitbake-layers-script'>
            <title>Adding a Layer Using the <filename>bitbake-layers</filename> Script</title>

            <para>
                Once you create your general layer, you must add it to your
                <filename>bblayers.conf</filename> file.
                Adding the layer to this configuration file makes the
                OpenEmbedded build system aware of your layer so that it can
                search it for metadata.
            </para>

            <para>
                Add your layer by using the
                <filename>bitbake-layers add-layer</filename> command:
                <literallayout class='monospaced'>
     $ bitbake-layers add-layer <replaceable>your_layer_name</replaceable>
                </literallayout>
                Here is an example that adds a layer named
                <filename>meta-scottrif</filename> to the configuration file.
                Following the command that adds the layer is another
                <filename>bitbake-layers</filename> command that shows the
                layers that are in your <filename>bblayers.conf</filename>
                file:
                <literallayout class='monospaced'>
     $ bitbake-layers add-layer meta-scottrif
     NOTE: Starting bitbake server...
     Parsing recipes: 100% |##########################################################| Time: 0:00:49
     Parsing of 1441 .bb files complete (0 cached, 1441 parsed). 2055 targets, 56 skipped, 0 masked, 0 errors.
     $ bitbake-layers show-layers
     NOTE: Starting bitbake server...
     layer                 path                                      priority
     ==========================================================================
     meta                  /home/scottrif/poky/meta                  5
     meta-poky             /home/scottrif/poky/meta-poky             5
     meta-yocto-bsp        /home/scottrif/poky/meta-yocto-bsp        5
     workspace             /home/scottrif/poky/build/workspace       99
     meta-scottrif         /home/scottrif/poky/build/meta-scottrif   6
                </literallayout>
                Adding the layer to this file enables the build system to
                locate the layer during the build.
                <note>
                    During a build, the OpenEmbedded build system looks in
                    the layers from the top of the list down to the bottom
                    in that order.
                </note>
                </para>
        </section>
    </section>

    <section id='usingpoky-extend-customimage'>
        <title>Customizing Images</title>

        <para>
            You can customize images to satisfy particular requirements.
            This section describes several methods and provides guidelines for each.
        </para>

        <section id='usingpoky-extend-customimage-localconf'>
            <title>Customizing Images Using <filename>local.conf</filename></title>

            <para>
                Probably the easiest way to customize an image is to add a
                package by way of the <filename>local.conf</filename>
                configuration file.
                Because it is limited to local use, this method generally only
                allows you to add packages and is not as flexible as creating
                your own customized image.
                When you add packages using local variables this way, you need
                to realize that these variable changes are in effect for every
                build and consequently affect all images, which might not
                be what you require.
            </para>

            <para>
                To add a package to your image using the local configuration
                file, use the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'>IMAGE_INSTALL</ulink></filename>
                variable with the <filename>_append</filename> operator:
                <literallayout class='monospaced'>
     IMAGE_INSTALL_append = " strace"
                </literallayout>
                Use of the syntax is important - specifically, the space between
                the quote and the package name, which is
                <filename>strace</filename> in this example.
                This space is required since the <filename>_append</filename>
                operator does not add the space.
            </para>

            <para>
                Furthermore, you must use <filename>_append</filename> instead
                of the <filename>+=</filename> operator if you want to avoid
                ordering issues.
                The reason for this is because doing so unconditionally appends
                to the variable and avoids ordering problems due to the
                variable being set in image recipes and
                <filename>.bbclass</filename> files with operators like
                <filename>?=</filename>.
                Using <filename>_append</filename> ensures the operation takes
                affect.
            </para>

            <para>
                As shown in its simplest use,
                <filename>IMAGE_INSTALL_append</filename> affects all images.
                It is possible to extend the syntax so that the variable
                applies to a specific image only.
                Here is an example:
                <literallayout class='monospaced'>
     IMAGE_INSTALL_append_pn-core-image-minimal = " strace"
                </literallayout>
                This example adds <filename>strace</filename> to the
                <filename>core-image-minimal</filename> image only.
            </para>

            <para>
                You can add packages using a similar approach through the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-CORE_IMAGE_EXTRA_INSTALL'>CORE_IMAGE_EXTRA_INSTALL</ulink></filename>
                variable.
                If you use this variable, only
                <filename>core-image-*</filename> images are affected.
            </para>
        </section>

        <section id='usingpoky-extend-customimage-imagefeatures'>
            <title>Customizing Images Using Custom <filename>IMAGE_FEATURES</filename> and
                <filename>EXTRA_IMAGE_FEATURES</filename></title>

            <para>
                Another method for customizing your image is to enable or
                disable high-level image features by using the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_FEATURES'><filename>IMAGE_FEATURES</filename></ulink>
                and <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTRA_IMAGE_FEATURES'><filename>EXTRA_IMAGE_FEATURES</filename></ulink>
                variables.
                Although the functions for both variables are nearly equivalent,
                best practices dictate using <filename>IMAGE_FEATURES</filename>
                from within a recipe and using
                <filename>EXTRA_IMAGE_FEATURES</filename> from within
                your <filename>local.conf</filename> file, which is found in the
                <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
            </para>

            <para>
                To understand how these features work, the best reference is
                <filename>meta/classes/core-image.bbclass</filename>.
                This class lists out the available
                <filename>IMAGE_FEATURES</filename> of which most map to
                package groups while some, such as
                <filename>debug-tweaks</filename> and
                <filename>read-only-rootfs</filename>, resolve as general
                configuration settings.
            </para>

            <para>
                In summary, the file looks at the contents of the
                <filename>IMAGE_FEATURES</filename> variable and then maps
                or configures the feature accordingly.
                Based on this information, the build system automatically
                adds the appropriate packages or configurations to the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'><filename>IMAGE_INSTALL</filename></ulink>
                variable.
                Effectively, you are enabling extra features by extending the
                class or creating a custom class for use with specialized image
                <filename>.bb</filename> files.
            </para>

            <para>
                Use the <filename>EXTRA_IMAGE_FEATURES</filename> variable
                from within your local configuration file.
                Using a separate area from which to enable features with
                this variable helps you avoid overwriting the features in the
                image recipe that are enabled with
                <filename>IMAGE_FEATURES</filename>.
                The value of <filename>EXTRA_IMAGE_FEATURES</filename> is added
                to <filename>IMAGE_FEATURES</filename> within
                <filename>meta/conf/bitbake.conf</filename>.
            </para>

            <para>
                To illustrate how you can use these variables to modify your
                image, consider an example that selects the SSH server.
                The Yocto Project ships with two SSH servers you can use
                with your images: Dropbear and OpenSSH.
                Dropbear is a minimal SSH server appropriate for
                resource-constrained environments, while OpenSSH is a
                well-known standard SSH server implementation.
                By default, the <filename>core-image-sato</filename> image
                is configured to use Dropbear.
                The <filename>core-image-full-cmdline</filename> and
                <filename>core-image-lsb</filename> images both
                include OpenSSH.
                The <filename>core-image-minimal</filename> image does not
                contain an SSH server.
            </para>

            <para>
                You can customize your image and change these defaults.
                Edit the <filename>IMAGE_FEATURES</filename> variable
                in your recipe or use the
                <filename>EXTRA_IMAGE_FEATURES</filename> in your
                <filename>local.conf</filename> file so that it configures the
                image you are working with to include
                <filename>ssh-server-dropbear</filename> or
                <filename>ssh-server-openssh</filename>.
            </para>

            <note>
                See the
                "<ulink url='&YOCTO_DOCS_REF_URL;#ref-images'>Images</ulink>"
                section in the Yocto Project Reference Manual for a complete
                list of image features that ship with the Yocto Project.
            </note>
        </section>

        <section id='usingpoky-extend-customimage-custombb'>
            <title>Customizing Images Using Custom .bb Files</title>

            <para>
                You can also customize an image by creating a custom recipe
                that defines additional software as part of the image.
                The following example shows the form for the two lines you need:
                <literallayout class='monospaced'>
     IMAGE_INSTALL = "packagegroup-core-x11-base package1 package2"

     inherit core-image
                </literallayout>
            </para>

            <para>
                Defining the software using a custom recipe gives you total
                control over the contents of the image.
                It is important to use the correct names of packages in the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'>IMAGE_INSTALL</ulink></filename>
                variable.
                You must use the OpenEmbedded notation and not the Debian notation for the names
                (e.g. <filename>glibc-dev</filename> instead of <filename>libc6-dev</filename>).
            </para>

            <para>
                The other method for creating a custom image is to base it on an existing image.
                For example, if you want to create an image based on <filename>core-image-sato</filename>
                but add the additional package <filename>strace</filename> to the image,
                copy the <filename>meta/recipes-sato/images/core-image-sato.bb</filename> to a
                new <filename>.bb</filename> and add the following line to the end of the copy:
                <literallayout class='monospaced'>
     IMAGE_INSTALL += "strace"
                </literallayout>
            </para>
        </section>

        <section id='usingpoky-extend-customimage-customtasks'>
            <title>Customizing Images Using Custom Package Groups</title>

            <para>
                For complex custom images, the best approach for customizing
                an image is to create a custom package group recipe that is
                used to build the image or images.
                A good example of a package group recipe is
                <filename>meta/recipes-core/packagegroups/packagegroup-base.bb</filename>.
            </para>

            <para>
                If you examine that recipe, you see that the
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'>PACKAGES</ulink></filename>
                variable lists the package group packages to produce.
                The <filename>inherit packagegroup</filename> statement
                sets appropriate default values and automatically adds
                <filename>-dev</filename>, <filename>-dbg</filename>, and
                <filename>-ptest</filename> complementary packages for each
                package specified in the <filename>PACKAGES</filename>
                statement.
                <note>
                    The <filename>inherit packages</filename> should be
                    located near the top of the recipe, certainly before
                    the <filename>PACKAGES</filename> statement.
                </note>
            </para>

            <para>
                For each package you specify in <filename>PACKAGES</filename>,
                you can use
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'>RDEPENDS</ulink></filename>
                and
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'>RRECOMMENDS</ulink></filename>
                entries to provide a list of packages the parent task package
                should contain.
                You can see examples of these further down in the
                <filename>packagegroup-base.bb</filename> recipe.
            </para>

            <para>
                Here is a short, fabricated example showing the same basic
                pieces:
                <literallayout class='monospaced'>
     DESCRIPTION = "My Custom Package Groups"

     inherit packagegroup

     PACKAGES = "\
         packagegroup-custom-apps \
         packagegroup-custom-tools \
         "

     RDEPENDS_packagegroup-custom-apps = "\
         dropbear \
         portmap \
         psplash"

     RDEPENDS_packagegroup-custom-tools = "\
         oprofile \
         oprofileui-server \
         lttng-tools"

     RRECOMMENDS_packagegroup-custom-tools = "\
         kernel-module-oprofile"
                </literallayout>
            </para>

            <para>
                In the previous example, two package group packages are created with their dependencies and their
                recommended package dependencies listed: <filename>packagegroup-custom-apps</filename>, and
                <filename>packagegroup-custom-tools</filename>.
                To build an image using these package group packages, you need to add
                <filename>packagegroup-custom-apps</filename> and/or
                <filename>packagegroup-custom-tools</filename> to
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'>IMAGE_INSTALL</ulink></filename>.
                For other forms of image dependencies see the other areas of this section.
            </para>
        </section>

        <section id='usingpoky-extend-customimage-image-name'>
            <title>Customizing an Image Hostname</title>

            <para>
                By default, the configured hostname (i.e.
                <filename>/etc/hostname</filename>) in an image is the
                same as the machine name.
                For example, if
                <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
                equals "qemux86", the configured hostname written to
                <filename>/etc/hostname</filename> is "qemux86".
            </para>

            <para>
                You can customize this name by altering the value of the
                "hostname" variable in the
                <filename>base-files</filename> recipe using either
                an append file or a configuration file.
                Use the following in an append file:
                <literallayout class='monospaced'>
     hostname="myhostname"
                </literallayout>
                Use the following in a configuration file:
                <literallayout class='monospaced'>
     hostname_pn-base-files = "myhostname"
                </literallayout>
            </para>

            <para>
                Changing the default value of the variable "hostname" can be
                useful in certain situations.
                For example, suppose you need to do extensive testing on an
                image and you would like to easily identify the image
                under test from existing images with typical default
                hostnames.
                In this situation, you could change the default hostname to
                "testme", which results in all the images using the name
                "testme".
                Once testing is complete and you do not need to rebuild the
                image for test any longer, you can easily reset the default
                hostname.
            </para>

            <para>
                Another point of interest is that if you unset the variable,
                the image will have no default hostname in the filesystem.
                Here is an example that unsets the variable in a
                configuration file:
                <literallayout class='monospaced'>
     hostname_pn-base-files = ""
                </literallayout>
                Having no default hostname in the filesystem is suitable for
                environments that use dynamic hostnames such as virtual
                machines.
            </para>
        </section>
    </section>

    <section id='new-recipe-writing-a-new-recipe'>
        <title>Writing a New Recipe</title>

        <para>
            Recipes (<filename>.bb</filename> files) are fundamental components
            in the Yocto Project environment.
            Each software component built by the OpenEmbedded build system
            requires a recipe to define the component.
            This section describes how to create, write, and test a new
            recipe.
            <note>
                For information on variables that are useful for recipes and
                for information about recipe naming issues, see the
                "<ulink url='&YOCTO_DOCS_REF_URL;#ref-varlocality-recipe-required'>Required</ulink>"
                section of the Yocto Project Reference Manual.
            </note>
        </para>

        <section id='new-recipe-overview'>
            <title>Overview</title>

            <para>
                The following figure shows the basic process for creating a
                new recipe.
                The remainder of the section provides details for the steps.
                <imagedata fileref="figures/recipe-workflow.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>
        </section>

        <section id='new-recipe-locate-or-automatically-create-a-base-recipe'>
            <title>Locate or Automatically Create a Base Recipe</title>

            <para>
                You can always write a recipe from scratch.
                However, three choices exist that can help you quickly get a
                start on a new recipe:
                <itemizedlist>
                    <listitem><para>
                        <emphasis><filename>devtool add</filename>:</emphasis>
                        A command that assists in creating a recipe and
                        an environment conducive to development.
                        </para></listitem>
                    <listitem><para>
                        <emphasis><filename>recipetool create</filename>:</emphasis>
                        A command provided by the Yocto Project that automates
                        creation of a base recipe based on the source
                        files.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Existing Recipes:</emphasis>
                        Location and modification of an existing recipe that is
                        similar in function to the recipe you need.
                        </para></listitem>
                </itemizedlist>
                <note>
                    For information on recipe syntax, see the
                    "<link linkend='recipe-syntax'>Recipe Syntax</link>"
                    section.
                </note>
            </para>

            <section id='new-recipe-creating-the-base-recipe-using-devtool'>
                <title>Creating the Base Recipe Using <filename>devtool add</filename></title>

                <para>
                    The <filename>devtool add</filename> command uses the same
                    logic for auto-creating the recipe as
                    <filename>recipetool create</filename>, which is listed
                    below.
                    Additionally, however, <filename>devtool add</filename>
                    sets up an environment that makes it easy for you to
                    patch the source and to make changes to the recipe as
                    is often necessary when adding a recipe to build a new
                    piece of software to be included in a build.
                </para>

                <para>
                    You can find a complete description of the
                    <filename>devtool add</filename> command in the
                    "<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-a-closer-look-at-devtool-add'>A Closer Look at <filename>devtool</filename> add</ulink>"
                    section in the Yocto Project Application Development
                    and the Extensible Software Development Kit (eSDK) manual.
                </para>
            </section>

            <section id='new-recipe-creating-the-base-recipe-using-recipetool'>
                <title>Creating the Base Recipe Using <filename>recipetool create</filename></title>

                <para>
                    <filename>recipetool create</filename> automates creation
                    of a base recipe given a set of source code files.
                    As long as you can extract or point to the source files,
                    the tool will construct a recipe and automatically
                    configure all pre-build information into the recipe.
                    For example, suppose you have an application that builds
                    using Autotools.
                    Creating the base recipe using
                    <filename>recipetool</filename> results in a recipe
                    that has the pre-build dependencies, license requirements,
                    and checksums configured.
                </para>

                <para>
                    To run the tool, you just need to be in your
                    <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
                    and have sourced the build environment setup script
                    (i.e.
                    <ulink url='&YOCTO_DOCS_REF_URL;#structure-core-script'><filename>oe-init-build-env</filename></ulink>).
                    To get help on the tool, use the following command:
                    <literallayout class='monospaced'>
     $ recipetool -h
     NOTE: Starting bitbake server...
     usage: recipetool [-d] [-q] [--color COLOR] [-h] &lt;subcommand&gt; ...

     OpenEmbedded recipe tool

     options:
       -d, --debug     Enable debug output
       -q, --quiet     Print only errors
       --color COLOR   Colorize output (where COLOR is auto, always, never)
       -h, --help      show this help message and exit

     subcommands:
       create          Create a new recipe
       newappend       Create a bbappend for the specified target in the specified
                       layer
       setvar          Set a variable within a recipe
       appendfile      Create/update a bbappend to replace a target file
       appendsrcfiles  Create/update a bbappend to add or replace source files
       appendsrcfile   Create/update a bbappend to add or replace a source file
     Use recipetool &lt;subcommand&gt; --help to get help on a specific command
                    </literallayout>
                </para>

                <para>
                    Running
                    <filename>recipetool create -o</filename>&nbsp;<replaceable>OUTFILE</replaceable>
                    creates the base recipe and locates it properly in the
                    layer that contains your source files.
                    Following are some syntax examples:
                </para>

                <para>
                    Use this syntax to generate a recipe based on
                    <replaceable>source</replaceable>.
                    Once generated, the recipe resides in the existing source
                    code layer:
                    <literallayout class='monospaced'>
     recipetool create -o <replaceable>OUTFILE</replaceable>&nbsp;<replaceable>source</replaceable>
                    </literallayout>
                    Use this syntax to generate a recipe using code that you
                    extract from <replaceable>source</replaceable>.
                    The extracted code is placed in its own layer defined
                    by <replaceable>EXTERNALSRC</replaceable>.
                    <literallayout class='monospaced'>
     recipetool create -o <replaceable>OUTFILE</replaceable> -x <replaceable>EXTERNALSRC</replaceable> <replaceable>source</replaceable>
                    </literallayout>
                    Use this syntax to generate a recipe based on
                    <replaceable>source</replaceable>.
                    The options direct <filename>recipetool</filename> to
                    generate debugging information.
                    Once generated, the recipe resides in the existing source
                    code layer:
                    <literallayout class='monospaced'>
     recipetool create -d -o <replaceable>OUTFILE</replaceable> <replaceable>source</replaceable>
                    </literallayout>
                </para>
            </section>

            <section id='new-recipe-locating-and-using-a-similar-recipe'>
                <title>Locating and Using a Similar Recipe</title>

                <para>
                    Before writing a recipe from scratch, it is often useful to
                    discover whether someone else has already written one that
                    meets (or comes close to meeting) your needs.
                    The Yocto Project and OpenEmbedded communities maintain many
                    recipes that might be candidates for what you are doing.
                    You can find a good central index of these recipes in the
                    <ulink url='http://layers.openembedded.org'>OpenEmbedded Layer Index</ulink>.
                </para>

                <para>
                    Working from an existing recipe or a skeleton recipe is the
                    best way to get started.
                    Here are some points on both methods:
                    <itemizedlist>
                        <listitem><para><emphasis>Locate and modify a recipe that
                            is close to what you want to do:</emphasis>
                            This method works when you are familiar with the
                            current recipe space.
                            The method does not work so well for those new to
                            the Yocto Project or writing recipes.</para>
                            <para>Some risks associated with this method are
                            using a recipe that has areas totally unrelated to
                            what you are trying to accomplish with your recipe,
                            not recognizing areas of the recipe that you might
                            have to add from scratch, and so forth.
                            All these risks stem from unfamiliarity with the
                            existing recipe space.</para></listitem>
                        <listitem><para><emphasis>Use and modify the following
                            skeleton recipe:</emphasis>
                            If for some reason you do not want to use
                            <filename>recipetool</filename> and you cannot
                            find an existing recipe that is close to meeting
                            your needs, you can use the following structure to
                            provide the fundamental areas of a new recipe.
                            <literallayout class='monospaced'>
     DESCRIPTION = ""
     HOMEPAGE = ""
     LICENSE = ""
     SECTION = ""
     DEPENDS = ""
     LIC_FILES_CHKSUM = ""

     SRC_URI = ""
                            </literallayout>
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>
        </section>

        <section id='new-recipe-storing-and-naming-the-recipe'>
            <title>Storing and Naming the Recipe</title>

            <para>
                Once you have your base recipe, you should put it in your
                own layer and name it appropriately.
                Locating it correctly ensures that the OpenEmbedded build
                system can find it when you use BitBake to process the
                recipe.
            </para>

            <itemizedlist>
                <listitem><para><emphasis>Storing Your Recipe:</emphasis>
                    The OpenEmbedded build system locates your recipe
                    through the layer's <filename>conf/layer.conf</filename>
                    file and the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-BBFILES'><filename>BBFILES</filename></ulink>
                    variable.
                    This variable sets up a path from which the build system can
                    locate recipes.
                    Here is the typical use:
                    <literallayout class='monospaced'>
     BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
                 ${LAYERDIR}/recipes-*/*/*.bbappend"
                    </literallayout>
                    Consequently, you need to be sure you locate your new recipe
                    inside your layer such that it can be found.</para>
                    <para>You can find more information on how layers are
                    structured in the
                    "<link linkend='understanding-and-creating-layers'>Understanding and Creating Layers</link>"
                    section.</para></listitem>
                <listitem><para><emphasis>Naming Your Recipe:</emphasis>
                    When you name your recipe, you need to follow this naming
                    convention:
                    <literallayout class='monospaced'>
     <replaceable>basename</replaceable>_<replaceable>version</replaceable>.bb
                    </literallayout>
                    Use lower-cased characters and do not include the reserved
                    suffixes <filename>-native</filename>,
                    <filename>-cross</filename>, <filename>-initial</filename>,
                    or <filename>-dev</filename> casually (i.e. do not use them
                    as part of your recipe name unless the string applies).
                    Here are some examples:
                    <literallayout class='monospaced'>
     cups_1.7.0.bb
     gawk_4.0.2.bb
     irssi_0.8.16-rc1.bb
                    </literallayout></para></listitem>
            </itemizedlist>
        </section>

        <section id='new-recipe-running-a-build-on-the-recipe'>
            <title>Running a Build on the Recipe</title>

            <para>
                Creating a new recipe is usually an iterative process that
                requires using BitBake to process the recipe multiple times in
                order to progressively discover and add information to the
                recipe file.
            </para>

            <para>
                Assuming you have sourced the build environment setup script (i.e.
                <ulink url='&YOCTO_DOCS_REF_URL;#structure-core-script'><filename>&OE_INIT_FILE;</filename></ulink>)
                and you are in the
                <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>,
                use BitBake to process your recipe.
                All you need to provide is the
                <filename><replaceable>basename</replaceable></filename> of the recipe as described
                in the previous section:
                <literallayout class='monospaced'>
     $ bitbake <replaceable>basename</replaceable>
                </literallayout>

            </para>

            <para>
                During the build, the OpenEmbedded build system creates a
                temporary work directory for each recipe
                (<filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}</filename>)
                where it keeps extracted source files, log files, intermediate
                compilation and packaging files, and so forth.
            </para>

            <para>
                The path to the per-recipe temporary work directory depends
                on the context in which it is being built.
                The quickest way to find this path is to have BitBake return it
                by running the following:
                <literallayout class='monospaced'>
     $ bitbake -e <replaceable>basename</replaceable> | grep ^WORKDIR=
                </literallayout>
                As an example, assume a Source Directory top-level folder named
                <filename>poky</filename>, a default Build Directory at
                <filename>poky/build</filename>, and a
                <filename>qemux86-poky-linux</filename> machine target system.
                Furthermore, suppose your recipe is named
                <filename>foo_1.3.0.bb</filename>.
                In this case, the work directory the build system uses to
                build the package would be as follows:
                <literallayout class='monospaced'>
     poky/build/tmp/work/qemux86-poky-linux/foo/1.3.0-r0
                </literallayout>
                Inside this directory you can find sub-directories such as
                <filename>image</filename>, <filename>packages-split</filename>,
                and <filename>temp</filename>.
                After the build, you can examine these to determine how well
                the build went.
                <note>
                    You can find log files for each task in the recipe's
                    <filename>temp</filename> directory (e.g.
                    <filename>poky/build/tmp/work/qemux86-poky-linux/foo/1.3.0-r0/temp</filename>).
                    Log files are named <filename>log.<replaceable>taskname</replaceable></filename>
                    (e.g. <filename>log.do_configure</filename>,
                    <filename>log.do_fetch</filename>, and
                    <filename>log.do_compile</filename>).
                </note>
            </para>

            <para>
                You can find more information about the build process in
                "<ulink url='&YOCTO_DOCS_OM_URL;#overview-development-environment'>The Yocto Project Development Environment</ulink>"
                chapter of the Yocto Project Overview and Concepts Manual.
            </para>
        </section>

        <section id='new-recipe-fetching-code'>
            <title>Fetching Code</title>

            <para>
                The first thing your recipe must do is specify how to fetch
                the source files.
                Fetching is controlled mainly through the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                variable.
                Your recipe must have a <filename>SRC_URI</filename> variable
                that points to where the source is located.
                For a graphical representation of source locations, see the
                "<ulink url='&YOCTO_DOCS_OM_URL;#sources-dev-environment'>Sources</ulink>"
                section in the Yocto Project Overview and Concepts Manual.
            </para>

            <para>
                The
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-fetch'><filename>do_fetch</filename></ulink>
                task uses the prefix of each entry in the
                <filename>SRC_URI</filename> variable value to determine which
                fetcher to use to get your source files.
                It is the <filename>SRC_URI</filename> variable that triggers
                the fetcher.
                The
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-patch'><filename>do_patch</filename></ulink>
                task uses the variable after source is fetched to apply
                patches.
                The OpenEmbedded build system uses
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESOVERRIDES'><filename>FILESOVERRIDES</filename></ulink>
                for scanning directory locations for local files in
                <filename>SRC_URI</filename>.
            </para>

            <para>
                The <filename>SRC_URI</filename> variable in your recipe must
                define each unique location for your source files.
                It is good practice to not hard-code pathnames in an URL used
                in <filename>SRC_URI</filename>.
                Rather than hard-code these paths, use
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink><filename>}</filename>,
                which causes the fetch process to use the version specified in
                the recipe filename.
                Specifying the version in this manner means that upgrading the
                recipe to a future version is as simple as renaming the recipe
                to match the new version.
            </para>

            <para>
                Here is a simple example from the
                <filename>meta/recipes-devtools/cdrtools/cdrtools-native_3.01a20.bb</filename>
                recipe where the source comes from a single tarball.
                Notice the use of the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
                variable:
                <literallayout class='monospaced'>
     SRC_URI = "ftp://ftp.berlios.de/pub/cdrecord/alpha/cdrtools-${PV}.tar.bz2"
                </literallayout>
            </para>

            <para>
                Files mentioned in <filename>SRC_URI</filename> whose names end
                in a typical archive extension (e.g. <filename>.tar</filename>,
                <filename>.tar.gz</filename>, <filename>.tar.bz2</filename>,
                <filename>.zip</filename>, and so forth), are automatically
                extracted during the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-unpack'><filename>do_unpack</filename></ulink>
                task.
                For another example that specifies these types of files, see
                the
                "<link linkend='new-recipe-autotooled-package'>Autotooled Package</link>"
                section.
            </para>

            <para>
                Another way of specifying source is from an SCM.
                For Git repositories, you must specify
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>
                and you should specify
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
                to include the revision with
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRCPV'><filename>SRCPV</filename></ulink>.
                Here is an example from the recipe
                <filename>meta/recipes-kernel/blktrace/blktrace_git.bb</filename>:
                <literallayout class='monospaced'>
     SRCREV = "d6918c8832793b4205ed3bfede78c2f915c23385"

     PR = "r6"
     PV = "1.0.5+git${SRCPV}"

     SRC_URI = "git://git.kernel.dk/blktrace.git \
                file://ldflags.patch"
                </literallayout>
            </para>

            <para>
                If your <filename>SRC_URI</filename> statement includes
                URLs pointing to individual files fetched from a remote server
                other than a version control system, BitBake attempts to
                verify the files against checksums defined in your recipe to
                ensure they have not been tampered with or otherwise modified
                since the recipe was written.
                Two checksums are used:
                <filename>SRC_URI[md5sum]</filename> and
                <filename>SRC_URI[sha256sum]</filename>.
            </para>

            <para>
                If your <filename>SRC_URI</filename> variable points to
                more than a single URL (excluding SCM URLs), you need to
                provide the <filename>md5</filename> and
                <filename>sha256</filename> checksums for each URL.
                For these cases, you provide a name for each URL as part of
                the <filename>SRC_URI</filename> and then reference that name
                in the subsequent checksum statements.
                Here is an example:
                <literallayout class='monospaced'>
     SRC_URI = "${DEBIAN_MIRROR}/main/a/apmd/apmd_3.2.2.orig.tar.gz;name=tarball \
                ${DEBIAN_MIRROR}/main/a/apmd/apmd_${PV}.diff.gz;name=patch"

     SRC_URI[tarball.md5sum] = "b1e6309e8331e0f4e6efd311c2d97fa8"
     SRC_URI[tarball.sha256sum] = "7f7d9f60b7766b852881d40b8ff91d8e39fccb0d1d913102a5c75a2dbb52332d"

     SRC_URI[patch.md5sum] = "57e1b689264ea80f78353519eece0c92"
     SRC_URI[patch.sha256sum] = "7905ff96be93d725544d0040e425c42f9c05580db3c272f11cff75b9aa89d430"
                </literallayout>
            </para>

            <para>
                Proper values for <filename>md5</filename> and
                <filename>sha256</filename> checksums might be available
                with other signatures on the download page for the upstream
                source (e.g. <filename>md5</filename>,
                <filename>sha1</filename>, <filename>sha256</filename>,
                <filename>GPG</filename>, and so forth).
                Because the OpenEmbedded build system only deals with
                <filename>sha256sum</filename> and <filename>md5sum</filename>,
                you should verify all the signatures you find by hand.
            </para>

            <para>
                If no <filename>SRC_URI</filename> checksums are specified
                when you attempt to build the recipe, or you provide an
                incorrect checksum, the build will produce an error for each
                missing or incorrect checksum.
                As part of the error message, the build system provides
                the checksum string corresponding to the fetched file.
                Once you have the correct checksums, you can copy and paste
                them into your recipe and then run the build again to continue.
                <note>
                    As mentioned, if the upstream source provides signatures
                    for verifying the downloaded source code, you should
                    verify those manually before setting the checksum values
                    in the recipe and continuing with the build.
                </note>
            </para>

            <para>
                This final example is a bit more complicated and is from the
                <filename>meta/recipes-sato/rxvt-unicode/rxvt-unicode_9.20.bb</filename>
                recipe.
                The example's <filename>SRC_URI</filename> statement identifies
                multiple files as the source files for the recipe: a tarball, a
                patch file, a desktop file, and an icon.
                <literallayout class='monospaced'>
     SRC_URI = "http://dist.schmorp.de/rxvt-unicode/Attic/rxvt-unicode-${PV}.tar.bz2 \
                file://xwc.patch \
                file://rxvt.desktop \
                file://rxvt.png"
                </literallayout>
            </para>

            <para>
                When you specify local files using the
                <filename>file://</filename> URI protocol, the build system
                fetches files from the local machine.
                The path is relative to the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-FILESPATH'><filename>FILESPATH</filename></ulink>
                variable and searches specific directories in a certain order:
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BP'><filename>BP</filename></ulink><filename>}</filename>,
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BPN'><filename>BPN</filename></ulink><filename>}</filename>,
                and <filename>files</filename>.
                The directories are assumed to be subdirectories of the
                directory in which the recipe or append file resides.
                For another example that specifies these types of files, see the
                "<link linkend='new-recipe-single-c-file-package-hello-world'>Single .c File Package (Hello World!)</link>"
                section.
            </para>

            <para>
                The previous example also specifies a patch file.
                Patch files are files whose names usually end in
                <filename>.patch</filename> or <filename>.diff</filename> but
                can end with compressed suffixes such as
                <filename>diff.gz</filename> and
                <filename>patch.bz2</filename>, for example.
                The build system automatically applies patches as described
                in the
                "<link linkend='new-recipe-patching-code'>Patching Code</link>" section.
            </para>
        </section>

        <section id='new-recipe-unpacking-code'>
            <title>Unpacking Code</title>

            <para>
                During the build, the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-unpack'><filename>do_unpack</filename></ulink>
                task unpacks the source with
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink><filename>}</filename>
                pointing to where it is unpacked.
            </para>

            <para>
                If you are fetching your source files from an upstream source
                archived tarball and the tarball's internal structure matches
                the common convention of a top-level subdirectory named
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-BPN'><filename>BPN</filename></ulink><filename>}-${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink><filename>}</filename>,
                then you do not need to set <filename>S</filename>.
                However, if <filename>SRC_URI</filename> specifies to fetch
                source from an archive that does not use this convention,
                or from an SCM like Git or Subversion, your recipe needs to
                define <filename>S</filename>.
            </para>

            <para>
                If processing your recipe using BitBake successfully unpacks
                the source files, you need to be sure that the directory
                pointed to by <filename>${S}</filename> matches the structure
                of the source.
            </para>
        </section>

        <section id='new-recipe-patching-code'>
            <title>Patching Code</title>

            <para>
                Sometimes it is necessary to patch code after it has been
                fetched.
                Any files mentioned in <filename>SRC_URI</filename> whose
                names end in <filename>.patch</filename> or
                <filename>.diff</filename> or compressed versions of these
                suffixes (e.g. <filename>diff.gz</filename> are treated as
                patches.
                The
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-patch'><filename>do_patch</filename></ulink>
                task automatically applies these patches.
            </para>

            <para>
                The build system should be able to apply patches with the "-p1"
                option (i.e. one directory level in the path will be stripped
                off).
                If your patch needs to have more directory levels stripped off,
                specify the number of levels using the "striplevel" option in
                the <filename>SRC_URI</filename> entry for the patch.
                Alternatively, if your patch needs to be applied in a specific
                subdirectory that is not specified in the patch file, use the
                "patchdir" option in the entry.
            </para>

            <para>
                As with all local files referenced in
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                using <filename>file://</filename>, you should place
                patch files in a directory next to the recipe either
                named the same as the base name of the recipe
                (<ulink url='&YOCTO_DOCS_REF_URL;#var-BP'><filename>BP</filename></ulink>
                and
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BPN'><filename>BPN</filename></ulink>)
                or "files".
            </para>
        </section>

        <section id='new-recipe-licensing'>
            <title>Licensing</title>

            <para>
                Your recipe needs to have both the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE'><filename>LICENSE</filename></ulink>
                and
                <ulink url='&YOCTO_DOCS_REF_URL;#var-LIC_FILES_CHKSUM'><filename>LIC_FILES_CHKSUM</filename></ulink>
                variables:
                <itemizedlist>
                    <listitem><para><emphasis><filename>LICENSE</filename>:</emphasis>
                        This variable specifies the license for the software.
                        If you do not know the license under which the software
                        you are building is distributed, you should go to the
                        source code and look for that information.
                        Typical files containing this information include
                        <filename>COPYING</filename>,
                        <filename>LICENSE</filename>, and
                        <filename>README</filename> files.
                        You could also find the information near the top of
                        a source file.
                        For example, given a piece of software licensed under
                        the GNU General Public License version 2, you would
                        set <filename>LICENSE</filename> as follows:
                        <literallayout class='monospaced'>
     LICENSE = "GPLv2"
                        </literallayout></para>
                        <para>The licenses you specify within
                        <filename>LICENSE</filename> can have any name as long
                        as you do not use spaces, since spaces are used as
                        separators between license names.
                        For standard licenses, use the names of the files in
                        <filename>meta/files/common-licenses/</filename>
                        or the <filename>SPDXLICENSEMAP</filename> flag names
                        defined in <filename>meta/conf/licenses.conf</filename>.
                        </para></listitem>
                    <listitem><para><emphasis><filename>LIC_FILES_CHKSUM</filename>:</emphasis>
                        The OpenEmbedded build system uses this variable to
                        make sure the license text has not changed.
                        If it has, the build produces an error and it affords
                        you the chance to figure it out and correct the problem.
                        </para>
                        <para>You need to specify all applicable licensing
                        files for the software.
                        At the end of the configuration step, the build process
                        will compare the checksums of the files to be sure
                        the text has not changed.
                        Any differences result in an error with the message
                        containing the current checksum.
                        For more explanation and examples of how to set the
                        <filename>LIC_FILES_CHKSUM</filename> variable, see the
                        "<link link='usingpoky-configuring-LIC_FILES_CHKSUM'>Tracking License Changes</link>"
                        section.</para>

                        <para>To determine the correct checksum string, you
                        can list the appropriate files in the
                        <filename>LIC_FILES_CHKSUM</filename> variable with
                        incorrect md5 strings, attempt to build the software,
                        and then note the resulting error messages that will
                        report the correct md5 strings.
                        See the
                        "<link linkend='new-recipe-fetching-code'>Fetching Code</link>"
                        section for additional information.
                    </para>

                    <para>
                        Here is an example that assumes the software has a
                        <filename>COPYING</filename> file:
                        <literallayout class='monospaced'>
     LIC_FILES_CHKSUM = "file://COPYING;md5=xxx"
                        </literallayout>
                        When you try to build the software, the build system
                        will produce an error and give you the correct string
                        that you can substitute into the recipe file for a
                        subsequent build.
                        </para></listitem>
                </itemizedlist>
            </para>

<!--

            <para>
                For trying this out I created a new recipe named
                <filename>htop_1.0.2.bb</filename> and put it in
                <filename>poky/meta/recipes-extended/htop</filename>.
                There are two license type statements in my very simple
                recipe:
                <literallayout class='monospaced'>
     LICENSE = ""

     LIC_FILES_CHKSUM = ""

     SRC_URI[md5sum] = ""
     SRC_URI[sha256sum] = ""
                </literallayout>
                Evidently, you need to run a <filename>bitbake -c cleanall htop</filename>.
                Next, you delete or comment out the two <filename>SRC_URI</filename>
                lines at the end and then attempt to build the software with
                <filename>bitbake htop</filename>.
                Doing so causes BitBake to report some errors and and give
                you the actual strings you need for the last two
                <filename>SRC_URI</filename> lines.
                Prior to this, you have to dig around in the home page of the
                source for <filename>htop</filename> and determine that the
                software is released under GPLv2.
                You can provide that in the <filename>LICENSE</filename>
                statement.
                Now you edit your recipe to have those two strings for
                the <filename>SRC_URI</filename> statements:
                <literallayout class='monospaced'>
     LICENSE = "GPLv2"

     LIC_FILES_CHKSUM = ""

     SRC_URI = "${SOURCEFORGE_MIRROR}/htop/htop-${PV}.tar.gz"
     SRC_URI[md5sum] = "0d01cca8df3349c74569cefebbd9919e"
     SRC_URI[sha256sum] = "ee60657b044ece0df096c053060df7abf3cce3a568ab34d260049e6a37ccd8a1"
                </literallayout>
                At this point, you can build the software again using the
                <filename>bitbake htop</filename> command.
                There is just a set of errors now associated with the
                empty <filename>LIC_FILES_CHKSUM</filename> variable now.
            </para>
-->

        </section>

        <section id='new-dependencies'>
            <title>Dependencies</title>

            <para>
                Most software packages have a short list of other packages
                that they require, which are called dependencies.
                These dependencies fall into two main categories: build-time
                dependencies, which are required when the software is built;
                and runtime dependencies, which are required to be installed
                on the target in order for the software to run.
            </para>

            <para>
                Within a recipe, you specify build-time dependencies using the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>
                variable.
                Although nuances exist, items specified in
                <filename>DEPENDS</filename> should be names of other recipes.
                It is important that you specify all build-time dependencies
                explicitly.
                If you do not, due to the parallel nature of BitBake's
                execution, you can end up with a race condition where the
                dependency is present for one task of a recipe (e.g.
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-configure'><filename>do_configure</filename></ulink>)
                and then gone when the next task runs (e.g.
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-compile'><filename>do_compile</filename></ulink>).
            </para>

            <para>
                Another consideration is that configure scripts might
                automatically check for optional dependencies and enable
                corresponding functionality if those dependencies are found.
                This behavior means that to ensure deterministic results and
                thus avoid more race conditions, you need to either explicitly
                specify these dependencies as well, or tell the configure
                script explicitly to disable the functionality.
                If you wish to make a recipe that is more generally useful
                (e.g. publish the recipe in a layer for others to use),
                instead of hard-disabling the functionality, you can use the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGECONFIG'><filename>PACKAGECONFIG</filename></ulink>
                variable to allow functionality and the corresponding
                dependencies to be enabled and disabled easily by other
                users of the recipe.
            </para>

            <para>
                Similar to build-time dependencies, you specify runtime
                dependencies through a variable -
                <ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>,
                which is package-specific.
                All variables that are package-specific need to have the name
                of the package added to the end as an override.
                Since the main package for a recipe has the same name as the
                recipe, and the recipe's name can be found through the
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink><filename>}</filename>
                variable, then you specify the dependencies for the main
                package by setting <filename>RDEPENDS_${PN}</filename>.
                If the package were named <filename>${PN}-tools</filename>,
                then you would set <filename>RDEPENDS_${PN}-tools</filename>,
                and so forth.
            </para>

            <para>
                Some runtime dependencies will be set automatically at
                packaging time.
                These dependencies include any shared library dependencies
                (i.e. if a package "example" contains "libexample" and
                another package "mypackage" contains a binary that links to
                "libexample" then the OpenEmbedded build system will
                automatically add a runtime dependency to "mypackage" on
                "example").
                See the
                "<ulink url='&YOCTO_DOCS_OM_URL;#automatically-added-runtime-dependencies'>Automatically Added Runtime Dependencies</ulink>"
                section in the Yocto Project Overview and Concepts Manual for
                further details.
            </para>
        </section>

        <section id='new-recipe-configuring-the-recipe'>
            <title>Configuring the Recipe</title>

            <para>
                Most software provides some means of setting build-time
                configuration options before compilation.
                Typically, setting these options is accomplished by running a
                configure script with some options, or by modifying a build
                configuration file.
                <note>
                    As of Yocto Project Release 1.7, some of the core recipes
                    that package binary configuration scripts now disable the
                    scripts due to the scripts previously requiring error-prone
                    path substitution.
                    The OpenEmbedded build system uses
                    <filename>pkg-config</filename> now, which is much more
                    robust.
                    You can find a list of the <filename>*-config</filename>
                    scripts that are disabled list in the
                    "<ulink url='&YOCTO_DOCS_REF_URL;#migration-1.7-binary-configuration-scripts-disabled'>Binary Configuration Scripts Disabled</ulink>"
                    section in the Yocto Project Reference Manual.
                </note>
            </para>

            <para>
                A major part of build-time configuration is about checking for
                build-time dependencies and possibly enabling optional
                functionality as a result.
                You need to specify any build-time dependencies for the
                software you are building in your recipe's
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>
                value, in terms of other recipes that satisfy those
                dependencies.
                You can often find build-time or runtime
                dependencies described in the software's documentation.
            </para>

            <para>
                The following list provides configuration items of note based
                on how your software is built:
                <itemizedlist>
                    <listitem><para><emphasis>Autotools:</emphasis>
                        If your source files have a
                        <filename>configure.ac</filename> file, then your
                        software is built using Autotools.
                        If this is the case, you just need to worry about
                        modifying the configuration.</para>
                        <para>When using Autotools, your recipe needs to inherit
                        the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-autotools'><filename>autotools</filename></ulink>
                        class and your recipe does not have to contain a
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-configure'><filename>do_configure</filename></ulink>
                        task.
                        However, you might still want to make some adjustments.
                        For example, you can set
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTRA_OECONF'><filename>EXTRA_OECONF</filename></ulink>
                        or
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGECONFIG_CONFARGS'><filename>PACKAGECONFIG_CONFARGS</filename></ulink>
                        to pass any needed configure options that are specific
                        to the recipe.</para></listitem>
                    <listitem><para><emphasis>CMake:</emphasis>
                        If your source files have a
                        <filename>CMakeLists.txt</filename> file, then your
                        software is built using CMake.
                        If this is the case, you just need to worry about
                        modifying the configuration.</para>
                        <para>When you use CMake, your recipe needs to inherit
                        the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-cmake'><filename>cmake</filename></ulink>
                        class and your recipe does not have to contain a
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-configure'><filename>do_configure</filename></ulink>
                        task.
                        You can make some adjustments by setting
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTRA_OECMAKE'><filename>EXTRA_OECMAKE</filename></ulink>
                        to pass any needed configure options that are specific
                        to the recipe.</para></listitem>
                    <listitem><para><emphasis>Other:</emphasis>
                        If your source files do not have a
                        <filename>configure.ac</filename> or
                        <filename>CMakeLists.txt</filename> file, then your
                        software is built using some method other than Autotools
                        or CMake.
                        If this is the case, you normally need to provide a
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-configure'><filename>do_configure</filename></ulink>
                        task in your recipe
                        unless, of course, there is nothing to configure.
                        </para>
                        <para>Even if your software is not being built by
                        Autotools or CMake, you still might not need to deal
                        with any configuration issues.
                        You need to determine if configuration is even a required step.
                        You might need to modify a Makefile or some configuration file
                        used for the build to specify necessary build options.
                        Or, perhaps you might need to run a provided, custom
                        configure script with the appropriate options.</para>
                        <para>For the case involving a custom configure
                        script, you would run
                        <filename>./configure --help</filename> and look for
                        the options you need to set.</para></listitem>
                </itemizedlist>
            </para>

            <para>
                Once configuration succeeds, it is always good practice to
                look at the <filename>log.do_configure</filename> file to
                ensure that the appropriate options have been enabled and no
                additional build-time dependencies need to be added to
                <filename>DEPENDS</filename>.
                For example, if the configure script reports that it found
                something not mentioned in <filename>DEPENDS</filename>, or
                that it did not find something that it needed for some
                desired optional functionality, then you would need to add
                those to <filename>DEPENDS</filename>.
                Looking at the log might also reveal items being checked for,
                enabled, or both that you do not want, or items not being found
                that are in <filename>DEPENDS</filename>, in which case
                you would need to look at passing extra options to the
                configure script as needed.
                For reference information on configure options specific to the
                software you are building, you can consult the output of the
                <filename>./configure --help</filename> command within
                <filename>${S}</filename> or consult the software's upstream
                documentation.
            </para>
        </section>

        <section id='new-recipe-using-headers-to-interface-with-devices'>
            <title>Using Headers to Interface with Devices</title>

            <para>
                If your recipe builds an application that needs to
                communicate with some device or needs an API into a custom
                kernel, you will need to provide appropriate header files.
                Under no circumstances should you ever modify the existing
                <filename>meta/recipes-kernel/linux-libc-headers/linux-libc-headers.inc</filename>
                file.
                These headers are used to build <filename>libc</filename> and
                must not be compromised with custom or machine-specific
                header information.
                If you customize <filename>libc</filename> through modified
                headers all other applications that use
                <filename>libc</filename> thus become affected.
                <note><title>Warning</title>
                    Never copy and customize the <filename>libc</filename>
                    header file (i.e.
                    <filename>meta/recipes-kernel/linux-libc-headers/linux-libc-headers.inc</filename>).
                </note>
                The correct way to interface to a device or custom kernel is
                to use a separate package that provides the additional headers
                for the driver or other unique interfaces.
                When doing so, your application also becomes responsible for
                creating a dependency on that specific provider.
            </para>

            <para>
                Consider the following:
                <itemizedlist>
                    <listitem><para>
                        Never modify
                        <filename>linux-libc-headers.inc</filename>.
                        Consider that file to be part of the
                        <filename>libc</filename> system, and not something
                        you use to access the kernel directly.
                        You should access <filename>libc</filename> through
                        specific <filename>libc</filename> calls.
                        </para></listitem>
                    <listitem><para>
                        Applications that must talk directly to devices
                        should either provide necessary headers themselves,
                        or establish a dependency on a special headers package
                        that is specific to that driver.
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                For example, suppose you want to modify an existing header
                that adds I/O control or network support.
                If the modifications are used by a small number programs,
                providing a unique version of a header is easy and has little
                impact.
                When doing so, bear in mind the guidelines in the previous
                list.
                <note>
                    If for some reason your changes need to modify the behavior
                    of the <filename>libc</filename>, and subsequently all
                    other applications on the system, use a
                    <filename>.bbappend</filename> to modify the
                    <filename>linux-kernel-headers.inc</filename> file.
                    However, take care to not make the changes
                    machine specific.
                </note>
            </para>

            <para>
                Consider a case where your kernel is older and you need
                an older <filename>libc</filename> ABI.
                The headers installed by your recipe should still be a
                standard mainline kernel, not your own custom one.
            </para>

            <para>
                When you use custom kernel headers you need to get them from
                <ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_KERNEL_DIR'><filename>STAGING_KERNEL_DIR</filename></ulink>,
                which is the directory with kernel headers that are
                required to build out-of-tree modules.
                Your recipe will also need the following:
                <literallayout class='monospaced'>
     do_configure[depends] += "virtual/kernel:do_shared_workdir"
                </literallayout>
            </para>
        </section>

        <section id='new-recipe-compilation'>
            <title>Compilation</title>

            <para>
                During a build, the <filename>do_compile</filename> task
                happens after source is fetched, unpacked, and configured.
                If the recipe passes through <filename>do_compile</filename>
                successfully, nothing needs to be done.
            </para>

            <para>
                However, if the compile step fails, you need to diagnose the
                failure.
                Here are some common issues that cause failures.
                <note>
                    For cases where improper paths are detected for
                    configuration files or for when libraries/headers cannot
                    be found, be sure you are using the more robust
                    <filename>pkg-config</filename>.
                    See the note in section
                    "<link linkend='new-recipe-configuring-the-recipe'>Configuring the Recipe</link>"
                    for additional information.
                </note>
                <itemizedlist>
                    <listitem><para><emphasis>Parallel build failures:</emphasis>
                        These failures manifest themselves as intermittent
                        errors, or errors reporting that a file or directory
                        that should be created by some other part of the build
                        process could not be found.
                        This type of failure can occur even if, upon inspection,
                        the file or directory does exist after the build has
                        failed, because that part of the build process happened
                        in the wrong order.</para>
                        <para>To fix the problem, you need to either satisfy
                        the missing dependency in the Makefile or whatever
                        script produced the Makefile, or (as a workaround)
                        set
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PARALLEL_MAKE'><filename>PARALLEL_MAKE</filename></ulink>
                        to an empty string:
                        <literallayout class='monospaced'>
     PARALLEL_MAKE = ""
                        </literallayout></para>
                        <para>
                            For information on parallel Makefile issues, see the
                            "<link linkend='debugging-parallel-make-races'>Debugging Parallel Make Races</link>"
                            section.
                            </para></listitem>
                    <listitem><para><emphasis>Improper host path usage:</emphasis>
                        This failure applies to recipes building for the target
                        or <filename>nativesdk</filename> only.
                        The failure occurs when the compilation process uses
                        improper headers, libraries, or other files from the
                        host system when cross-compiling for the target.
                        </para>
                        <para>To fix the problem, examine the
                        <filename>log.do_compile</filename> file to identify
                        the host paths being used (e.g.
                        <filename>/usr/include</filename>,
                        <filename>/usr/lib</filename>, and so forth) and then
                        either add configure options, apply a patch, or do both.
                        </para></listitem>
                    <listitem><para><emphasis>Failure to find required
                        libraries/headers:</emphasis>
                        If a build-time dependency is missing because it has
                        not been declared in
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>,
                        or because the dependency exists but the path used by
                        the build process to find the file is incorrect and the
                        configure step did not detect it, the compilation
                        process could fail.
                        For either of these failures, the compilation process
                        notes that files could not be found.
                        In these cases, you need to go back and add additional
                        options to the configure script as well as possibly
                        add additional build-time dependencies to
                        <filename>DEPENDS</filename>.</para>
                        <para>Occasionally, it is necessary to apply a patch
                        to the source to ensure the correct paths are used.
                        If you need to specify paths to find files staged
                        into the sysroot from other recipes, use the variables
                        that the OpenEmbedded build system provides
                        (e.g.
                        <filename>STAGING_BINDIR</filename>,
                        <filename>STAGING_INCDIR</filename>,
                        <filename>STAGING_DATADIR</filename>, and so forth).
<!--
                        (e.g.
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_BINDIR'><filename>STAGING_BINDIR</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_INCDIR'><filename>STAGING_INCDIR</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_DATADIR'><filename>STAGING_DATADIR</filename></ulink>,
                        and so forth).
-->
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='new-recipe-installing'>
            <title>Installing</title>

            <para>
                During <filename>do_install</filename>, the task copies the
                built files along with their hierarchy to locations that
                would mirror their locations on the target device.
                The installation process copies files from the
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink><filename>}</filename>,
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-B'><filename>B</filename></ulink><filename>}</filename>,
                and
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}</filename>
                directories to the
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-D'><filename>D</filename></ulink><filename>}</filename>
                directory to create the structure as it should appear on the
                target system.
            </para>

            <para>
                How your software is built affects what you must do to be
                sure your software is installed correctly.
                The following list describes what you must do for installation
                depending on the type of build system used by the software
                being built:
                <itemizedlist>
                    <listitem><para><emphasis>Autotools and CMake:</emphasis>
                        If the software your recipe is building uses Autotools
                        or CMake, the OpenEmbedded build
                        system understands how to install the software.
                        Consequently, you do not have to have a
                        <filename>do_install</filename> task as part of your
                        recipe.
                        You just need to make sure the install portion of the
                        build completes with no issues.
                        However, if you wish to install additional files not
                        already being installed by
                        <filename>make install</filename>, you should do this
                        using a <filename>do_install_append</filename> function
                        using the install command as described in
                        the "Manual" bulleted item later in this list.
                        </para></listitem>
                    <listitem><para><emphasis>Other (using
                        <filename>make install</filename>):</emphasis>
                        You need to define a
                        <filename>do_install</filename> function in your
                        recipe.
                        The function should call
                        <filename>oe_runmake install</filename> and will likely
                        need to pass in the destination directory as well.
                        How you pass that path is dependent on how the
                        <filename>Makefile</filename> being run is written
                        (e.g. <filename>DESTDIR=${D}</filename>,
                        <filename>PREFIX=${D}</filename>,
                        <filename>INSTALLROOT=${D}</filename>, and so forth).
                        </para>
                        <para>For an example recipe using
                        <filename>make install</filename>, see the
                        "<link linkend='new-recipe-makefile-based-package'>Makefile-Based Package</link>"
                        section.</para></listitem>
                    <listitem><para><emphasis>Manual:</emphasis>
                        You need to define a
                        <filename>do_install</filename> function in your
                        recipe.
                        The function must first use
                        <filename>install -d</filename> to create the
                        directories under
                        <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-D'><filename>D</filename></ulink><filename>}</filename>.
                        Once the directories exist, your function can use
                        <filename>install</filename> to manually install the
                        built software into the directories.</para>
                        <para>You can find more information on
                        <filename>install</filename> at
                        <ulink url='http://www.gnu.org/software/coreutils/manual/html_node/install-invocation.html'></ulink>.
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                For the scenarios that do not use Autotools or
                CMake, you need to track the installation
                and diagnose and fix any issues until everything installs
                correctly.
                You need to look in the default location of
                <filename>${D}</filename>, which is
                <filename>${WORKDIR}/image</filename>, to be sure your
                files have been installed correctly.
            </para>

            <note><title>Notes</title>
                <itemizedlist>
                    <listitem><para>
                        During the installation process, you might need to
                        modify some of the installed files to suit the target
                        layout.
                        For example, you might need to replace hard-coded paths
                        in an initscript with values of variables provided by
                        the build system, such as replacing
                        <filename>/usr/bin/</filename> with
                        <filename>${bindir}</filename>.
                        If you do perform such modifications during
                        <filename>do_install</filename>, be sure to modify the
                        destination file after copying rather than before
                        copying.
                        Modifying after copying ensures that the build system
                        can re-execute <filename>do_install</filename> if
                        needed.
                        </para></listitem>
                    <listitem><para>
                        <filename>oe_runmake install</filename>, which can be
                        run directly or can be run indirectly by the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-autotools'><filename>autotools</filename></ulink>
                        and
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-cmake'><filename>cmake</filename></ulink>
                        classes, runs <filename>make install</filename> in
                        parallel.
                        Sometimes, a Makefile can have missing dependencies
                        between targets that can result in race conditions.
                        If you experience intermittent failures during
                        <filename>do_install</filename>, you might be able to
                        work around them by disabling parallel Makefile
                        installs by adding the following to the recipe:
                        <literallayout class='monospaced'>
     PARALLEL_MAKEINST = ""
                        </literallayout>
                        See
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PARALLEL_MAKEINST'><filename>PARALLEL_MAKEINST</filename></ulink>
                        for additional information.
                        </para></listitem>
                </itemizedlist>
            </note>
        </section>

        <section id='new-recipe-enabling-system-services'>
            <title>Enabling System Services</title>

            <para>
                If you want to install a service, which is a process that
                usually starts on boot and runs in the background, then
                you must include some additional definitions in your recipe.
            </para>

            <para>
                If you are adding services and the service initialization
                script or the service file itself is not installed, you must
                provide for that installation in your recipe using a
                <filename>do_install_append</filename> function.
                If your recipe already has a <filename>do_install</filename>
                function, update the function near its end rather than
                adding an additional <filename>do_install_append</filename>
                function.
            </para>

            <para>
                When you create the installation for your services, you need
                to accomplish what is normally done by
                <filename>make install</filename>.
                In other words, make sure your installation arranges the output
                similar to how it is arranged on the target system.
            </para>

            <para>
                The OpenEmbedded build system provides support for starting
                services two different ways:
                <itemizedlist>
                    <listitem><para><emphasis>SysVinit:</emphasis>
                        SysVinit is a system and service manager that
                        manages the init system used to control the very basic
                        functions of your system.
                        The init program is the first program
                        started by the Linux kernel when the system boots.
                        Init then controls the startup, running and shutdown
                        of all other programs.</para>
                        <para>To enable a service using SysVinit, your recipe
                        needs to inherit the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-update-rc.d'><filename>update-rc.d</filename></ulink>
                        class.
                        The class helps facilitate safely installing the
                        package on the target.</para>
                        <para>You will need to set the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-INITSCRIPT_PACKAGES'><filename>INITSCRIPT_PACKAGES</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-INITSCRIPT_NAME'><filename>INITSCRIPT_NAME</filename></ulink>,
                        and
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-INITSCRIPT_PARAMS'><filename>INITSCRIPT_PARAMS</filename></ulink>
                        variables within your recipe.</para></listitem>
                    <listitem><para><emphasis>systemd:</emphasis>
                        System Management Daemon (systemd) was designed to
                        replace SysVinit and to provide
                        enhanced management of services.
                        For more information on systemd, see the systemd
                        homepage at
                        <ulink url='http://freedesktop.org/wiki/Software/systemd/'></ulink>.
                        </para>
                        <para>To enable a service using systemd, your recipe
                        needs to inherit the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-systemd'><filename>systemd</filename></ulink>
                        class.
                        See the <filename>systemd.bbclass</filename> file
                        located in your
                        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>.
                        section for more information.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='new-recipe-packaging'>
            <title>Packaging</title>

            <para>
                Successful packaging is a combination of automated processes
                performed by the OpenEmbedded build system and some
                specific steps you need to take.
                The following list describes the process:
                <itemizedlist>
                    <listitem><para><emphasis>Splitting Files</emphasis>:
                        The <filename>do_package</filename> task splits the
                        files produced by the recipe into logical components.
                        Even software that produces a single binary might
                        still have debug symbols, documentation, and other
                        logical components that should be split out.
                        The <filename>do_package</filename> task ensures
                        that files are split up and packaged correctly.
                        </para></listitem>
                    <listitem><para><emphasis>Running QA Checks</emphasis>:
                        The
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-insane'><filename>insane</filename></ulink>
                        class adds a step to
                        the package generation process so that output quality
                        assurance checks are generated by the OpenEmbedded
                        build system.
                        This step performs a range of checks to be sure the
                        build's output is free of common problems that show
                        up during runtime.
                        For information on these checks, see the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-insane'><filename>insane</filename></ulink>
                        class and the
                        "<ulink url='&YOCTO_DOCS_REF_URL;#ref-qa-checks'>QA Error and Warning Messages</ulink>"
                        chapter in the Yocto Project Reference Manual.
                        </para></listitem>
                    <listitem><para><emphasis>Hand-Checking Your Packages</emphasis>:
                        After you build your software, you need to be sure
                        your packages are correct.
                        Examine the
                        <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}/packages-split</filename>
                        directory and make sure files are where you expect
                        them to be.
                        If you discover problems, you can set
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'><filename>PACKAGES</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'><filename>FILES</filename></ulink>,
                        <filename>do_install(_append)</filename>, and so forth as
                        needed.
                        </para></listitem>
                    <listitem><para><emphasis>Splitting an Application into Multiple Packages</emphasis>:
                        If you need to split an application into several
                        packages, see the
                        "<link linkend='splitting-an-application-into-multiple-packages'>Splitting an Application into Multiple Packages</link>"
                        section for an example.
                        </para></listitem>
                    <listitem><para><emphasis>Installing a Post-Installation Script</emphasis>:
                        For an example showing how to install a
                        post-installation script, see the
                        "<link linkend='new-recipe-post-installation-scripts'>Post-Installation Scripts</link>"
                        section.
                        </para></listitem>
                    <listitem><para><emphasis>Marking Package Architecture</emphasis>:
                        Depending on what your recipe is building and how it
                        is configured, it might be important to mark the
                        packages produced as being specific to a particular
                        machine, or to mark them as not being specific to
                        a particular machine or architecture at all.</para>
                        <para>By default, packages apply to any machine with the
                        same architecture as the target machine.
                        When a recipe produces packages that are
                        machine-specific (e.g. the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
                        value is passed into the configure script or a patch
                        is applied only for a particular machine), you should
                        mark them as such by adding the following to the
                        recipe:
                        <literallayout class='monospaced'>
     PACKAGE_ARCH = "${MACHINE_ARCH}"
                        </literallayout></para>
                        <para>On the other hand, if the recipe produces packages
                        that do not contain anything specific to the target
                        machine or architecture at all (e.g. recipes
                        that simply package script files or configuration
                        files), you should use the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-allarch'><filename>allarch</filename></ulink>
                        class to do this for you by adding this to your
                        recipe:
                        <literallayout class='monospaced'>
     inherit allarch
                        </literallayout>
                        Ensuring that the package architecture is correct is
                        not critical while you are doing the first few builds
                        of your recipe.
                        However, it is important in order
                        to ensure that your recipe rebuilds (or does not
                        rebuild) appropriately in response to changes in
                        configuration, and to ensure that you get the
                        appropriate packages installed on the target machine,
                        particularly if you run separate builds for more
                        than one target machine.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='new-sharing-files-between-recipes'>
            <title>Sharing Files Between Recipes</title>

            <para>
                Recipes often need to use files provided by other recipes on
                the build host.
                For example, an application linking to a common library needs
                access to the library itself and its associated headers.
                The way this access is accomplished is by populating a sysroot
                with files.
                Each recipe has two sysroots in its work directory, one for
                target files
                (<filename>recipe-sysroot</filename>) and one for files that
                are native to the build host
                (<filename>recipe-sysroot-native</filename>).
                <note>
                    You could find the term "staging" used within the Yocto
                    project regarding files populating sysroots (e.g. the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_DIR'><filename>STAGING_DIR</filename></ulink>
                    variable).
                </note>
            </para>

            <para>
                Recipes should never populate the sysroot directly (i.e. write
                files into sysroot).
                Instead, files should be installed into standard locations
                during the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>
                task within the
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-D'><filename>D</filename></ulink><filename>}</filename>
                directory.
                The reason for this limitation is that almost all files that
                populate the sysroot are cataloged in manifests in order to
                ensure the files can be removed later when a recipe is either
                modified or removed.
                Thus, the sysroot is able to remain free from stale files.
            </para>

            <para>
                A subset of the files installed by the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>
                task are used by the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-populate_sysroot'><filename>do_populate_sysroot</filename></ulink>
                task as defined by the the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SYSROOT_DIRS'><filename>SYSROOT_DIRS</filename></ulink>
                variable to automatically populate the sysroot.
                It is possible to modify the list of directories that populate
                the sysroot.
                The following example shows how you could add the
                <filename>/opt</filename> directory to the list of
                directories within a recipe:
                <literallayout class='monospaced'>
     SYSROOT_DIRS += "/opt"
                </literallayout>
            </para>

            <para>
                For a more complete description of the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-populate_sysroot'><filename>do_populate_sysroot</filename></ulink>
                task and its associated functions, see the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-staging'><filename>staging</filename></ulink>
                class.
            </para>
        </section>

        <section id='metadata-virtual-providers'>
            <title>Using Virtual Providers</title>

            <para>
                Prior to a build, if you know that several different recipes
                provide the same functionality, you can use a virtual provider
                (i.e. <filename>virtual/*</filename>) as a placeholder for the
                actual provider.
                The actual provider is determined at build-time.
            </para>

            <para>
                A common scenario where a virtual provider is used would be
                for the kernel recipe.
                Suppose you have three kernel recipes whose
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink>
                values map to <filename>kernel-big</filename>,
                <filename>kernel-mid</filename>, and
                <filename>kernel-small</filename>.
                Furthermore, each of these recipes in some way uses a
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PROVIDES'><filename>PROVIDES</filename></ulink>
                statement that essentially identifies itself as being able
                to provide <filename>virtual/kernel</filename>.
                Here is one way through the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-kernel'><filename>kernel</filename></ulink>
                class:
                <literallayout class='monospaced'>
     PROVIDES += "${@ "virtual/kernel" if (d.getVar("KERNEL_PACKAGE_NAME") == "kernel") else "" }"
                </literallayout>
                Any recipe that inherits the <filename>kernel</filename> class
                is going to utilize a <filename>PROVIDES</filename> statement
                that identifies that recipe as being able to provide the
                <filename>virtual/kernel</filename> item.
            </para>

            <para>
                Now comes the time to actually build an image and you need a
                kernel recipe, but which one?
                You can configure your build to call out the kernel recipe
                you want by using the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PREFERRED_PROVIDER'><filename>PREFERRED_PROVIDER</filename></ulink>
                variable.
                As an example, consider the
                <ulink url='https://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta/conf/machine/include/x86-base.inc'><filename>x86-base.inc</filename></ulink>
                include file, which is a machine
                (i.e. <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>)
                configuration file.
                This include file is the reason all x86-based machines use the
                <filename>linux-yocto</filename> kernel.
                Here are the relevant lines from the include file:
                <literallayout class='monospaced'>
     PREFERRED_PROVIDER_virtual/kernel ??= "linux-yocto"
     PREFERRED_VERSION_linux-yocto ??= "4.15%"
                </literallayout>
            </para>

            <para>
                When you use a virtual provider, you do not have to
                "hard code" a recipe name as a build dependency.
                You can use the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>
                variable to state the build is dependent on
                <filename>virtual/kernel</filename> for example:
                <literallayout class='monospaced'>
     DEPENDS = "virtual/kernel"
                </literallayout>
                During the build, the OpenEmbedded build system picks
                the correct recipe needed for the
                <filename>virtual/kernel</filename> dependency based on the
                <filename>PREFERRED_PROVIDER</filename> variable.
                If you want to use the small kernel mentioned at the beginning
                of this section, configure your build as follows:
                <literallayout class='monospaced'>
     PREFERRED_PROVIDER_virtual/kernel ??= "kernel-small"
                </literallayout>
                <note>
                    Any recipe that
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PROVIDES'><filename>PROVIDES</filename></ulink>
                    a <filename>virtual/*</filename> item that is ultimately
                    not selected through
                    <filename>PREFERRED_PROVIDER</filename> does not get built.
                    Preventing these recipes from building is usually the
                    desired behavior since this mechanism's purpose is to
                    select between mutually exclusive alternative providers.
                </note>
            </para>

            <para>
                The following lists specific examples of virtual providers:
                <itemizedlist>
                    <listitem><para>
                        <filename>virtual/kernel</filename>:
                        Provides the name of the kernel recipe to use when
                        building a kernel image.
                        </para></listitem>
                    <listitem><para>
                        <filename>virtual/bootloader</filename>:
                        Provides the name of the bootloader to use when
                        building an image.
                        </para></listitem>
                    <listitem><para>
                        <filename>virtual/mesa</filename>:
                        Provides <filename>gbm.pc</filename>.
                        </para></listitem>
                    <listitem><para>
                        <filename>virtual/egl</filename>:
                        Provides <filename>egl.pc</filename> and possibly
                        <filename>wayland-egl.pc</filename>.
                        </para></listitem>
                    <listitem><para>
                        <filename>virtual/libgl</filename>:
                        Provides <filename>gl.pc</filename> (i.e. libGL).
                        </para></listitem>
                    <listitem><para>
                        <filename>virtual/libgles1</filename>:
                        Provides <filename>glesv1_cm.pc</filename>
                        (i.e. libGLESv1_CM).
                        </para></listitem>
                    <listitem><para>
                        <filename>virtual/libgles2</filename>:
                        Provides <filename>glesv2.pc</filename>
                        (i.e. libGLESv2).
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='properly-versioning-pre-release-recipes'>
            <title>Properly Versioning Pre-Release Recipes</title>

            <para>
                Sometimes the name of a recipe can lead to versioning
                problems when the recipe is upgraded to a final release.
                For example, consider the
                <filename>irssi_0.8.16-rc1.bb</filename> recipe file in
                the list of example recipes in the
                "<link linkend='new-recipe-storing-and-naming-the-recipe'>Storing and Naming the Recipe</link>"
                section.
                This recipe is at a release candidate stage (i.e.
                "rc1").
                When the recipe is released, the recipe filename becomes
                <filename>irssi_0.8.16.bb</filename>.
                The version change from <filename>0.8.16-rc1</filename>
                to <filename>0.8.16</filename> is seen as a decrease by the
                build system and package managers, so the resulting packages
                will not correctly trigger an upgrade.
            </para>

            <para>
                In order to ensure the versions compare properly, the
                recommended convention is to set
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
                within the recipe to
                "<replaceable>previous_version</replaceable>+<replaceable>current_version</replaceable>".
                You can use an additional variable so that you can use the
                current version elsewhere.
                Here is an example:
                <literallayout class='monospaced'>
     REALPV = "0.8.16-rc1"
     PV = "0.8.15+${REALPV}"
                </literallayout>
            </para>
        </section>

        <section id='new-recipe-post-installation-scripts'>
            <title>Post-Installation Scripts</title>

            <para>
                Post-installation scripts run immediately after installing
                a package on the target or during image creation when a
                package is included in an image.
                To add a post-installation script to a package, add a
                <filename>pkg_postinst_</filename><replaceable>PACKAGENAME</replaceable><filename>()</filename> function to
                the recipe file (<filename>.bb</filename>) and replace
                <replaceable>PACKAGENAME</replaceable> with the name of the package
                you want to attach to the <filename>postinst</filename>
                script.
                To apply the post-installation script to the main package
                for the recipe, which is usually what is required, specify
                <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink><filename>}</filename>
                in place of <replaceable>PACKAGENAME</replaceable>.
            </para>

            <para>
                A post-installation function has the following structure:
                <literallayout class='monospaced'>
     pkg_postinst_<replaceable>PACKAGENAME</replaceable>() {
     # Commands to carry out
     }
                </literallayout>
            </para>

            <para>
                The script defined in the post-installation function is
                called when the root filesystem is created.
                If the script succeeds, the package is marked as installed.
                If the script fails, the package is marked as unpacked and
                the script is executed when the image boots again.
                <note>
                    Any RPM post-installation script that runs on the target
                    should return a 0 exit code.
                    RPM does not allow non-zero exit codes for these scripts,
                    and the RPM package manager will cause the package to fail
                    installation on the target.
                </note>
            </para>

            <para>
                Sometimes it is necessary for the execution of a
                post-installation script to be delayed until the first boot.
                For example, the script might need to be executed on the
                device itself.
                To delay script execution until boot time, you must explicitly
                mark post installs to defer to the target.
                You can use <filename>pkg_postinst_ontarget()</filename> or
                call
                <filename>postinst-intercepts defer_to_first_boot</filename>
                from <filename>pkg_postinst()</filename>.
                Any failure of a <filename>pkg_postinst()</filename> script
                (including exit 1) triggers an error during the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-rootfs'><filename>do_rootfs</filename></ulink>
                task.
            </para>

            <para>
                If you have recipes that use
                <filename>pkg_postinst</filename> function
                and they require the use of non-standard native
                tools that have dependencies during rootfs construction, you
                need to use the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_WRITE_DEPS'><filename>PACKAGE_WRITE_DEPS</filename></ulink>
                variable in your recipe to list these tools.
                If you do not use this variable, the tools might be missing and
                execution of the post-installation script is deferred until
                first boot.
                Deferring the script to first boot is undesirable and for
                read-only rootfs impossible.
            </para>

            <note>
                Equivalent support for pre-install, pre-uninstall, and
                post-uninstall scripts exist by way of
                <filename>pkg_preinst</filename>,
                <filename>pkg_prerm</filename>, and
                <filename>pkg_postrm</filename>, respectively.
                These scrips work in exactly the same way as does
                <filename>pkg_postinst</filename> with the exception
                that they run at different times.
                Also, because of when they run, they are not applicable to
                being run at image creation time like
                <filename>pkg_postinst</filename>.
            </note>
        </section>

        <section id='new-recipe-testing'>
            <title>Testing</title>

            <para>
                The final step for completing your recipe is to be sure that
                the software you built runs correctly.
                To accomplish runtime testing, add the build's output
                packages to your image and test them on the target.
            </para>

            <para>
                For information on how to customize your image by adding
                specific packages, see the
                "<link linkend='usingpoky-extend-customimage'>Customizing Images</link>"
                section.
            </para>
        </section>

        <section id='new-recipe-testing-examples'>
            <title>Examples</title>

            <para>
                To help summarize how to write a recipe, this section provides
                some examples given various scenarios:
                <itemizedlist>
                    <listitem><para>Recipes that use local files</para></listitem>
                    <listitem><para>Using an Autotooled package</para></listitem>
                    <listitem><para>Using a Makefile-based package</para></listitem>
                    <listitem><para>Splitting an application into multiple packages</para></listitem>
                    <listitem><para>Adding binaries to an image</para></listitem>
                </itemizedlist>
            </para>

            <section id='new-recipe-single-c-file-package-hello-world'>
                <title>Single .c File Package (Hello World!)</title>

                <para>
                    Building an application from a single file that is stored
                    locally (e.g. under <filename>files</filename>) requires
                    a recipe that has the file listed in the
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename>
                    variable.
                    Additionally, you need to manually write the
                    <filename>do_compile</filename> and
                    <filename>do_install</filename> tasks.
                    The <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'>S</ulink></filename>
                    variable defines the directory containing the source code,
                    which is set to
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>
                    in this case - the directory BitBake uses for the build.
                    <literallayout class='monospaced'>
     SUMMARY = "Simple helloworld application"
     SECTION = "examples"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"

     SRC_URI = "file://helloworld.c"

     S = "${WORKDIR}"

     do_compile() {
     	${CC} helloworld.c -o helloworld
     }

     do_install() {
     	install -d ${D}${bindir}
     	install -m 0755 helloworld ${D}${bindir}
     }
                    </literallayout>
                </para>

                <para>
                    By default, the <filename>helloworld</filename>,
                    <filename>helloworld-dbg</filename>, and
                    <filename>helloworld-dev</filename> packages are built.
                    For information on how to customize the packaging process,
                    see the
                    "<link linkend='splitting-an-application-into-multiple-packages'>Splitting an Application into Multiple Packages</link>"
                    section.
                </para>
            </section>

            <section id='new-recipe-autotooled-package'>
                <title>Autotooled Package</title>
                <para>
                    Applications that use Autotools such as <filename>autoconf</filename> and
                    <filename>automake</filename> require a recipe that has a source archive listed in
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename> and
                    also inherit the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-autotools'><filename>autotools</filename></ulink>
                    class, which contains the definitions of all the steps
                    needed to build an Autotool-based application.
                    The result of the build is automatically packaged.
                    And, if the application uses NLS for localization, packages with local information are
                    generated (one package per language).
                    Following is one example: (<filename>hello_2.3.bb</filename>)
                    <literallayout class='monospaced'>
     SUMMARY = "GNU Helloworld application"
     SECTION = "examples"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=751419260aa954499f7abaabaa882bbe"

     SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.gz"

     inherit autotools gettext
                     </literallayout>
                </para>

                <para>
                    The variable
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-LIC_FILES_CHKSUM'>LIC_FILES_CHKSUM</ulink></filename>
                    is used to track source license changes as described in the
                    "<link linkend='usingpoky-configuring-LIC_FILES_CHKSUM'>Tracking License Changes</link>"
                    section in the Yocto Project Overview and Concepts Manual.
                    You can quickly create Autotool-based recipes in a manner
                    similar to the previous example.
                </para>
            </section>

            <section id='new-recipe-makefile-based-package'>
                <title>Makefile-Based Package</title>

                <para>
                    Applications that use GNU <filename>make</filename> also require a recipe that has
                    the source archive listed in
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename>.
                    You do not need to add a <filename>do_compile</filename> step since by default BitBake
                    starts the <filename>make</filename> command to compile the application.
                    If you need additional <filename>make</filename> options, you should store them in the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTRA_OEMAKE'><filename>EXTRA_OEMAKE</filename></ulink>
                    or
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGECONFIG_CONFARGS'><filename>PACKAGECONFIG_CONFARGS</filename></ulink>
                    variables.
                    BitBake passes these options into the GNU <filename>make</filename> invocation.
                    Note that a <filename>do_install</filename> task is still required.
                    Otherwise, BitBake runs an empty <filename>do_install</filename> task by default.
                </para>

               <para>
                    Some applications might require extra parameters to be passed to the compiler.
                    For example, the application might need an additional header path.
                    You can accomplish this by adding to the
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-CFLAGS'>CFLAGS</ulink></filename> variable.
                    The following example shows this:
                    <literallayout class='monospaced'>
     CFLAGS_prepend = "-I ${S}/include "
                    </literallayout>
                </para>

                <para>
                In the following example, <filename>mtd-utils</filename> is a makefile-based package:
                    <literallayout class='monospaced'>
     SUMMARY = "Tools for managing memory technology devices"
     SECTION = "base"
     DEPENDS = "zlib lzo e2fsprogs util-linux"
     HOMEPAGE = "http://www.linux-mtd.infradead.org/"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=0636e73ff0215e8d672dc4c32c317bb3 \
                         file://include/common.h;beginline=1;endline=17;md5=ba05b07912a44ea2bf81ce409380049c"

     # Use the latest version at 26 Oct, 2013
     SRCREV = "9f107132a6a073cce37434ca9cda6917dd8d866b"
     SRC_URI = "git://git.infradead.org/mtd-utils.git \
                     file://add-exclusion-to-mkfs-jffs2-git-2.patch \
     "

     PV = "1.5.1+git${SRCPV}"

     S = "${WORKDIR}/git"

     EXTRA_OEMAKE = "'CC=${CC}' 'RANLIB=${RANLIB}' 'AR=${AR}' 'CFLAGS=${CFLAGS} -I${S}/include -DWITHOUT_XATTR' 'BUILDDIR=${S}'"

     do_install () {
             oe_runmake install DESTDIR=${D} SBINDIR=${sbindir} MANDIR=${mandir} INCLUDEDIR=${includedir}
     }

     PACKAGES =+ "mtd-utils-jffs2 mtd-utils-ubifs mtd-utils-misc"

     FILES_mtd-utils-jffs2 = "${sbindir}/mkfs.jffs2 ${sbindir}/jffs2dump ${sbindir}/jffs2reader ${sbindir}/sumtool"
     FILES_mtd-utils-ubifs = "${sbindir}/mkfs.ubifs ${sbindir}/ubi*"
     FILES_mtd-utils-misc = "${sbindir}/nftl* ${sbindir}/ftl* ${sbindir}/rfd* ${sbindir}/doc* ${sbindir}/serve_image ${sbindir}/recv_image"

     PARALLEL_MAKE = ""

     BBCLASSEXTEND = "native"
                    </literallayout>
                </para>
            </section>

            <section id='splitting-an-application-into-multiple-packages'>
                <title>Splitting an Application into Multiple Packages</title>

                <para>
                    You can use the variables
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'>PACKAGES</ulink></filename> and
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'>FILES</ulink></filename>
                    to split an application into multiple packages.
                </para>

                <para>
                    Following is an example that uses the <filename>libxpm</filename> recipe.
                    By default, this recipe generates a single package that contains the library along
                    with a few binaries.
                    You can modify the recipe to split the binaries into separate packages:
                    <literallayout class='monospaced'>
     require xorg-lib-common.inc

     SUMMARY = "Xpm: X Pixmap extension library"
     LICENSE = "BSD"
     LIC_FILES_CHKSUM = "file://COPYING;md5=51f4270b012ecd4ab1a164f5f4ed6cf7"
     DEPENDS += "libxext libsm libxt"
     PE = "1"

     XORG_PN = "libXpm"

     PACKAGES =+ "sxpm cxpm"
     FILES_cxpm = "${bindir}/cxpm"
     FILES_sxpm = "${bindir}/sxpm"
                    </literallayout>
                </para>

                <para>
                    In the previous example, we want to ship the <filename>sxpm</filename>
                    and <filename>cxpm</filename> binaries in separate packages.
                    Since <filename>bindir</filename> would be packaged into the main
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'>PN</ulink></filename>
                    package by default, we prepend the <filename>PACKAGES</filename>
                    variable so additional package names are added to the start of list.
                    This results in the extra <filename>FILES_*</filename>
                    variables then containing information that define which files and
                    directories go into which packages.
                    Files included by earlier packages are skipped by latter packages.
                    Thus, the main <filename>PN</filename> package
                    does not include the above listed files.
                </para>
            </section>

            <section id='packaging-externally-produced-binaries'>
                <title>Packaging Externally Produced Binaries</title>

                <para>
                    Sometimes, you need to add pre-compiled binaries to an
                    image.
                    For example, suppose that binaries for proprietary code
                    exist, which are created by a particular division of a
                    company.
                    Your part of the company needs to use those binaries as
                    part of an image that you are building using the
                    OpenEmbedded build system.
                    Since you only have the binaries and not the source code,
                    you cannot use a typical recipe that expects to fetch the
                    source specified in
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                    and then compile it.
                </para>

                <para>
                    One method is to package the binaries and then install them
                    as part of the image.
                    Generally, it is not a good idea to package binaries
                    since, among other things, it can hinder the ability to
                    reproduce builds and could lead to compatibility problems
                    with ABI in the future.
                    However, sometimes you have no choice.
                </para>

                <para>
                    The easiest solution is to create a recipe that uses
                    the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-bin-package'><filename>bin_package</filename></ulink>
                    class and to be sure that you are using default locations
                    for build artifacts.
                    In most cases, the <filename>bin_package</filename> class
                    handles "skipping" the configure and compile steps as well
                    as sets things up to grab packages from the appropriate
                    area.
                    In particular, this class sets <filename>noexec</filename>
                    on both the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-configure'><filename>do_configure</filename></ulink>
                    and
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-compile'><filename>do_compile</filename></ulink>
                    tasks, sets
                    <filename>FILES_${PN}</filename> to "/" so that it picks
                    up all files, and sets up a
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>
                    task, which effectively copies all files from
                    <filename>${S}</filename> to <filename>${D}</filename>.
                    The <filename>bin_package</filename> class works well when
                    the files extracted into <filename>${S}</filename> are
                    already laid out in the way they should be laid out
                    on the target.
                    For more information on these variables, see the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'><filename>FILES</filename></ulink>,
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink>,
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink>,
                    and
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-D'><filename>D</filename></ulink>
                    variables in the Yocto Project Reference Manual's variable
                    glossary.
                    <note><title>Notes</title>
                        <itemizedlist>
                            <listitem><para>
                                Using
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>
                                is a good idea even for components distributed
                                in binary form, and is often necessary for
                                shared libraries.
                                For a shared library, listing the library
                                dependencies in
                                <filename>DEPENDS</filename> makes sure that
                                the libraries are available in the staging
                                sysroot when other recipes link against the
                                library, which might be necessary for
                                successful linking.
                                </para></listitem>
                            <listitem><para>
                                Using <filename>DEPENDS</filename> also
                                allows runtime dependencies between packages
                                to be added automatically.
                                See the
                                "<ulink url='&YOCTO_DOCS_OM_URL;#automatically-added-runtime-dependencies'>Automatically Added Runtime Dependencies</ulink>"
                                section in the Yocto Project Overview and
                                Concepts Manual for more information.
                                </para></listitem>
                        </itemizedlist>
                    </note>
                </para>

                <para>
                    If you cannot use the <filename>bin_package</filename>
                    class, you need to be sure you are doing the following:
                    <itemizedlist>
                        <listitem><para>
                            Create a recipe where the
                            <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-configure'><filename>do_configure</filename></ulink>
                            and
                            <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-compile'><filename>do_compile</filename></ulink>
                            tasks do nothing:
                            It is usually sufficient to just not define these
                            tasks in the recipe, because the default
                            implementations do nothing unless a Makefile is
                            found in
                            <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink><filename>}</filename>.
                            </para>

                            <para>If
                            <filename>${S}</filename> might contain a Makefile,
                            or if you inherit some class that replaces
                            <filename>do_configure</filename> and
                            <filename>do_compile</filename> with custom
                            versions, then you can use the
                            <filename>[</filename><ulink url='&YOCTO_DOCS_BB_URL;#variable-flags'><filename>noexec</filename></ulink><filename>]</filename>
                            flag to turn the tasks into no-ops, as follows:
                            <literallayout class='monospaced'>
     do_configure[noexec] = "1"
     do_compile[noexec] = "1"
                            </literallayout>
                            Unlike
                            <ulink url='&YOCTO_DOCS_BB_URL;#deleting-a-task'><filename>deleting the tasks</filename></ulink>,
                            using the flag preserves the dependency chain from
                            the
                            <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-fetch'><filename>do_fetch</filename></ulink>,                     <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-unpack'><filename>do_unpack</filename></ulink>,
                            and
                            <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-patch'><filename>do_patch</filename></ulink>
                            tasks to the
                            <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>
                            task.
                            </para></listitem>
                        <listitem><para>Make sure your
                            <filename>do_install</filename> task installs the
                            binaries appropriately.
                            </para></listitem>
                        <listitem><para>Ensure that you set up
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'><filename>FILES</filename></ulink>
                            (usually
                            <filename>FILES_${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink><filename>}</filename>)
                            to point to the files you have installed, which of
                            course depends on where you have installed them
                            and whether those files are in different locations
                            than the defaults.
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>
        </section>

        <section id="following-recipe-style-guidelines">
            <title>Following Recipe Style Guidelines</title>

            <para>
                When writing recipes, it is good to conform to existing
                style guidelines.
                The
                <ulink url='http://www.openembedded.org/wiki/Styleguide'>OpenEmbedded Styleguide</ulink>
                wiki page provides rough guidelines for preferred recipe style.
            </para>

            <para>
                It is common for existing recipes to deviate a bit from this
                style.
                However, aiming for at least a consistent style is a good idea.
                Some practices, such as omitting spaces around
                <filename>=</filename> operators in assignments or ordering
                recipe components in an erratic way, are widely seen as poor
                style.
            </para>
        </section>

        <section id='recipe-syntax'>
            <title>Recipe Syntax</title>

            <para>
                Understanding recipe file syntax is important for writing
                recipes.
                The following list overviews the basic items that make up a
                BitBake recipe file.
                For more complete BitBake syntax descriptions, see the
                "<ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual-metadata'>Syntax and Operators</ulink>"
                chapter of the BitBake User Manual.
                <itemizedlist>
                    <listitem><para>
                        <emphasis>Variable Assignments and Manipulations:</emphasis>
                        Variable assignments allow a value to be assigned to a
                        variable.
                        The assignment can be static text or might include
                        the contents of other variables.
                        In addition to the assignment, appending and prepending
                        operations are also supported.</para>

                        <para>The following example shows some of the ways
                        you can use variables in recipes:
                        <literallayout class='monospaced'>
     S = "${WORKDIR}/postfix-${PV}"
     CFLAGS += "-DNO_ASM"
     SRC_URI_append = " file://fixup.patch"
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Functions:</emphasis>
                        Functions provide a series of actions to be performed.
                        You usually use functions to override the default
                        implementation of a task function or to complement
                        a default function (i.e. append or prepend to an
                        existing function).
                        Standard functions use <filename>sh</filename> shell
                        syntax, although access to OpenEmbedded variables and
                        internal methods are also available.</para>

                        <para>The following is an example function from the
                        <filename>sed</filename> recipe:
                        <literallayout class='monospaced'>
     do_install () {
         autotools_do_install
         install -d ${D}${base_bindir}
         mv ${D}${bindir}/sed ${D}${base_bindir}/sed
         rmdir ${D}${bindir}/
     }
                        </literallayout>
                        It is also possible to implement new functions that
                        are called between existing tasks as long as the
                        new functions are not replacing or complementing the
                        default functions.
                        You can implement functions in Python
                        instead of shell.
                        Both of these options are not seen in the majority of
                        recipes.
                        </para></listitem>
                    <listitem><para><emphasis>Keywords:</emphasis>
                        BitBake recipes use only a few keywords.
                        You use keywords to include common
                        functions (<filename>inherit</filename>), load parts
                        of a recipe from other files
                        (<filename>include</filename> and
                        <filename>require</filename>) and export variables
                        to the environment (<filename>export</filename>).
                        </para>

                        <para>The following example shows the use of some of
                        these keywords:
                        <literallayout class='monospaced'>
     export POSTCONF = "${STAGING_BINDIR}/postconf"
     inherit autoconf
     require otherfile.inc
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Comments (#):</emphasis>
                        Any lines that begin with the hash character
                        (<filename>#</filename>) are treated as comment lines
                        and are ignored:
                        <literallayout class='monospaced'>
     # This is a comment
                        </literallayout>
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                This next list summarizes the most important and most commonly
                used parts of the recipe syntax.
                For more information on these parts of the syntax, you can
                reference the
                <ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual-metadata'>Syntax and Operators</ulink>
                chapter in the BitBake User Manual.
                <itemizedlist>
                    <listitem><para>
                        <emphasis>Line Continuation (\):</emphasis>
                        Use the backward slash (<filename>\</filename>)
                        character to split a statement over multiple lines.
                        Place the slash character at the end of the line that
                        is to be continued on the next line:
                        <literallayout class='monospaced'>
     VAR = "A really long \
            line"
                        </literallayout>
                        <note>
                            You cannot have any characters including spaces
                            or tabs after the slash character.
                        </note>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Using Variables (${<replaceable>VARNAME</replaceable>}):</emphasis>
                        Use the <filename>${<replaceable>VARNAME</replaceable>}</filename>
                        syntax to access the contents of a variable:
                        <literallayout class='monospaced'>
     SRC_URI = "${SOURCEFORGE_MIRROR}/libpng/zlib-${PV}.tar.gz"
                        </literallayout>
                        <note>
                            It is important to understand that the value of a
                            variable expressed in this form does not get
                            substituted automatically.
                            The expansion of these expressions happens
                            on-demand later (e.g. usually when a function that
                            makes reference to the variable executes).
                            This behavior ensures that the values are most
                            appropriate for the context in which they are
                            finally used.
                            On the rare occasion that you do need the variable
                            expression to be expanded immediately, you can use
                            the <filename>:=</filename> operator instead of
                            <filename>=</filename> when you make the
                            assignment, but this is not generally needed.
                        </note>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Quote All Assignments ("<replaceable>value</replaceable>"):</emphasis>
                        Use double quotes around values in all variable
                        assignments (e.g.
                        <filename>"<replaceable>value</replaceable>"</filename>).
                        Following is an example:
                        <literallayout class='monospaced'>
     VAR1 = "${OTHERVAR}"
     VAR2 = "The version is ${PV}"
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Conditional Assignment (?=):</emphasis>
                        Conditional assignment is used to assign a
                        value to a variable, but only when the variable is
                        currently unset.
                        Use the question mark followed by the equal sign
                        (<filename>?=</filename>) to make a "soft" assignment
                        used for conditional assignment.
                        Typically, "soft" assignments are used in the
                        <filename>local.conf</filename> file for variables
                        that are allowed to come through from the external
                        environment.
                        </para>

                        <para>Here is an example where
                        <filename>VAR1</filename> is set to "New value" if
                        it is currently empty.
                        However, if <filename>VAR1</filename> has already been
                        set, it remains unchanged:
                        <literallayout class='monospaced'>
     VAR1 ?= "New value"
                        </literallayout>
                        In this next example, <filename>VAR1</filename>
                        is left with the value "Original value":
                        <literallayout class='monospaced'>
     VAR1 = "Original value"
     VAR1 ?= "New value"
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Appending (+=):</emphasis>
                        Use the plus character followed by the equals sign
                        (<filename>+=</filename>) to append values to existing
                        variables.
                        <note>
                            This operator adds a space between the existing
                            content of the variable and the new content.
                        </note></para>

                        <para>Here is an example:
                        <literallayout class='monospaced'>
     SRC_URI += "file://fix-makefile.patch"
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Prepending (=+):</emphasis>
                        Use the equals sign followed by the plus character
                        (<filename>=+</filename>) to prepend values to existing
                        variables.
                        <note>
                            This operator adds a space between the new content
                            and the existing content of the variable.
                        </note></para>

                        <para>Here is an example:
                        <literallayout class='monospaced'>
     VAR =+ "Starts"
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Appending (_append):</emphasis>
                        Use the <filename>_append</filename> operator to
                        append values to existing variables.
                        This operator does not add any additional space.
                        Also, the operator is applied after all the
                        <filename>+=</filename>, and
                        <filename>=+</filename> operators have been applied and
                        after all <filename>=</filename> assignments have
                        occurred.
                        </para>

                        <para>The following example shows the space being
                        explicitly added to the start to ensure the appended
                        value is not merged with the existing value:
                        <literallayout class='monospaced'>
     SRC_URI_append = " file://fix-makefile.patch"
                        </literallayout>
                        You can also use the <filename>_append</filename>
                        operator with overrides, which results in the actions
                        only being performed for the specified target or
                        machine:
                        <literallayout class='monospaced'>
     SRC_URI_append_sh4 = " file://fix-makefile.patch"
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Prepending (_prepend):</emphasis>
                        Use the <filename>_prepend</filename> operator to
                        prepend values to existing variables.
                        This operator does not add any additional space.
                        Also, the operator is applied after all the
                        <filename>+=</filename>, and
                        <filename>=+</filename> operators have been applied and
                        after all <filename>=</filename> assignments have
                        occurred.
                        </para>

                        <para>The following example shows the space being
                        explicitly added to the end to ensure the prepended
                        value is not merged with the existing value:
                        <literallayout class='monospaced'>
     CFLAGS_prepend = "-I${S}/myincludes "
                        </literallayout>
                        You can also use the <filename>_prepend</filename>
                        operator with overrides, which results in the actions
                        only being performed for the specified target or
                        machine:
                        <literallayout class='monospaced'>
     CFLAGS_prepend_sh4 = "-I${S}/myincludes "
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Overrides:</emphasis>
                        You can use overrides to set a value conditionally,
                        typically based on how the recipe is being built.
                        For example, to set the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-KBRANCH'><filename>KBRANCH</filename></ulink>
                        variable's value to "standard/base" for any target
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>,
                        except for qemuarm where it should be set to
                        "standard/arm-versatile-926ejs", you would do the
                        following:
                        <literallayout class='monospaced'>
     KBRANCH = "standard/base"
     KBRANCH_qemuarm  = "standard/arm-versatile-926ejs"
                        </literallayout>
                        Overrides are also used to separate alternate values
                        of a variable in other situations.
                        For example, when setting variables such as
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'><filename>FILES</filename></ulink>
                        and
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>
                        that are specific to individual packages produced by
                        a recipe, you should always use an override that
                        specifies the name of the package.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Indentation:</emphasis>
                        Use spaces for indentation rather than than tabs.
                        For shell functions, both currently work.
                        However, it is a policy decision of the Yocto Project
                        to use tabs in shell functions.
                        Realize that some layers have a policy to use spaces
                        for all indentation.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Using Python for Complex Operations:</emphasis>
                        For more advanced processing, it is possible to use
                        Python code during variable assignments (e.g.
                        search and replacement on a variable).</para>

                        <para>You indicate Python code using the
                        <filename>${@<replaceable>python_code</replaceable>}</filename>
                        syntax for the variable assignment:
                        <literallayout class='monospaced'>
     SRC_URI = "ftp://ftp.info-zip.org/pub/infozip/src/zip${@d.getVar('PV',1).replace('.', '')}.tgz
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Shell Function Syntax:</emphasis>
                        Write shell functions as if you were writing a shell
                        script when you describe a list of actions to take.
                        You should ensure that your script works with a generic
                        <filename>sh</filename> and that it does not require
                        any <filename>bash</filename> or other shell-specific
                        functionality.
                        The same considerations apply to various system
                        utilities (e.g. <filename>sed</filename>,
                        <filename>grep</filename>, <filename>awk</filename>,
                        and so forth) that you might wish to use.
                        If in doubt, you should check with multiple
                        implementations - including those from BusyBox.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>
    </section>

    <section id="platdev-newmachine">
        <title>Adding a New Machine</title>

        <para>
            Adding a new machine to the Yocto Project is a straightforward
            process.
            This section describes how to add machines that are similar
            to those that the Yocto Project already supports.
            <note>
                Although well within the capabilities of the Yocto Project,
                adding a totally new architecture might require
                changes to <filename>gcc/glibc</filename> and to the site
                information, which is beyond the scope of this manual.
            </note>
        </para>

        <para>
            For a complete example that shows how to add a new machine,
            see the
            "<ulink url='&YOCTO_DOCS_BSP_URL;#creating-a-new-bsp-layer-using-the-bitbake-layers-script'>Creating a New BSP Layer Using the <filename>bitbake-layers</filename> Script</ulink>"
            section in the Yocto Project Board Support Package (BSP)
            Developer's Guide.
        </para>

        <section id="platdev-newmachine-conffile">
            <title>Adding the Machine Configuration File</title>

            <para>
                To add a new machine, you need to add a new machine
                configuration file to the layer's
                <filename>conf/machine</filename> directory.
                This configuration file provides details about the device
                you are adding.
            </para>

            <para>
                The OpenEmbedded build system uses the root name of the
                machine configuration file to reference the new machine.
                For example, given a machine configuration file named
                <filename>crownbay.conf</filename>, the build system
                recognizes the machine as "crownbay".
            </para>

            <para>
                The most important variables you must set in your machine
                configuration file or include from a lower-level configuration
                file are as follows:
                <itemizedlist>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-TARGET_ARCH'>TARGET_ARCH</ulink></filename>
                        (e.g. "arm")</para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-PREFERRED_PROVIDER'>PREFERRED_PROVIDER</ulink>_virtual/kernel</filename>
                        </para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_FEATURES'>MACHINE_FEATURES</ulink></filename>
                        (e.g. "apm screen wifi")</para></listitem>
                </itemizedlist>
            </para>

            <para>
                You might also need these variables:
                <itemizedlist>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SERIAL_CONSOLES'>SERIAL_CONSOLES</ulink></filename>
                        (e.g. "115200;ttyS0 115200;ttyS1")</para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-KERNEL_IMAGETYPE'>KERNEL_IMAGETYPE</ulink></filename>
                        (e.g. "zImage")</para></listitem>
                    <listitem><para><filename><ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_FSTYPES'>IMAGE_FSTYPES</ulink></filename>
                        (e.g. "tar.gz jffs2")</para></listitem>
                </itemizedlist>
            </para>

            <para>
                You can find full details on these variables in the reference
                section.
                You can leverage existing machine <filename>.conf</filename>
                files from <filename>meta-yocto-bsp/conf/machine/</filename>.
            </para>
        </section>

        <section id="platdev-newmachine-kernel">
            <title>Adding a Kernel for the Machine</title>

            <para>
                The OpenEmbedded build system needs to be able to build a kernel
                for the machine.
                You need to either create a new kernel recipe for this machine,
                or extend an existing kernel recipe.
                You can find several kernel recipe examples in the
                Source Directory at
                <filename>meta/recipes-kernel/linux</filename>
                that you can use as references.
            </para>

            <para>
                If you are creating a new kernel recipe, normal recipe-writing
                rules apply for setting up a
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename>.
                Thus, you need to specify any necessary patches and set
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'>S</ulink></filename>
                to point at the source code.
                You need to create a <filename>do_configure</filename> task that
                configures the unpacked kernel with a
                <filename>defconfig</filename> file.
                You can do this by using a <filename>make defconfig</filename>
                command or, more commonly, by copying in a suitable
                <filename>defconfig</filename> file and then running
                <filename>make oldconfig</filename>.
                By making use of <filename>inherit kernel</filename> and
                potentially some of the <filename>linux-*.inc</filename> files,
                most other functionality is centralized and the defaults of the
                class normally work well.
            </para>

            <para>
                If you are extending an existing kernel recipe, it is usually
                a matter of adding a suitable <filename>defconfig</filename>
                file.
                The file needs to be added into a location similar to
                <filename>defconfig</filename> files used for other machines
                in a given kernel recipe.
                A possible way to do this is by listing the file in the
                <filename>SRC_URI</filename> and adding the machine to the
                expression in
                <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-COMPATIBLE_MACHINE'>COMPATIBLE_MACHINE</ulink></filename>:
                <literallayout class='monospaced'>
     COMPATIBLE_MACHINE = '(qemux86|qemumips)'
                </literallayout>
                For more information on <filename>defconfig</filename> files,
                see the
                "<ulink url='&YOCTO_DOCS_KERNEL_DEV_URL;#changing-the-configuration'>Changing the Configuration</ulink>"
                section in the Yocto Project Linux Kernel Development Manual.
            </para>
        </section>

        <section id="platdev-newmachine-formfactor">
            <title>Adding a Formfactor Configuration File</title>

            <para>
                A formfactor configuration file provides information about the
                target hardware for which the image is being built and information that
                the build system cannot obtain from other sources such as the kernel.
                Some examples of information contained in a formfactor configuration file include
                framebuffer orientation, whether or not the system has a keyboard,
                the positioning of the keyboard in relation to the screen, and
                the screen resolution.
            </para>

            <para>
                The build system uses reasonable defaults in most cases.
                However, if customization is
                necessary, you need to create a <filename>machconfig</filename> file
                in the <filename>meta/recipes-bsp/formfactor/files</filename>
                directory.
                This directory contains directories for specific machines such as
                <filename>qemuarm</filename> and <filename>qemux86</filename>.
                For information about the settings available and the defaults, see the
                <filename>meta/recipes-bsp/formfactor/files/config</filename> file found in the
                same area.
            </para>

            <para>
                Following is an example for "qemuarm" machine:
                <literallayout class='monospaced'>
     HAVE_TOUCHSCREEN=1
     HAVE_KEYBOARD=1

     DISPLAY_CAN_ROTATE=0
     DISPLAY_ORIENTATION=0
     #DISPLAY_WIDTH_PIXELS=640
     #DISPLAY_HEIGHT_PIXELS=480
     #DISPLAY_BPP=16
     DISPLAY_DPI=150
     DISPLAY_SUBPIXEL_ORDER=vrgb
                </literallayout>
            </para>
        </section>
    </section>

    <section id='gs-upgrading-recipes'>
        <title>Upgrading Recipes</title>

        <para>
            Over time, upstream developers publish new versions for software
            built by layer recipes.
            It is recommended to keep recipes up-to-date with upstream
            version releases.
        </para>

        <para>
            While several methods exist that allow you upgrade a recipe,
            you might consider checking on the upgrade status of a recipe
            first.
            You can do so using the
            <filename>devtool check-upgrade-status</filename> command.
            See the
            "<ulink url='&YOCTO_DOCS_REF_URL;#devtool-checking-on-the-upgrade-status-of-a-recipe'>Checking on the Upgrade Status of a Recipe</ulink>"
            section in the Yocto Project Reference Manual for more information.
        </para>

        <para>
            The remainder of this section describes three ways you can
            upgrade a recipe.
            You can use the Automated Upgrade Helper (AUH) to set up
            automatic version upgrades.
            Alternatively, you can use <filename>devtool upgrade</filename>
            to set up semi-automatic version upgrades.
            Finally, you can manually upgrade a recipe by editing the
            recipe itself.
        </para>

        <section id='gs-using-the-auto-upgrade-helper'>
            <title>Using the Auto Upgrade Helper (AUH)</title>

            <para>
                The AUH utility works in conjunction with the
                OpenEmbedded build system in order to automatically generate
                upgrades for recipes based on new versions being
                published upstream.
                Use AUH when you want to create a service that performs the
                upgrades automatically and optionally sends you an email with
                the results.
            </para>

            <para>
                AUH allows you to update several recipes with a single use.
                You can also optionally perform build and integration tests
                using images with the results saved to your hard drive and
                emails of results optionally sent to recipe maintainers.
                Finally, AUH creates Git commits with appropriate commit
                messages in the layer's tree for the changes made to recipes.
                <note>
                    Conditions do exist when you should not use AUH to upgrade
                    recipes and you should instead use either
                    <filename>devtool upgrade</filename> or upgrade your
                    recipes manually:
                    <itemizedlist>
                        <listitem><para>
                            When AUH cannot complete the upgrade sequence.
                            This situation usually results because custom
                            patches carried by the recipe cannot be
                            automatically rebased to the new version.
                            In this case, <filename>devtool upgrade</filename>
                            allows you to manually resolve conflicts.
                            </para></listitem>
                        <listitem><para>
                            When for any reason you want fuller control over
                            the upgrade process.
                            For example, when you want special arrangements
                            for testing.
                            </para></listitem>
                    </itemizedlist>
                </note>
            </para>

            <para>
                The following steps describe how to set up the AUH utility:
                <orderedlist>
                    <listitem><para>
                        <emphasis>Be Sure the Development Host is Set Up:</emphasis>
                        You need to be sure that your development host is
                        set up to use the Yocto Project.
                        For information on how to set up your host, see the
                        "<link linkend='dev-preparing-the-build-host'>Preparing the Build Host</link>"
                        section.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Make Sure Git is Configured:</emphasis>
                        The AUH utility requires Git to be configured because
                        AUH uses Git to save upgrades.
                        Thus, you must have Git user and email configured.
                        The following command shows your configurations:
                        <literallayout class='monospaced'>
     $ git config --list
                        </literallayout>
                        If you do not have the user and email configured, you
                        can use the following commands to do so:
                        <literallayout class='monospaced'>
     $ git config --global user.name <replaceable>some_name</replaceable>
     $ git config --global user.email <replaceable>username</replaceable>@<replaceable>domain</replaceable>.com
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Clone the AUH Repository:</emphasis>
                        To use AUH, you must clone the repository onto your
                        development host.
                        The following command uses Git to create a local
                        copy of the repository on your system:
                        <literallayout class='monospaced'>
     $ git clone git://git.yoctoproject.org/auto-upgrade-helper
     Cloning into 'auto-upgrade-helper'...
     remote: Counting objects: 768, done.
     remote: Compressing objects: 100% (300/300), done.
     remote: Total 768 (delta 499), reused 703 (delta 434)
     Receiving objects: 100% (768/768), 191.47 KiB | 98.00 KiB/s, done.
     Resolving deltas: 100% (499/499), done.
     Checking connectivity... done.
                        </literallayout>
                        AUH is not part of the
                        <ulink url='&YOCTO_DOCS_REF_URL;#oe-core'>OpenEmbedded-Core (OE-Core)</ulink>
                        or
                        <ulink url='&YOCTO_DOCS_REF_URL;#poky'>Poky</ulink>
                        repositories.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Create a Dedicated Build Directory:</emphasis>
                        Run the
                        <ulink url='&YOCTO_DOCS_REF_URL;#structure-core-script'><filename>oe-init-build-env</filename></ulink>
                        script to create a fresh build directory that you
                        use exclusively for running the AUH utility:
                        <literallayout class='monospaced'>
     $ cd ~/poky
     $ source oe-init-build-env <replaceable>your_AUH_build_directory</replaceable>
                        </literallayout>
                        Re-using an existing build directory and its
                        configurations is not recommended as existing settings
                        could cause AUH to fail or behave undesirably.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Make Configurations in Your Local Configuration File:</emphasis>
                        Several settings need to exist in the
                        <filename>local.conf</filename> file in the build
                        directory you just created for AUH.
                        Make these following configurations:
                        <itemizedlist>
                            <listitem><para>
                                If you want to enable
                                <ulink url='&YOCTO_DOCS_DEV_URL;#maintaining-build-output-quality'>Build History</ulink>,
                                which is optional, you need the following
                                lines in the
                                <filename>conf/local.conf</filename> file:
                                <literallayout class='monospaced'>
     INHERIT =+ "buildhistory"
     BUILDHISTORY_COMMIT = "1"
                                </literallayout>
                                With this configuration and a successful
                                upgrade, a build history "diff" file appears in
                                the
                                <filename>upgrade-helper/work/recipe/buildhistory-diff.txt</filename>
                                file found in your build directory.
                                </para></listitem>
                            <listitem><para>
                                If you want to enable testing through the
                                <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-testimage*'><filename>testimage</filename></ulink>
                                class, which is optional, you need to have the
                                following set in your
                                <filename>conf/local.conf</filename> file:
                                <literallayout class='monospaced'>
     INHERIT += "testimage"
                                </literallayout>
                                <note>
                                    If your distro does not enable by default
                                    ptest, which Poky does, you need the
                                    following in your
                                    <filename>local.conf</filename> file:
                                    <literallayout class='monospaced'>
     DISTRO_FEATURES_append = " ptest"
                                    </literallayout>
                                </note>
                                </para></listitem>
                        </itemizedlist>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Optionally Start a vncserver:</emphasis>
                        If you are running in a server without an X11 session,
                        you need to start a vncserver:
                        <literallayout class='monospaced'>
     $ vncserver :1
     $ export DISPLAY=:1
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Create and Edit an AUH Configuration File:</emphasis>
                        You need to have the
                        <filename>upgrade-helper/upgrade-helper.conf</filename>
                        configuration file in your build directory.
                        You can find a sample configuration file in the
                        <ulink url='http://git.yoctoproject.org/cgit/cgit.cgi/auto-upgrade-helper/tree/'>AUH source repository</ulink>.
                        </para>

                        <para>Read through the sample file and make
                        configurations as needed.
                        For example, if you enabled build history in your
                        <filename>local.conf</filename> as described earlier,
                        you must enable it in
                        <filename>upgrade-helper.conf</filename>.</para>

                        <para>Also, if you are using the default
                        <filename>maintainers.inc</filename> file supplied
                        with Poky and located in
                        <filename>meta-yocto</filename> and you do not set a
                        "maintainers_whitelist" or "global_maintainer_override"
                        in the <filename>upgrade-helper.conf</filename>
                        configuration, and you specify "-e all" on the
                        AUH command-line, the utility automatically sends out
                        emails to all the default maintainers.
                        Please avoid this.
                        </para></listitem>
                </orderedlist>
            </para>

            <para>
                This next set of examples describes how to use the AUH:
                <itemizedlist>
                    <listitem><para>
                        <emphasis>Upgrading a Specific Recipe:</emphasis>
                        To upgrade a specific recipe, use the following
                        form:
                        <literallayout class='monospaced'>
     $ upgrade-helper.py <replaceable>recipe_name</replaceable>
                        </literallayout>
                        For example, this command upgrades the
                        <filename>xmodmap</filename> recipe:
                        <literallayout class='monospaced'>
     $ upgrade-helper.py xmodmap
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Upgrading a Specific Recipe to a Particular Version:</emphasis>
                        To upgrade a specific recipe to a particular version,
                        use the following form:
                        <literallayout class='monospaced'>
     $ upgrade-helper.py <replaceable>recipe_name</replaceable> -t <replaceable>version</replaceable>
                        </literallayout>
                        For example, this command upgrades the
                        <filename>xmodmap</filename> recipe to version
                        1.2.3:
                        <literallayout class='monospaced'>
     $ upgrade-helper.py xmodmap -t 1.2.3
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Upgrading all Recipes to the Latest Versions and Suppressing Email Notifications:</emphasis>
                        To upgrade all recipes to their most recent versions
                        and suppress the email notifications, use the following
                        command:
                        <literallayout class='monospaced'>
     $ upgrade-helper.py all
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Upgrading all Recipes to the Latest Versions and Send Email Notifications:</emphasis>
                        To upgrade all recipes to their most recent versions
                        and send email messages to maintainers for each
                        attempted recipe as well as a status email, use the
                        following command:
                        <literallayout class='monospaced'>
     $ upgrade-helper.py -e all
                        </literallayout>
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                Once you have run the AUH utility, you can find the results
                in the AUH build directory:
                <literallayout class='monospaced'>
     ${BUILDDIR}/upgrade-helper/<replaceable>timestamp</replaceable>
                </literallayout>
                The AUH utility also creates recipe update commits from
                successful upgrade attempts in the layer tree.
            </para>

            <para>
                You can easily set up to run the AUH utility on a regular
                basis by using a cron job.
                See the
                <ulink url='http://git.yoctoproject.org/cgit/cgit.cgi/auto-upgrade-helper/tree/weeklyjob.sh'><filename>weeklyjob.sh</filename></ulink>
                file distributed with the utility for an example.
            </para>
        </section>

        <section id='gs-using-devtool-upgrade'>
            <title>Using <filename>devtool upgrade</filename></title>

            <para>
                As mentioned earlier, an alternative method for upgrading
                recipes to newer versions is to use
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-devtool-reference'><filename>devtool upgrade</filename></ulink>.
                You can read about <filename>devtool upgrade</filename> in
                general in the
                "<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-devtool-use-devtool-upgrade-to-create-a-version-of-the-recipe-that-supports-a-newer-version-of-the-software'>Use <filename>devtool upgrade</filename> to Create a Version of the Recipe that Supports a Newer Version of the Software</ulink>"
                section in the Yocto Project Application Development and the
                Extensible Software Development Kit (eSDK) Manual.
            </para>

            <para>
                To see all the command-line options available with
                <filename>devtool upgrade</filename>, use the following help
                command:
                <literallayout class='monospaced'>
     $ devtool upgrade -h
                </literallayout>
            </para>

            <para>
                If you want to find out what version a recipe is currently at
                upstream without any attempt to upgrade your local version of
                the recipe, you can use the following command:
                <literallayout class='monospaced'>
     $ devtool latest-version <replaceable>recipe_name</replaceable>
                </literallayout>
            </para>

            <para>
                As mentioned in the previous section describing AUH,
                <filename>devtool upgrade</filename> works in a
                less-automated manner than AUH.
                Specifically, <filename>devtool upgrade</filename> only
                works on a single recipe that you name on the command line,
                cannot perform build and integration testing using images,
                and does not automatically generate commits for changes in
                the source tree.
                Despite all these "limitations",
                <filename>devtool upgrade</filename> updates the recipe file
                to the new upstream version and attempts to rebase custom
                patches contained by the recipe as needed.
                <note>
                    AUH uses much of <filename>devtool upgrade</filename>
                    behind the scenes making AUH somewhat of a "wrapper"
                    application for <filename>devtool upgrade</filename>.
                </note>
            </para>

            <para>
                A typical scenario involves having used Git to clone an
                upstream repository that you use during build operations.
                Because you are (or have) built the recipe in the past, the
                layer is likely added to your configuration already.
                If for some reason, the layer is not added, you could add
                it easily using the
                <ulink url='&YOCTO_DOCS_BSP_URL;#creating-a-new-bsp-layer-using-the-bitbake-layers-script'><filename>bitbake-layers</filename></ulink>
                script.
                For example, suppose you use the <filename>nano.bb</filename>
                recipe from the <filename>meta-oe</filename> layer in the
                <filename>meta-openembedded</filename> repository.
                For this example, assume that the layer has been cloned into
                following area:
                <literallayout class='monospaced'>
     /home/scottrif/meta-openembedded
                </literallayout>
                The following command from your
                <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
                adds the layer to your build configuration (i.e.
                <filename>${BUILDDIR}/conf/bblayers.conf</filename>):
                <literallayout class='monospaced'>
     $ bitbake-layers add-layer /home/scottrif/meta-openembedded/meta-oe
     NOTE: Starting bitbake server...
     Parsing recipes: 100% |##########################################| Time: 0:00:55
     Parsing of 1431 .bb files complete (0 cached, 1431 parsed). 2040 targets, 56 skipped, 0 masked, 0 errors.
     Removing 12 recipes from the x86_64 sysroot: 100% |##############| Time: 0:00:00
     Removing 1 recipes from the x86_64_i586 sysroot: 100% |##########| Time: 0:00:00
     Removing 5 recipes from the i586 sysroot: 100% |#################| Time: 0:00:00
     Removing 5 recipes from the qemux86 sysroot: 100% |##############| Time: 0:00:00
                </literallayout>
                For this example, assume that the <filename>nano.bb</filename>
                recipe that is upstream has a 2.9.3 version number.
                However, the version in the local repository is 2.7.4.
                The following command from your build directory automatically
                upgrades the recipe for you:
                <note>
                    Using the <filename>-V</filename> option is not necessary.
                    Omitting the version number causes
                    <filename>devtool upgrade</filename> to upgrade the recipe
                    to the most recent version.
                </note>
                <literallayout class='monospaced'>
     $ devtool upgrade nano -V 2.9.3
     NOTE: Starting bitbake server...
     NOTE: Creating workspace layer in /home/scottrif/poky/build/workspace
     Parsing recipes: 100% |##########################################| Time: 0:00:46
     Parsing of 1431 .bb files complete (0 cached, 1431 parsed). 2040 targets, 56 skipped, 0 masked, 0 errors.
     NOTE: Extracting current version source...
     NOTE: Resolving any missing task queue dependencies
            .
            .
            .
     NOTE: Executing SetScene Tasks
     NOTE: Executing RunQueue Tasks
     NOTE: Tasks Summary: Attempted 74 tasks of which 72 didn't need to be rerun and all succeeded.
     Adding changed files: 100% |#####################################| Time: 0:00:00
     NOTE: Upgraded source extracted to /home/scottrif/poky/build/workspace/sources/nano
     NOTE: New recipe is /home/scottrif/poky/build/workspace/recipes/nano/nano_2.9.3.bb
                </literallayout>
                Continuing with this example, you can use
                <filename>devtool build</filename> to build the newly upgraded
                recipe:
                <literallayout class='monospaced'>
     $ devtool build nano
     NOTE: Starting bitbake server...
     Loading cache: 100% |################################################################################################| Time: 0:00:01
     Loaded 2040 entries from dependency cache.
     Parsing recipes: 100% |##############################################################################################| Time: 0:00:00
     Parsing of 1432 .bb files complete (1431 cached, 1 parsed). 2041 targets, 56 skipped, 0 masked, 0 errors.
     NOTE: Resolving any missing task queue dependencies
            .
            .
            .
     NOTE: Executing SetScene Tasks
     NOTE: Executing RunQueue Tasks
     NOTE: nano: compiling from external source tree /home/scottrif/poky/build/workspace/sources/nano
     NOTE: Tasks Summary: Attempted 520 tasks of which 304 didn't need to be rerun and all succeeded.
                </literallayout>
                Within the <filename>devtool upgrade</filename> workflow,
                opportunity exists to deploy and test your rebuilt software.
                For this example, however, running
                <filename>devtool finish</filename> cleans up the workspace
                once the source in your workspace is clean.
                This usually means using Git to stage and submit commits
                for the changes generated by the upgrade process.
            </para>

            <para>
                Once the tree is clean, you can clean things up in this
                example with the following command from the
                <filename>${BUILDDIR}/workspace/sources/nano</filename>
                directory:
                <literallayout class='monospaced'>
     $ devtool finish nano meta-oe
     NOTE: Starting bitbake server...
     Loading cache: 100% |################################################################################################| Time: 0:00:00
     Loaded 2040 entries from dependency cache.
     Parsing recipes: 100% |##############################################################################################| Time: 0:00:01
     Parsing of 1432 .bb files complete (1431 cached, 1 parsed). 2041 targets, 56 skipped, 0 masked, 0 errors.
     NOTE: Adding new patch 0001-nano.bb-Stuff-I-changed-when-upgrading-nano.bb.patch
     NOTE: Updating recipe nano_2.9.3.bb
     NOTE: Removing file /home/scottrif/meta-openembedded/meta-oe/recipes-support/nano/nano_2.7.4.bb
     NOTE: Moving recipe file to /home/scottrif/meta-openembedded/meta-oe/recipes-support/nano
     NOTE: Leaving source tree /home/scottrif/poky/build/workspace/sources/nano as-is; if you no longer need it then please delete it manually
                </literallayout>
                Using the <filename>devtool finish</filename> command cleans
                up the workspace and creates a patch file based on your
                commits.
                The tool puts all patch files back into the source directory
                in a sub-directory named <filename>nano</filename> in this
                case.
            </para>
        </section>

        <section id='dev-manually-upgrading-a-recipe'>
            <title>Manually Upgrading a Recipe</title>

            <para>
                If for some reason you choose not to upgrade recipes using the
                <link linkend='gs-using-the-auto-upgrade-helper'>Auto Upgrade Helper (AUH)</link>
                or by using
                <link linkend='gs-using-devtool-upgrade'><filename>devtool upgrade</filename></link>,
                you can manually edit the recipe files to upgrade the versions.
                <note><title>Caution</title>
                    Manually updating multiple recipes scales poorly and
                    involves many steps.
                    The recommendation to upgrade recipe versions is through
                    AUH or <filename>devtool upgrade</filename>, both of which
                    automate some steps and provide guidance for others needed
                    for the manual process.
                </note>
            </para>

            <para>
                To manually upgrade recipe versions, follow these general steps:
                <orderedlist>
                    <listitem><para>
                        <emphasis>Change the Version:</emphasis>
                        Rename the recipe such that the version (i.e. the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>
                        part of the recipe name) changes appropriately.
                        If the version is not part of the recipe name, change
                        the value as it is set for <filename>PV</filename>
                        within the recipe itself.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Update <filename>SRCREV</filename> if Needed:</emphasis>
                        If the source code your recipe builds is fetched from
                        Git or some other version control system, update
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>
                        to point to the commit hash that matches the new
                        version.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Build the Software:</emphasis>
                        Try to build the recipe using BitBake.
                        Typical build failures include the following:
                        <itemizedlist>
                            <listitem><para>
                                License statements were updated for the new
                                version.
                                For this case, you need to review any changes
                                to the license and update the values of
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE'><filename>LICENSE</filename></ulink>
                                and
                                <ulink url='&YOCTO_DOCS_REF_URL;#var-LIC_FILES_CHKSUM'><filename>LIC_FILES_CHKSUM</filename></ulink>
                                as needed.
                                <note>
                                    License changes are often inconsequential.
                                    For example, the license text's copyright
                                    year might have changed.
                                </note>
                                </para></listitem>
                            <listitem><para>
                                Custom patches carried by the older version of
                                the recipe might fail to apply to the new
                                version.
                                For these cases, you need to review the
                                failures.
                                Patches might not be necessary for the new
                                version of the software if the upgraded version
                                has fixed those issues.
                                If a patch is necessary and failing, you need
                                to rebase it into the new version.
                                </para></listitem>
                        </itemizedlist>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Optionally Attempt to Build for Several Architectures:</emphasis>
                        Once you successfully build the new software for a
                        given architecture, you could test the build for
                        other architectures by changing the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
                        variable and rebuilding the software.
                        This optional step is especially important if the
                        recipe is to be released publicly.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Check the Upstream Change Log or Release Notes:</emphasis>
                        Checking both these reveals if new features exist that
                        could break backwards-compatibility.
                        If so, you need to take steps to mitigate or eliminate
                        that situation.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Optionally Create a Bootable Image and Test:</emphasis>
                        If you want, you can test the new software by booting
                        it onto actual hardware.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Create a Commit with the Change in the Layer Repository:</emphasis>
                        After all builds work and any testing is successful,
                        you can create commits for any changes in the layer
                        holding your upgraded recipe.
                        </para></listitem>
                </orderedlist>
            </para>
        </section>
    </section>

    <section id='finding-the-temporary-source-code'>
        <title>Finding Temporary Source Code</title>

        <para>
            You might find it helpful during development to modify the
            temporary source code used by recipes to build packages.
            For example, suppose you are developing a patch and you need to
            experiment a bit to figure out your solution.
            After you have initially built the package, you can iteratively
            tweak the source code, which is located in the
            <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>,
            and then you can force a re-compile and quickly test your altered
            code.
            Once you settle on a solution, you can then preserve your changes
            in the form of patches.
        </para>

        <para>
            During a build, the unpacked temporary source code used by recipes
            to build packages is available in the Build Directory as
            defined by the
            <ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink>
            variable.
            Below is the default value for the <filename>S</filename> variable
            as defined in the
            <filename>meta/conf/bitbake.conf</filename> configuration file
            in the
            <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>:
            <literallayout class='monospaced'>
     S = "${WORKDIR}/${BP}"
            </literallayout>
            You should be aware that many recipes override the
            <filename>S</filename> variable.
            For example, recipes that fetch their source from Git usually set
            <filename>S</filename> to <filename>${WORKDIR}/git</filename>.
            <note>
                The
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BP'><filename>BP</filename></ulink>
                represents the base recipe name, which consists of the name
                and version:
                <literallayout class='monospaced'>
     BP = "${BPN}-${PV}"
                </literallayout>
            </note>
        </para>

        <para>
            The path to the work directory for the recipe
            (<ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>)
            is defined as follows:
            <literallayout class='monospaced'>
     ${TMPDIR}/work/${MULTIMACH_TARGET_SYS}/${PN}/${EXTENDPE}${PV}-${PR}
            </literallayout>
            The actual directory depends on several things:
            <itemizedlist>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>:
                    The top-level build output directory.
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-MULTIMACH_TARGET_SYS'><filename>MULTIMACH_TARGET_SYS</filename></ulink>:
                    The target system identifier.
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink>:
                    The recipe name.
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTENDPE'><filename>EXTENDPE</filename></ulink>:
                    The epoch - (if
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PE'><filename>PE</filename></ulink>
                    is not specified, which is usually the case for most
                    recipes, then <filename>EXTENDPE</filename> is blank).
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PV'><filename>PV</filename></ulink>:
                    The recipe version.
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>:
                    The recipe revision.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            As an example, assume a Source Directory top-level folder
            named <filename>poky</filename>, a default Build Directory at
            <filename>poky/build</filename>, and a
            <filename>qemux86-poky-linux</filename> machine target
            system.
            Furthermore, suppose your recipe is named
            <filename>foo_1.3.0.bb</filename>.
            In this case, the work directory the build system uses to
            build the package would be as follows:
            <literallayout class='monospaced'>
     poky/build/tmp/work/qemux86-poky-linux/foo/1.3.0-r0
            </literallayout>
        </para>
    </section>

    <section id="using-a-quilt-workflow">
        <title>Using Quilt in Your Workflow</title>

        <para>
            <ulink url='http://savannah.nongnu.org/projects/quilt'>Quilt</ulink>
            is a powerful tool that allows you to capture source code changes
            without having a clean source tree.
            This section outlines the typical workflow you can use to modify
            source code, test changes, and then preserve the changes in the
            form of a patch all using Quilt.
            <note><title>Tip</title>
                With regard to preserving changes to source files, if you
                clean a recipe or have <filename>rm_work</filename> enabled,
                the
                <ulink url='&YOCTO_DOCS_SDK_URL;#using-devtool-in-your-sdk-workflow'><filename>devtool</filename> workflow</ulink>
                as described in the Yocto Project Application Development
                and the Extensible Software Development Kit (eSDK) manual
                is a safer development flow than the flow that uses Quilt.
            </note>
        </para>

        <para>
            Follow these general steps:
            <orderedlist>
                <listitem><para>
                    <emphasis>Find the Source Code:</emphasis>
                    Temporary source code used by the OpenEmbedded build system
                    is kept in the
                    <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
                    See the
                    "<link linkend='finding-the-temporary-source-code'>Finding Temporary Source Code</link>"
                    section to learn how to locate the directory that has the
                    temporary source code for a particular package.
                    </para></listitem>
                <listitem><para>
                    <emphasis>Change Your Working Directory:</emphasis>
                    You need to be in the directory that has the temporary
                    source code.
                    That directory is defined by the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink>
                    variable.</para></listitem>
                <listitem><para>
                    <emphasis>Create a New Patch:</emphasis>
                    Before modifying source code, you need to create a new
                    patch.
                    To create a new patch file, use
                    <filename>quilt new</filename> as below:
                    <literallayout class='monospaced'>
     $ quilt new my_changes.patch
                    </literallayout>
                    </para></listitem>
                <listitem><para>
                    <emphasis>Notify Quilt and Add Files:</emphasis>
                    After creating the patch, you need to notify Quilt about
                    the files you plan to edit.
                    You notify Quilt by adding the files to the patch you
                    just created:
                    <literallayout class='monospaced'>
     $ quilt add file1.c file2.c file3.c
                    </literallayout>
                    </para></listitem>
                <listitem><para>
                    <emphasis>Edit the Files:</emphasis>
                    Make your changes in the source code to the files you added
                    to the patch.
                    </para></listitem>
                <listitem><para>
                    <emphasis>Test Your Changes:</emphasis>
                    Once you have modified the source code, the easiest way to
                    test your changes is by calling the
                    <filename>do_compile</filename> task as shown in the
                    following example:
                    <literallayout class='monospaced'>
     $ bitbake -c compile -f <replaceable>package</replaceable>
                    </literallayout>
                    The <filename>-f</filename> or <filename>--force</filename>
                    option forces the specified task to execute.
                    If you find problems with your code, you can just keep
                    editing and re-testing iteratively until things work
                    as expected.
                    <note>
                        All the modifications you make to the temporary
                        source code disappear once you run the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-clean'><filename>do_clean</filename></ulink>
                        or
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-cleanall'><filename>do_cleanall</filename></ulink>
                        tasks using BitBake (i.e.
                        <filename>bitbake -c clean <replaceable>package</replaceable></filename>
                        and
                        <filename>bitbake -c cleanall <replaceable>package</replaceable></filename>).
                        Modifications will also disappear if you use the
                        <filename>rm_work</filename> feature as described
                        in the
                        "<ulink url='&YOCTO_DOCS_DEV_URL;#dev-saving-memory-during-a-build'>Conserving Disk Space During Builds</ulink>"
                        section.
                    </note>
                    </para></listitem>
                <listitem><para>
                    <emphasis>Generate the Patch:</emphasis>
                    Once your changes work as expected, you need to use Quilt
                    to generate the final patch that contains all your
                    modifications.
                    <literallayout class='monospaced'>
     $ quilt refresh
                    </literallayout>
                    At this point, the <filename>my_changes.patch</filename>
                    file has all your edits made to the
                    <filename>file1.c</filename>, <filename>file2.c</filename>,
                    and <filename>file3.c</filename> files.</para>

                    <para>You can find the resulting patch file in the
                    <filename>patches/</filename> subdirectory of the source
                    (<filename>S</filename>) directory.
                    </para></listitem>
                <listitem><para>
                    <emphasis>Copy the Patch File:</emphasis>
                    For simplicity, copy the patch file into a directory
                    named <filename>files</filename>, which you can create
                    in the same directory that holds the recipe
                    (<filename>.bb</filename>) file or the append
                    (<filename>.bbappend</filename>) file.
                    Placing the patch here guarantees that the OpenEmbedded
                    build system will find the patch.
                    Next, add the patch into the
                    <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'>SRC_URI</ulink></filename>
                    of the recipe.
                    Here is an example:
                    <literallayout class='monospaced'>
     SRC_URI += "file://my_changes.patch"
                    </literallayout>
                    </para></listitem>
            </orderedlist>
        </para>
    </section>

    <section id="platdev-appdev-devshell">
        <title>Using a Development Shell</title>

        <para>
            When debugging certain commands or even when just editing packages,
            <filename>devshell</filename> can be a useful tool.
            When you invoke <filename>devshell</filename>, all tasks up to and
            including
            <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-patch'><filename>do_patch</filename></ulink>
            are run for the specified target.
            Then, a new terminal is opened and you are placed in
            <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink><filename>}</filename>,
            the source directory.
            In the new terminal, all the OpenEmbedded build-related environment variables are
            still defined so you can use commands such as <filename>configure</filename> and
            <filename>make</filename>.
            The commands execute just as if the OpenEmbedded build system were executing them.
            Consequently, working this way can be helpful when debugging a build or preparing
            software to be used with the OpenEmbedded build system.
        </para>

        <para>
            Following is an example that uses <filename>devshell</filename> on a target named
            <filename>matchbox-desktop</filename>:
            <literallayout class='monospaced'>
     $ bitbake matchbox-desktop -c devshell
            </literallayout>
        </para>

        <para>
            This command spawns a terminal with a shell prompt within the OpenEmbedded build environment.
            The <ulink url='&YOCTO_DOCS_REF_URL;#var-OE_TERMINAL'><filename>OE_TERMINAL</filename></ulink>
            variable controls what type of shell is opened.
        </para>

        <para>
            For spawned terminals, the following occurs:
            <itemizedlist>
                <listitem><para>The <filename>PATH</filename> variable includes the
                    cross-toolchain.</para></listitem>
                <listitem><para>The <filename>pkgconfig</filename> variables find the correct
                    <filename>.pc</filename> files.</para></listitem>
                <listitem><para>The <filename>configure</filename> command finds the
                    Yocto Project site files as well as any other necessary files.</para></listitem>
            </itemizedlist>
        </para>

        <para>
            Within this environment, you can run configure or compile
            commands as if they were being run by
            the OpenEmbedded build system itself.
            As noted earlier, the working directory also automatically changes to the
            Source Directory (<ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink>).
        </para>

        <para>
            To manually run a specific task using <filename>devshell</filename>,
            run the corresponding <filename>run.*</filename> script in
            the
            <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}/temp</filename>
            directory (e.g.,
            <filename>run.do_configure.</filename><replaceable>pid</replaceable>).
            If a task's script does not exist, which would be the case if the task was
            skipped by way of the sstate cache, you can create the task by first running
            it outside of the <filename>devshell</filename>:
            <literallayout class='monospaced'>
     $ bitbake -c <replaceable>task</replaceable>
            </literallayout>
            <note><title>Notes</title>
                <itemizedlist>
                    <listitem><para>Execution of a task's <filename>run.*</filename>
                        script and BitBake's execution of a task are identical.
                        In other words, running the script re-runs the task
                        just as it would be run using the
                        <filename>bitbake -c</filename> command.
                        </para></listitem>
                    <listitem><para>Any <filename>run.*</filename> file that does not
                        have a <filename>.pid</filename> extension is a
                        symbolic link (symlink) to the most recent version of that
                        file.
                        </para></listitem>
                </itemizedlist>
            </note>
        </para>

        <para>
            Remember, that the <filename>devshell</filename> is a mechanism that allows
            you to get into the BitBake task execution environment.
            And as such, all commands must be called just as BitBake would call them.
            That means you need to provide the appropriate options for
            cross-compilation and so forth as applicable.
        </para>

        <para>
            When you are finished using <filename>devshell</filename>, exit the shell
            or close the terminal window.
        </para>

        <note><title>Notes</title>
            <itemizedlist>
                <listitem><para>
                    It is worth remembering that when using <filename>devshell</filename>
                    you need to use the full compiler name such as <filename>arm-poky-linux-gnueabi-gcc</filename>
                    instead of just using <filename>gcc</filename>.
                    The same applies to other applications such as <filename>binutils</filename>,
                    <filename>libtool</filename> and so forth.
                    BitBake sets up environment variables such as <filename>CC</filename>
                    to assist applications, such as <filename>make</filename> to find the correct tools.
                    </para></listitem>
                <listitem><para>
                    It is also worth noting that <filename>devshell</filename> still works over
                    X11 forwarding and similar situations.
                    </para></listitem>
            </itemizedlist>
        </note>
    </section>

    <section id="platdev-appdev-devpyshell">
        <title>Using a Development Python Shell</title>

        <para>
            Similar to working within a development shell as described in
            the previous section, you can also spawn and work within an
            interactive Python development shell.
            When debugging certain commands or even when just editing packages,
            <filename>devpyshell</filename> can be a useful tool.
            When you invoke <filename>devpyshell</filename>, all tasks up to and
            including
            <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-patch'><filename>do_patch</filename></ulink>
            are run for the specified target.
            Then a new terminal is opened.
            Additionally, key Python objects and code are available in the same
            way they are to BitBake tasks, in particular, the data store 'd'.
            So, commands such as the following are useful when exploring the data
            store and running functions:
            <literallayout class='monospaced'>
     pydevshell> d.getVar("STAGING_DIR")
     '/media/build1/poky/build/tmp/sysroots'
     pydevshell> d.getVar("STAGING_DIR")
     '${TMPDIR}/sysroots'
     pydevshell> d.setVar("FOO", "bar")
     pydevshell> d.getVar("FOO")
     'bar'
     pydevshell> d.delVar("FOO")
     pydevshell> d.getVar("FOO")
     pydevshell> bb.build.exec_func("do_unpack", d)
     pydevshell>
            </literallayout>
            The commands execute just as if the OpenEmbedded build system were executing them.
            Consequently, working this way can be helpful when debugging a build or preparing
            software to be used with the OpenEmbedded build system.
        </para>

        <para>
            Following is an example that uses <filename>devpyshell</filename> on a target named
            <filename>matchbox-desktop</filename>:
            <literallayout class='monospaced'>
     $ bitbake matchbox-desktop -c devpyshell
            </literallayout>
        </para>

        <para>
            This command spawns a terminal and places you in an interactive
            Python interpreter within the OpenEmbedded build environment.
            The <ulink url='&YOCTO_DOCS_REF_URL;#var-OE_TERMINAL'><filename>OE_TERMINAL</filename></ulink>
            variable controls what type of shell is opened.
        </para>

        <para>
            When you are finished using <filename>devpyshell</filename>, you
            can exit the shell either by using Ctrl+d or closing the terminal
            window.
        </para>
    </section>

    <section id='dev-building'>
        <title>Building</title>

        <para>
            This section describes various build procedures.
            For example, the steps needed for a simple build, a target that
            uses multiple configurations, building an image for more than
            one machine, and so forth.
        </para>

        <section id='dev-building-a-simple-image'>
            <title>Building a Simple Image</title>

            <para>
                In the development environment, you need to build an image
                whenever you change hardware support, add or change system
                libraries, or add or change services that have dependencies.
                Several methods exist that allow you to build an image within
                the Yocto Project.
                This section presents the basic steps you need to build a
                simple image using BitBake from a build host running Linux.
                <note><title>Notes</title>
                    <itemizedlist>
                        <listitem><para>
                            For information on how to build an image using
                            <ulink url='&YOCTO_DOCS_REF_URL;#toaster-term'>Toaster</ulink>,
                            see the
                            <ulink url='&YOCTO_DOCS_TOAST_URL;'>Toaster User Manual</ulink>.
                            </para></listitem>
                        <listitem><para>
                            For information on how to use
                            <filename>devtool</filename> to build images, see
                            the
                            "<ulink url='&YOCTO_DOCS_SDK_URL;#using-devtool-in-your-sdk-workflow'>Using <filename>devtool</filename> in Your SDK Workflow</ulink>"
                            section in the Yocto Project Application
                            Development and the Extensible Software Development
                            Kit (eSDK) manual.
                            </para></listitem>
                        <listitem><para>
                            For a quick example on how to build an image using
                            the OpenEmbedded build system, see the
                            <ulink url='&YOCTO_DOCS_BRIEF_URL;'>Yocto Project Quick Build</ulink>
                            document.
                            </para></listitem>
                    </itemizedlist>
                </note>
            </para>

            <para>
                The build process creates an entire Linux distribution from
                source and places it in your
                <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
                under <filename>tmp/deploy/images</filename>.
                For detailed information on the build process using BitBake,
                see the
                "<ulink url='&YOCTO_DOCS_OM_URL;#images-dev-environment'>Images</ulink>"
                section in the Yocto Project Overview and Concepts Manual.
            </para>

            <para>
                The following figure and list overviews the build process:
                <imagedata fileref="figures/bitbake-build-flow.png" width="7in" depth="4in" align="center" scalefit="1" />
                <orderedlist>
                    <listitem><para>
                        <emphasis>Set up Your Host Development System to Support
                        Development Using the Yocto Project</emphasis>:
                        See the
                        "<link linkend='dev-manual-start'>Setting Up to Use the Yocto Project</link>"
                        section for options on how to get a build host ready to
                        use the Yocto Project.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Initialize the Build Environment:</emphasis>
                        Initialize the build environment by sourcing the build
                        environment script (i.e.
                        <ulink url='&YOCTO_DOCS_REF_URL;#structure-core-script'><filename>&OE_INIT_FILE;</filename></ulink>):
                        <literallayout class='monospaced'>
     $ source &OE_INIT_FILE; [<replaceable>build_dir</replaceable>]
                        </literallayout></para>

                        <para>When you use the initialization script, the
                        OpenEmbedded build system uses
                        <filename>build</filename> as the default Build
                        Directory in your current work directory.
                        You can use a <replaceable>build_dir</replaceable>
                        argument with the script to specify a different build
                        directory.
                        <note><title>Tip</title>
                            A common practice is to use a different Build
                            Directory for different targets.
                            For example, <filename>~/build/x86</filename> for a
                            <filename>qemux86</filename> target, and
                            <filename>~/build/arm</filename> for a
                            <filename>qemuarm</filename> target.
                        </note>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Make Sure Your <filename>local.conf</filename>
                        File is Correct:</emphasis>
                        Ensure the <filename>conf/local.conf</filename>
                        configuration file, which is found in the Build
                        Directory, is set up how you want it.
                        This file defines many aspects of the build environment
                        including the target machine architecture through the
                        <filename><ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'>MACHINE</ulink></filename> variable,
                        the packaging format used during the build
                        (<ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></ulink>),
                        and a centralized tarball download directory through the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-DL_DIR'><filename>DL_DIR</filename></ulink> variable.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Build the Image:</emphasis>
                        Build the image using the <filename>bitbake</filename>
                        command:
                        <literallayout class='monospaced'>
     $ bitbake <replaceable>target</replaceable>
                        </literallayout>
                        <note>
                            For information on BitBake, see the
                            <ulink url='&YOCTO_DOCS_BB_URL;'>BitBake User Manual</ulink>.
                        </note>
                        The <replaceable>target</replaceable> is the name of the
                        recipe you want to build.
                        Common targets are the images in
                        <filename>meta/recipes-core/images</filename>,
                        <filename>meta/recipes-sato/images</filename>, and so
                        forth all found in the
                        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>.
                        Or, the target can be the name of a recipe for a
                        specific piece of software such as BusyBox.
                        For more details about the images the OpenEmbedded build
                        system supports, see the
                        "<ulink url='&YOCTO_DOCS_REF_URL;#ref-images'>Images</ulink>"
                        chapter in the Yocto Project Reference Manual.</para>

                        <para>As an example, the following command builds the
                        <filename>core-image-minimal</filename> image:
                        <literallayout class='monospaced'>
     $ bitbake core-image-minimal
                        </literallayout>
                        Once an image has been built, it often needs to be
                        installed.
                        The images and kernels built by the OpenEmbedded
                        build system are placed in the Build Directory in
                        <filename class="directory">tmp/deploy/images</filename>.
                        For information on how to run pre-built images such as
                        <filename>qemux86</filename> and <filename>qemuarm</filename>,
                        see the
                        <ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Application Development and the Extensible Software Development Kit (eSDK)</ulink>
                        manual.
                        For information about how to install these images,
                        see the documentation for your particular board or
                        machine.
                        </para></listitem>
                </orderedlist>
            </para>
        </section>

        <section id='dev-building-images-for-multiple-targets-using-multiple-configurations'>
            <title>Building Images for Multiple Targets Using Multiple Configurations</title>

            <para>
                You can use a single <filename>bitbake</filename> command
                to build multiple images or packages for different targets
                where each image or package requires a different configuration
                (multiple configuration builds).
                The builds, in this scenario, are sometimes referred to as
                "multiconfigs", and this section uses that term throughout.
            </para>

            <para>
                This section describes how to set up for multiple
                configuration builds and how to account for cross-build
                dependencies between the multiconfigs.
            </para>

            <section id='dev-setting-up-and-running-a-multiple-configuration-build'>
                <title>Setting Up and Running a Multiple Configuration Build</title>

                <para>
                    To accomplish a multiple configuration build, you must
                    define each target's configuration separately using
                    a parallel configuration file in the
                    <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>,
                    and you must follow a required file hierarchy.
                    Additionally, you must enable the multiple configuration
                    builds in your <filename>local.conf</filename> file.
                </para>

                <para>
                    Follow these steps to set up and execute multiple
                    configuration builds:
                    <itemizedlist>
                        <listitem><para>
                            <emphasis>Create Separate Configuration Files</emphasis>:
                            You need to create a single configuration file for
                            each build target (each multiconfig).
                            Minimally, each configuration file must define the
                            machine and the temporary directory BitBake uses
                            for the build.
                            Suggested practice dictates that you do not
                            overlap the temporary directories
                            used during the builds.
                            However, it is possible that you can share the
                            temporary directory
                            (<ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>).
                            For example, consider a scenario with two
                            different multiconfigs for the same
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>: "qemux86" built for
                            two distributions such as "poky" and "poky-lsb".
                            In this case, you might want to use the same
                            <filename>TMPDIR</filename>.</para>

                            <para>Here is an example showing the minimal
                            statements needed in a configuration file for
                            a "qemux86" target whose temporary build directory
                            is <filename>tmpmultix86</filename>:
                            <literallayout class='monospaced'>
     MACHINE="qemux86"
     TMPDIR="${TOPDIR}/tmpmultix86"
                            </literallayout></para>

                            <para>The location for these multiconfig
                            configuration files is specific.
                            They must reside in the current build directory in
                            a sub-directory of <filename>conf</filename> named
                            <filename>multiconfig</filename>.
                            Following is an example that defines two
                            configuration files for the "x86" and "arm"
                            multiconfigs:
                            <imagedata fileref="figures/multiconfig_files.png" align="center" width="4in" depth="3in" />
                            </para>

                            <para>The reason for this required file hierarchy
                            is because the <filename>BBPATH</filename> variable
                            is not constructed until the layers are parsed.
                            Consequently, using the configuration file as a
                            pre-configuration file is not possible unless it is
                            located in the current working directory.
                            </para></listitem>
                        <listitem><para>
                            <emphasis>Add the BitBake Multi-configuration Variable to the Local Configuration File</emphasis>:
                            Use the
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-BBMULTICONFIG'><filename>BBMULTICONFIG</filename></ulink>
                            variable in your
                            <filename>conf/local.conf</filename> configuration
                            file to specify each multiconfig.
                            Continuing with the example from the previous
                            figure, the <filename>BBMULTICONFIG</filename>
                            variable needs to enable two multiconfigs: "x86"
                            and "arm" by specifying each configuration file:
                            <literallayout class='monospaced'>
     BBMULTICONFIG = "x86 arm"
                            </literallayout>
                            </para></listitem>
                        <listitem><para>
                            <emphasis>Launch BitBake</emphasis>:
                            Use the following BitBake command form to launch the
                            multiple configuration build:
                            <literallayout class='monospaced'>
     $ bitbake [multiconfig:<replaceable>multiconfigname</replaceable>:]<replaceable>target</replaceable> [[[multiconfig:<replaceable>multiconfigname</replaceable>:]<replaceable>target</replaceable>] ... ]
                            </literallayout>
                            For the example in this section, the following
                            command applies:
                            <literallayout class='monospaced'>
     $ bitbake multiconfig:x86:core-image-minimal multiconfig:arm:core-image-sato
                            </literallayout>
                            The previous BitBake command builds a
                            <filename>core-image-minimal</filename> image that
                            is configured through the
                            <filename>x86.conf</filename> configuration file
                            and builds a <filename>core-image-sato</filename>
                            image that is configured through the
                            <filename>arm.conf</filename> configuration file.
                            </para></listitem>
                    </itemizedlist>
                    <note>
                        Support for multiple configuration builds in the
                        Yocto Project &DISTRO; (&DISTRO_NAME;) Release does
                        not include Shared State (sstate) optimizations.
                        Consequently, if a build uses the same object twice
                        in, for example, two different
                        <filename>TMPDIR</filename> directories, the build
                        either loads from an existing sstate cache for that
                        build at the start or builds the object fresh.
                    </note>
                </para>
            </section>

            <section id='dev-enabling-multiple-configuration-build-dependencies'>
                <title>Enabling Multiple Configuration Build Dependencies</title>

                <para>
                    Sometimes dependencies can exist between targets
                    (multiconfigs) in a multiple configuration build.
                    For example, suppose that in order to build a
                    <filename>core-image-sato</filename> image for an "x86"
                    multiconfig, the root filesystem of an "arm"
                    multiconfig must exist.
                    This dependency is essentially that the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-image'><filename>do_image</filename></ulink>
                    task in the <filename>core-image-sato</filename> recipe
                    depends on the completion of the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-rootfs'><filename>do_rootfs</filename></ulink>
                    task of the <filename>core-image-minimal</filename>
                    recipe.
                </para>

                <para>
                    To enable dependencies in a multiple configuration
                    build, you must declare the dependencies in the recipe
                    using the following statement form:
                    <literallayout class='monospaced'>
     <replaceable>task_or_package</replaceable>[mcdepends] = "multiconfig:<replaceable>from_multiconfig</replaceable>:<replaceable>to_multiconfig</replaceable>:<replaceable>recipe_name</replaceable>:<replaceable>task_on_which_to_depend</replaceable>"
                    </literallayout>
                    To better show how to use this statement, consider the
                    example scenario from the first paragraph of this section.
                    The following statement needs to be added to the recipe
                    that builds the <filename>core-image-sato</filename>
                    image:
                    <literallayout class='monospaced'>
     do_image[mcdepends] = "multiconfig:x86:arm:core-image-minimal:do_rootfs"
                    </literallayout>
                    In this example, the
                    <replaceable>from_multiconfig</replaceable> is "x86".
                    The <replaceable>to_multiconfig</replaceable> is "arm".
                    The task on which the <filename>do_image</filename> task
                    in the recipe depends is the <filename>do_rootfs</filename>
                    task from the <filename>core-image-minimal</filename>
                    recipe associated with the "arm" multiconfig.
               </para>

               <para>
                   Once you set up this dependency, you can build the
                   "x86" multiconfig using a BitBake command as follows:
                   <literallayout class='monospaced'>
     $ bitbake multiconfig:x86:core-image-sato
                   </literallayout>
                   This command executes all the tasks needed to create
                   the <filename>core-image-sato</filename> image for the
                   "x86" multiconfig.
                   Because of the dependency, BitBake also executes through
                   the <filename>do_rootfs</filename> task for the "arm"
                   multiconfig build.
               </para>

               <para>
                   Having a recipe depend on the root filesystem of another
                   build might not seem that useful.
                   Consider this change to the statement in the
                   <filename>core-image-sato</filename> recipe:
                   <literallayout class='monospaced'>
     do_image[mcdepends] = "multiconfig:x86:arm:core-image-minimal:do_image"
                   </literallayout>
                   In this case, BitBake must create the
                   <filename>core-image-minimal</filename> image for the
                   "arm" build since the "x86" build depends on it.
               </para>

               <para>
                   Because "x86" and "arm" are enabled for multiple
                   configuration builds and have separate configuration
                   files, BitBake places the artifacts for each build in the
                   respective temporary build directories (i.e.
                   <ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>).
               </para>
            </section>
        </section>

        <section id='building-an-initramfs-image'>
            <title>Building an Initial RAM Filesystem (initramfs) Image</title>

            <para>
                An initial RAM filesystem (initramfs) image provides a temporary
                root filesystem used for early system initialization (e.g.
                loading of modules needed to locate and mount the "real" root
                filesystem).
                <note>
                    The initramfs image is the successor of initial RAM disk
                    (initrd).
                    It is a "copy in and out" (cpio) archive of the initial
                    filesystem that gets loaded into memory during the Linux
                    startup process.
                    Because Linux uses the contents of the archive during
                    initialization, the initramfs image needs to contain all of the
                    device drivers and tools needed to mount the final root
                    filesystem.
                </note>
            </para>

            <para>
                Follow these steps to create an initramfs image:
                <orderedlist>
                    <listitem><para>
                        <emphasis>Create the initramfs Image Recipe:</emphasis>
                        You can reference the
                        <filename>core-image-minimal-initramfs.bb</filename>
                        recipe found in the <filename>meta/recipes-core</filename>
                        directory of the
                        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
                        as an example from which to work.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Decide if You Need to Bundle the initramfs Image
                        Into the Kernel Image:</emphasis>
                        If you want the initramfs image that is built to be
                        bundled in with the kernel image, set the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-INITRAMFS_IMAGE_BUNDLE'><filename>INITRAMFS_IMAGE_BUNDLE</filename></ulink>
                        variable to "1" in your <filename>local.conf</filename>
                        configuration file and set the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-INITRAMFS_IMAGE'><filename>INITRAMFS_IMAGE</filename></ulink>
                        variable in the recipe that builds the kernel image.
                        <note><title>Tip</title>
                            It is recommended that you do bundle the initramfs
                            image with the kernel image to avoid circular
                            dependencies between the kernel recipe and the
                            initramfs recipe should the initramfs image
                            include kernel modules.
                        </note>
                        Setting the <filename>INITRAMFS_IMAGE_BUNDLE</filename>
                        flag causes the initramfs image to be unpacked
                        into the <filename>${B}/usr/</filename> directory.
                        The unpacked initramfs image is then passed to the kernel's
                        <filename>Makefile</filename> using the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-CONFIG_INITRAMFS_SOURCE'><filename>CONFIG_INITRAMFS_SOURCE</filename></ulink>
                        variable, allowing the initramfs image to be built into
                        the kernel normally.
                        <note>
                            If you choose to not bundle the initramfs image with
                            the kernel image, you are essentially using an
                            <ulink url='https://en.wikipedia.org/wiki/Initrd'>Initial RAM Disk (initrd)</ulink>.
                            Creating an initrd is handled primarily through the
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-INITRD_IMAGE'><filename>INITRD_IMAGE</filename></ulink>,
                            <filename>INITRD_LIVE</filename>, and
                            <filename>INITRD_IMAGE_LIVE</filename> variables.
                            For more information, see the
                            <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta/classes/image-live.bbclass'><filename>image-live.bbclass</filename></ulink>
                            file.
                        </note>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Optionally Add Items to the initramfs Image
                        Through the initramfs Image Recipe:</emphasis>
                        If you add items to the initramfs image by way of its
                        recipe, you should use
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_INSTALL'><filename>PACKAGE_INSTALL</filename></ulink>
                        rather than
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL'><filename>IMAGE_INSTALL</filename></ulink>.
                        <filename>PACKAGE_INSTALL</filename> gives more direct
                        control of what is added to the image as compared to
                        the defaults you might not necessarily want that are
                        set by the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-image'><filename>image</filename></ulink>
                        or
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-core-image'><filename>core-image</filename></ulink>
                        classes.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Build the Kernel Image and the initramfs
                        Image:</emphasis>
                        Build your kernel image using BitBake.
                        Because the initramfs image recipe is a dependency of the
                        kernel image, the initramfs image is built as well and
                        bundled with the kernel image if you used the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-INITRAMFS_IMAGE_BUNDLE'><filename>INITRAMFS_IMAGE_BUNDLE</filename></ulink>
                        variable described earlier.
                        </para></listitem>
                </orderedlist>
            </para>
        </section>

        <section id='building-a-tiny-system'>
            <title>Building a Tiny System</title>

            <para>
                Very small distributions have some significant advantages such
                as requiring less on-die or in-package memory (cheaper), better
                performance through efficient cache usage, lower power requirements
                due to less memory, faster boot times, and reduced development
                overhead.
                Some real-world examples where a very small distribution gives
                you distinct advantages are digital cameras, medical devices,
                and small headless systems.
            </para>

            <para>
                This section presents information that shows you how you can
                trim your distribution to even smaller sizes than the
                <filename>poky-tiny</filename> distribution, which is around
                5 Mbytes, that can be built out-of-the-box using the Yocto Project.
            </para>

            <section id='tiny-system-overview'>
                <title>Overview</title>

                <para>
                    The following list presents the overall steps you need to
                    consider and perform to create distributions with smaller
                    root filesystems, achieve faster boot times, maintain your critical
                    functionality, and avoid initial RAM disks:
                    <itemizedlist>
                        <listitem><para>
                            <link linkend='goals-and-guiding-principles'>Determine your goals and guiding principles.</link>
                            </para></listitem>
                        <listitem><para>
                            <link linkend='understand-what-gives-your-image-size'>Understand what contributes to your image size.</link>
                            </para></listitem>
                        <listitem><para>
                            <link linkend='trim-the-root-filesystem'>Reduce the size of the root filesystem.</link>
                            </para></listitem>
                        <listitem><para>
                            <link linkend='trim-the-kernel'>Reduce the size of the kernel.</link>
                            </para></listitem>
                        <listitem><para>
                            <link linkend='remove-package-management-requirements'>Eliminate packaging requirements.</link>
                            </para></listitem>
                        <listitem><para>
                            <link linkend='look-for-other-ways-to-minimize-size'>Look for other ways to minimize size.</link>
                            </para></listitem>
                        <listitem><para>
                            <link linkend='iterate-on-the-process'>Iterate on the process.</link>
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>

            <section id='goals-and-guiding-principles'>
                <title>Goals and Guiding Principles</title>

                <para>
                    Before you can reach your destination, you need to know
                    where you are going.
                    Here is an example list that you can use as a guide when
                    creating very small distributions:
                    <itemizedlist>
                        <listitem><para>Determine how much space you need
                            (e.g. a kernel that is 1 Mbyte or less and
                            a root filesystem that is 3 Mbytes or less).
                            </para></listitem>
                        <listitem><para>Find the areas that are currently
                            taking 90% of the space and concentrate on reducing
                            those areas.
                            </para></listitem>
                        <listitem><para>Do not create any difficult "hacks"
                            to achieve your goals.</para></listitem>
                        <listitem><para>Leverage the device-specific
                            options.</para></listitem>
                        <listitem><para>Work in a separate layer so that you
                            keep changes isolated.
                            For information on how to create layers, see
                            the "<link linkend='understanding-and-creating-layers'>Understanding and Creating Layers</link>" section.
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>

            <section id='understand-what-gives-your-image-size'>
                <title>Understand What Contributes to Your Image Size</title>

                <para>
                    It is easiest to have something to start with when creating
                    your own distribution.
                    You can use the Yocto Project out-of-the-box to create the
                    <filename>poky-tiny</filename> distribution.
                    Ultimately, you will want to make changes in your own
                    distribution that are likely modeled after
                    <filename>poky-tiny</filename>.
                    <note>
                        To use <filename>poky-tiny</filename> in your build,
                        set the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO'><filename>DISTRO</filename></ulink>
                        variable in your
                        <filename>local.conf</filename> file to "poky-tiny"
                        as described in the
                        "<link linkend='creating-your-own-distribution'>Creating Your Own Distribution</link>"
                        section.
                    </note>
                </para>

                <para>
                    Understanding some memory concepts will help you reduce the
                    system size.
                    Memory consists of static, dynamic, and temporary memory.
                    Static memory is the TEXT (code), DATA (initialized data
                    in the code), and BSS (uninitialized data) sections.
                    Dynamic memory represents memory that is allocated at runtime:
                    stacks, hash tables, and so forth.
                    Temporary memory is recovered after the boot process.
                    This memory consists of memory used for decompressing
                    the kernel and for the <filename>__init__</filename>
                    functions.
                </para>

                <para>
                    To help you see where you currently are with kernel and root
                    filesystem sizes, you can use two tools found in the
                    <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink> in
                    the <filename>scripts/tiny/</filename> directory:
                    <itemizedlist>
                        <listitem><para><filename>ksize.py</filename>: Reports
                            component sizes for the kernel build objects.
                            </para></listitem>
                        <listitem><para><filename>dirsize.py</filename>: Reports
                            component sizes for the root filesystem.</para></listitem>
                    </itemizedlist>
                    This next tool and command help you organize configuration
                    fragments and view file dependencies in a human-readable form:
                    <itemizedlist>
                        <listitem><para><filename>merge_config.sh</filename>:
                            Helps you manage configuration files and fragments
                            within the kernel.
                            With this tool, you can merge individual configuration
                            fragments together.
                            The tool allows you to make overrides and warns you
                            of any missing configuration options.
                            The tool is ideal for allowing you to iterate on
                            configurations, create minimal configurations, and
                            create configuration files for different machines
                            without having to duplicate your process.</para>
                            <para>The <filename>merge_config.sh</filename> script is
                            part of the Linux Yocto kernel Git repositories
                            (i.e. <filename>linux-yocto-3.14</filename>,
                            <filename>linux-yocto-3.10</filename>,
                            <filename>linux-yocto-3.8</filename>, and so forth)
                            in the
                            <filename>scripts/kconfig</filename> directory.</para>
                            <para>For more information on configuration fragments,
                            see the
                            "<ulink url='&YOCTO_DOCS_KERNEL_DEV_URL;#creating-config-fragments'>Creating Configuration Fragments</ulink>"
                            section in the Yocto Project Linux Kernel Development
                            Manual.
                            </para></listitem>
                        <listitem><para><filename>bitbake -u taskexp -g <replaceable>bitbake_target</replaceable></filename>:
                            Using the BitBake command with these options brings up
                            a Dependency Explorer from which you can view file
                            dependencies.
                            Understanding these dependencies allows you to make
                            informed decisions when cutting out various pieces of the
                            kernel and root filesystem.</para></listitem>
                    </itemizedlist>
                </para>
            </section>

            <section id='trim-the-root-filesystem'>
                <title>Trim the Root Filesystem</title>

                <para>
                    The root filesystem is made up of packages for booting,
                    libraries, and applications.
                    To change things, you can configure how the packaging happens,
                    which changes the way you build them.
                    You can also modify the filesystem itself or select a different
                    filesystem.
                </para>

                <para>
                    First, find out what is hogging your root filesystem by running the
                    <filename>dirsize.py</filename> script from your root directory:
                    <literallayout class='monospaced'>
     $ cd <replaceable>root-directory-of-image</replaceable>
     $ dirsize.py 100000 > dirsize-100k.log
     $ cat dirsize-100k.log
                    </literallayout>
                    You can apply a filter to the script to ignore files under
                    a certain size.
                    The previous example filters out any files below 100 Kbytes.
                    The sizes reported by the tool are uncompressed, and thus
                    will be smaller by a relatively constant factor in a
                    compressed root filesystem.
                    When you examine your log file, you can focus on areas of the
                    root filesystem that take up large amounts of memory.
                </para>

                <para>
                    You need to be sure that what you eliminate does not cripple
                    the functionality you need.
                    One way to see how packages relate to each other is by using
                    the Dependency Explorer UI with the BitBake command:
                    <literallayout class='monospaced'>
     $ cd <replaceable>image-directory</replaceable>
     $ bitbake -u taskexp -g <replaceable>image</replaceable>
                    </literallayout>
                    Use the interface to select potential packages you wish to
                    eliminate and see their dependency relationships.
                </para>

                <para>
                    When deciding how to reduce the size, get rid of packages that
                    result in minimal impact on the feature set.
                    For example, you might not need a VGA display.
                    Or, you might be able to get by with <filename>devtmpfs</filename>
                    and <filename>mdev</filename> instead of
                    <filename>udev</filename>.
                </para>

                <para>
                    Use your <filename>local.conf</filename> file to make changes.
                    For example, to eliminate <filename>udev</filename> and
                    <filename>glib</filename>, set the following in the
                    local configuration file:
                    <literallayout class='monospaced'>
     VIRTUAL-RUNTIME_dev_manager = ""
                    </literallayout>
                </para>

                <para>
                    Finally, you should consider exactly the type of root
                    filesystem you need to meet your needs while also reducing
                    its size.
                    For example, consider <filename>cramfs</filename>,
                    <filename>squashfs</filename>, <filename>ubifs</filename>,
                    <filename>ext2</filename>, or an <filename>initramfs</filename>
                    using <filename>initramfs</filename>.
                    Be aware that <filename>ext3</filename> requires a 1 Mbyte
                    journal.
                    If you are okay with running read-only, you do not need this
                    journal.
                </para>

                <note>
                    After each round of elimination, you need to rebuild your
                    system and then use the tools to see the effects of your
                    reductions.
                </note>
            </section>

            <section id='trim-the-kernel'>
                <title>Trim the Kernel</title>

                <para>
                    The kernel is built by including policies for hardware-independent
                    aspects.
                    What subsystems do you enable?
                    For what architecture are you building?
                    Which drivers do you build by default?
                    <note>You can modify the kernel source if you want to help
                        with boot time.
                    </note>
                </para>

                <para>
                    Run the <filename>ksize.py</filename> script from the top-level
                    Linux build directory to get an idea of what is making up
                    the kernel:
                    <literallayout class='monospaced'>
     $ cd <replaceable>top-level-linux-build-directory</replaceable>
     $ ksize.py > ksize.log
     $ cat ksize.log
                    </literallayout>
                    When you examine the log, you will see how much space is
                    taken up with the built-in <filename>.o</filename> files for
                    drivers, networking, core kernel files, filesystem, sound,
                    and so forth.
                    The sizes reported by the tool are uncompressed, and thus
                    will be smaller by a relatively constant factor in a compressed
                    kernel image.
                    Look to reduce the areas that are large and taking up around
                    the "90% rule."
                </para>

                <para>
                    To examine, or drill down, into any particular area, use the
                    <filename>-d</filename> option with the script:
                    <literallayout class='monospaced'>
     $ ksize.py -d > ksize.log
                    </literallayout>
                    Using this option breaks out the individual file information
                    for each area of the kernel (e.g. drivers, networking, and
                    so forth).
                </para>

                <para>
                    Use your log file to see what you can eliminate from the kernel
                    based on features you can let go.
                    For example, if you are not going to need sound, you do not
                    need any drivers that support sound.
                </para>

                <para>
                    After figuring out what to eliminate, you need to reconfigure
                    the kernel to reflect those changes during the next build.
                    You could run <filename>menuconfig</filename> and make all your
                    changes at once.
                    However, that makes it difficult to see the effects of your
                    individual eliminations and also makes it difficult to replicate
                    the changes for perhaps another target device.
                    A better method is to start with no configurations using
                    <filename>allnoconfig</filename>, create configuration
                    fragments for individual changes, and then manage the
                    fragments into a single configuration file using
                    <filename>merge_config.sh</filename>.
                    The tool makes it easy for you to iterate using the
                    configuration change and build cycle.
                </para>

                <para>
                    Each time you make configuration changes, you need to rebuild
                    the kernel and check to see what impact your changes had on
                    the overall size.
                </para>
            </section>

            <section id='remove-package-management-requirements'>
                <title>Remove Package Management Requirements</title>

                <para>
                    Packaging requirements add size to the image.
                    One way to reduce the size of the image is to remove all the
                    packaging requirements from the image.
                    This reduction includes both removing the package manager
                    and its unique dependencies as well as removing the package
                    management data itself.
                </para>

                <para>
                    To eliminate all the packaging requirements for an image,
                    be sure that "package-management" is not part of your
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-IMAGE_FEATURES'><filename>IMAGE_FEATURES</filename></ulink>
                    statement for the image.
                    When you remove this feature, you are removing the package
                    manager as well as its dependencies from the root filesystem.
                </para>
            </section>

            <section id='look-for-other-ways-to-minimize-size'>
                <title>Look for Other Ways to Minimize Size</title>

                <para>
                    Depending on your particular circumstances, other areas that you
                    can trim likely exist.
                    The key to finding these areas is through tools and methods
                    described here combined with experimentation and iteration.
                    Here are a couple of areas to experiment with:
                    <itemizedlist>
                        <listitem><para><filename>glibc</filename>:
                            In general, follow this process:
                            <orderedlist>
                                <listitem><para>Remove <filename>glibc</filename>
                                    features from
                                    <ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></ulink>
                                    that you think you do not need.</para></listitem>
                                <listitem><para>Build your distribution.
                                    </para></listitem>
                                <listitem><para>If the build fails due to missing
                                    symbols in a package, determine if you can
                                    reconfigure the package to not need those
                                    features.
                                    For example, change the configuration to not
                                    support wide character support as is done for
                                    <filename>ncurses</filename>.
                                    Or, if support for those characters is needed,
                                    determine what <filename>glibc</filename>
                                    features provide the support and restore the
                                    configuration.
                                    </para></listitem>
                                <listitem><para>Rebuild and repeat the process.
                                    </para></listitem>
                            </orderedlist></para></listitem>
                        <listitem><para><filename>busybox</filename>:
                            For BusyBox, use a process similar as described for
                            <filename>glibc</filename>.
                            A difference is you will need to boot the resulting
                            system to see if you are able to do everything you
                            expect from the running system.
                            You need to be sure to integrate configuration fragments
                            into Busybox because BusyBox handles its own core
                            features and then allows you to add configuration
                            fragments on top.
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>

            <section id='iterate-on-the-process'>
                <title>Iterate on the Process</title>

                <para>
                    If you have not reached your goals on system size, you need
                    to iterate on the process.
                    The process is the same.
                    Use the tools and see just what is taking up 90% of the root
                    filesystem and the kernel.
                    Decide what you can eliminate without limiting your device
                    beyond what you need.
                </para>

                <para>
                    Depending on your system, a good place to look might be
                    Busybox, which provides a stripped down
                    version of Unix tools in a single, executable file.
                    You might be able to drop virtual terminal services or perhaps
                    ipv6.
                </para>
            </section>
        </section>

        <section id='building-images-for-more-than-one-machine'>
            <title>Building Images for More than One Machine</title>

            <para>
                A common scenario developers face is creating images for several
                different machines that use the same software environment.
                In this situation, it is tempting to set the
                tunings and optimization flags for each build specifically for
                the targeted hardware (i.e. "maxing out" the tunings).
                Doing so can considerably add to build times and package feed
                maintenance collectively for the machines.
                For example, selecting tunes that are extremely specific to a
                CPU core used in a system might enable some micro optimizations
                in GCC for that particular system but would otherwise not gain
                you much of a performance difference across the other systems
                as compared to using a more general tuning across all the builds
                (e.g. setting
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEFAULTTUNE'><filename>DEFAULTTUNE</filename></ulink>
                specifically for each machine's build).
                Rather than "max out" each build's tunings, you can take steps that
                cause the OpenEmbedded build system to reuse software across the
                various machines where it makes sense.
            </para>

            <para>
                If build speed and package feed maintenance are considerations,
                you should consider the points in this section that can help you
                optimize your tunings to best consider build times and package
                feed maintenance.
                <itemizedlist>
                    <listitem><para>
                        <emphasis>Share the Build Directory:</emphasis>
                        If at all possible, share the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>
                        across builds.
                        The Yocto Project supports switching between different
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>
                        values in the same <filename>TMPDIR</filename>.
                        This practice is well supported and regularly used by
                        developers when building for multiple machines.
                        When you use the same <filename>TMPDIR</filename> for
                        multiple machine builds, the OpenEmbedded build system can
                        reuse the existing native and often cross-recipes for
                        multiple machines.
                        Thus, build time decreases.
                        <note>
                            If
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO'><filename>DISTRO</filename></ulink>
                            settings change or fundamental configuration settings
                            such as the filesystem layout, you need to work with
                            a clean <filename>TMPDIR</filename>.
                            Sharing <filename>TMPDIR</filename> under these
                            circumstances might work but since it is not
                            guaranteed, you should use a clean
                            <filename>TMPDIR</filename>.
                        </note>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Enable the Appropriate Package Architecture:</emphasis>
                        By default, the OpenEmbedded build system enables three
                        levels of package architectures: "all", "tune" or "package",
                        and "machine".
                        Any given recipe usually selects one of these package
                        architectures (types) for its output.
                        Depending for what a given recipe creates packages, making
                        sure you enable the appropriate package architecture can
                        directly impact the build time.</para>

                        <para>A recipe that just generates scripts can enable
                        "all" architecture because there are no binaries to build.
                        To specifically enable "all" architecture, be sure your
                        recipe inherits the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-allarch'><filename>allarch</filename></ulink>
                        class.
                        This class is useful for "all" architectures because it
                        configures many variables so packages can be used across
                        multiple architectures.</para>

                        <para>If your recipe needs to generate packages that are
                        machine-specific or when one of the build or runtime
                        dependencies is already machine-architecture dependent,
                        which makes your recipe also machine-architecture dependent,
                        make sure your recipe enables the "machine" package
                        architecture through the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_ARCH'><filename>MACHINE_ARCH</filename></ulink>
                        variable:
                        <literallayout class='monospaced'>
     PACKAGE_ARCH = "${MACHINE_ARCH}"
                        </literallayout>
                        When you do not specifically enable a package
                        architecture through the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></ulink>,
                        The OpenEmbedded build system defaults to the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-TUNE_PKGARCH'><filename>TUNE_PKGARCH</filename></ulink>
                        setting:
                        <literallayout class='monospaced'>
     PACKAGE_ARCH = "${TUNE_PKGARCH}"
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Choose a Generic Tuning File if Possible:</emphasis>
                        Some tunes are more generic and can run on multiple targets
                        (e.g. an <filename>armv5</filename> set of packages could
                        run on <filename>armv6</filename> and
                        <filename>armv7</filename> processors in most cases).
                        Similarly, <filename>i486</filename> binaries could work
                        on <filename>i586</filename> and higher processors.
                        You should realize, however, that advances on newer
                        processor versions would not be used.</para>

                        <para>If you select the same tune for several different
                        machines, the OpenEmbedded build system reuses software
                        previously built, thus speeding up the overall build time.
                        Realize that even though a new sysroot for each machine is
                        generated, the software is not recompiled and only one
                        package feed exists.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Manage Granular Level Packaging:</emphasis>
                        Sometimes cases exist where injecting another level of
                        package architecture beyond the three higher levels noted
                        earlier can be useful.
                        For example, consider how NXP (formerly Freescale) allows
                        for the easy reuse of binary packages in their layer
                        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/meta-freescale/'><filename>meta-freescale</filename></ulink>.
                        In this example, the
                        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/meta-freescale/tree/classes/fsl-dynamic-packagearch.bbclass'><filename>fsl-dynamic-packagearch</filename></ulink>
                        class shares GPU packages for i.MX53 boards because
                        all boards share the AMD GPU.
                        The i.MX6-based boards can do the same because all boards
                        share the Vivante GPU.
                        This class inspects the BitBake datastore to identify if
                        the package provides or depends on one of the
                        sub-architecture values.
                        If so, the class sets the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></ulink>
                        value based on the <filename>MACHINE_SUBARCH</filename>
                        value.
                        If the package does not provide or depend on one of the
                        sub-architecture values but it matches a value in the
                        machine-specific filter, it sets
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_ARCH'><filename>MACHINE_ARCH</filename></ulink>.
                        This behavior reduces the number of packages built and
                        saves build time by reusing binaries.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Use Tools to Debug Issues:</emphasis>
                        Sometimes you can run into situations where software is
                        being rebuilt when you think it should not be.
                        For example, the OpenEmbedded build system might not be
                        using shared state between machines when you think it
                        should be.
                        These types of situations are usually due to references
                        to machine-specific variables such as
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-SERIAL_CONSOLES'><filename>SERIAL_CONSOLES</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-XSERVER'><filename>XSERVER</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_FEATURES'><filename>MACHINE_FEATURES</filename></ulink>,
                        and so forth in code that is supposed to only be
                        tune-specific or when the recipe depends
                        (<ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'><filename>RRECOMMENDS</filename></ulink>,
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-RSUGGESTS'><filename>RSUGGESTS</filename></ulink>,
                        and so forth) on some other recipe that already has
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></ulink>
                        defined as "${MACHINE_ARCH}".
                        <note>
                            Patches to fix any issues identified are most welcome
                            as these issues occasionally do occur.
                        </note></para>

                        <para>For such cases, you can use some tools to help you
                        sort out the situation:
                        <itemizedlist>
                            <listitem><para>
                                <emphasis><filename>sstate-diff-machines.sh</filename>:</emphasis>
                                You can find this tool in the
                                <filename>scripts</filename> directory of the
                                Source Repositories.
                                See the comments in the script for information on
                                how to use the tool.
                                </para></listitem>
                            <listitem><para>
                                <emphasis>BitBake's "-S printdiff" Option:</emphasis>
                                Using this option causes BitBake to try to
                                establish the closest signature match it can
                                (e.g. in the shared state cache) and then run
                                <filename>bitbake-diffsigs</filename> over the
                                matches to determine the stamps and delta where
                                these two stamp trees diverge.
                                </para></listitem>
                        </itemizedlist>
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id="building-software-from-an-external-source">
            <title>Building Software from an External Source</title>

            <para>
                By default, the OpenEmbedded build system uses the
                <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
                when building source code.
                The build process involves fetching the source files, unpacking
                them, and then patching them if necessary before the build takes
                place.
            </para>

            <para>
                Situations exist where you might want to build software from source
                files that are external to and thus outside of the
                OpenEmbedded build system.
                For example, suppose you have a project that includes a new BSP with
                a heavily customized kernel.
                And, you want to minimize exposing the build system to the
                development team so that they can focus on their project and
                maintain everyone's workflow as much as possible.
                In this case, you want a kernel source directory on the development
                machine where the development occurs.
                You want the recipe's
                <ulink url='&YOCTO_DOCS_REF_URL;#var-SRC_URI'><filename>SRC_URI</filename></ulink>
                variable to point to the external directory and use it as is, not
                copy it.
            </para>

            <para>
                To build from software that comes from an external source, all you
                need to do is inherit the
                <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-externalsrc'><filename>externalsrc</filename></ulink>
                class and then set the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTERNALSRC'><filename>EXTERNALSRC</filename></ulink>
                variable to point to your external source code.
                Here are the statements to put in your
                <filename>local.conf</filename> file:
                <literallayout class='monospaced'>
     INHERIT += "externalsrc"
     EXTERNALSRC_pn-<replaceable>myrecipe</replaceable> = "<replaceable>path-to-your-source-tree</replaceable>"
                </literallayout>
            </para>

            <para>
                This next example shows how to accomplish the same thing by setting
                <filename>EXTERNALSRC</filename> in the recipe itself or in the
                recipe's append file:
                <literallayout class='monospaced'>
     EXTERNALSRC = "<replaceable>path</replaceable>"
     EXTERNALSRC_BUILD = "<replaceable>path</replaceable>"
                </literallayout>
                <note>
                    In order for these settings to take effect, you must globally
                    or locally inherit the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-externalsrc'><filename>externalsrc</filename></ulink>
                    class.
                </note>
            </para>

            <para>
                By default, <filename>externalsrc.bbclass</filename> builds
                the source code in a directory separate from the external source
                directory as specified by
                <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTERNALSRC'><filename>EXTERNALSRC</filename></ulink>.
                If you need to have the source built in the same directory in
                which it resides, or some other nominated directory, you can set
                <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTERNALSRC_BUILD'><filename>EXTERNALSRC_BUILD</filename></ulink>
                to point to that directory:
                <literallayout class='monospaced'>
     EXTERNALSRC_BUILD_pn-<replaceable>myrecipe</replaceable> = "<replaceable>path-to-your-source-tree</replaceable>"
                </literallayout>
            </para>
        </section>

        <section id="replicating-a-build-offline">
            <title>Replicating a Build Offline</title>

            <para>
                It can be useful to take a "snapshot" of upstream sources
                used in a build and then use that "snapshot" later to
                replicate the build offline.
                To do so, you need to first prepare and populate your downloads
                directory your "snapshot" of files.
                Once your downloads directory is ready, you can use it at
                any time and from any machine to replicate your build.
            </para>

            <para>
                Follow these steps to populate your Downloads directory:
                <orderedlist>
                    <listitem><para>
                        <emphasis>Create a Clean Downloads Directory:</emphasis>
                        Start with an empty downloads directory
                        (<ulink url='&YOCTO_DOCS_REF_URL;#var-DL_DIR'><filename>DL_DIR</filename></ulink>).
                        You start with an empty downloads directory by either
                        removing the files in the existing directory or by
                        setting
                        <filename>DL_DIR</filename> to point to either an
                        empty location or one that does not yet exist.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Generate Tarballs of the Source Git Repositories:</emphasis>
                        Edit your <filename>local.conf</filename> configuration
                        file as follows:
                        <literallayout class='monospaced'>
     DL_DIR = "/home/<replaceable>your-download-dir</replaceable>/"
     BB_GENERATE_MIRROR_TARBALLS = "1"
                        </literallayout>
                        During the fetch process in the next step, BitBake
                        gathers the source files and creates tarballs in
                        the directory pointed to by <filename>DL_DIR</filename>.
                        See the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-BB_GENERATE_MIRROR_TARBALLS'><filename>BB_GENERATE_MIRROR_TARBALLS</filename></ulink>
                        variable for more information.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Populate Your Downloads Directory Without Building:</emphasis>
                        Use BitBake to fetch your sources but inhibit the
                        build:
                        <literallayout class='monospaced'>
     $ bitbake <replaceable>target</replaceable> --runonly=fetch
                        </literallayout>
                        The downloads directory (i.e.
                        <filename>${DL_DIR}</filename>) now has a "snapshot" of
                        the source files in the form of tarballs, which can
                        be used for the build.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Optionally Remove Any Git or other SCM Subdirectories From the Downloads Directory:</emphasis>
                        If you want, you can clean up your downloads directory
                        by removing any Git or other Source Control Management
                        (SCM) subdirectories such as
                        <filename>${DL_DIR}/git2/*</filename>.
                        The tarballs already contain these subdirectories.
                        </para></listitem>
                </orderedlist>
            </para>

            <para>
                Once your downloads directory has everything it needs regarding
                source files, you can create your "own-mirror" and build
                your target.
                Understand that you can use the files to build the target
                offline from any machine and at any time.
            </para>

            <para>
                Follow these steps to build your target using the files in the
                downloads directory:
                <orderedlist>
                    <listitem><para>
                        <emphasis>Using Local Files Only:</emphasis>
                        Inside your <filename>local.conf</filename> file, add
                        the
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-SOURCE_MIRROR_URL'><filename>SOURCE_MIRROR_URL</filename></ulink>
                        variable,
                        inherit the <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-own-mirrors'><filename>own-mirrors</filename></ulink>
                        class, and use the
                        <ulink url='&YOCTO_DOCS_BB_URL;#var-bb-BB_NO_NETWORK'><filename>BB_NO_NETWORK</filename></ulink>
                        variable to your <filename>local.conf</filename>.
                        <literallayout class='monospaced'>
     SOURCE_MIRROR_URL ?= "file:///home/<replaceable>your-download-dir</replaceable>/"
     INHERIT += "own-mirrors"
     BB_NO_NETWORK = "1"
                        </literallayout>
                        The <filename>SOURCE_MIRROR_URL</filename> and
                        <filename>own-mirror</filename> class set up the system
                        to use the downloads directory as your "own mirror".
                        Using the <filename>BB_NO_NETWORK</filename>
                        variable makes sure that BitBake's fetching process
                        in step 3 stays local, which means files from
                        your "own-mirror" are used.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Start With a Clean Build:</emphasis>
                        You can start with a clean build by removing the
                        <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink><filename>}</filename>
                        directory or using a new
                        <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
                        </para></listitem>
                    <listitem><para>
                        <emphasis>Build Your Target:</emphasis>
                        Use BitBake to build your target:
                        <literallayout class='monospaced'>
     $ bitbake <replaceable>target</replaceable>
                        </literallayout>
                        The build completes using the known local "snapshot" of
                        source files from your mirror.
                        The resulting tarballs for your "snapshot" of source
                        files are in the downloads directory.
                        <note>
                            <para>The offline build does not work if recipes
                            attempt to find the latest version of software
                            by setting
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>
                            to
                            <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-AUTOREV'><filename>AUTOREV</filename></ulink><filename>}</filename>:
                            <literallayout class='monospaced'>
     SRCREV = "${AUTOREV}"
                            </literallayout>
                            When a recipe sets
                            <filename>SRCREV</filename> to
                            <filename>${AUTOREV}</filename>, the build system
                            accesses the network in an attempt to determine the
                            latest version of software from the SCM.
                            Typically, recipes that use
                            <filename>AUTOREV</filename> are custom or
                            modified recipes.
                            Recipes that reside in public repositories
                            usually do not use <filename>AUTOREV</filename>.
                            </para>

                            <para>If you do have recipes that use
                            <filename>AUTOREV</filename>, you can take steps to
                            still use the recipes in an offline build.
                            Do the following:
                                <orderedlist>
                                    <listitem><para>
                                        Use a configuration generated by
                                        enabling
                                        <link linkend='maintaining-build-output-quality'>build history</link>.
                                        </para></listitem>
                                    <listitem><para>
                                        Use the
                                        <filename>buildhistory-collect-srcrevs</filename>
                                        command to collect the stored
                                        <filename>SRCREV</filename> values from
                                        the build's history.
                                        For more information on collecting these
                                        values, see the
                                        "<link linkend='build-history-package-information'>Build History Package Information</link>"
                                        section.
                                        </para></listitem>
                                    <listitem><para>
                                        Once you have the correct source
                                        revisions, you can modify those recipes
                                        to to set <filename>SRCREV</filename>
                                        to specific versions of the software.
                                        </para></listitem>
                                </orderedlist>
                            </para>
                        </note>
                        </para></listitem>
                </orderedlist>
            </para>
        </section>
    </section>

    <section id='speeding-up-a-build'>
        <title>Speeding Up a Build</title>

        <para>
            Build time can be an issue.
            By default, the build system uses simple controls to try and maximize
            build efficiency.
            In general, the default settings for all the following variables
            result in the most efficient build times when dealing with single
            socket systems (i.e. a single CPU).
            If you have multiple CPUs, you might try increasing the default
            values to gain more speed.
            See the descriptions in the glossary for each variable for more
            information:
            <itemizedlist>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-BB_NUMBER_THREADS'><filename>BB_NUMBER_THREADS</filename>:</ulink>
                    The maximum number of threads BitBake simultaneously executes.
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_NUMBER_PARSE_THREADS'><filename>BB_NUMBER_PARSE_THREADS</filename>:</ulink>
                    The number of threads BitBake uses during parsing.
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PARALLEL_MAKE'><filename>PARALLEL_MAKE</filename>:</ulink>
                    Extra options passed to the <filename>make</filename> command
                    during the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-compile'><filename>do_compile</filename></ulink>
                    task in order to specify parallel compilation on the
                    local build host.
                    </para></listitem>
                <listitem><para>
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PARALLEL_MAKEINST'><filename>PARALLEL_MAKEINST</filename>:</ulink>
                    Extra options passed to the <filename>make</filename> command
                    during the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>
                    task in order to specify parallel installation on the
                    local build host.
                    </para></listitem>
            </itemizedlist>
            As mentioned, these variables all scale to the number of processor
            cores available on the build system.
            For single socket systems, this auto-scaling ensures that the build
            system fundamentally takes advantage of potential parallel operations
            during the build based on the build machine's capabilities.
        </para>

        <para>
            Following are additional factors that can affect build speed:
            <itemizedlist>
                <listitem><para>
                    File system type:
                    The file system type that the build is being performed on can
                    also influence performance.
                    Using <filename>ext4</filename> is recommended as compared
                    to <filename>ext2</filename> and <filename>ext3</filename>
                    due to <filename>ext4</filename> improved features
                    such as extents.
                    </para></listitem>
                <listitem><para>
                    Disabling the updating of access time using
                    <filename>noatime</filename>:
                    The <filename>noatime</filename> mount option prevents the
                    build system from updating file and directory access times.
                    </para></listitem>
                <listitem><para>
                    Setting a longer commit:
                    Using the "commit=" mount option increases the interval
                    in seconds between disk cache writes.
                    Changing this interval from the five second default to
                    something longer increases the risk of data loss but decreases
                    the need to write to the disk, thus increasing the build
                    performance.
                    </para></listitem>
                <listitem><para>
                    Choosing the packaging backend:
                    Of the available packaging backends, IPK is the fastest.
                    Additionally, selecting a singular packaging backend also
                    helps.
                    </para></listitem>
                <listitem><para>
                    Using <filename>tmpfs</filename> for
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>
                    as a temporary file system:
                    While this can help speed up the build, the benefits are
                    limited due to the compiler using
                    <filename>-pipe</filename>.
                    The build system goes to some lengths to avoid
                    <filename>sync()</filename> calls into the
                    file system on the principle that if there was a significant
                    failure, the
                    <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
                    contents could easily be rebuilt.
                    </para></listitem>
                <listitem><para>
                    Inheriting the
                    <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-rm-work'><filename>rm_work</filename></ulink>
                    class:
                    Inheriting this class has shown to speed up builds due to
                    significantly lower amounts of data stored in the data
                    cache as well as on disk.
                    Inheriting this class also makes cleanup of
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>
                    faster, at the expense of being easily able to dive into the
                    source code.
                    File system maintainers have recommended that the fastest way
                    to clean up large numbers of files is to reformat partitions
                    rather than delete files due to the linear nature of
                    partitions.
                    This, of course, assumes you structure the disk partitions and
                    file systems in a way that this is practical.
                    </para></listitem>
            </itemizedlist>
            Aside from the previous list, you should keep some trade offs in
            mind that can help you speed up the build:
            <itemizedlist>
                <listitem><para>
                    Remove items from
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></ulink>
                    that you might not need.
                    </para></listitem>
                <listitem><para>
                    Exclude debug symbols and other debug information:
                    If you do not need these symbols and other debug information,
                    disabling the <filename>*-dbg</filename> package generation
                    can speed up the build.
                    You can disable this generation by setting the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-INHIBIT_PACKAGE_DEBUG_SPLIT'><filename>INHIBIT_PACKAGE_DEBUG_SPLIT</filename></ulink>
                    variable to "1".
                    </para></listitem>
                <listitem><para>
                    Disable static library generation for recipes derived from
                    <filename>autoconf</filename> or <filename>libtool</filename>:
                    Following is an example showing how to disable static
                    libraries and still provide an override to handle exceptions:
                    <literallayout class='monospaced'>
     STATICLIBCONF = "--disable-static"
     STATICLIBCONF_sqlite3-native = ""
     EXTRA_OECONF += "${STATICLIBCONF}"
                    </literallayout>
                    <note><title>Notes</title>
                        <itemizedlist>
                            <listitem><para>
                                Some recipes need static libraries in order to work
                                correctly (e.g. <filename>pseudo-native</filename>
                                needs <filename>sqlite3-native</filename>).
                                Overrides, as in the previous example, account for
                                these kinds of exceptions.
                                </para></listitem>
                            <listitem><para>
                                Some packages have packaging code that assumes the
                                presence of the static libraries.
                                If so, you might need to exclude them as well.
                                </para></listitem>
                        </itemizedlist>
                    </note>
                </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id="platdev-working-with-libraries">
        <title>Working With Libraries</title>

        <para>
            Libraries are an integral part of your system.
            This section describes some common practices you might find
            helpful when working with libraries to build your system:
            <itemizedlist>
                <listitem><para><link linkend='including-static-library-files'>How to include static library files</link>
                    </para></listitem>
                <listitem><para><link linkend='combining-multiple-versions-library-files-into-one-image'>How to use the Multilib feature to combine multiple versions of library files into a single image</link>
                    </para></listitem>
                <listitem><para><link linkend='installing-multiple-versions-of-the-same-library'>How to install multiple versions of the same library in parallel on the same system</link>
                    </para></listitem>
            </itemizedlist>
        </para>

        <section id='including-static-library-files'>
            <title>Including Static Library Files</title>

            <para>
                If you are building a library and the library offers static linking, you can control
                which static library files (<filename>*.a</filename> files) get included in the
                built library.
            </para>

            <para>
                The <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'><filename>PACKAGES</filename></ulink>
                and <ulink url='&YOCTO_DOCS_REF_URL;#var-FILES'><filename>FILES_*</filename></ulink>
                variables in the
                <filename>meta/conf/bitbake.conf</filename> configuration file define how files installed
                by the <filename>do_install</filename> task are packaged.
                By default, the <filename>PACKAGES</filename> variable includes
                <filename>${PN}-staticdev</filename>, which represents all static library files.
                <note>
                    Some previously released versions of the Yocto Project
                    defined the static library files through
                    <filename>${PN}-dev</filename>.
                </note>
                Following is part of the BitBake configuration file, where
                you can see how the static library files are defined:
                <literallayout class='monospaced'>
     PACKAGE_BEFORE_PN ?= ""
     PACKAGES = "${PN}-dbg ${PN}-staticdev ${PN}-dev ${PN}-doc ${PN}-locale ${PACKAGE_BEFORE_PN} ${PN}"
     PACKAGES_DYNAMIC = "^${PN}-locale-.*"
     FILES = ""

     FILES_${PN} = "${bindir}/* ${sbindir}/* ${libexecdir}/* ${libdir}/lib*${SOLIBS} \
                 ${sysconfdir} ${sharedstatedir} ${localstatedir} \
                 ${base_bindir}/* ${base_sbindir}/* \
                 ${base_libdir}/*${SOLIBS} \
                 ${base_prefix}/lib/udev/rules.d ${prefix}/lib/udev/rules.d \
                 ${datadir}/${BPN} ${libdir}/${BPN}/* \
                 ${datadir}/pixmaps ${datadir}/applications \
                 ${datadir}/idl ${datadir}/omf ${datadir}/sounds \
                 ${libdir}/bonobo/servers"

     FILES_${PN}-bin = "${bindir}/* ${sbindir}/*"

     FILES_${PN}-doc = "${docdir} ${mandir} ${infodir} ${datadir}/gtk-doc \
                 ${datadir}/gnome/help"
     SECTION_${PN}-doc = "doc"

     FILES_SOLIBSDEV ?= "${base_libdir}/lib*${SOLIBSDEV} ${libdir}/lib*${SOLIBSDEV}"
     FILES_${PN}-dev = "${includedir} ${FILES_SOLIBSDEV} ${libdir}/*.la \
                     ${libdir}/*.o ${libdir}/pkgconfig ${datadir}/pkgconfig \
                     ${datadir}/aclocal ${base_libdir}/*.o \
                     ${libdir}/${BPN}/*.la ${base_libdir}/*.la"
     SECTION_${PN}-dev = "devel"
     ALLOW_EMPTY_${PN}-dev = "1"
     RDEPENDS_${PN}-dev = "${PN} (= ${EXTENDPKGV})"

     FILES_${PN}-staticdev = "${libdir}/*.a ${base_libdir}/*.a ${libdir}/${BPN}/*.a"
     SECTION_${PN}-staticdev = "devel"
     RDEPENDS_${PN}-staticdev = "${PN}-dev (= ${EXTENDPKGV})"
                </literallayout>
            </para>
        </section>

        <section id="combining-multiple-versions-library-files-into-one-image">
            <title>Combining Multiple Versions of Library Files into One Image</title>

            <para>
                The build system offers the ability to build libraries with different
                target optimizations or architecture formats and combine these together
                into one system image.
                You can link different binaries in the image
                against the different libraries as needed for specific use cases.
                This feature is called "Multilib."
            </para>

            <para>
                An example would be where you have most of a system compiled in 32-bit
                mode using 32-bit libraries, but you have something large, like a database
                engine, that needs to be a 64-bit application and uses 64-bit libraries.
                Multilib allows you to get the best of both 32-bit and 64-bit libraries.
            </para>

            <para>
                While the Multilib feature is most commonly used for 32 and 64-bit differences,
                the approach the build system uses facilitates different target optimizations.
                You could compile some binaries to use one set of libraries and other binaries
                to use a different set of libraries.
                The libraries could differ in architecture, compiler options, or other
                optimizations.
            </para>

            <para>
                Several examples exist in the
                <filename>meta-skeleton</filename> layer found in the
               <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>:
                <itemizedlist>
                    <listitem><para><filename>conf/multilib-example.conf</filename>
                        configuration file</para></listitem>
                    <listitem><para><filename>conf/multilib-example2.conf</filename>
                        configuration file</para></listitem>
                    <listitem><para><filename>recipes-multilib/images/core-image-multilib-example.bb</filename>
                        recipe</para></listitem>
                </itemizedlist>
            </para>

            <section id='preparing-to-use-multilib'>
                <title>Preparing to Use Multilib</title>

                <para>
                    User-specific requirements drive the Multilib feature.
                    Consequently, there is no one "out-of-the-box" configuration that likely
                    exists to meet your needs.
                </para>

                <para>
                    In order to enable Multilib, you first need to ensure your recipe is
                    extended to support multiple libraries.
                    Many standard recipes are already extended and support multiple libraries.
                    You can check in the <filename>meta/conf/multilib.conf</filename>
                    configuration file in the
                    <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink> to see how this is
                    done using the
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-BBCLASSEXTEND'><filename>BBCLASSEXTEND</filename></ulink>
                    variable.
                    Eventually, all recipes will be covered and this list will
                    not be needed.
                </para>

                <para>
                    For the most part, the Multilib class extension works automatically to
                    extend the package name from <filename>${PN}</filename> to
                    <filename>${MLPREFIX}${PN}</filename>, where <filename>MLPREFIX</filename>
                    is the particular multilib (e.g. "lib32-" or "lib64-").
                    Standard variables such as
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>,
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>,
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-RPROVIDES'><filename>RPROVIDES</filename></ulink>,
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'><filename>RRECOMMENDS</filename></ulink>,
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'><filename>PACKAGES</filename></ulink>, and
                    <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES_DYNAMIC'><filename>PACKAGES_DYNAMIC</filename></ulink>
                    are automatically extended by the system.
                    If you are extending any manual code in the recipe, you can use the
                    <filename>${MLPREFIX}</filename> variable to ensure those names are extended
                    correctly.
                    This automatic extension code resides in <filename>multilib.bbclass</filename>.
                </para>
            </section>

            <section id='using-multilib'>
                <title>Using Multilib</title>

                <para>
                    After you have set up the recipes, you need to define the actual
                    combination of multiple libraries you want to build.
                    You accomplish this through your <filename>local.conf</filename>
                    configuration file in the
                    <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
                    An example configuration would be as follows:
                    <literallayout class='monospaced'>
     MACHINE = "qemux86-64"
     require conf/multilib.conf
     MULTILIBS = "multilib:lib32"
     DEFAULTTUNE_virtclass-multilib-lib32 = "x86"
     IMAGE_INSTALL_append = " lib32-glib-2.0"
                    </literallayout>
                    This example enables an
                    additional library named <filename>lib32</filename> alongside the
                    normal target packages.
                    When combining these "lib32" alternatives, the example uses "x86" for tuning.
                    For information on this particular tuning, see
                    <filename>meta/conf/machine/include/ia32/arch-ia32.inc</filename>.
                </para>

                <para>
                    The example then includes <filename>lib32-glib-2.0</filename>
                    in all the images, which illustrates one method of including a
                    multiple library dependency.
                    You can use a normal image build to include this dependency,
                    for example:
                    <literallayout class='monospaced'>
     $ bitbake core-image-sato
                    </literallayout>
                    You can also build Multilib packages specifically with a command like this:
                    <literallayout class='monospaced'>
     $ bitbake lib32-glib-2.0
                    </literallayout>
                </para>
            </section>

            <section id='additional-implementation-details'>
                <title>Additional Implementation Details</title>

                <para>
                    Generic implementation details as well as details that are
                    specific to package management systems exist.
                    Following are implementation details that exist regardless
                    of the package management system:
                    <itemizedlist>
                        <listitem><para>The typical convention used for the
                            class extension code as used by
                            Multilib assumes that all package names specified
                            in
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGES'><filename>PACKAGES</filename></ulink>
                            that contain <filename>${PN}</filename> have
                            <filename>${PN}</filename> at the start of the name.
                            When that convention is not followed and
                            <filename>${PN}</filename> appears at
                            the middle or the end of a name, problems occur.
                            </para></listitem>
                        <listitem><para>The
                            <ulink url='&YOCTO_DOCS_REF_URL;#var-TARGET_VENDOR'><filename>TARGET_VENDOR</filename></ulink>
                            value under Multilib will be extended to
                            "-<replaceable>vendor</replaceable>ml<replaceable>multilib</replaceable>"
                            (e.g. "-pokymllib32" for a "lib32" Multilib with
                            Poky).
                            The reason for this slightly unwieldy contraction
                            is that any "-" characters in the vendor
                            string presently break Autoconf's
                            <filename>config.sub</filename>, and
                            other separators are problematic for different
                            reasons.
                            </para></listitem>
                    </itemizedlist>
                </para>

                <para>
                    For the RPM Package Management System, the following implementation details
                    exist:
                    <itemizedlist>
                        <listitem><para>A unique architecture is defined for the Multilib packages,
                            along with creating a unique deploy folder under
                            <filename>tmp/deploy/rpm</filename> in the
                            <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
                            For example, consider <filename>lib32</filename> in a
                            <filename>qemux86-64</filename> image.
                            The possible architectures in the system are "all", "qemux86_64",
                            "lib32_qemux86_64", and "lib32_x86".</para></listitem>
                        <listitem><para>The <filename>${MLPREFIX}</filename> variable is stripped from
                            <filename>${PN}</filename> during RPM packaging.
                            The naming for a normal RPM package and a Multilib RPM package in a
                            <filename>qemux86-64</filename> system resolves to something similar to
                            <filename>bash-4.1-r2.x86_64.rpm</filename> and
                            <filename>bash-4.1.r2.lib32_x86.rpm</filename>, respectively.
                            </para></listitem>
                        <listitem><para>When installing a Multilib image, the RPM backend first
                            installs the base image and then installs the Multilib libraries.
                            </para></listitem>
                        <listitem><para>The build system relies on RPM to resolve the identical files in the
                            two (or more) Multilib packages.</para></listitem>
                    </itemizedlist>
                </para>

                <para>
                    For the IPK Package Management System, the following implementation details exist:
                    <itemizedlist>
                        <listitem><para>The <filename>${MLPREFIX}</filename> is not stripped from
                            <filename>${PN}</filename> during IPK packaging.
                            The naming for a normal RPM package and a Multilib IPK package in a
                            <filename>qemux86-64</filename> system resolves to something like
                            <filename>bash_4.1-r2.x86_64.ipk</filename> and
                            <filename>lib32-bash_4.1-rw_x86.ipk</filename>, respectively.
                            </para></listitem>
                        <listitem><para>The IPK deploy folder is not modified with
                            <filename>${MLPREFIX}</filename> because packages with and without
                            the Multilib feature can exist in the same folder due to the
                            <filename>${PN}</filename> differences.</para></listitem>
                        <listitem><para>IPK defines a sanity check for Multilib installation
                            using certain rules for file comparison, overridden, etc.
                            </para></listitem>
                    </itemizedlist>
                </para>
            </section>
        </section>

        <section id='installing-multiple-versions-of-the-same-library'>
            <title>Installing Multiple Versions of the Same Library</title>

            <para>
                Situations can exist where you need to install and use
                multiple versions of the same library on the same system
                at the same time.
                These situations almost always exist when a library API
                changes and you have multiple pieces of software that
                depend on the separate versions of the library.
                To accommodate these situations, you can install multiple
                versions of the same library in parallel on the same system.
            </para>

            <para>
                The process is straightforward as long as the libraries use
                proper versioning.
                With properly versioned libraries, all you need to do to
                individually specify the libraries is create separate,
                appropriately named recipes where the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-PN'><filename>PN</filename></ulink> part of the
                name includes a portion that differentiates each library version
                (e.g.the major part of the version number).
                Thus, instead of having a single recipe that loads one version
                of a library (e.g. <filename>clutter</filename>), you provide
                multiple recipes that result in different versions
                of the libraries you want.
                As an example, the following two recipes would allow the
                two separate versions of the <filename>clutter</filename>
                library to co-exist on the same system:
                <literallayout class='monospaced'>
     clutter-1.6_1.6.20.bb
     clutter-1.8_1.8.4.bb
                </literallayout>
                Additionally, if you have other recipes that depend on a given
                library, you need to use the
                <ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>
                variable to create the dependency.
                Continuing with the same example, if you want to have a recipe
                depend on the 1.8 version of the <filename>clutter</filename>
                library, use the following in your recipe:
                <literallayout class='monospaced'>
     DEPENDS = "clutter-1.8"
                </literallayout>
            </para>
        </section>
    </section>

    <section id='using-x32-psabi'>
        <title>Using x32 psABI</title>

        <para>
            x32 processor-specific Application Binary Interface
            (<ulink url='https://software.intel.com/en-us/node/628948'>x32 psABI</ulink>)
            is a native 32-bit processor-specific ABI for
            <trademark class='registered'>Intel</trademark> 64 (x86-64)
            architectures.
            An ABI defines the calling conventions between functions in a
            processing environment.
            The interface determines what registers are used and what the
            sizes are for various C data types.
        </para>

        <para>
            Some processing environments prefer using 32-bit applications even
            when running on Intel 64-bit platforms.
            Consider the i386 psABI, which is a very old 32-bit ABI for Intel
            64-bit platforms.
            The i386 psABI does not provide efficient use and access of the
            Intel 64-bit processor resources, leaving the system underutilized.
            Now consider the x86_64 psABI.
            This ABI is newer and uses 64-bits for data sizes and program
            pointers.
            The extra bits increase the footprint size of the programs,
            libraries, and also increases the memory and file system size
            requirements.
            Executing under the x32 psABI enables user programs to utilize CPU
            and system resources more efficiently while keeping the memory
            footprint of the applications low.
            Extra bits are used for registers but not for addressing mechanisms.
        </para>

        <para>
            The Yocto Project supports the final specifications of x32 psABI
            as follows:
            <itemizedlist>
                <listitem><para>
                    You can create packages and images in x32 psABI format on
                    x86_64 architecture targets.
                    </para></listitem>
                <listitem><para>
                    You can successfully build recipes with the x32 toolchain.
                    </para></listitem>
                <listitem><para>
                    You can create and boot
                    <filename>core-image-minimal</filename> and
                    <filename>core-image-sato</filename> images.
                    </para></listitem>
                <listitem><para>
                    RPM Package Manager (RPM) support exists for x32 binaries.
                    </para></listitem>
                <listitem><para>
                    Support for large images exists.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            To use the x32 psABI, you need to edit your
            <filename>conf/local.conf</filename> configuration file as
            follows:
            <literallayout class='monospaced'>
     MACHINE = "qemux86-64"
     DEFAULTTUNE = "x86-64-x32"
     baselib = "${@d.getVar('BASE_LIB_tune-' + (d.getVar('DEFAULTTUNE') \
        or 'INVALID')) or 'lib'}"
            </literallayout>
            Once you have set up your configuration file, use BitBake to
            build an image that supports the x32 psABI.
            Here is an example:
            <literallayout class='monospaced'>
     $ bitbake core-image-sato
            </literallayout>
        </para>
    </section>

    <section id='enabling-gobject-introspection-support'>
        <title>Enabling GObject Introspection Support</title>

        <para>
            <ulink url='https://wiki.gnome.org/Projects/GObjectIntrospection'>GObject introspection</ulink>
            is the standard mechanism for accessing GObject-based software
            from runtime environments.
            GObject is a feature of the GLib library that provides an object
            framework for the GNOME desktop and related software.
            GObject Introspection adds information to GObject that allows
            objects created within it to be represented across different
            programming languages.
            If you want to construct GStreamer pipelines using Python, or
            control UPnP infrastructure using Javascript and GUPnP,
            GObject introspection is the only way to do it.
        </para>

        <para>
            This section describes the Yocto Project support for generating
            and packaging GObject introspection data.
            GObject introspection data is a description of the
            API provided by libraries built on top of GLib framework,
            and, in particular, that framework's GObject mechanism.
            GObject Introspection Repository (GIR) files go to
            <filename>-dev</filename> packages,
            <filename>typelib</filename> files go to main packages as they
            are packaged together with libraries that are introspected.
        </para>

        <para>
            The data is generated when building such a library, by linking
            the library with a small executable binary that asks the library
            to describe itself, and then executing the binary and
            processing its output.
        </para>

        <para>
            Generating this data in a cross-compilation environment
            is difficult because the library is produced for the target
            architecture, but its code needs to be executed on the build host.
            This problem is solved with the OpenEmbedded build system by
            running the code through QEMU, which allows precisely that.
            Unfortunately, QEMU does not always work perfectly as mentioned
            in the xxx section.
        </para>

        <section id='enabling-the-generation-of-introspection-data'>
            <title>Enabling the Generation of Introspection Data</title>

            <para>
                Enabling the generation of introspection data (GIR files)
                in your library package involves the following:
                <orderedlist>
                    <listitem><para>
                        Inherit the
                        <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-gobject-introspection'><filename>gobject-introspection</filename></ulink>
                        class.
                        </para></listitem>
                    <listitem><para>
                        Make sure introspection is not disabled anywhere in
                        the recipe or from anything the recipe includes.
                        Also, make sure that "gobject-introspection-data" is
                        not in
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></ulink>
                        and that "qemu-usermode" is not in
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></ulink>.
                        If either of these conditions exist, nothing will
                        happen.
                        </para></listitem>
                    <listitem><para>
                        Try to build the recipe.
                        If you encounter build errors that look like
                        something is unable to find
                        <filename>.so</filename> libraries, check where these
                        libraries are located in the source tree and add
                        the following to the recipe:
                        <literallayout class='monospaced'>
     GIR_EXTRA_LIBS_PATH = "${B}/<replaceable>something</replaceable>/.libs"
                        </literallayout>
                        <note>
                            See recipes in the <filename>oe-core</filename>
                            repository that use that
                            <filename>GIR_EXTRA_LIBS_PATH</filename> variable
                            as an example.
                        </note>
                        </para></listitem>
                    <listitem><para>
                        Look for any other errors, which probably mean that
                        introspection support in a package is not entirely
                        standard, and thus breaks down in a cross-compilation
                        environment.
                        For such cases, custom-made fixes are needed.
                        A good place to ask and receive help in these cases
                        is the
                        <ulink url='&YOCTO_DOCS_REF_URL;#resources-mailinglist'>Yocto Project mailing lists</ulink>.
                        </para></listitem>
                </orderedlist>
                <note>
                    Using a library that no longer builds against the latest
                    Yocto Project release and prints introspection related
                    errors is a good candidate for the previous procedure.
                </note>
            </para>
        </section>

        <section id='disabling-the-generation-of-introspection-data'>
            <title>Disabling the Generation of Introspection Data</title>

            <para>
                You might find that you do not want to generate
                introspection data.
                Or, perhaps QEMU does not work on your build host and
                target architecture combination.
                If so, you can use either of the following methods to
                disable GIR file generations:
                <itemizedlist>
                    <listitem><para>
                        Add the following to your distro configuration:
                        <literallayout class='monospaced'>
     DISTRO_FEATURES_BACKFILL_CONSIDERED = "gobject-introspection-data"
                        </literallayout>
                        Adding this statement disables generating
                        introspection data using QEMU but will still enable
                        building introspection tools and libraries
                        (i.e. building them does not require the use of QEMU).
                        </para></listitem>
                    <listitem><para>
                        Add the following to your machine configuration:
                        <literallayout class='monospaced'>
     MACHINE_FEATURES_BACKFILL_CONSIDERED = "qemu-usermode"
                        </literallayout>
                        Adding this statement disables the use of QEMU
                        when building packages for your machine.
                        Currently, this feature is used only by introspection
                        recipes and has the same effect as the previously
                        described option.
                        <note>
                            Future releases of the Yocto Project might have
                            other features affected by this option.
                        </note>
                        </para></listitem>
                </itemizedlist>
                If you disable introspection data, you can still
                obtain it through other means such as copying the data
                from a suitable sysroot, or by generating it on the
                target hardware.
                The OpenEmbedded build system does not currently
                provide specific support for these techniques.
            </para>
        </section>

        <section id='testing-that-introspection-works-in-an-image'>
            <title>Testing that Introspection Works in an Image</title>

            <para>
                Use the following procedure to test if generating
                introspection data is working in an image:
                <orderedlist>
                    <listitem><para>
                        Make sure that "gobject-introspection-data" is not in
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></ulink>
                        and that "qemu-usermode" is not in
                        <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></ulink>.
                        </para></listitem>
                    <listitem><para>
                        Build <filename>core-image-sato</filename>.
                        </para></listitem>
                    <listitem><para>
                        Launch a Terminal and then start Python in the
                        terminal.
                        </para></listitem>
                    <listitem><para>
                        Enter the following in the terminal:
                        <literallayout class='monospaced'>
     >>> from gi.repository import GLib
     >>> GLib.get_host_name()
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        For something a little more advanced, enter the
                        following:
                        <literallayout class='monospaced'>
     http://python-gtk-3-tutorial.readthedocs.org/en/latest/introduction.html
                        </literallayout>
                        </