import-layers/yocto-poky/documentation/ref-manual/closer-look.xml

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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<chapter id='closer-look'>
<title>A Closer Look at the Yocto Project Development Environment</title>

    <para>
        This chapter 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 chapter 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 id='general-yocto-environment-figure'>
        <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
                <ulink url='&YOCTO_DOCS_DEV_URL;#bitbake-term'>BitBake</ulink>.
                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.
                </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.
        </para>

        <para>
            The following figure shows an expanded representation of the
            "User Configuration" box of the
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>:
        </para>

        <para>
            <imagedata fileref="figures/user-configuration.png" align="center" />
        </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>.
            <note>
                The Poky repository is primarily an aggregation of existing
                repositories.
                It is not a canonical upstream source.
            </note>
        </para>

        <para>
            The <filename>meta-poky</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
            (i.e.
            <link linkend='structure-core-script'><filename>&OE_INIT_FILE;</filename></link>
            or
            <link linkend='structure-memres-core-script'><filename>oe-init-build-env-memres</filename></link>).
        </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 build environment setup 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_FILE;</filename> or
            <filename>oe-init-build-env-memres</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-poky</filename> layer:
            <itemizedlist>
                <listitem><para><emphasis>Parallelism Options:</emphasis>
                    Controlled by the
                    <link linkend='var-BB_NUMBER_THREADS'><filename>BB_NUMBER_THREADS</filename></link>,
                    <link linkend='var-PARALLEL_MAKE'><filename>PARALLEL_MAKE</filename></link>,
                    and
                    <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_NUMBER_PARSE_THREADS'><filename>BB_NUMBER_PARSE_THREADS</filename></ulink>
                    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 the <filename>site.conf</filename> file, you need to
            create that yourself.
            The <filename>auto.conf</filename> file is typically created by
            an autobuilder:
            <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 packaging format to use
                    through the
                    <link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>
                    variable.</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>
                    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 <replaceable>target</replaceable></filename>
            command, BitBake sorts out the configurations to ultimately
            define your build environment.
            It is important to understand that the OpenEmbedded build system
            reads the configuration files in a specific order:
            <filename>site.conf</filename>, <filename>auto.conf</filename>,
            and <filename>local.conf</filename>.
            And, the build system applies the normal assignment statement
            rules.
            Because the files are parsed in a specific order, variable
            assignments for the same variable could be affected.
            For example, if the <filename>auto.conf</filename> file and
            the <filename>local.conf</filename> set
            <replaceable>variable1</replaceable> to different values, because
            the build system parses <filename>local.conf</filename> after
            <filename>auto.conf</filename>,
            <replaceable>variable1</replaceable> is assigned the value from
            the <filename>local.conf</filename> file.
        </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 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
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>:
        </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/distro/<replaceable>distro</replaceable>.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>) hold
                        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/<replaceable>distro</replaceable>.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 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/<replaceable>machine</replaceable>.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>recipes-*</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
            <link linkend='general-yocto-environment-figure'>general 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
            come 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.
            You can also instruct the OpenEmbedded build system to create
            tarballs from Git repositories, which is not the default behavior,
            and store them in the <filename>DL_DIR</filename> by using the
            <link linkend='var-BB_GENERATE_MIRROR_TARBALLS'><filename>BB_GENERATE_MIRROR_TARBALLS</filename></link>
            variable.
        </para>

        <para>
            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</filename></link>
                class to include that 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</filename> class, 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
                <link linkend='ref-tasks-fetch'><filename>do_fetch</filename></link>
                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>

            <note>
                For information on how to have the OpenEmbedded build system
                generate tarballs for Git repositories and place them in the
                <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
                directory, see the
                <link linkend='var-BB_GENERATE_MIRROR_TARBALLS'><filename>BB_GENERATE_MIRROR_TARBALLS</filename></link>
                variable.
            </note>

            <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
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>
            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.
            The OpenEmbedded build system provides classes to generate
            different package types, and you specify which classes to enable
            through 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</filename></link>
            class.
        </para>

        <para>
            The package feed area resides in the Build Directory.
            The directory the build system uses to temporarily store packages
            is determined by a combination of variables and the particular
            package manager in use.
            See the "Package Feeds" box in the illustration and note the
            information to the right of that area.
            In particular, the following defines where package files are
            kept:
            <itemizedlist>
                <listitem><para><link linkend='var-DEPLOY_DIR'><filename>DEPLOY_DIR</filename></link>:
                    Defined as <filename>tmp/deploy</filename> in the Build
                    Directory.
                    </para></listitem>
                <listitem><para><filename>DEPLOY_DIR_*</filename>:
                    Depending on the package manager used, the package type
                    sub-folder.
                    Given RPM, IPK, or DEB packaging and tarball creation, the
                    <link linkend='var-DEPLOY_DIR_RPM'><filename>DEPLOY_DIR_RPM</filename></link>,
                    <link linkend='var-DEPLOY_DIR_IPK'><filename>DEPLOY_DIR_IPK</filename></link>,
                    <link linkend='var-DEPLOY_DIR_DEB'><filename>DEPLOY_DIR_DEB</filename></link>,
                    or
                    <link linkend='var-DEPLOY_DIR_TAR'><filename>DEPLOY_DIR_TAR</filename></link>,
                    variables are used, respectively.
                    </para></listitem>
                <listitem><para><link linkend='var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></link>:
                    Defines architecture-specific sub-folders.
                    For example, packages could exist for the i586 or qemux86
                    architectures.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            BitBake uses the <filename>do_package_write_*</filename> tasks to
            generate packages and place them into the package holding area (e.g.
            <filename>do_package_write_ipk</filename> for IPK packages).
            See the
            "<link linkend='ref-tasks-package_write_deb'><filename>do_package_write_deb</filename></link>",
            "<link linkend='ref-tasks-package_write_ipk'><filename>do_package_write_ipk</filename></link>",
            "<link linkend='ref-tasks-package_write_rpm'><filename>do_package_write_rpm</filename></link>",
            and
            "<link linkend='ref-tasks-package_write_tar'><filename>do_package_write_tar</filename></link>"
            sections for additional information.
            As an example, consider a scenario where an IPK packaging manager
            is being used and package architecture support for both i586
            and qemux86 exist.
            Packages for the i586 architecture are placed in
            <filename>build/tmp/deploy/ipk/i586</filename>, while packages for
            the qemux86 architecture are placed in
            <filename>build/tmp/deploy/ipk/qemux86</filename>.
        </para>
    </section>

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

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

        <para>
            Separate documentation exists for the BitBake tool.
            See the
            <ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual'>BitBake User Manual</ulink>
            for reference material on BitBake.
        </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="6.5in" depth="5in" />
            </para>

            <para>
                The
                <link linkend='ref-tasks-fetch'><filename>do_fetch</filename></link>
                and
                <link linkend='ref-tasks-unpack'><filename>do_unpack</filename></link>
                tasks fetch the source files and unpack them into the work
                directory.
                <note>
                    For every local file (e.g. <filename>file://</filename>)
                    that is part of a recipe's
                    <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
                    statement, the OpenEmbedded build system takes a checksum
                    of the file for the recipe and inserts the checksum into
                    the signature for the <filename>do_fetch</filename>.
                    If any local file has been modified, the
                    <filename>do_fetch</filename> task and all tasks that
                    depend on it are re-executed.
                </note>
                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 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 that 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> -
                        The base directory where the OpenEmbedded build system
                        performs all its work during the build.
                        </para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></link> -
                        The architecture of the built package or packages.
                        </para></listitem>
                    <listitem><para><link linkend='var-TARGET_OS'><filename>TARGET_OS</filename></link> -
                        The operating system of the target device.
                        </para></listitem>
                    <listitem><para><link linkend='var-PN'><filename>PN</filename></link> -
                        The name of the built package.
                        </para></listitem>
                    <listitem><para><link linkend='var-PV'><filename>PV</filename></link> -
                        The version of the recipe used to build the package.
                        </para></listitem>
                    <listitem><para><link linkend='var-PR'><filename>PR</filename></link> -
                        The revision of the recipe used to build the package.
                        </para></listitem>
                    <listitem><para><link linkend='var-WORKDIR'><filename>WORKDIR</filename></link> -
                        The location within <filename>TMPDIR</filename> where
                        a specific package is built.
                        </para></listitem>
                    <listitem><para><link linkend='var-S'><filename>S</filename></link> -
                        Contains the unpacked source files for a given recipe.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='patching-dev-environment'>
            <title>Patching</title>

            <para>
                Once source code is fetched and unpacked, BitBake locates
                patch files and applies them to the source files:
                <imagedata fileref="figures/patching.png" align="center" width="6in" depth="5in" />
            </para>

            <para>
                The
                <link linkend='ref-tasks-patch'><filename>do_patch</filename></link>
                task processes recipes by
                using the
                <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
                variable to locate applicable patch files, which by default
                are <filename>*.patch</filename> or
                <filename>*.diff</filename> files, or any file if
                "apply=yes" is specified for the file in
                <filename>SRC_URI</filename>.
            </para>

            <para>
                BitBake finds and applies multiple patches for a single recipe
                in the order in which it finds the patches.
                Patches are applied to the recipe's source files located in the
                <link linkend='var-S'><filename>S</filename></link> directory.
            </para>

            <para>
                For more information on how the source directories are
                created, see the
                "<link linkend='source-fetching-dev-environment'>Source Fetching</link>"
                section.
            </para>
        </section>

        <section id='configuration-and-compilation-dev-environment'>
            <title>Configuration and Compilation</title>

            <para>
                After source code is patched, BitBake executes tasks that
                configure and compile the source code:
                <imagedata fileref="figures/configuration-compile-autoreconf.png" align="center" width="7in" depth="5in" />
            </para>

            <para>
                This step in the build process consists of three tasks:
                <itemizedlist>
                    <listitem><para>
                        <emphasis><link linkend='ref-tasks-prepare_recipe_sysroot'><filename>do_prepare_recipe_sysroot</filename></link>:</emphasis>
                        This task sets up the two sysroots in
                        <filename>${</filename><link linkend='var-WORKDIR'><filename>WORKDIR</filename></link><filename>}</filename>
                        (i.e. <filename>recipe-sysroot</filename> and
                        <filename>recipe-sysroot-native</filename>) so that
                        the sysroots contain the contents of the
                        <link linkend='ref-tasks-populate_sysroot'><filename>do_populate_sysroot</filename></link>
                        tasks of the recipes on which the recipe
                        containing the tasks depends.
                        A sysroot exists for both the target and for the native
                        binaries, which run on the host system.
                        </para></listitem>
                    <listitem><para><emphasis><filename>do_configure</filename>:</emphasis>
                        This task configures the source by enabling and
                        disabling any build-time and configuration options for
                        the software being built.
                        Configurations can come from the recipe itself as well
                        as from an inherited class.
                        Additionally, the software itself might configure itself
                        depending on the target for which it is being built.
                        </para>

                        <para>The configurations handled by the
                        <link linkend='ref-tasks-configure'><filename>do_configure</filename></link>
                        task are specific
                        to source code configuration for the source code
                        being built by the recipe.</para>

                        <para>If you are using the
                        <link linkend='ref-classes-autotools'><filename>autotools</filename></link>
                        class,
                        you can add additional configuration options by using
                        the <link linkend='var-EXTRA_OECONF'><filename>EXTRA_OECONF</filename></link>
                        or
                        <link linkend='var-PACKAGECONFIG_CONFARGS'><filename>PACKAGECONFIG_CONFARGS</filename></link>
                        variables.
                        For information on how this variable works within
                        that class, see the
                        <filename>meta/classes/autotools.bbclass</filename> file.
                        </para></listitem>
                    <listitem><para><emphasis><filename>do_compile</filename>:</emphasis>
                        Once a configuration task has been satisfied, BitBake
                        compiles the source using the
                        <link linkend='ref-tasks-compile'><filename>do_compile</filename></link>
                        task.
                        Compilation occurs in the directory pointed to by the
                        <link linkend='var-B'><filename>B</filename></link>
                        variable.
                        Realize that the <filename>B</filename> directory is, by
                        default, the same as the
                        <link linkend='var-S'><filename>S</filename></link>
                        directory.</para></listitem>
                    <listitem><para><emphasis><filename>do_install</filename>:</emphasis>
                        Once compilation is done, BitBake executes the
                        <link linkend='ref-tasks-install'><filename>do_install</filename></link>
                        task.
                        This task copies files from the <filename>B</filename>
                        directory and places them in a holding area pointed to
                        by the
                        <link linkend='var-D'><filename>D</filename></link>
                        variable.</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='package-splitting-dev-environment'>
            <title>Package Splitting</title>

            <para>
                After source code is configured and compiled, the
                OpenEmbedded build system analyzes
                the results and splits the output into packages:
                <imagedata fileref="figures/analysis-for-package-splitting.png" align="center" width="7in" depth="7in" />
            </para>

            <para>
                The
                <link linkend='ref-tasks-package'><filename>do_package</filename></link>
                and
                <link linkend='ref-tasks-packagedata'><filename>do_packagedata</filename></link>
                tasks combine to analyze
                the files found in the
                <link linkend='var-D'><filename>D</filename></link> directory
                and split them into subsets based on available packages and
                files.
                The analyzing process involves the following as well as other
                items: splitting out debugging symbols,
                looking at shared library dependencies between packages,
                and looking at package relationships.
                The <filename>do_packagedata</filename> task creates package
                metadata based on the analysis such that the
                OpenEmbedded build system can generate the final packages.
                Working, staged, and intermediate results of the analysis
                and package splitting process use these areas:
                <itemizedlist>
                    <listitem><para><link linkend='var-PKGD'><filename>PKGD</filename></link> -
                        The destination directory for packages before they are
                        split.
                        </para></listitem>
                    <listitem><para><link linkend='var-PKGDATA_DIR'><filename>PKGDATA_DIR</filename></link> -
                        A shared, global-state directory that holds data
                        generated during the packaging process.
                        </para></listitem>
                    <listitem><para><link linkend='var-PKGDESTWORK'><filename>PKGDESTWORK</filename></link> -
                        A temporary work area used by the
                        <filename>do_package</filename> task.
                        </para></listitem>
                    <listitem><para><link linkend='var-PKGDEST'><filename>PKGDEST</filename></link> -
                        The parent directory for packages after they have
                        been split.
                        </para></listitem>
                </itemizedlist>
                The <link linkend='var-FILES'><filename>FILES</filename></link>
                variable defines the files that go into each package in
                <link linkend='var-PACKAGES'><filename>PACKAGES</filename></link>.
                If you want details on how this is accomplished, you can
                look at the
                <link linkend='ref-classes-package'><filename>package</filename></link>
                class.
            </para>

            <para>
                Depending on the type of packages being created (RPM, DEB, or
                IPK), the <filename>do_package_write_*</filename> task
                creates the actual packages and places them in the
                Package Feed area, which is
                <filename>${TMPDIR}/deploy</filename>.
                You can see the
                "<link linkend='package-feeds-dev-environment'>Package Feeds</link>"
                section for more detail on that part of the build process.
                <note>
                    Support for creating feeds directly from the
                    <filename>deploy/*</filename> directories does not exist.
                    Creating such feeds usually requires some kind of feed
                    maintenance mechanism that would upload the new packages
                    into an official package feed (e.g. the
                    Ångström distribution).
                    This functionality is highly distribution-specific
                    and thus is not provided out of the box.
                </note>
            </para>
        </section>

        <section id='image-generation-dev-environment'>
            <title>Image Generation</title>

            <para>
                Once packages are split and stored in the Package Feeds area,
                the OpenEmbedded build system uses BitBake to generate the
                root filesystem image:
                <imagedata fileref="figures/image-generation.png" align="center" width="6in" depth="7in" />
            </para>

            <para>
                The image generation process consists of several stages and
                depends on several tasks and variables.
                The
                <link linkend='ref-tasks-rootfs'><filename>do_rootfs</filename></link>
                task creates the root filesystem (file and directory structure)
                for an image.
                This task uses several key variables to help create the list
                of packages to actually install:
                <itemizedlist>
                    <listitem><para><link linkend='var-IMAGE_INSTALL'><filename>IMAGE_INSTALL</filename></link>:
                        Lists out the base set of packages to install from
                        the Package Feeds area.</para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_EXCLUDE'><filename>PACKAGE_EXCLUDE</filename></link>:
                        Specifies packages that should not be installed.
                        </para></listitem>
                    <listitem><para><link linkend='var-IMAGE_FEATURES'><filename>IMAGE_FEATURES</filename></link>:
                        Specifies features to include in the image.
                        Most of these features map to additional packages for
                        installation.</para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>:
                        Specifies the package backend to use and consequently
                        helps determine where to locate packages within the
                        Package Feeds area.</para></listitem>
                    <listitem><para><link linkend='var-IMAGE_LINGUAS'><filename>IMAGE_LINGUAS</filename></link>:
                        Determines the language(s) for which additional
                        language support packages are installed.
                        </para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_INSTALL'><filename>PACKAGE_INSTALL</filename></link>:
                        The final list of packages passed to the package manager
                        for installation into the image.
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                With
                <link linkend='var-IMAGE_ROOTFS'><filename>IMAGE_ROOTFS</filename></link>
                pointing to the location of the filesystem under construction and
                the <filename>PACKAGE_INSTALL</filename> variable providing the
                final list of packages to install, the root file system is
                created.
            </para>

            <para>
                Package installation is under control of the package manager
                (e.g. dnf/rpm, opkg, or apt/dpkg) regardless of whether or
                not package management is enabled for the target.
                At the end of the process, if package management is not
                enabled for the target, the package manager's data files
                are deleted from the root filesystem.
                As part of the final stage of package installation, postinstall
                scripts that are part of the packages are run.
                Any scripts that fail to run
                on the build host are run on the target when the target system
                is first booted.
                If you are using a
                <ulink url='&YOCTO_DOCS_DEV_URL;#creating-a-read-only-root-filesystem'>read-only root filesystem</ulink>,
                all the post installation scripts must succeed during the
                package installation phase since the root filesystem is
                read-only.
            </para>

            <para>
                The final stages of the <filename>do_rootfs</filename> task
                handle post processing.
                Post processing includes creation of a manifest file and
                optimizations.
            </para>

            <para>
                The manifest file (<filename>.manifest</filename>) resides
                in the same directory as the root filesystem image.
                This file lists out, line-by-line, the installed packages.
                The manifest file is useful for the
                <link linkend='ref-classes-testimage*'><filename>testimage</filename></link>
                class, for example, to determine whether or not to run
                specific tests.
                See the
                <link linkend='var-IMAGE_MANIFEST'><filename>IMAGE_MANIFEST</filename></link>
                variable for additional information.
            </para>

            <para>
                Optimizing processes run across the image include
                <filename>mklibs</filename>, <filename>prelink</filename>,
                and any other post-processing commands as defined by the
                <link linkend='var-ROOTFS_POSTPROCESS_COMMAND'><filename>ROOTFS_POSTPROCESS_COMMAND</filename></link>
                variable.
                The <filename>mklibs</filename> process optimizes the size
                of the libraries, while the
                <filename>prelink</filename> process optimizes the dynamic
                linking of shared libraries to reduce start up time of
                executables.
            </para>

            <para>
                After the root filesystem is built, processing begins on
                the image through the <filename>do_image</filename> task.
                The build system runs any pre-processing commands as defined
                by the
                <link linkend='var-IMAGE_PREPROCESS_COMMAND'><filename>IMAGE_PREPROCESS_COMMAND</filename></link>
                variable.
                This variable specifies a list of functions to call before
                the OpenEmbedded build system creates the final image output
                files.
            </para>

            <para>
                The <filename>do_image</filename> task dynamically creates
                other <filename>do_image_*</filename> tasks as needed, which
                include compressing the root filesystem image to reduce the
                overall size of the image.
                The process turns everything into an image file or a set of
                image files.
                The formats used for the root filesystem depend on the
                <link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
                variable.
            </para>

            <para>
                The final task involved in image creation is the
                <filename>do_image_complete</filename> task.
                This task completes the image by applying any image
                post processing as defined through the
                <link linkend='var-IMAGE_POSTPROCESS_COMMAND'><filename>IMAGE_POSTPROCESS_COMMAND</filename></link>
                variable.
                The variable specifies a list of functions to call once the
                OpenEmbedded build system has created the final image output
                files.
            </para>

            <note>
                The entire image generation process is run under Pseudo.
                Running under Pseudo ensures that the files in the root
                filesystem have correct ownership.
            </note>
        </section>

        <section id='sdk-generation-dev-environment'>
            <title>SDK Generation</title>

            <para>
                The OpenEmbedded build system uses BitBake to generate the
                Software Development Kit (SDK) installer script for both the
                standard and extensible SDKs:
                <imagedata fileref="figures/sdk-generation.png" align="center" />
            </para>

            <note>
                For more information on the cross-development toolchain
                generation, see the
                "<link linkend='cross-development-toolchain-generation'>Cross-Development Toolchain Generation</link>"
                section.
                For information on advantages gained when building a
                cross-development toolchain using the
                <link linkend='ref-tasks-populate_sdk'><filename>do_populate_sdk</filename></link>
                task, see the
                "<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-building-an-sdk-installer'>Building an SDK Installer</ulink>"
                section in the Yocto Project Software Development Kit (SDK)
                Developer's Guide.
            </note>

            <para>
                Like image generation, the SDK script process consists of
                several stages and depends on many variables.
                The <filename>do_populate_sdk</filename> and
                <filename>do_populate_sdk_ext</filename> tasks use these
                key variables to help create the list of packages to actually
                install.
                For information on the variables listed in the figure, see the
                "<link linkend='sdk-dev-environment'>Application Development SDK</link>"
                section.
            </para>

            <para>
                The <filename>do_populate_sdk</filename> task helps create
                the standard SDK and handles two parts: a target part and a
                host part.
                The target part is the part built for the target hardware and
                includes libraries and headers.
                The host part is the part of the SDK that runs on the
                <link linkend='var-SDKMACHINE'><filename>SDKMACHINE</filename></link>.
            </para>

            <para>
                The <filename>do_populate_sdk_ext</filename> task helps create
                the extensible SDK and handles host and target parts
                differently than its counter part does for the standard SDK.
                For the extensible SDK, the task encapsulates the build system,
                which includes everything needed (host and target) for the SDK.
            </para>

            <para>
                Regardless of the type of SDK being constructed, the
                tasks perform some cleanup after which a cross-development
                environment setup script and any needed configuration files
                are created.
                The final output is the Cross-development
                toolchain installation script (<filename>.sh</filename> file),
                which includes the environment setup script.
            </para>
        </section>

        <section id='stamp-files-and-the-rerunning-of-tasks'>
            <title>Stamp Files and the Rerunning of Tasks</title>

            <para>
                For each task that completes successfully, BitBake writes a
                stamp file into the
                <link linkend='var-STAMPS_DIR'><filename>STAMPS_DIR</filename></link>
                directory.
                The beginning of the stamp file's filename is determined by the
                <link linkend='var-STAMP'><filename>STAMP</filename></link>
                variable, and the end of the name consists of the task's name
                and current
                <ulink url='&YOCTO_DOCS_BB_URL;#checksums'>input checksum</ulink>.
                <note>
                    This naming scheme assumes that
                    <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_SIGNATURE_HANDLER'><filename>BB_SIGNATURE_HANDLER</filename></ulink>
                    is "OEBasicHash", which is almost always the case in
                    current OpenEmbedded.
                </note>
                To determine if a task needs to be rerun, BitBake checks if a
                stamp file with a matching input checksum exists for the task.
                If such a stamp file exists, the task's output is assumed to
                exist and still be valid.
                If the file does not exist, the task is rerun.
                <note>
                    <para>The stamp mechanism is more general than the shared
                    state (sstate) cache mechanism described in the
                    "<link linkend='setscene-tasks-and-shared-state'>Setscene Tasks and Shared State</link>"
                    section.
                    BitBake avoids rerunning any task that has a valid
                    stamp file, not just tasks that can be accelerated through
                    the sstate cache.</para>
                    <para>However, you should realize that stamp files only
                    serve as a marker that some work has been done and that
                    these files do not record task output.
                    The actual task output would usually be somewhere in
                    <link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>
                    (e.g. in some recipe's
                    <link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>.)
                    What the sstate cache mechanism adds is a way to cache task
                    output that can then be shared between build machines.
                    </para>
                </note>
                Since <filename>STAMPS_DIR</filename> is usually a subdirectory
                of <filename>TMPDIR</filename>, removing
                <filename>TMPDIR</filename> will also remove
                <filename>STAMPS_DIR</filename>, which means tasks will
                properly be rerun to repopulate <filename>TMPDIR</filename>.
            </para>

            <para>
                If you want some task to always be considered "out of date",
                you can mark it with the
                <ulink url='&YOCTO_DOCS_BB_URL;#variable-flags'><filename>nostamp</filename></ulink>
                varflag.
                If some other task depends on such a task, then that task will
                also always be considered out of date, which might not be what
                you want.
            </para>

            <para>
                For details on how to view information about a task's
                signature, see the
                "<link linkend='usingpoky-viewing-task-variable-dependencies'>Viewing Task Variable Dependencies</link>"
                section.
            </para>
        </section>

        <section id='setscene-tasks-and-shared-state'>
            <title>Setscene Tasks and Shared State</title>

            <para>
                The description of tasks so far assumes that BitBake needs to
                build everything and there are no prebuilt objects available.
                BitBake does support skipping tasks if prebuilt objects are
                available.
                These objects are usually made available in the form of a
                shared state (sstate) cache.
                <note>
                    For information on variables affecting sstate, see the
                    <link linkend='var-SSTATE_DIR'><filename>SSTATE_DIR</filename></link>
                    and
                    <link linkend='var-SSTATE_MIRRORS'><filename>SSTATE_MIRRORS</filename></link>
                    variables.
                </note>
            </para>

            <para>
                The idea of a setscene task (i.e
                <filename>do_</filename><replaceable>taskname</replaceable><filename>_setscene</filename>)
                is a version of the task where
                instead of building something, BitBake can skip to the end
                result and simply place a set of files into specific locations
                as needed.
                In some cases, it makes sense to have a setscene task variant
                (e.g. generating package files in the
                <filename>do_package_write_*</filename> task).
                In other cases, it does not make sense, (e.g. a
                <link linkend='ref-tasks-patch'><filename>do_patch</filename></link>
                task or
                <link linkend='ref-tasks-unpack'><filename>do_unpack</filename></link>
                task) since the work involved would be equal to or greater than
                the underlying task.
            </para>

            <para>
                In the OpenEmbedded build system, the common tasks that have
                setscene variants are <link linkend='ref-tasks-package'><filename>do_package</filename></link>,
                <filename>do_package_write_*</filename>,
                <link linkend='ref-tasks-deploy'><filename>do_deploy</filename></link>,
                <link linkend='ref-tasks-packagedata'><filename>do_packagedata</filename></link>,
                and
                <link linkend='ref-tasks-populate_sysroot'><filename>do_populate_sysroot</filename></link>.
                Notice that these are most of the tasks whose output is an
                end result.
            </para>

            <para>
                The OpenEmbedded build system has knowledge of the relationship
                between these tasks and other tasks that precede them.
                For example, if BitBake runs
                <filename>do_populate_sysroot_setscene</filename> for
                something, there is little point in running any of the
                <filename>do_fetch</filename>, <filename>do_unpack</filename>,
                <filename>do_patch</filename>,
                <filename>do_configure</filename>,
                <filename>do_compile</filename>, and
                <filename>do_install</filename> tasks.
                However, if <filename>do_package</filename> needs to be run,
                BitBake would need to run those other tasks.
            </para>

            <para>
                It becomes more complicated if everything can come from an
                sstate cache because some objects are simply not required at
                all.
                For example, you do not need a compiler or native tools, such
                as quilt, if there is nothing to compile or patch.
                If the <filename>do_package_write_*</filename> packages are
                available from sstate, BitBake does not need the
                <filename>do_package</filename> task data.
            </para>

            <para>
                To handle all these complexities, BitBake runs in two phases.
                The first is the "setscene" stage.
                During this stage, BitBake first checks the sstate cache for
                any targets it is planning to build.
                BitBake does a fast check to see if the object exists rather
                than a complete download.
                If nothing exists, the second phase, which is the setscene
                stage, completes and the main build proceeds.
            </para>

            <para>
                If objects are found in the sstate cache, the OpenEmbedded
                build system works backwards from the end targets specified
                by the user.
                For example, if an image is being built, the OpenEmbedded build
                system first looks for the packages needed for that image and
                the tools needed to construct an image.
                If those are available, the compiler is not needed.
                Thus, the compiler is not even downloaded.
                If something was found to be unavailable, or the download or
                setscene task fails, the OpenEmbedded build system then tries
                to install dependencies, such as the compiler, from the cache.
            </para>

            <para>
                The availability of objects in the sstate cache is handled by
                the function specified by the
                <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_HASHCHECK_FUNCTION'><filename>BB_HASHCHECK_FUNCTION</filename></ulink>
                variable and returns a list of the objects that are available.
                The function specified by the
                <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_SETSCENE_DEPVALID'><filename>BB_SETSCENE_DEPVALID</filename></ulink>
                variable is the function that determines whether a given
                dependency needs to be followed, and whether for any given
                relationship the function needs to be passed.
                The function returns a True or False value.
            </para>
        </section>
    </section>

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

        <para>
            The images produced by the OpenEmbedded build system
            are compressed forms of the
            root filesystem that are ready to boot on a target device.
            You can see from the
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>
            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="5.5in" depth="5.5in" />
        </para>

        <para>
            For a list of example images that the Yocto Project provides,
            see 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>tmp/deploy/images/<replaceable>machine</replaceable>/</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,
            while the
            <link linkend='var-DEPLOY_DIR_IMAGE'><filename>DEPLOY_DIR_IMAGE</filename></link>
            variable points to the appropriate directory containing images for
            the current configuration.
            <itemizedlist>
                <listitem><para><filename><replaceable>kernel-image</replaceable></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/<replaceable>machine</replaceable></filename>
                    directory can contain multiple image files for the
                    machine.</para></listitem>
                <listitem><para><filename><replaceable>root-filesystem-image</replaceable></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/<replaceable>machine</replaceable></filename>
                    directory can contain multiple root filesystems for the
                    machine.</para></listitem>
                <listitem><para><filename><replaceable>kernel-modules</replaceable></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/<replaceable>machine</replaceable></filename>
                    directory can contain multiple kernel module tarballs
                    for the machine.</para></listitem>
                <listitem><para><filename><replaceable>bootloaders</replaceable></filename>:
                    Bootloaders supporting the image, if applicable to the
                    target machine.
                    The <filename>deploy/images/<replaceable>machine</replaceable></filename>
                    directory can contain multiple bootloaders for the
                    machine.</para></listitem>
                <listitem><para><filename><replaceable>symlinks</replaceable></filename>:
                    The <filename>deploy/images/<replaceable>machine</replaceable></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
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>,
            the output labeled "Application Development SDK" represents an
            SDK.
            The SDK generation process differs depending on whether you build
            a standard SDK
            (e.g. <filename>bitbake -c populate_sdk</filename> <replaceable>imagename</replaceable>)
            or an extensible SDK
            (e.g. <filename>bitbake -c populate_sdk_ext</filename> <replaceable>imagename</replaceable>).
            This section is going to take a closer look at this output:
            <imagedata fileref="figures/sdk.png" align="center" width="9in" depth="7.25in" />
        </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 environment setup script is added so that you can initialize
            the environment before using the tools.
        </para>

        <note><title>Notes</title>
            <itemizedlist>
                <listitem><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
                    or building and installing your own SDK installer.
                    </para></listitem>
                <listitem><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.
                    </para></listitem>
                <listitem><para>
                    For information on setting up a cross-development
                    environment, see the
                    <ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Software Development Kit (SDK) Developer's Guide</ulink>.
                    </para></listitem>
            </itemizedlist>
        </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.
            Depending on the type of SDK, several variables exist that help
            configure these files.
            The following list shows the variables associated with a standard
            SDK:
            <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 <replaceable>imagename</replaceable></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>
                <listitem><para><link linkend='var-SDKPATH'><filename>SDKPATH</filename></link>:
                    Defines the default SDK installation path offered by the
                    installation script.
                    </para></listitem>
            </itemizedlist>
            This next list, shows the variables associated with an extensible
            SDK:
            <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-SDK_EXT_TYPE'><filename>SDK_EXT_TYPE</filename></link>:
                    Controls whether or not shared state artifacts are copied
                    into the extensible SDK.
                    By default, all required shared state artifacts are copied
                    into the SDK.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_INCLUDE_PKGDATA'><filename>SDK_INCLUDE_PKGDATA</filename></link>:
                    Specifies whether or not packagedata will be included in
                    the extensible SDK for all recipes in the "world" target.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_INCLUDE_TOOLCHAIN'><filename>SDK_INCLUDE_TOOLCHAIN</filename></link>:
                    Specifies whether or not the toolchain will be included
                    when building the extensible SDK.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_LOCAL_CONF_WHITELIST'><filename>SDK_LOCAL_CONF_WHITELIST</filename></link>:
                    A list of variables allowed through from the build system
                    configuration into the extensible SDK configuration.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_LOCAL_CONF_BLACKLIST'><filename>SDK_LOCAL_CONF_BLACKLIST</filename></link>:
                    A list of variables not allowed through from the build
                    system configuration into the extensible SDK configuration.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_INHERIT_BLACKLIST'><filename>SDK_INHERIT_BLACKLIST</filename></link>:
                    A list of classes to remove from the
                    <link linkend='var-INHERIT'><filename>INHERIT</filename></link>
                    value globally within the extensible SDK configuration.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

</chapter>
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