openembedded-core-contrib - OpenEmbedded Core user contribution trees

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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" width="5.5in" 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>.
            <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
            (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-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 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/&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>) 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/&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 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>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.
            BitBake generates packages whose types are 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</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> tasks to
            place generated packages 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.
        </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><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>
                        variable.
                        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 many variables.
                The
                <link linkend='ref-tasks-rootfs'><filename>do_rootfs</filename></link>
                task uses these 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>
                </itemizedlist>
            </para>

            <para>
                Package installation is under control of the package manager
                (e.g. smart/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.
            </para>

            <para>
                During image generation, the build system attempts to run
                all post-installation scripts.
                Any 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>
                During Optimization, optimizing processes are run across
                the image.
                These processes include <filename>mklibs</filename> and
                <filename>prelink</filename>.
                The <filename>mklibs</filename> process optimizes the size
                of the libraries.
                A <filename>prelink</filename> process optimizes the dynamic
                linking of shared libraries to reduce start up time of
                executables.
            </para>

            <para>
                Along with writing out the root filesystem image, the
                <filename>do_rootfs</filename> task creates a manifest file
                (<filename>.manifest</filename>) in the same directory as
                the root filesystem image that lists out, line-by-line, the
                installed packages.
                This 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>
                Part of the image generation process includes compressing the
                root filesystem image.
                Compression is accomplished through several optimization
                routines designed to reduce the overall size of the image.
            </para>

            <para>
                After the root filesystem has been constructed, the image
                generation 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>

            <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:
                <imagedata fileref="figures/sdk-generation.png" align="center" width="6in" depth="7in" />
            </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_ADT_URL;#optionally-building-a-toolchain-installer'>Optionally Building a Toolchain Installer</ulink>"
                section in the Yocto Project Application 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> task uses 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 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>
                Once both parts are constructed, the
                <filename>do_populate_sdk</filename> task performs some cleanup
                on both parts.
                After the cleanup, the task creates a cross-development
                environment setup script and any configuration files that
                might be needed.
            </para>

            <para>
                The final output of the task is the Cross-development
                toolchain installation script (<filename>.sh</filename> file),
                which includes the environment setup script.
            </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/&lt;machine&gt;/</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>&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/&lt;machine&gt;</filename>
                    directory can contain multiple image files for the
                    machine.</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/&lt;machine&gt;</filename>
                    directory can contain multiple root filesystems for the
                    machine.</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/&lt;machine&gt;</filename>
                    directory can contain multiple kernel module tarballs
                    for the machine.</para></listitem>
                <listitem><para><filename>&lt;bootloaders&gt;</filename>:
                    Bootloaders supporting the image, if applicable to the
                    target machine.
                    The <filename>deploy/images/&lt;machine&gt;</filename>
                    directory can contain multiple bootloaders for the
                    machine.</para></listitem>
                <listitem><para><filename>&lt;symlinks&gt;</filename>:
                    The <filename>deploy/images/&lt;machine&gt;</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.
            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>
                <listitem><para><link linkend='var-SDKPATH'><filename>SDKPATH</filename></link>:
                    Defines the default SDK installation path offered by the
                    installation script.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

</chapter>
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