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<chapter id='technical-details'>
<title>Technical Details</title>

    <para>
        This chapter provides technical details for various parts of the Yocto Project.
        Currently, topics include Yocto Project components,
        shared state (sstate) cache, x32, and Licenses.
    </para>

<section id='usingpoky-components'>
    <title>Yocto Project Components</title>

    <para>
        The BitBake task executor together with various types of configuration files form the
        OpenEmbedded Core.
        This section overviews these by describing what they are used for
        and how they interact.
    </para>

    <para>
        BitBake handles the parsing and execution of the data files.
        The data itself is of various types:
    <itemizedlist>
        <listitem><para><emphasis>Recipes:</emphasis>  Provides details about particular
            pieces of software.</para></listitem>
        <listitem><para><emphasis>Class Data:</emphasis>  Abstracts common build
            information (e.g. how to build a Linux kernel).</para></listitem>
        <listitem><para><emphasis>Configuration Data:</emphasis>  Defines machine-specific settings,
            policy decisions, and so forth.
            Configuration data acts as the glue to bind everything together.</para></listitem>
    </itemizedlist>
        For more information on data, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-terms'>Yocto Project Terms</ulink>"
        section in the Yocto Project Development Manual.
    </para>

    <para>
        BitBake knows how to combine multiple data sources together and refers to each data source
        as a layer.
        For information on layers, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and
        Creating Layers</ulink>" section of the Yocto Project Development Manual.
    </para>

    <para>
        Following are some brief details on these core components.
        For more detailed information on these components, see the
        "<link linkend='ref-structure'>Source Directory Structure</link>" chapter.
    </para>

    <section id='usingpoky-components-bitbake'>
        <title>BitBake</title>

        <para>
            BitBake is the tool at the heart of the OpenEmbedded build system
            and is responsible for parsing the
            <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink>,
            generating a list of tasks from it, and then executing those tasks.
            To see a list of the options BitBake supports, use the following
            help command:
            <literallayout class='monospaced'>
     $ bitbake --help
            </literallayout>
        </para>

        <para>
            The most common usage for BitBake is <filename>bitbake &lt;packagename&gt;</filename>, where
            <filename>packagename</filename> is the name of the package you want to build
            (referred to as the "target" in this manual).
            The target often equates to the first part of a <filename>.bb</filename> filename.
            So, to run the <filename>matchbox-desktop_1.2.3.bb</filename> file, you
            might type the following:
            <literallayout class='monospaced'>
     $ bitbake matchbox-desktop
            </literallayout>
            Several different versions of <filename>matchbox-desktop</filename> might exist.
            BitBake chooses the one selected by the distribution configuration.
            You can get more details about how BitBake chooses between different
            target versions and providers in the
            "<link linkend='ref-bitbake-providers'>Preferences and Providers</link>" section.
        </para>

        <para>
            BitBake also tries to execute any dependent tasks first.
            So for example, before building <filename>matchbox-desktop</filename>, BitBake
            would build a cross compiler and <filename>eglibc</filename> if they had not already
            been built.
            <note>This release of the Yocto Project does not support the <filename>glibc</filename>
                GNU version of the Unix standard C library.  By default, the OpenEmbedded build system
                builds with <filename>eglibc</filename>.</note>
        </para>

        <para>
            A useful BitBake option to consider is the <filename>-k</filename> or
            <filename>--continue</filename> option.
            This option instructs BitBake to try and continue processing the job as much
            as possible even after encountering an error.
            When an error occurs, the target that
            failed and those that depend on it cannot be remade.
            However, when you use this option other dependencies can still be processed.
        </para>
    </section>

    <section id='usingpoky-components-metadata'>
        <title>Metadata (Recipes)</title>

        <para>
            The <filename>.bb</filename> files are usually referred to as "recipes."
            In general, a recipe contains information about a single piece of software.
            The information includes the location from which to download the source patches
            (if any are needed), which special configuration options to apply,
            how to compile the source files, and how to package the compiled output.
        </para>

        <para>
            The term "package" can also be used to describe recipes.
            However, since the same word is used for the packaged output from the OpenEmbedded
            build system (i.e. <filename>.ipk</filename> or <filename>.deb</filename> files),
            this document avoids using the term "package" when referring to recipes.
        </para>
    </section>

    <section id='usingpoky-components-classes'>
        <title>Classes</title>

        <para>
            Class files (<filename>.bbclass</filename>) contain information that
            is useful to share between
            <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink> files.
            An example is the Autotools class, which contains
            common settings for any application that Autotools uses.
            The "<link linkend='ref-classes'>Classes</link>" chapter provides details
            about common classes and how to use them.
        </para>
    </section>

    <section id='usingpoky-components-configuration'>
        <title>Configuration</title>

        <para>
            The configuration files (<filename>.conf</filename>) define various configuration variables
            that govern the OpenEmbedded build process.
            These files fall into several areas that define machine configuration options,
            distribution configuration options, compiler tuning options, general common configuration
            options, and user configuration options in <filename>local.conf</filename>, which is found
            in the
            <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>.
        </para>
    </section>
</section>

<section id="a-closer-look-at-the-yocto-project-development-environment">
    <title>A Closer Look at the Yocto Project Development Environment</title>

    <para>
        This section takes a more detailed look at the Yocto Project
        development environment.
        The following diagram represents the development environment at a
        high level.
        The remainder of this section expands on the fundamental input, output,
        process, and
        <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink>) blocks
        in the Yocto Project development environment.
    </para>

    <para>
        <imagedata fileref="figures/yocto-environment-ref.png" align="center" width="8in" depth="4.25in" />
    </para>

    <para>
        The generalized Yocto Project Development Environment consists of
        several functional areas:
        <itemizedlist>
            <listitem><para><emphasis>User Configuration:</emphasis>
                Metadata you can use to control the build process.
                </para></listitem>
            <listitem><para><emphasis>Metadata Layers:</emphasis>
                Various layers that provide software, machine, and
                distro Metadata.</para></listitem>
            <listitem><para><emphasis>Source Files:</emphasis>
                Upstream releases, local projects, and SCMs.</para></listitem>
            <listitem><para><emphasis>Build System:</emphasis>
                Processes under the control of BitBake.
                This block expands on how BitBake fetches source, applies
                patches, completes compilation, analyzes output for package
                generation, creates and tests packages, generates images, and
                generates cross-development tools.</para></listitem>
            <listitem><para><emphasis>Package Feeds:</emphasis>
                Directories containing output packages (rpm, deb or ipk),
                which are subsequently used in the construction of an image or
                SDK, produced by the build system.
                These feeds can also be copied and shared using a web server or
                other means to facilitate extending or updating existing
                images on devices at runtime if runtime package management is
                enabled.</para></listitem>
            <listitem><para><emphasis>Images:</emphasis>
                Images produced by the development process.
                Where do they go?
                Can you mess with them (i.e. freely delete them or move them?).
                </para></listitem>
            <listitem><para><emphasis>Application Development SDK:</emphasis>
                Cross-development tools that are produced along with an image
                or separately with BitBake.</para></listitem>
        </itemizedlist>
    </para>

    <section id="user-configuration">
        <title>User Configuration</title>

        <para>
            User configuration helps define the build.
            Through user configuration, you can tell BitBake the
            target architecture for which you are building the image,
            where to store downloaded source, and other build properties.
            The following figure shows an expanded representation of the
            user configuration box of the Yocto Project development
            environment:
        </para>

        <para>
            <imagedata fileref="figures/user-configuration.png" align="center" width="6in" depth="3.5in" />
        </para>

        <para>
            BitBake needs some basic configuration files in order to complete
            a build.
            These files are <filename>*.conf</filename> files.
            The minimally necessary ones reside as example files in the
            <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>.
            For simplicity, this section refers to the Source Directory as
            the "Poky Directory."
        </para>

        <para>
            When you clone the <filename>poky</filename> Git repository or you
            download and unpack a Yocto Project release, you can set up the
            Source Directory to be named anything you want.
            For this discussion, the cloned repository uses the default
            name <filename>poky</filename>.
            Within the figure, layers appear inside the Source Directory using
            a bold typeface.
            <note>
                The Poky repository is primarily an aggregation of existing
                repositories.
                It is not a canonical upstream source.
            </note>
        </para>

        <para>
            The <filename>meta-yocto</filename> layer inside Poky contains
            a <filename>conf</filename> directory that has example
            configuration files.
            These example files are used as a basis for creating actual
            configuration files when you source the build environment
            script <filename>oe-init-build-env</filename>.
        </para>

        <para>
            Sourcing the build environment script creates a
            <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>
            if one does not already exist.
            BitBake uses the Build Directory for all its work during builds.
            The Build Directory has a <filename>conf</filename> directory that
            contains default versions of your <filename>local.conf</filename>
            and <filename>bblayers.conf</filename> configuration files.
            These default configuration files are created only if versions
            do not already exist in the Build Directory at the time you
            source the <filename>oe-init-build-env</filename> script.
        </para>

        <para>
            Because the Poky repository is fundamentally an aggregation of
            existing repositories, some users might be familiar with running
            the <filename>oe-init-build-env</filename> script in the context of
            separate OpenEmbedded-Core and BitBake repositories rather than a
            single Poky repository.
            This discussion assumes the script is executed from within a cloned
            or unpacked version of Poky.
        </para>

        <para>
            Depending on where the script is sourced, different sub-scripts
            are called to set up the Build Directory (Yocto or OpenEmbedded).
            Specifically, the script
            <filename>scripts/oe-setup-builddir</filename> inside the
            poky directory sets up the Build Directory and seeds the directory
            (if necessary) with configuration files appropriate for the
            Yocto Project development environment.
            <note>
                The <filename>scripts/oe-setup-builddir</filename> script
                uses the <filename>$TEMPLATECONF</filename> variable to
                determine which sample configuration files to locate.
            </note>
        </para>

        <para>
            The <filename>local.conf</filename> file provides many
            basic variables that define a build environment.
            Here is a list of a few.
            To see the default configurations in a <filename>local.conf</filename>
            file created by the build environment script, see the
            <filename>local.conf.sample</filename> in the
            <filename>meta-yocto</filename> layer:
            <itemizedlist>
                <listitem><para><emphasis>Parallelism Options:</emphasis>
                    Controlled by the
                    <link linkend='var-BB_NUMBER_THREADS'><filename>BB_NUMBER_THREADS</filename></link>
                    and
                    <link linkend='var-PARALLEL_MAKE'><filename>PARALLEL_MAKE</filename></link>
                    variables.</para></listitem>
                <listitem><para><emphasis>Target Machine Selection:</emphasis>
                    Controlled by the
                    <link linkend='var-MACHINE'><filename>MACHINE</filename></link>
                    variable.</para></listitem>
                <listitem><para><emphasis>Download Directory:</emphasis>
                    Controlled by the
                    <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
                    variable.</para></listitem>
                <listitem><para><emphasis>Shared State Directory:</emphasis>
                    Controlled by the
                    <link linkend='var-SSTATE_DIR'><filename>SSTATE_DIR</filename></link>
                    variable.</para></listitem>
                <listitem><para><emphasis>Build Output:</emphasis>
                    Controlled by the
                    <link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>
                    variable.</para></listitem>
            </itemizedlist>
            <note>
                Configurations set in the <filename>conf/local.conf</filename>
                file can also be set in the
                <filename>conf/site.conf</filename> and
                <filename>conf/auto.conf</filename> configuration files.
            </note>
        </para>

        <para>
            The <filename>bblayers.conf</filename> file tells BitBake what
            layers you want considered during the build.
            By default, the layers listed in this file include layers
            minimally needed by the build system.
            However, you must manually add any custom layers you have created.
            You can find more information on working with the
            <filename>bblayers.conf</filename> file in the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#enabling-your-layer'>Enabling Your Layer</ulink>"
            section in the Yocto Project Development Manual.
        </para>

        <para>
            The files <filename>site.conf</filename> and
            <filename>auto.conf</filename> are not created by the environment
            initialization script.
            If you want these configuration files, you must create them
            yourself:
            <itemizedlist>
                <listitem><para><emphasis><filename>site.conf</filename>:</emphasis>
                    You can use the <filename>conf/site.conf</filename>
                    configuration file to configure multiple build directories.
                    For example, suppose you had several build environments and
                    they shared some common features.
                    You can set these default build properties here.
                    A good example is perhaps the level of parallelism you want
                    to use through the
                    <link linkend='var-BB_NUMBER_THREADS'><filename>BB_NUMBER_THREADS</filename></link>
                    and
                    <link linkend='var-PARALLEL_MAKE'><filename>PARALLEL_MAKE</filename></link>
                    variables.</para>
                    <para>One useful scenario for using the
                    <filename>conf/site.conf</filename> file is to extend your
                    <link linkend='var-BBPATH'><filename>BBPATH</filename></link>
                    variable to include the path to a
                    <filename>conf/site.conf</filename>.
                    Then, when BitBake looks for Metadata using
                    <filename>BBPATH</filename>, it finds the
                    <filename>conf/site.conf</filename> file and applies your
                    common configurations found in the file.
                    To override configurations in a particular build directory,
                    alter the similar configurations within that build
                    directory's <filename>conf/local.conf</filename> file.
                    </para></listitem>
                <listitem><para><emphasis><filename>auto.conf</filename>:</emphasis>
                    This file is not hand-created.
                    Rather, the file is usually created and written to by
                    an autobuilder.
                    The settings put into the file are typically the same as
                    you would find in the <filename>conf/local.conf</filename>
                    or the <filename>conf/site.conf</filename> files.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            You can edit all configuration files to further define
            any particular build environment.
            This process is represented by the "User Configuration Edits"
            box in the figure.
        </para>

        <para>
            When you launch your build with the
            <filename>bitbake &lt;target&gt;</filename> command, BitBake
            sorts out the configurations to ultimately define your build
            environment.
        </para>
    </section>

    <section id="metadata-machine-configuration-and-policy-configuration">
        <title>Metadata, Machine Configuration, and Policy Configuration</title>

        <para>
            The previous section described the user configurations that
            define the BitBake's global behavior.
            This section takes a closer look at the layers the build system
            uses to further control the build.
            These layers provide Metadata for the software, machine, and
            policy.
        </para>

        <para>
            In general, three types of layer input exist:
            <itemizedlist>
                <listitem><para><emphasis>Policy Configuration:</emphasis>
                    Distribution Layers provide top-level or general
                    policies for the image or SDK being built.
                    For example, this layer would dictate whether BitBake
                    produces RPM or IPK packages.</para></listitem>
                <listitem><para><emphasis>Machine Configuration:</emphasis>
                    Board Support Package (BSP) layers provide machine
                    configurations.
                    This type of information is specific to a particular
                    target architecture.</para></listitem>
                <listitem><para><emphasis>Metadata:</emphasis>
                    Software layers contain user-supplied recipe files,
                    patches, and append files.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            The following figure shows an expanded representation of the
            Metadata, Machine Configuration, and Policy Configuration input
            (layers) boxes of the Yocto Project development environment:
        </para>

        <para>
            <imagedata fileref="figures/layer-input.png" align="center" width="8in" depth="7.5in" />
        </para>

        <para>
            In general, all layers have a similar structure.
            They all contain a licensing file
            (e.g. <filename>COPYING</filename>) if the layer is to be
            distributed, a <filename>README</filename> file as good practice
            and especially if the layer is to be distributed, a
            configuration directory, and recipe directories.
        </para>

        <para>
            The Yocto Project has many layers that can be used.
            You can see a web-interface listing of them on the
            <ulink url="http://git.yoctoproject.org/">Source Repositories</ulink>
            page.
            The layers are shown at the bottom categorized under
            "Yocto Metadata Layers."
            These layers are fundamentally a subset of the
            <ulink url="http://layers.openembedded.org/layerindex/layers/">OpenEmbedded Metadata Index</ulink>,
            which lists all layers provided by the OpenEmbedded community.
            <note>
                Layers exist in the Yocto Project Source Repositories that
                cannot be found in the OpenEmbedded Metadata Index.
                These layers are either deprecated or experimental in nature.
            </note>
        </para>

        <para>
            BitBake uses the <filename>conf/bblayers.conf</filename> file,
            which is part of the user configuration, to find what layers it
            should be using as part of the build.
        </para>

        <para>
            For more information on layers, see the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>"
            section in the Yocto Project Development Manual.
        </para>

        <section id="distro-layer">
            <title>Distro Layer</title>

            <para>
                The distribution layer provides policy configurations for your
                distribution.
                Best practices dictate that you isolate these types of
                configurations into their own layer.
                Settings you provide in
                <filename>conf/&lt;distro&gt;.conf</filename> override similar
                settings that BitBake finds in your
                <filename>conf/local.conf</filename> file in the Build
                Directory.
            </para>

            <para>
                The following list provides some explanation and references
                for what you typically find in the distribution layer:
                <itemizedlist>
                    <listitem><para><emphasis>classes:</emphasis>
                        Class files (<filename>.bbclass</filename>) holds
                        common functionality that can be shared among
                        recipes in the distribution.
                        When your recipes inherit a class, they take on the
                        settings and functions for that class.
                        You can read more about class files in the
                        "<link linkend='ref-classes'>Classes</link>" section.
                        </para></listitem>
                    <listitem><para><emphasis>conf:</emphasis>
                        This area holds configuration files for the
                        layer (<filename>conf/layer.conf</filename>),
                        the distribution
                        (<filename>conf/distro/&lt;distro&gt;.conf</filename>),
                        and any distribution-wide include files.
                        </para></listitem>
                    <listitem><para><emphasis>recipes-*:</emphasis>
                        Recipes and append files that affect common
                        functionality across the distribution.
                        This area could include recipes and append files to
                        to add distribution-specific configuration,
                        initialization scripts, custom image recipes,
                        and so forth.</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id="bsp-layer">
            <title>BSP Layer</title>

            <para>
                The BSP Layer provides machine configurations.
                Everything in this layer is specific to the machine for which
                you are building the image or the SDK.
                A common structure or form is defined for BSP layers.
                You can learn more about this structure in the
                <ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
                <note>
                    In order for a BSP layer to be considered compliant with the
                    Yocto Project, it must meet some structural requirements.
                </note>
            </para>

            <para>
                The BSP Layer's configuration directory contains
                configuration files for the machine
                (<filename>conf/machine/&lt;machine&gt;.conf</filename>) and,
                of course, the layer (<filename>conf/layer.conf</filename>).
            </para>

            <para>
                The remainder of the layer is dedicated to specific recipes
                by function: <filename>recipes-bsp</filename>,
                <filename>recipes-core</filename>,
                <filename>recipes-graphics</filename>, and
                <filename>recipes-kernel</filename>.
                Metadata can exist for multiple formfactors, graphics
                support systems, and so forth.
                <note>
                    While the figure shows several <filename>recipe-*</filename>
                    directories, not all these directories appear in all
                    BSP layers.
                </note>
            </para>
        </section>

        <section id="software-layer">
            <title>Software Layer</title>

            <para>
                The software layer provides the Metadata for additional
                software packages used during the build.
                This layer does not include Metadata that is specific to the
                distribution or the machine, which are found in their
                respective layers.
            </para>

            <para>
                This layer contains any new recipes that your project needs
                in the form of recipe files.
            </para>
        </section>
    </section>

    <section id="sources-dev-environment">
        <title>Sources</title>

        <para>
            In order for the OpenEmbedded build system to create an image or
            any target, it must be able to access source files.
            The main
            <link linkend='a-closer-look-at-the-yocto-project-development-environment'>Yocto Project Development Environment figure</link>
            represents source files using the "Upstream Project Releases",
            "Local Projects", and "SCMs (optional)" boxes.
            The figure represents mirrors, which also play a role in locating
            source files, with the "Source Mirror(s)" box.
        </para>

        <para>
            The method by which source files are ultimately organized is
            a function of the project.
            For example, for released software, projects tend to use tarballs
            or other archived files that can capture the state of a release
            guaranteeing that it is statically represented.
            On the other hand, for a project that is more dynamic or
            experimental in nature, a project might keep source files in a
            repository controlled by a Source Control Manager (SCM) such as
            Git.
            Pulling source from a repository allows you to control
            the point in the repository (the revision) from which you want to
            build software.
            Finally, a combination of the two might exist, which would give the
            consumer a choice when deciding where to get source files.
        </para>

        <para>
            BitBake uses the
            <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
            variable to point to source files regardless of their location.
            Each recipe must have a <filename>SRC_URI</filename> variable
            that points to the source.
        </para>

        <para>
            Another area that plays a significant role in where source files
            comes from is pointed to by the
            <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
            variable.
            This area is a cache that can hold previously downloaded source.
            Judicious use of a <filename>DL_DIR</filename> directory can
            save the build system a trip across the Internet when looking
            for files.
            A good method for using a download directory is to have
            <filename>DL_DIR</filename> point to an area outside of your
            Build Directory.
            Doing so allows you to safely delete the Build Directory
            if needed without fear of removing any downloaded source file.
        </para>

        <para>
            The remainder of this section provides a deeper look into the
            source files and the mirrors.
            Here is a more detailed look at the source file area of the
            base figure:
            <imagedata fileref="figures/source-input.png" align="center" width="7in" depth="7.5in" />
        </para>

        <section id='upstream-project-releases'>
            <title>Upstream Project Releases</title>

            <para>
                Upstream project releases exist anywhere in the form of an
                archived file (e.g. tarball or zip file).
                These files correspond to individual recipes.
                For example, the figure uses specific releases each for
                BusyBox, Qt, and Dbus.
                An archive file can be for any released product that can be
                built using a recipe.
            </para>
        </section>

        <section id='local-projects'>
            <title>Local Projects</title>

            <para>
                Local projects are custom bits of software the user provides.
                These bits reside somewhere local to a project - perhaps
                a directory into which the user checks in items (e.g.
                a local directory containing a development source tree
                used by the group).
            </para>

            <para>
                The canonical method through which to include a local project
                is to use the
                <link linkend='ref-classes-externalsrc'><filename>externalsrc.bbclass</filename></link>
                class to include local project.
                You use either the <filename>local.conf</filename> or a
                recipe's append file to override or set the
                recipe to point to the local directory on your disk to pull
                in the whole source tree.
            </para>

            <para>
                For information on how to use the
                <filename>externalsrc.bbclass</filename>, see the
                "<link linkend='ref-classes-externalsrc'><filename>externalsrc.bbclass</filename></link>"
                section.
            </para>
        </section>

        <section id='scms'>
            <title>Source Control Managers (Optional)</title>

            <para>
                Another place the build system can get source files from is
                through an SCM such as Git or Subversion.
                In this case, a repository is cloned or checked out.
                The <filename>do_fetch</filename> task inside BitBake uses
                the <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
                variable and the argument's prefix to determine the correct
                fetcher module.
            </para>

            <para>
                When fetching a repository, BitBake uses the
                <link linkend='var-SRCREV'><filename>SRCREV</filename></link>
                variable to determine the specific revision from which to
                build.
            </para>
        </section>

        <section id='source-mirrors'>
            <title>Source Mirror(s)</title>

            <para>
                Two kinds of mirrors exist: pre-mirrors and regular mirrors.
                The <link linkend='var-PREMIRRORS'><filename>PREMIRRORS</filename></link>
                and
                <link linkend='var-MIRRORS'><filename>MIRRORS</filename></link>
                variables point to these, respectively.
                BitBake checks pre-mirrors before looking upstream for any
                source files.
                Pre-mirrors are appropriate when you have a shared directory
                that is not a directory defined by the
                <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
                variable.
                A Pre-mirror typically points to a shared directory that is
                local to your organization.
            </para>

            <para>
                Regular mirrors can be any site across the Internet that is
                used as an alternative location for source code should the
                primary site not be functioning for some reason or another.
            </para>
        </section>
    </section>

    <section id="package-feeds-dev-environment">
        <title>Package Feeds</title>

        <para>
            When the OpenEmbedded build system generates an image or an SDK,
            it gets the packages from a package feed area located in the
            <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>.
            The main
            <link linkend='a-closer-look-at-the-yocto-project-development-environment'>Yocto Project Development Environment</link>
            figure shows this package feeds area in the upper-right corner.
        </para>

        <para>
            This section looks a little closer into the package feeds area used
            by the build system.
            Here is a more detailed look at the area:
            <imagedata fileref="figures/package-feeds.png" align="center" width="7in" depth="6in" />
        </para>

        <para>
            Package feeds are an intermediary step in the build process.
            BitBake generates packages whose type is defined by the
            <link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>
            variable.
            Before placing the packages into package feeds,
            the build process validates them with generated output quality
            assurance checks through the
            <link linkend='ref-classes-insane'><filename>insane.bbclass</filename></link>
            class.
        </para>

        <para>
            The package feed area resides in
            <filename>tmp/deploy</filename> of the Build Directory.
            Folders are created that correspond to the package type
            (IPK, DEB, or RPM) created.
            Further organization is derived through the value of the
            <link linkend='var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></link>
            variable for each package.
            For example, packages can exist for the i586 or qemux86
            architectures.
            The package files themselves reside within the appropriate
            architecture folder.
        </para>

        <para>
            BitBake uses the <filename>do_package_write_*</filename> task to
            place generated packages into the package holding area (e.g.
            <filename>do_package_write_ipk</filename> for IPK packages).
        </para>
    </section>

    <section id='images-dev-environment'>
        <title>Images</title>

        <para>
            The images produced by the OpenEmbedded build system
            are compressed forms of the
            root filesystems that are ready to boot on a target device.
            You can see from the main
            <link linkend='a-closer-look-at-the-yocto-project-development-environment'>Yocto Project Development Environment</link>
            figure that BitBake output in part consists of images.
            This section is going to look more closely at this output:
            <imagedata fileref="figures/images.png" align="center" width="5in" depth="4in" />
        </para>

        <para>
            For a list of example images that the Yocto Project provides,
            the
            "<link linkend='ref-images'>Images</link>" chapter.
        </para>

        <para>
            Images are written out to the
            <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>
            inside the <filename>deploy/images</filename> folder as shown
            in the figure.
            This folder contains any files expected to be loaded on the
            target device.
            The
            <link linkend='var-DEPLOY_DIR'><filename>DEPLOY_DIR</filename></link>
            variable points to the <filename>deploy</filename> directory.
            <itemizedlist>
                <listitem><para><filename>&lt;kernel-image&gt;</filename>:
                    A kernel binary file.
                    The <link linkend='var-KERNEL_IMAGETYPE'><filename>KERNEL_IMAGETYPE</filename></link>
                    variable setting determines the naming scheme for the
                    kernel image file.
                    Depending on that variable, the file could begin with
                    a variety of naming strings.
                    The <filename>deploy/images</filename> directory can
                    contain multiple image files.</para></listitem>
                <listitem><para><filename>&lt;root-filesystem-image&gt;</filename>:
                    Root filesystems for the target device (e.g.
                    <filename>*.ext3</filename> or <filename>*.bz2</filename>
                    files).
                    The <link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
                    variable setting determines the root filesystem image
                    type.
                    The <filename>deploy/images</filename> directory can
                    contain multiple root filesystems.</para></listitem>
                <listitem><para><filename>&lt;kernel-modules&gt;</filename>:
                    Tarballs that contain all the modules built for the kernel.
                    Kernel module tarballs exist for legacy purposes and
                    can be suppressed by setting the
                    <link linkend='var-MODULE_TARBALL_DEPLOY'><filename>MODULE_TARBALL_DEPLOY</filename></link>
                    variable to "0".
                    The <filename>deploy/images</filename> directory can
                    contain multiple kernel module tarballs.
                    </para></listitem>
                <listitem><para><filename>&lt;bootloaders&gt;</filename>:
                    Bootloaders supporting the image, if applicable to the
                    target machine.
                    The <filename>deploy/images</filename> directory can
                    contain multiple bootloaders.
                    </para></listitem>
                <listitem><para><filename>&lt;symlinks&gt;</filename>:
                    The <filename>deploy/images</filename> folder contains
                    a symbolic link that points to the most recently built file
                    for each machine.
                    These links might be useful for external scripts that
                    need to obtain the latest version of each file.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='sdk-dev-environment'>
        <title>Application Development SDK</title>

        <para>
            In the overview figure of the
            <link linkend='a-closer-look-at-the-yocto-project-development-environment'>Yocto Project Development Environment</link>
            the output labeled "Application Development SDK" represents an
            SDK.
            This section is going to take a closer look at this output:
            <imagedata fileref="figures/sdk.png" align="center" width="5in" depth="4in" />
        </para>

        <para>
            The specific form of this output is a self-extracting
            SDK installer (<filename>*.sh</filename>) that, when run,
            installs the SDK, which consists of a cross-development
            toolchain, a set of libraries and headers, and an SDK
            environment setup script.
            Running this installer essentially sets up your
            cross-development environment.
            You can think of the cross-toolchain as the "host"
            part because it runs on the SDK machine.
            You can think of the libraries and headers as the "target"
            part because they are built for the target hardware.
            The setup script is added so that you can initialize the
            environment before using the tools.
        </para>

        <note>
            <para>
                The Yocto Project supports several methods by which you can
                set up this cross-development environment.
                These methods include downloading pre-built SDK installers,
                building and installing your own SDK installer, or running
                an Application Development Toolkit (ADT) installer to
                install not just cross-development toolchains
                but also additional tools to help in this type of
                development.
            </para>

            <para>
                For background information on cross-development toolchains
                in the Yocto Project development environment, see the
                "<link linkend='cross-development-toolchain-generation'>Cross-Development Toolchain Generation</link>"
                section.
                For information on setting up a cross-development
                environment, see the
                "<ulink url='&YOCTO_DOCS_ADT_URL;#installing-the-adt'>Installing the ADT and Toolchains</ulink>"
                section in the Yocto Project Application Developer's Guide.
            </para>
        </note>

        <para>
            Once built, the SDK installers are written out to the
            <filename>deploy/sdk</filename> folder inside the
            <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>
            as shown in the figure at the beginning of this section.
            Several variables exist that help configure these files:
            <itemizedlist>
                <listitem><para><link linkend='var-DEPLOY_DIR'><filename>DEPLOY_DIR</filename></link>:
                    Points to the <filename>deploy</filename>
                    directory.</para></listitem>
                <listitem><para><link linkend='var-SDKMACHINE'><filename>SDKMACHINE</filename></link>:
                    Specifies the architecture of the machine
                    on which the cross-development tools are run to
                    create packages for the target hardware.
                    </para></listitem>
                <listitem><para><link linkend='var-SDKIMAGE_FEATURES'><filename>SDKIMAGE_FEATURES</filename></link>:
                    Lists the features to include in the "target" part
                    of the SDK.
                    </para></listitem>
                <listitem><para><link linkend='var-TOOLCHAIN_HOST_TASK'><filename>TOOLCHAIN_HOST_TASK</filename></link>:
                    Lists packages that make up the host
                    part of the SDK (i.e. the part that runs on
                    the <filename>SDKMACHINE</filename>).
                    When you use
                    <filename>bitbake -c populate_sdk &lt;imagename&gt;</filename>
                    to create the SDK, a set of default packages
                    apply.
                    This variable allows you to add more packages.
                    </para></listitem>
                <listitem><para><link linkend='var-TOOLCHAIN_TARGET_TASK'><filename>TOOLCHAIN_TARGET_TASK</filename></link>:
                    Lists packages that make up the target part
                    of the SDK (i.e. the part built for the
                    target hardware).
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='bitbake-dev-environment'>
        <title>BitBake</title>

        <para>
            The OpenEmbedded build system uses BitBake to produce images.
            You can see from the
            <link linkend='a-closer-look-at-the-yocto-project-development-environment'>Yocto Project Development Environment</link>
            figure, the BitBake area consists of several functional areas.
            This section takes a closer look at each of those areas.
        </para>

        <section id='source-fetching-dev-environment'>
            <title>Source Fetching</title>

            <para>
                The first stages of building a recipe are to fetch and unpack
                the source code:
                <imagedata fileref="figures/source-fetching.png" align="center" width="6in" depth="4in" />
            </para>

            <para>
                The <filename>do_fetch</filename> and
                <filename>do_unpack</filename> tasks fetch the source files
                and unpack them into a working directory.
                By default, everything is accomplished in the
                <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>,
                which has a defined structure.
                For additional general information on the Build Directory,
                see the
                "<link linkend='structure-core-build'><filename>build/</filename></link>"
                section.
            </para>

            <para>
                Unpacked source source files are pointed to by the
                <link linkend='var-S'><filename>S</filename></link> variable.
                Each recipe has an area in the Build Directory where the
                unpacked source code resides.
                The name of directory for any given recipe is defined from
                several different variables.
                You can see the variables that define these directories
                by looking at the figure:
                <itemizedlist>
                    <listitem><para><link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>
                        </para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></link>
                        </para></listitem>
                    <listitem><para><link linkend='var-TARGET_OS'><filename>TARGET_OS</filename></link>
                        </para></listitem>
                    <listitem><para><link linkend='var-PN'><filename>PN</filename></link>
                        </para></listitem>
                    <listitem><para><link linkend='var-PV'><filename>PV</filename></link>
                        </para></listitem>
                    <listitem><para><link linkend='var-PR'><filename>PR</filename></link>
                        </para></listitem>
                    <listitem><para><link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>
                        </para></listitem>
                    <listitem><para><link linkend='var-S'><filename>S</filename></link>
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                Briefly, the <filename>S</filename> directory contains the
                unpacked source files for a recipe.
                The <filename>WORKDIR</filename> directory is where all the
                building goes on for a given recipe.
            </para>
        </section>
    </section>
</section>

<section id="cross-development-toolchain-generation">
    <title>Cross-Development Toolchain Generation</title>

    <para>
        The Yocto Project does most of the work for you when it comes to
        creating
        <ulink url='&YOCTO_DOCS_DEV_URL;#cross-development-toolchain'>cross-development toolchains</ulink>.
        This section provides some technical background information on how
        cross-development toolchains are created and used.
        For more information on these toolchain, you can also see the
        <ulink url='&YOCTO_DOCS_ADT_URL;'>the Yocto Project Application Developer's Guide</ulink>.
    </para>

    <para>
        In the Yocto Project development environment, cross-development
        toolchains are used to build the image and applications that run on the
        target hardware.
        With just a few commands, the OpenEmbedded build system creates
        these necessary toolchains for you.
    </para>

    <para>
        The following figure shows a high-level build environment regarding
        toolchain construction and use.
    </para>

    <para>
        <imagedata fileref="figures/cross-development-toolchains.png" width="8in" depth="6in" align="center" />
    </para>

    <para>
        Most of the work occurs on the Build Host.
        This is the machine used to build images and generally work within the
        the Yocto Project environment.
        When you run BitBake to create an image, the OpenEmbedded build system
        uses the host <filename>gcc</filename> compiler to bootstrap a
        cross-compiler named <filename>gcc-cross</filename>.
        The <filename>gcc-cross</filename> compiler is what BitBake uses to
        compile source files when creating the target image.
        You can think of <filename>gcc-cross</filename> simply as an
        automatically generated cross-compiler that is used internally within
        BitBake only.
    </para>

    <para>
        The chain of events that occurs when <filename>gcc-cross</filename> is
        bootstrapped is as follows:
        <literallayout class='monospaced'>
     gcc -> binutils-cross -> gcc-cross-initial -> linux-libc-headers -> eglibc-initial -> eglibc -> gcc-cross -> gcc-runtime
        </literallayout>
        <itemizedlist>
            <listitem><para><filename>gcc</filename>:
                The build host's GNU Compiler Collection (GCC).
                </para></listitem>
            <listitem><para><filename>binutils-cross</filename>:
                The bare minimum binary utilities needed in order to run
                the <filename>gcc-cross-initial</filename> phase of the
                bootstrap operation.
                </para></listitem>
            <listitem><para><filename>gcc-cross-initial</filename>:
                An early stage of the bootstrap process for creating
                the cross-compiler.
                This stage builds enough of the <filename>gcc-cross</filename>,
                the C library, and other pieces needed to finish building the
                final cross-compiler in later stages.
                This tool is a "native" package (i.e. it is designed to run on
                the build host).
                </para></listitem>
            <listitem><para><filename>linux-libc-headers</filename>:
                Headers needed for the cross-compiler.
                </para></listitem>
            <listitem><para><filename>eglibc-initial</filename>:
                An initial version of the Embedded GLIBC needed to bootstrap
                <filename>eglibc</filename>.
                </para></listitem>
            <listitem><para><filename>gcc-cross</filename>:
                The final stage of the bootstrap process for the
                cross-compiler.
                This stage results in the actual cross-compiler that
                BitBake uses when it builds an image for a targeted
                device.
                <note>
                    If you are replacing this cross compiler toolchain
                    with a custom version, you must replace
                    <filename>gcc-cross</filename>.
                </note>
                This tool is also a "native" package (i.e. it is
                designed to run on the build host).
                </para></listitem>
            <listitem><para><filename>gcc-runtime</filename>:
                Runtime libraries resulting from the toolchain bootstrapping
                process.
                This tool produces a binary that consists of the
                runtime libraries need for the targeted device.
                </para></listitem>
        </itemizedlist>
    </para>

    <para>
        You can use the OpenEmbedded build system to build an installer for
        the relocatable SDK used to develop applications.
        When you run the installer, it installs the toolchain, which contains
        the development tools (e.g., the
        <filename>gcc-cross-canadian</filename>),
        <filename>binutils-cross-canadian</filename>, and other
        <filename>nativesdk-*</filename> tools you need to cross-compile and
        test your software.
        The figure shows the commands you use to easily build out this
        toolchain.
        This cross-development toolchain is built to execute on the
        <link linkend='var-SDKMACHINE'><filename>SDKMACHINE</filename></link>,
        which might or might not be the same
        machine as the Build Host.
        <note>
            If your target architecture is supported by the Yocto Project,
            you can take advantage of pre-built images that ship with the
            Yocto Project and already contain cross-development toolchain
            installers.
        </note>
    </para>

    <para>
        Here is the bootstrap process for the relocatable toolchain:
        <literallayout class='monospaced'>
     gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers -> eglibc-initial -> nativesdk-eglibc -> gcc-crosssdk -> gcc-cross-canadian
        </literallayout>
        <itemizedlist>
            <listitem><para><filename>gcc</filename>:
                The build host's GNU Compiler Collection (GCC).
                </para></listitem>
            <listitem><para><filename>binutils-crosssdk</filename>:
                The bare minimum binary utilities needed in order to run
                the <filename>gcc-crosssdk-initial</filename> phase of the
                bootstrap operation.
                </para></listitem>
            <listitem><para><filename>gcc-crosssdk-initial</filename>:
                An early stage of the bootstrap process for creating
                the cross-compiler.
                This stage builds enough of the
                <filename>gcc-crosssdk</filename> and supporting pieces so that
                the final stage of the bootstrap process can produce the
                finished cross-compiler.
                This tool is a "native" binary that runs on the build host.
                </para></listitem>
            <listitem><para><filename>linux-libc-headers</filename>:
                Headers needed for the cross-compiler.
                </para></listitem>
            <listitem><para><filename>eglibc-initial</filename>:
                An initial version of the Embedded GLIBC needed to bootstrap
                <filename>nativesdk-eglibc</filename>.
                </para></listitem>
            <listitem><para><filename>nativesdk-eglibc</filename>:
                The Embedded GLIBC needed to bootstrap the
                <filename>gcc-crosssdk</filename>.
                </para></listitem>
            <listitem><para><filename>gcc-crosssdk</filename>:
                The final stage of the bootstrap process for the
                relocatable cross-compiler.
                The <filename>gcc-crosssdk</filename> is a transitory compiler
                and never leaves the build host.
                Its purpose is to help in the bootstrap process to create the
                eventual relocatable <filename>gcc-cross-canadian</filename>
                compiler, which is relocatable.
                This tool is also a "native" package (i.e. it is
                designed to run on the build host).
                </para></listitem>
            <listitem><para><filename>gcc-cross-canadian</filename>:
                The final relocatable cross-compiler.
                When run on the
                <link linkend='var-SDKMACHINE'><filename>SDKMACHINE</filename></link>,
                this tool
                produces executable code that runs on the target device.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id="shared-state-cache">
    <title>Shared State Cache</title>

    <para>
        By design, the OpenEmbedded build system builds everything from scratch unless
        BitBake can determine that parts do not need to be rebuilt.
        Fundamentally, building from scratch is attractive as it means all parts are
        built fresh and there is no possibility of stale data causing problems.
        When developers hit problems, they typically default back to building from scratch
        so they know the state of things from the start.
    </para>

    <para>
        Building an image from scratch is both an advantage and a disadvantage to the process.
        As mentioned in the previous paragraph, building from scratch ensures that
        everything is current and starts from a known state.
        However, building from scratch also takes much longer as it generally means
        rebuilding things that do not necessarily need rebuilt.
    </para>

    <para>
        The Yocto Project implements shared state code that supports incremental builds.
        The implementation of the shared state code answers the following questions that
        were fundamental roadblocks within the OpenEmbedded incremental build support system:
        <itemizedlist>
            <listitem>What pieces of the system have changed and what pieces have not changed?</listitem>
            <listitem>How are changed pieces of software removed and replaced?</listitem>
            <listitem>How are pre-built components that do not need to be rebuilt from scratch
                used when they are available?</listitem>
        </itemizedlist>
    </para>

    <para>
        For the first question, the build system detects changes in the "inputs" to a given task by
        creating a checksum (or signature) of the task's inputs.
        If the checksum changes, the system assumes the inputs have changed and the task needs to be
        rerun.
        For the second question, the shared state (sstate) code tracks which tasks add which output
        to the build process.
        This means the output from a given task can be removed, upgraded or otherwise manipulated.
        The third question is partly addressed by the solution for the second question
        assuming the build system can fetch the sstate objects from remote locations and
        install them if they are deemed to be valid.
    </para>

    <note>
        The OpenEmbedded build system does not maintain
        <link linkend='var-PR'><filename>PR</filename></link> information
        as part of the shared state packages.
        Consequently, considerations exist that affect maintaining shared
        state feeds.
        For information on how the OpenEmbedded works with packages and can
        track incrementing <filename>PR</filename> information, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#incrementing-a-package-revision-number'>Incrementing a Package Revision Number</ulink>"
        section.
    </note>

    <para>
        The rest of this section goes into detail about the overall incremental build
        architecture, the checksums (signatures), shared state, and some tips and tricks.
    </para>

    <section id='overall-architecture'>
        <title>Overall Architecture</title>

        <para>
            When determining what parts of the system need to be built, BitBake
            uses a per-task basis and does not use a per-recipe basis.
            You might wonder why using a per-task basis is preferred over a per-recipe basis.
            To help explain, consider having the IPK packaging backend enabled and then switching to DEB.
            In this case, <filename>do_install</filename> and <filename>do_package</filename>
            output are still valid.
            However, with a per-recipe approach, the build would not include the
            <filename>.deb</filename> files.
            Consequently, you would have to invalidate the whole build and rerun it.
            Rerunning everything is not the best situation.
            Also in this case, the core must be "taught" much about specific tasks.
            This methodology does not scale well and does not allow users to easily add new tasks
            in layers or as external recipes without touching the packaged-staging core.
        </para>
    </section>

    <section id='checksums'>
        <title>Checksums (Signatures)</title>

        <para>
            The shared state code uses a checksum, which is a unique signature of a task's
            inputs, to determine if a task needs to be run again.
            Because it is a change in a task's inputs that triggers a rerun, the process
            needs to detect all the inputs to a given task.
            For shell tasks, this turns out to be fairly easy because
            the build process generates a "run" shell script for each task and
            it is possible to create a checksum that gives you a good idea of when
            the task's data changes.
        </para>

        <para>
            To complicate the problem, there are things that should not be included in
            the checksum.
            First, there is the actual specific build path of a given task -
            the <link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>.
            It does not matter if the working directory changes because it should not
            affect the output for target packages.
            Also, the build process has the objective of making native or cross packages relocatable.
            The checksum therefore needs to exclude <filename>WORKDIR</filename>.
            The simplistic approach for excluding the working directory is to set
            <filename>WORKDIR</filename> to some fixed value and create the checksum
            for the "run" script.
        </para>

        <para>
            Another problem results from the "run" scripts containing functions that
            might or might not get called.
            The incremental build solution contains code that figures out dependencies
            between shell functions.
            This code is used to prune the "run" scripts down to the minimum set,
            thereby alleviating this problem and making the "run" scripts much more
            readable as a bonus.
        </para>

        <para>
            So far we have solutions for shell scripts.
            What about Python tasks?
            The same approach applies even though these tasks are more difficult.
            The process needs to figure out what variables a Python function accesses
            and what functions it calls.
            Again, the incremental build solution contains code that first figures out
            the variable and function dependencies, and then creates a checksum for the data
            used as the input to the task.
        </para>

        <para>
            Like the <filename>WORKDIR</filename> case, situations exist where dependencies
            should be ignored.
            For these cases, you can instruct the build process to ignore a dependency
            by using a line like the following:
            <literallayout class='monospaced'>
     PACKAGE_ARCHS[vardepsexclude] = "MACHINE"
            </literallayout>
            This example ensures that the <filename>PACKAGE_ARCHS</filename> variable does not
            depend on the value of <filename>MACHINE</filename>, even if it does reference it.
        </para>

        <para>
            Equally, there are cases where we need to add dependencies BitBake is not able to find.
            You can accomplish this by using a line like the following:
            <literallayout class='monospaced'>
      PACKAGE_ARCHS[vardeps] = "MACHINE"
            </literallayout>
            This example explicitly adds the <filename>MACHINE</filename> variable as a
            dependency for <filename>PACKAGE_ARCHS</filename>.
        </para>

        <para>
            Consider a case with in-line Python, for example, where BitBake is not
            able to figure out dependencies.
            When running in debug mode (i.e. using <filename>-DDD</filename>), BitBake
            produces output when it discovers something for which it cannot figure out
            dependencies.
            The Yocto Project team has currently not managed to cover those dependencies
            in detail and is aware of the need to fix this situation.
        </para>

        <para>
            Thus far, this section has limited discussion to the direct inputs into a task.
            Information based on direct inputs is referred to as the "basehash" in the
            code.
            However, there is still the question of a task's indirect inputs - the
            things that were already built and present in the
            <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>.
            The checksum (or signature) for a particular task needs to add the hashes
            of all the tasks on which the particular task depends.
            Choosing which dependencies to add is a policy decision.
            However, the effect is to generate a master checksum that combines the basehash
            and the hashes of the task's dependencies.
        </para>

        <para>
            At the code level, there are a variety of ways both the basehash and the
            dependent task hashes can be influenced.
            Within the BitBake configuration file, we can give BitBake some extra information
            to help it construct the basehash.
            The following statements effectively result in a list of global variable
            dependency excludes - variables never included in any checksum:
            <literallayout class='monospaced'>
  BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH"
  BB_HASHBASE_WHITELIST += "DL_DIR SSTATE_DIR THISDIR FILESEXTRAPATHS"
  BB_HASHBASE_WHITELIST += "FILE_DIRNAME HOME LOGNAME SHELL TERM USER"
  BB_HASHBASE_WHITELIST += "FILESPATH USERNAME STAGING_DIR_HOST STAGING_DIR_TARGET"
            </literallayout>
            The previous example actually excludes
            <link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>
            since it is actually constructed as a path within
            <link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>, which is on
            the whitelist.
        </para>

        <para>
            The rules for deciding which hashes of dependent tasks to include through
            dependency chains are more complex and are generally accomplished with a
            Python function.
            The code in <filename>meta/lib/oe/sstatesig.py</filename> shows two examples
            of this and also illustrates how you can insert your own policy into the system
            if so desired.
            This file defines the two basic signature generators <filename>OE-Core</filename>
            uses:  "OEBasic" and "OEBasicHash".
            By default, there is a dummy "noop" signature handler enabled in BitBake.
            This means that behavior is unchanged from previous versions.
            <filename>OE-Core</filename> uses the "OEBasicHash" signature handler by default
            through this setting in the <filename>bitbake.conf</filename> file:
            <literallayout class='monospaced'>
  BB_SIGNATURE_HANDLER ?= "OEBasicHash"
            </literallayout>
            The "OEBasicHash" <filename>BB_SIGNATURE_HANDLER</filename> is the same as the
            "OEBasic" version but adds the task hash to the stamp files.
            This results in any
            <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink>
            change that changes the task hash, automatically
            causing the task to be run again.
            This removes the need to bump <link linkend='var-PR'><filename>PR</filename></link>
            values and changes to Metadata automatically ripple across the build.
        </para>

        <para>
            It is also worth noting that the end result of these signature generators is to
            make some dependency and hash information available to the build.
            This information includes:
            <literallayout class='monospaced'>
  BB_BASEHASH_task-&lt;taskname&gt; - the base hashes for each task in the recipe
  BB_BASEHASH_&lt;filename:taskname&gt; - the base hashes for each dependent task
  BBHASHDEPS_&lt;filename:taskname&gt; - The task dependencies for each task
  BB_TASKHASH - the hash of the currently running task
            </literallayout>
        </para>
    </section>

    <section id='shared-state'>
        <title>Shared State</title>

        <para>
            Checksums and dependencies, as discussed in the previous section, solve half the
            problem.
            The other part of the problem is being able to use checksum information during the build
            and being able to reuse or rebuild specific components.
        </para>

        <para>
            The shared state class (<filename>sstate.bbclass</filename>)
            is a relatively generic implementation of how to "capture" a snapshot of a given task.
            The idea is that the build process does not care about the source of a task's output.
            Output could be freshly built or it could be downloaded and unpacked from
            somewhere - the build process does not need to worry about its source.
        </para>

        <para>
            There are two types of output, one is just about creating a directory
            in <link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>.
            A good example is the output of either <filename>do_install</filename> or
            <filename>do_package</filename>.
            The other type of output occurs when a set of data is merged into a shared directory
            tree such as the sysroot.
        </para>

        <para>
            The Yocto Project team has tried to keep the details of the implementation hidden in
            <filename>sstate.bbclass</filename>.
            From a user's perspective, adding shared state wrapping to a task
            is as simple as this <filename>do_deploy</filename> example taken from
            <filename>do_deploy.bbclass</filename>:
            <literallayout class='monospaced'>
     DEPLOYDIR = "${WORKDIR}/deploy-${PN}"
     SSTATETASKS += "do_deploy"
     do_deploy[sstate-name] = "deploy"
     do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"
     do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"

     python do_deploy_setscene () {
         sstate_setscene(d)
     }
     addtask do_deploy_setscene
            </literallayout>
            In the example, we add some extra flags to the task, a name field ("deploy"), an
            input directory where the task sends data, and the output
            directory where the data from the task should eventually be copied.
            We also add a <filename>_setscene</filename> variant of the task and add the task
            name to the <filename>SSTATETASKS</filename> list.
        </para>

        <para>
            If you have a directory whose contents you need to preserve, you can do this with
            a line like the following:
            <literallayout class='monospaced'>
     do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}"
            </literallayout>
            This method, as well as the following example, also works for multiple directories.
            <literallayout class='monospaced'>
     do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}"
     do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}"
     do_package[sstate-lockfile] = "${PACKAGELOCK}"
            </literallayout>
            These methods also include the ability to take a lockfile when manipulating
            shared state directory structures since some cases are sensitive to file
            additions or removals.
        </para>

        <para>
            Behind the scenes, the shared state code works by looking in
            <link linkend='var-SSTATE_DIR'><filename>SSTATE_DIR</filename></link> and
            <link linkend='var-SSTATE_MIRRORS'><filename>SSTATE_MIRRORS</filename></link>
            for shared state files.
            Here is an example:
            <literallayout class='monospaced'>
     SSTATE_MIRRORS ?= "\
     file://.* http://someserver.tld/share/sstate/PATH \n \
     file://.* file:///some/local/dir/sstate/PATH"
            </literallayout>
            <note>
                The shared state directory (<filename>SSTATE_DIR</filename>) is
                organized into two-character subdirectories, where the subdirectory
                names are based on the first two characters of the hash.
                If the shared state directory structure for a mirror has the
                same structure as <filename>SSTATE_DIR</filename>, you must
                specify "PATH" as part of the URI to enable the build system
                to map to the appropriate subdirectory.
            </note>
        </para>

        <para>
            The shared state package validity can be detected just by looking at the
            filename since the filename contains the task checksum (or signature) as
            described earlier in this section.
            If a valid shared state package is found, the build process downloads it
            and uses it to accelerate the task.
        </para>

        <para>
            The build processes use the <filename>*_setscene</filename> tasks
            for the task acceleration phase.
            BitBake goes through this phase before the main execution code and tries
            to accelerate any tasks for which it can find shared state packages.
            If a shared state package for a task is available, the shared state
            package is used.
            This means the task and any tasks on which it is dependent are not
            executed.
        </para>

        <para>
            As a real world example, the aim is when building an IPK-based image,
            only the <filename>do_package_write_ipk</filename> tasks would have their
            shared state packages fetched and extracted.
            Since the sysroot is not used, it would never get extracted.
            This is another reason why a task-based approach is preferred over a
            recipe-based approach, which would have to install the output from every task.
        </para>
    </section>

    <section id='tips-and-tricks'>
        <title>Tips and Tricks</title>

        <para>
            The code in the build system that supports incremental builds is not
            simple code.
            This section presents some tips and tricks that help you work around
            issues related to shared state code.
        </para>

        <section id='debugging'>
            <title>Debugging</title>

            <para>
                When things go wrong, debugging needs to be straightforward.
                Because of this, the Yocto Project team included strong debugging
                tools:
                <itemizedlist>
                    <listitem><para>Whenever a shared state package is written out, so is a
                        corresponding <filename>.siginfo</filename> file.
                        This practice results in a pickled Python database of all
                        the metadata that went into creating the hash for a given shared state
                        package.</para></listitem>
                    <listitem><para>If you run BitBake with the <filename>--dump-signatures</filename>
                        (or <filename>-S</filename>) option, BitBake dumps out
                        <filename>.siginfo</filename> files in
                        the stamp directory for every task it would have executed instead of
                        building the specified target package.</para></listitem>
                    <listitem><para>There is a <filename>bitbake-diffsigs</filename> command that
                        can process <filename>.siginfo</filename> files.
                        If you specify one of these files, BitBake dumps out the dependency
                        information in the file.
                        If you specify two files, BitBake compares the two files and dumps out
                        the differences between the two.
                        This more easily helps answer the question of "What
                        changed between X and Y?"</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='invalidating-shared-state'>
            <title>Invalidating Shared State</title>

            <para>
                The shared state code uses checksums and shared state
                cache to avoid unnecessarily rebuilding tasks.
                As with all schemes, this one has some drawbacks.
                It is possible that you could make implicit changes that are not factored
                into the checksum calculation, but do affect a task's output.
                A good example is perhaps when a tool changes its output.
                Assume that the output of <filename>rpmdeps</filename> needed to change.
                The result of the change should be that all the
                <filename>package</filename>, <filename>package_write_rpm</filename>,
                and <filename>package_deploy-rpm</filename> shared state cache
                items would become invalid.
                But, because this is a change that is external to the code and therefore implicit,
                the associated shared state cache items do not become invalidated.
                In this case, the build process uses the cached items rather than running the
                task again.
                Obviously, these types of implicit changes can cause problems.
            </para>

            <para>
                To avoid these problems during the build, you need to understand the effects of any
                change you make.
                Note that any changes you make directly to a function automatically are factored into
                the checksum calculation and thus, will invalidate the associated area of sstate cache.
                You need to be aware of any implicit changes that are not obvious changes to the
                code and could affect the output of a given task.
                Once you are aware of such changes, you can take steps to invalidate the cache
                and force the tasks to run.
                The steps to take are as simple as changing function's comments in the source code.
                For example, to invalidate package shared state files, change the comment statements
                of <filename>do_package</filename> or the comments of one of the functions it calls.
                The change is purely cosmetic, but it causes the checksum to be recalculated and
                forces the task to be run again.
            </para>

            <note>
                For an example of a commit that makes a cosmetic change to invalidate
                a shared state, see this
                <ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/commit/meta/classes/package.bbclass?id=737f8bbb4f27b4837047cb9b4fbfe01dfde36d54'>commit</ulink>.
            </note>
        </section>
    </section>
</section>

<section id='x32'>
    <title>x32</title>

    <para>
        x32 is a processor-specific Application Binary Interface (psABI) for x86_64.
        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>

    <section id='support'>
        <title>Support</title>

        <para>
            While the x32 psABI specifications are not fully finalized, this Yocto Project
            release supports current development specifications of x32 psABI.
            As of this release of the Yocto Project, x32 psABI support exists 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 many 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>
            </itemizedlist>
        </para>
    </section>

    <section id='stabilizing-and-completing-x32'>
        <title>Stabilizing and Completing x32</title>

        <para>
            As of this Yocto Project release, the x32 psABI kernel and library
            interfaces specifications are not finalized.
        </para>

        <para>
            Future Plans for the x32 psABI in the Yocto Project include the following:
            <itemizedlist>
                <listitem><para>Enhance and fix the few remaining recipes so they
                    work with and support x32 toolchains.</para></listitem>
                <listitem><para>Enhance RPM Package Manager (RPM) support for x32 binaries.</para></listitem>
                <listitem><para>Support larger images.</para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='using-x32-right-now'>
        <title>Using x32 Right Now</title>

        <para>
            Follow these steps to use the x32 spABI:
            <itemizedlist>
                <listitem><para>Enable the x32 psABI tuning file for <filename>x86_64</filename>
                    machines by editing the <filename>conf/local.conf</filename> like this:
                    <literallayout class='monospaced'>
      MACHINE = "qemux86-64"
      DEFAULTTUNE = "x86-64-x32"
      baselib = "${@d.getVar('BASE_LIB_tune-' + (d.getVar('DEFAULTTUNE', True) \
         or 'INVALID'), True) or 'lib'}"
      #MACHINE = "atom-pc"
      #DEFAULTTUNE = "core2-64-x32"
                    </literallayout></para></listitem>
                <listitem><para>As usual, use BitBake to build an image that supports the x32 psABI.
                    Here is an example:
                    <literallayout class='monospaced'>
     $ bitbake core-image-sato
                    </literallayout></para></listitem>
                <listitem><para>As usual, run your image using QEMU:
                    <literallayout class='monospaced'>
     $ runqemu qemux86-64 core-image-sato
                    </literallayout></para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>

<section id="wayland">
    <title>Wayland</title>

    <para>
        <ulink url='http://en.wikipedia.org/wiki/Wayland_(display_server_protocol)#Weston'>Wayland</ulink>
        is a computer display server protocol that when implemented
        provides a method for compositing window managers to communicate
        directly with applications and video hardware and expects them to
        communicate with input hardware using other libraries.
        Using Wayland with supporting targets can result in better control
        over graphics frame rendering than an application might otherwise
        achieve.
    </para>

    <para>
        The Yocto Project provides the Wayland protocol libraries and the
        reference Weston compositor as part of it release.
        This section describes what you need to do to implement Wayland and
        use the compositor when building an image for a supporting target.
    </para>

    <section id="wayland-support">
        <title>Support</title>

        <para>
            The Wayland protocol libraries and the reference Weston compositor
            ship as integrated packages in the <filename>meta</filename> layer
            of the
            <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>.
            Specifically, you can find the recipes that build both Wayland
            and Weston at <filename>meta/recipes-graphics/wayland</filename>.
        </para>

        <para>
            You can build both the Wayland and Weston packages for use only
            with targets that accept the
            <ulink url='http://dri.freedesktop.org/wiki/'>Mesa 3D and Direct Rendering Infrastructure</ulink>,
            which is also known as Mesa DRI.
            This implies that you cannot build and use the packages if your
            target uses, for example, the
            <trademark class='registered'>Intel</trademark> Embedded Media and
            Graphics Driver (<trademark class='registered'>Intel</trademark>
            EMGD) that overrides Mesa DRI.
        </para>

        <note>
            Due to lack of EGL support, Weston 1.0.3 will not run directly on
            the emulated QEMU hardware.
            However, this version of Weston will run under X emulation without
            issues.
        </note>
    </section>

    <section id="enabling-wayland-in-an-image">
        <title>Enabling Wayland in an Image</title>

        <para>
            To enable Wayland, you need to enable it to be built and enable
            it to be included in the image.
        </para>

        <section id="enable-building">
            <title>Building</title>

            <para>
                To cause Mesa to build the <filename>wayland-egl</filename>
                platform and Weston to build Wayland with Kernel Mode
                Setting
                (<ulink url='https://wiki.archlinux.org/index.php/Kernel_Mode_Setting'>KMS</ulink>)
                support, include the "wayland" flag in the
                <link linkend="var-DISTRO_FEATURES"><filename>DISTRO_FEATURES</filename></link>
                statement in your <filename>local.conf</filename> file:
                <literallayout class='monospaced'>
     DISTRO_FEATURES_append = " wayland"
                </literallayout>
            </para>

            <note>
                If X11 has been enabled elsewhere, Weston will build Wayland
                with X11 support
            </note>
        </section>

        <section id="enable-installation-in-an-image">
            <title>Installing</title>

            <para>
                To install the Wayland feature into an image, you must
                include the following
                <link linkend='var-CORE_IMAGE_EXTRA_INSTALL'><filename>CORE_IMAGE_EXTRA_INSTALL</filename></link>
                statement in your <filename>local.conf</filename> file:
                <literallayout class='monospaced'>
     CORE_IMAGE_EXTRA_INSTALL += "wayland weston"
                </literallayout>
            </para>
        </section>
    </section>

    <section id="running-weston">
        <title>Running Weston</title>

        <para>
            To run Weston inside X11, enabling it as described earlier and
            building a Sato image is sufficient.
            If you are running your image under Sato, a Weston Launcher appears
            in the "Utility" category.
        </para>

        <para>
            Alternatively, you can run Weston through the command-line
            interpretor (CLI), which is better suited for development work.
            To run Weston under the CLI you need to do the following after
            your image is built:
            <orderedlist>
                <listitem><para>Run these commands to export
                    <filename>XDG_RUNTIME_DIR</filename>:
                    <literallayout class='monospaced'>
     mkdir -p /tmp/$USER-weston
     chmod 0700 /tmp/$USER-weston
     export XDG_RUNTIME_DIR=/tmp/$USER=weston
                    </literallayout></para></listitem>
                <listitem><para>Launch Weston in the shell:
                    <literallayout class='monospaced'>
     weston
                    </literallayout></para></listitem>
            </orderedlist>
        </para>
    </section>
</section>

<section id="licenses">
    <title>Licenses</title>

    <para>
        This section describes the mechanism by which the OpenEmbedded build system
        tracks changes to licensing text.
        The section also describes how to enable commercially licensed recipes,
        which by default are disabled.
    </para>

    <para>
        For information that can help you maintain compliance with various open
        source licensing during the lifecycle of the product, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#maintaining-open-source-license-compliance-during-your-products-lifecycle'>Maintaining Open Source License Compliance During Your Project's Lifecycle</ulink>" section
        in the Yocto Project Development Manual.
    </para>

    <section id="usingpoky-configuring-LIC_FILES_CHKSUM">
        <title>Tracking License Changes</title>

        <para>
            The license of an upstream project might change in the future.
            In order to prevent these changes going unnoticed, the
            <filename><link linkend='var-LIC_FILES_CHKSUM'>LIC_FILES_CHKSUM</link></filename>
            variable tracks changes to the license text. The checksums are validated at the end of the
            configure step, and if the checksums do not match, the build will fail.
        </para>

        <section id="usingpoky-specifying-LIC_FILES_CHKSUM">
            <title>Specifying the <filename>LIC_FILES_CHKSUM</filename> Variable</title>

            <para>
                The <filename>LIC_FILES_CHKSUM</filename>
                variable contains checksums of the license text in the source code for the recipe.
                Following is an example of how to specify <filename>LIC_FILES_CHKSUM</filename>:
                <literallayout class='monospaced'>
     LIC_FILES_CHKSUM = "file://COPYING;md5=xxxx \
                         file://licfile1.txt;beginline=5;endline=29;md5=yyyy \
                         file://licfile2.txt;endline=50;md5=zzzz \
                         ..."
                </literallayout>
            </para>

            <para>
                The build system uses the
                <filename><link linkend='var-S'>S</link></filename> variable as
                the default directory used when searching files listed in
                <filename>LIC_FILES_CHKSUM</filename>.
                The previous example employs the default directory.
            </para>

            <para>
                Consider this next example:
                <literallayout class='monospaced'>
     LIC_FILES_CHKSUM = "file://src/ls.c;beginline=5;endline=16;\
                                         md5=bb14ed3c4cda583abc85401304b5cd4e"
     LIC_FILES_CHKSUM = "file://${WORKDIR}/license.html;md5=5c94767cedb5d6987c902ac850ded2c6"
                </literallayout>
            </para>

            <para>
                The first line locates a file in
                <filename>${S}/src/ls.c</filename>.
                The second line refers to a file in
                <filename><link linkend='var-WORKDIR'>WORKDIR</link></filename>.
            </para>
            <para>
                Note that <filename>LIC_FILES_CHKSUM</filename> variable is
                mandatory for all recipes, unless the
                <filename>LICENSE</filename> variable is set to "CLOSED".
            </para>
        </section>

        <section id="usingpoky-LIC_FILES_CHKSUM-explanation-of-syntax">
            <title>Explanation of Syntax</title>
            <para>
                As mentioned in the previous section, the
                <filename>LIC_FILES_CHKSUM</filename> variable lists all the
                important files that contain the license text for the source code.
                It is possible to specify a checksum for an entire file, or a specific section of a
                file (specified by beginning and ending line numbers with the "beginline" and "endline"
                parameters, respectively).
                The latter is useful for source files with a license notice header,
                README documents, and so forth.
                If you do not use the "beginline" parameter, then it is assumed that the text begins on the
                first line of the file.
                Similarly, if you do not use the "endline" parameter, it is assumed that the license text
                ends with the last line of the file.
            </para>

            <para>
                The "md5" parameter stores the md5 checksum of the license text.
                If the license text changes in any way as compared to this parameter
                then a mismatch occurs.
                This mismatch triggers a build failure and notifies the developer.
                Notification allows the developer to review and address the license text changes.
                Also note that if a mismatch occurs during the build, the correct md5
                checksum is placed in the build log and can be easily copied to the recipe.
            </para>

            <para>
                There is no limit to how many files you can specify using the
                <filename>LIC_FILES_CHKSUM</filename> variable.
                Generally, however, every project requires a few specifications for license tracking.
                Many projects have a "COPYING" file that stores the license information for all the source
                code files.
                This practice allows you to just track the "COPYING" file as long as it is kept up to date.
            </para>

            <tip>
                If you specify an empty or invalid "md5" parameter, BitBake returns an md5 mis-match
                error and displays the correct "md5" parameter value during the build.
                The correct parameter is also captured in the build log.
            </tip>

            <tip>
                If the whole file contains only license text, you do not need to use the "beginline" and
                "endline" parameters.
            </tip>
        </section>
    </section>

    <section id="enabling-commercially-licensed-recipes">
        <title>Enabling Commercially Licensed Recipes</title>

        <para>
            By default, the OpenEmbedded build system disables
            components that have commercial or other special licensing
            requirements.
            Such requirements are defined on a
            recipe-by-recipe basis through the <filename>LICENSE_FLAGS</filename> variable
            definition in the affected recipe.
            For instance, the
            <filename>$HOME/poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly</filename>
            recipe contains the following statement:
            <literallayout class='monospaced'>
     LICENSE_FLAGS = "commercial"
            </literallayout>
            Here is a slightly more complicated example that contains both an
            explicit recipe name and version (after variable expansion):
            <literallayout class='monospaced'>
     LICENSE_FLAGS = "license_${PN}_${PV}"
            </literallayout>
	        In order for a component restricted by a <filename>LICENSE_FLAGS</filename>
	        definition to be enabled and included in an image, it
	        needs to have a matching entry in the global
	        <filename>LICENSE_FLAGS_WHITELIST</filename> variable, which is a variable
	        typically defined in your <filename>local.conf</filename> file.
            For example, to enable
	        the <filename>$HOME/poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly</filename>
	        package, you could add either the string
	        "commercial_gst-plugins-ugly" or the more general string
	        "commercial" to <filename>LICENSE_FLAGS_WHITELIST</filename>.
            See the
            "<link linkend='license-flag-matching'>License Flag Matching</link>" section
            for a full explanation of how <filename>LICENSE_FLAGS</filename> matching works.
            Here is the example:
            <literallayout class='monospaced'>
     LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly"
            </literallayout>
	        Likewise, to additionally enable the package built from the recipe containing
	        <filename>LICENSE_FLAGS = "license_${PN}_${PV}"</filename>, and assuming
	        that the actual recipe name was <filename>emgd_1.10.bb</filename>,
	        the following string would enable that package as well as
	        the original <filename>gst-plugins-ugly</filename> package:
            <literallayout class='monospaced'>
     LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly license_emgd_1.10"
            </literallayout>
	        As a convenience, you do not need to specify the complete license string
	        in the whitelist for every package.
            you can use an abbreviated form, which consists
	        of just the first portion or portions of the license string before
	        the initial underscore character or characters.
            A partial string will match
	        any license that contains the given string as the first
	        portion of its license.
            For example, the following
	        whitelist string will also match both of the packages
	        previously mentioned as well as any other packages that have
	        licenses starting with "commercial" or "license".
            <literallayout class='monospaced'>
     LICENSE_FLAGS_WHITELIST = "commercial license"
            </literallayout>
        </para>

        <section id="license-flag-matching">
            <title>License Flag Matching</title>

            <para>
		        License flag matching allows you to control what recipes the
                OpenEmbedded build system includes in the build.
                Fundamentally, the build system attempts to match
                <filename>LICENSE_FLAG</filename> strings found in
                recipes against <filename>LICENSE_FLAGS_WHITELIST</filename>
                strings found in the whitelist.
                A match, causes the build system to include a recipe in the
                build, while failure to find a match causes the build system to
                exclude a recipe.
            </para>

            <para>
                In general, license flag matching is simple.
                However, understanding some concepts will help you
                correctly and effectively use matching.
            </para>

            <para>
                Before a flag
                defined by a particular recipe is tested against the
                contents of the whitelist, the expanded string
                <filename>_${PN}</filename> is appended to the flag.
                This expansion makes each <filename>LICENSE_FLAGS</filename>
                value recipe-specific.
                After expansion, the string is then matched against the
                whitelist.
                Thus, specifying
                <filename>LICENSE_FLAGS = "commercial"</filename>
                in recipe "foo", for example, results in the string
                <filename>"commercial_foo"</filename>.
                And, to create a match, that string must appear in the
                whitelist.
            </para>

            <para>
                Judicious use of the <filename>LICENSE_FLAGS</filename>
                strings and the contents of the
                <filename>LICENSE_FLAGS_WHITELIST</filename> variable
                allows you a lot of flexibility for including or excluding
                recipes based on licensing.
                For example, you can broaden the matching capabilities by
                using license flags string subsets in the whitelist.
                <note>When using a string subset, be sure to use the part of
                    the expanded string that precedes the appended underscore
                    character (e.g. <filename>usethispart_1.3</filename>,
                    <filename>usethispart_1.4</filename>, and so forth).
                </note>
                For example, simply specifying the string "commercial" in
                the whitelist matches any expanded
                <filename>LICENSE_FLAGS</filename> definition that starts with
                the string "commercial" such as "commercial_foo" and
                "commercial_bar", which are the strings the build system
                automatically generates for hypothetical recipes named
                "foo" and "bar" assuming those recipes simply specify the
                following:
                <literallayout class='monospaced'>
     LICENSE_FLAGS = "commercial"
                </literallayout>
                Thus, you can choose to exhaustively
                enumerate each license flag in the whitelist and
                allow only specific recipes into the image, or
                you can use a string subset that causes a broader range of
                matches to allow a range of recipes into the image.
            </para>

            <para>
                This scheme works even if the
                <filename>LICENSE_FLAG</filename> string already
                has <filename>_${PN}</filename> appended.
                For example, the build system turns the license flag
                "commercial_1.2_foo" into "commercial_1.2_foo_foo" and would
                match both the general "commercial" and the specific
                "commercial_1.2_foo" strings found in the whitelist, as
                expected.
            </para>

            <para>
                Here are some other scenarios:
                <itemizedlist>
                    <listitem><para>You can specify a versioned string in the
                        recipe such as "commercial_foo_1.2" in a "foo" recipe.
                        The build system expands this string to
                        "commercial_foo_1.2_foo".
                        Combine this license flag with a whitelist that has
                        the string "commercial" and you match the flag along
                        with any other flag that starts with the string
                        "commercial".</para></listitem>
                    <listitem><para>Under the same circumstances, you can
                        use "commercial_foo" in the whitelist and the
                        build system not only matches "commercial_foo_1.2" but
                        also matches any license flag with the string
                        "commercial_foo", regardless of the version.
                        </para></listitem>
                    <listitem><para>You can be very specific and use both the
                        package and version parts in the whitelist (e.g.
                        "commercial_foo_1.2") to specifically match a
                        versioned recipe.</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id="other-variables-related-to-commercial-licenses">
            <title>Other Variables Related to Commercial Licenses</title>

            <para>
                Other helpful variables related to commercial
                license handling exist and are defined in the
                <filename>$HOME/poky/meta/conf/distro/include/default-distrovars.inc</filename> file:
                <literallayout class='monospaced'>
     COMMERCIAL_AUDIO_PLUGINS ?= ""
     COMMERCIAL_VIDEO_PLUGINS ?= ""
     COMMERCIAL_QT = ""
                </literallayout>
                If you want to enable these components, you can do so by making sure you have
                statements similar to the following
                in your <filename>local.conf</filename> configuration file:
                <literallayout class='monospaced'>
     COMMERCIAL_AUDIO_PLUGINS = "gst-plugins-ugly-mad \
        gst-plugins-ugly-mpegaudioparse"
     COMMERCIAL_VIDEO_PLUGINS = "gst-plugins-ugly-mpeg2dec \
        gst-plugins-ugly-mpegstream gst-plugins-bad-mpegvideoparse"
     COMMERCIAL_QT ?= "qmmp"
     LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly commercial_gst-plugins-bad commercial_qmmp"
                </literallayout>
                Of course, you could also create a matching whitelist
                for those components using the more general "commercial"
                in the whitelist, but that would also enable all the
                other packages with <filename>LICENSE_FLAGS</filename> containing
                "commercial", which you may or may not want:
                <literallayout class='monospaced'>
     LICENSE_FLAGS_WHITELIST = "commercial"
                </literallayout>
            </para>

            <para>
                Specifying audio and video plug-ins as part of the
                <filename>COMMERCIAL_AUDIO_PLUGINS</filename> and
                <filename>COMMERCIAL_VIDEO_PLUGINS</filename> statements
                or commercial Qt components as part of
                the <filename>COMMERCIAL_QT</filename> statement (along
                with the enabling <filename>LICENSE_FLAGS_WHITELIST</filename>) includes the
                plug-ins or components into built images, thus adding
                support for media formats or components.
            </para>
        </section>
    </section>
</section>
</chapter>
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