poky/documentation/ref-manual/technical-details.xml

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

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

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

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

    <para>
        The
        <link linkend='bitbake-term'>BitBake</link>
        task executor together with various types of configuration files form
        the OpenEmbedded Core.
        This section overviews these components by describing their use and
        how they interact.
    </para>

    <para>
        BitBake handles the parsing and execution of the data files.
        The data itself is of various types:
        <itemizedlist>
            <listitem><para><emphasis>Recipes:</emphasis> Provides details
                about particular pieces of software.
                </para></listitem>
            <listitem><para><emphasis>Class Data:</emphasis> Abstracts
                common build information (e.g. how to build a Linux kernel).
                </para></listitem>
            <listitem><para><emphasis>Configuration Data:</emphasis> Defines
                machine-specific settings, policy decisions, and so forth.
                Configuration data acts as the glue to bind everything
                together.
                </para></listitem>
        </itemizedlist>
    </para>

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

    <para>
        Following are some brief details on these core components.
        For additional information on how these components interact during
        a build, see the
        "<ulink url='&YOCTO_DOCS_OVERVIEW_URL;#development-concepts'>Development Concepts</ulink>"
        section in the Yocto Project Overview Manual.
    </para>

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

        <para>
            BitBake is the tool at the heart of the OpenEmbedded build system
            and is responsible for parsing the
            <link linkend='metadata'>Metadata</link>,
            generating a list of tasks from it, and then executing those tasks.
        </para>

        <para>
            This section briefly introduces BitBake.
            If you want more information on BitBake, see the
            <ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual'>BitBake User Manual</ulink>.
        </para>

        <para>
            To see a list of the options BitBake supports, use either of
            the following commands:
            <literallayout class='monospaced'>
     $ bitbake -h
     $ bitbake --help
            </literallayout>
        </para>

        <para>
            The most common usage for BitBake is <filename>bitbake <replaceable>packagename</replaceable></filename>, where
            <filename>packagename</filename> is the name of the package you want to build
            (referred to as the "target" in this manual).
            The target often equates to the first part of a recipe's filename
            (e.g. "foo" for a recipe named
            <filename>foo_1.3.0-r0.bb</filename>).
            So, to process the <filename>matchbox-desktop_1.2.3.bb</filename> recipe file, you
            might type the following:
            <literallayout class='monospaced'>
     $ bitbake matchbox-desktop
            </literallayout>
            Several different versions of <filename>matchbox-desktop</filename> might exist.
            BitBake chooses the one selected by the distribution configuration.
            You can get more details about how BitBake chooses between different
            target versions and providers in the
            "<ulink url='&YOCTO_DOCS_BB_URL;#bb-bitbake-preferences'>Preferences</ulink>"
            section of the BitBake User Manual.
        </para>

        <para>
            BitBake also tries to execute any dependent tasks first.
            So for example, before building <filename>matchbox-desktop</filename>, BitBake
            would build a cross compiler and <filename>glibc</filename> if they had not already
            been built.
        </para>

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

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

        <para>
            Files that have the <filename>.bb</filename> suffix are "recipes"
            files.
            In general, a recipe contains information about a single piece of
            software.
            This information includes the location from which to download the
            unaltered source, any source patches to be applied to that source
            (if needed), which special configuration options to apply,
            how to compile the source files, and how to package the compiled
            output.
        </para>

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

    <section id='metadata-virtual-providers'>
        <title>Metadata (Virtual Providers)</title>

        <para>
            Prior to the build, if you know that several different recipes
            provide the same functionality, you can use a virtual provider
            (i.e. <filename>virtual/*</filename>) as a placeholder for the
            actual provider.
            The actual provider would be determined at build
            time.
            In this case, you should add <filename>virtual/*</filename>
            to <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>,
            rather than listing the specified provider.
            You would select the actual provider by setting the
            <link linkend='var-PREFERRED_PROVIDER'><filename>PREFERRED_PROVIDER</filename></link>
            variable (i.e. <filename>PREFERRED_PROVIDER_virtual/*</filename>)
            in the build's configuration file (e.g.
            <filename>poky/build/conf/local.conf</filename>).
            <note>
                Any recipe that PROVIDES a <filename>virtual/*</filename> item
                that is ultimately not selected through
                <filename>PREFERRED_PROVIDER</filename> does not get built.
                Preventing these recipes from building is usually the desired
                behavior since this mechanism's purpose is to select between
                mutually exclusive alternative providers.
            </note>
        </para>

        <para>
            The following lists specific examples of virtual providers:
            <itemizedlist>
                <listitem><para>
                    <filename>virtual/mesa</filename>:
                    Provides <filename>gbm.pc</filename>.
                    </para></listitem>
                <listitem><para>
                    <filename>virtual/egl</filename>:
                    Provides <filename>egl.pc</filename> and possibly
                    <filename>wayland-egl.pc</filename>.
                    </para></listitem>
                <listitem><para>
                    <filename>virtual/libgl</filename>:
                    Provides <filename>gl.pc</filename> (i.e. libGL).
                    </para></listitem>
                <listitem><para>
                    <filename>virtual/libgles1</filename>:
                    Provides <filename>glesv1_cm.pc</filename>
                    (i.e. libGLESv1_CM).
                    </para></listitem>
                <listitem><para>
                    <filename>virtual/libgles2</filename>:
                    Provides <filename>glesv2.pc</filename> (i.e. libGLESv2).
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

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

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

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

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

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

    <para>
        The Yocto Project does most of the work for you when it comes to
        creating
        <link linkend='cross-development-toolchain'>cross-development toolchains</link>.
        This section provides some technical background on how
        cross-development toolchains are created and used.
        For more information on toolchains, you can also see the
        <ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Application Development and the Extensible Software Development Kit (eSDK)</ulink>
        manual.
    </para>

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

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

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

    <para>
        Most of the work occurs on the Build Host.
        This is the machine used to build images and generally work within the
        the Yocto Project environment.
        When you run BitBake to create an image, the OpenEmbedded build system
        uses the host <filename>gcc</filename> compiler to bootstrap a
        cross-compiler named <filename>gcc-cross</filename>.
        The <filename>gcc-cross</filename> compiler is what BitBake uses to
        compile source files when creating the target image.
        You can think of <filename>gcc-cross</filename> simply as an
        automatically generated cross-compiler that is used internally within
        BitBake only.
        <note>
            The extensible SDK does not use
            <filename>gcc-cross-canadian</filename> since this SDK
            ships a copy of the OpenEmbedded build system and the sysroot
            within it contains <filename>gcc-cross</filename>.
        </note>
    </para>

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

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

    <para>
        Here is the bootstrap process for the relocatable toolchain:
        <literallayout class='monospaced'>
     gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers ->
        glibc-initial -> nativesdk-glibc -> gcc-crosssdk -> gcc-cross-canadian
        </literallayout>
        <itemizedlist>
            <listitem><para><filename>gcc</filename>:
                The build host's GNU Compiler Collection (GCC).
                </para></listitem>
            <listitem><para><filename>binutils-crosssdk</filename>:
                The bare minimum binary utilities needed in order to run
                the <filename>gcc-crosssdk-initial</filename> phase of the
                bootstrap operation.
                </para></listitem>
            <listitem><para><filename>gcc-crosssdk-initial</filename>:
                An early stage of the bootstrap process for creating
                the cross-compiler.
                This stage builds enough of the
                <filename>gcc-crosssdk</filename> and supporting pieces so that
                the final stage of the bootstrap process can produce the
                finished cross-compiler.
                This tool is a "native" binary that runs on the build host.
                </para></listitem>
            <listitem><para><filename>linux-libc-headers</filename>:
                Headers needed for the cross-compiler.
                </para></listitem>
            <listitem><para><filename>glibc-initial</filename>:
                An initial version of the Embedded GLIBC needed to bootstrap
                <filename>nativesdk-glibc</filename>.
                </para></listitem>
            <listitem><para><filename>nativesdk-glibc</filename>:
                The Embedded GLIBC needed to bootstrap the
                <filename>gcc-crosssdk</filename>.
                </para></listitem>
            <listitem><para><filename>gcc-crosssdk</filename>:
                The final stage of the bootstrap process for the
                relocatable cross-compiler.
                The <filename>gcc-crosssdk</filename> is a transitory compiler
                and never leaves the build host.
                Its purpose is to help in the bootstrap process to create the
                eventual relocatable <filename>gcc-cross-canadian</filename>
                compiler, which is relocatable.
                This tool is also a "native" package (i.e. it is
                designed to run on the build host).
                </para></listitem>
            <listitem><para><filename>gcc-cross-canadian</filename>:
                The final relocatable cross-compiler.
                When run on the
                <link linkend='var-SDKMACHINE'><filename>SDKMACHINE</filename></link>,
                this tool
                produces executable code that runs on the target device.
                Only one cross-canadian compiler is produced per architecture
                since they can be targeted at different processor optimizations
                using configurations passed to the compiler through the
                compile commands.
                This circumvents the need for multiple compilers and thus
                reduces the size of the toolchains.
                </para></listitem>
        </itemizedlist>
    </para>

    <note>
        For information on advantages gained when building a
        cross-development toolchain installer, see the
        "<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-building-an-sdk-installer'>Building an SDK Installer</ulink>"
        section in the Yocto Project Application Development and the
        Extensible Software Development Kit (eSDK) manual.
    </note>
</section>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

     python do_deploy_setscene () {
         sstate_setscene(d)
     }
     addtask do_deploy_setscene
     do_deploy[dirs] = "${DEPLOYDIR} ${B}"
            </literallayout>
            The following list explains the previous example:
            <itemizedlist>
                <listitem><para>
                    Adding "do_deploy" to <filename>SSTATETASKS</filename>
                    adds some required sstate-related processing, which is
                    implemented in the
                    <link linkend='ref-classes-sstate'><filename>sstate</filename></link>
                    class, to before and after the
                    <link linkend='ref-tasks-deploy'><filename>do_deploy</filename></link>
                    task.
                    </para></listitem>
                <listitem><para>
                    The
                    <filename>do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"</filename>
                    declares that <filename>do_deploy</filename> places its
                    output in <filename>${DEPLOYDIR}</filename> when run
                    normally (i.e. when not using the sstate cache).
                    This output becomes the input to the shared state cache.
                    </para></listitem>
                <listitem><para>
                    The
                    <filename>do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"</filename>
                    line causes the contents of the shared state cache to be
                    copied to <filename>${DEPLOY_DIR_IMAGE}</filename>.
                    <note>
                        If <filename>do_deploy</filename> is not already in
                        the shared state cache or if its input checksum
                        (signature) has changed from when the output was
                        cached, the task will be run to populate the shared
                        state cache, after which the contents of the shared
                        state cache is copied to
                        <filename>${DEPLOY_DIR_IMAGE}</filename>.
                        If <filename>do_deploy</filename> is in the shared
                        state cache and its signature indicates that the
                        cached output is still valid (i.e. if no
                        relevant task inputs have changed), then the contents
                        of the shared state cache will be copied directly to
                        <filename>${DEPLOY_DIR_IMAGE}</filename> by the
                        <filename>do_deploy_setscene</filename> task instead,
                        skipping the <filename>do_deploy</filename> task.
                    </note>
                    </para></listitem>
                <listitem><para>
                    The following task definition is glue logic needed to make
                    the previous settings effective:
                    <literallayout class='monospaced'>
     python do_deploy_setscene () {
         sstate_setscene(d)
     }
     addtask do_deploy_setscene
                    </literallayout>
                    <filename>sstate_setscene()</filename> takes the flags
                    above as input and accelerates the
                    <filename>do_deploy</filename> task through the
                    shared state cache if possible.
                    If the task was accelerated,
                    <filename>sstate_setscene()</filename> returns True.
                    Otherwise, it returns False, and the normal
                    <filename>do_deploy</filename> task runs.
                    For more information, see the
                    "<ulink url='&YOCTO_DOCS_BB_URL;#setscene'>setscene</ulink>"
                    section in the BitBake User Manual.
                    </para></listitem>
                <listitem><para>
                    The <filename>do_deploy[dirs] = "${DEPLOYDIR} ${B}"</filename>
                    line creates <filename>${DEPLOYDIR}</filename> and
                    <filename>${B}</filename> before the
                    <filename>do_deploy</filename> task runs, and also sets
                    the current working directory of
                    <filename>do_deploy</filename> to
                    <filename>${B}</filename>.
                    For more information, see the
                    "<ulink url='&YOCTO_DOCS_BB_URL;#variable-flags'>Variable Flags</ulink>"
                    section in the BitBake User Manual.
                    <note>
                        In cases where
                        <filename>sstate-inputdirs</filename> and
                        <filename>sstate-outputdirs</filename> would be the
                        same, you can use
                        <filename>sstate-plaindirs</filename>.
                        For example, to preserve the
                        <filename>${PKGD}</filename> and
                        <filename>${PKGDEST}</filename> output from the
                        <link linkend='ref-tasks-package'><filename>do_package</filename></link>
                        task, use the following:
                        <literallayout class='monospaced'>
     do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}"
                        </literallayout>
                    </note>
                    </para></listitem>
                <listitem><para>
                     <filename>sstate-inputdirs</filename> and
                     <filename>sstate-outputdirs</filename> can also be used
                     with multiple directories.
                     For example, the following declares
                     <filename>PKGDESTWORK</filename> and
                     <filename>SHLIBWORK</filename> as shared state
                     input directories, which populates the shared state
                     cache, and <filename>PKGDATA_DIR</filename> and
                     <filename>SHLIBSDIR</filename> as the corresponding
                     shared state output directories:
                     <literallayout class='monospaced'>
     do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}"
     do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}"
                     </literallayout>
                     </para></listitem>
                 <listitem><para>
                     These methods also include the ability to take a lockfile
                     when manipulating shared state directory structures,
                     for cases where file additions or removals are sensitive:
                     <literallayout class='monospaced'>
     do_package[sstate-lockfile] = "${PACKAGELOCK}"
                     </literallayout>
                     </para></listitem>
            </itemizedlist>
        </para>

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

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

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

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

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

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

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

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

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

            <para>
                Seeing what metadata went into creating the input signature
                of a shared state (sstate) task can be a useful debugging aid.
                This information is available in signature information
                (<filename>siginfo</filename>) files in
                <link linkend='var-SSTATE_DIR'><filename>SSTATE_DIR</filename></link>.
                For information on how to view and interpret information in
                <filename>siginfo</filename> files, see the
                "<link linkend='usingpoky-viewing-task-variable-dependencies'>Viewing Task Variable Dependencies</link>"
                section.
            </para>
        </section>

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

            <para>
                The OpenEmbedded build system uses checksums and shared state
                cache to avoid unnecessarily rebuilding tasks.
                Collectively, this scheme is known as "shared state code."
            </para>

            <para>
                As with all schemes, this one has some drawbacks.
                It is possible that you could make implicit changes to your
                code that the checksum calculations do not take into
                account.
                These implicit changes affect a task's output but do not trigger
                the shared state code into rebuilding a recipe.
                Consider an example during which a tool changes its output.
                Assume that the output of <filename>rpmdeps</filename> changes.
                The result of the change should be that all the
                <filename>package</filename> and
                <filename>package_write_rpm</filename> shared state cache
                items become invalid.
                However, because the change to the output is
                external to the code and therefore implicit,
                the associated shared state cache items do not become
                invalidated.
                In this case, the build process uses the cached items rather
                than running the task again.
                Obviously, these types of implicit changes can cause problems.
            </para>

            <para>
                To avoid these problems during the build, you need to
                understand the effects of any changes you make.
                Realize that changes you make directly to a function
                are automatically factored into the checksum calculation.
                Thus, these explicit changes invalidate the associated area of
                shared state cache.
                However, you need to be aware of any implicit changes that
                are not obvious changes to the code and could affect the output
                of a given task.
            </para>

            <para>
                When you identify an implicit change, you can easily take steps
                to invalidate the cache and force the tasks to run.
                The steps you can take are as simple as changing a function's
                comments in the source code.
                For example, to invalidate package shared state files, change
                the comment statements of
                <link linkend='ref-tasks-package'><filename>do_package</filename></link>
                or the comments of one of the functions it calls.
                Even though the change is purely cosmetic, it causes the
                checksum to be recalculated and forces the OpenEmbedded build
                system to run the task again.
            </para>

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

<section id='automatically-added-runtime-dependencies'>
    <title>Automatically Added Runtime Dependencies</title>

    <para>
        The OpenEmbedded build system automatically adds common types of
        runtime dependencies between packages, which means that you do not
        need to explicitly declare the packages using
        <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>.
        Three automatic mechanisms exist (<filename>shlibdeps</filename>,
        <filename>pcdeps</filename>, and <filename>depchains</filename>) that
        handle shared libraries, package configuration (pkg-config) modules,
        and <filename>-dev</filename> and <filename>-dbg</filename> packages,
        respectively.
        For other types of runtime dependencies, you must manually declare
        the dependencies.
        <itemizedlist>
            <listitem><para>
                <filename>shlibdeps</filename>:
                During the
                <link linkend='ref-tasks-package'><filename>do_package</filename></link>
                task of each recipe, all shared libraries installed by the
                recipe are located.
                For each shared library, the package that contains the shared
                library is registered as providing the shared library.
                More specifically, the package is registered as providing the
                <ulink url='https://en.wikipedia.org/wiki/Soname'>soname</ulink>
                of the library.
                The resulting shared-library-to-package mapping
                is saved globally in
                <link linkend='var-PKGDATA_DIR'><filename>PKGDATA_DIR</filename></link>
                by the
                <link linkend='ref-tasks-packagedata'><filename>do_packagedata</filename></link>
                task.</para>

                <para>Simultaneously, all executables and shared libraries
                installed by the recipe are inspected to see what shared
                libraries they link against.
                For each shared library dependency that is found,
                <filename>PKGDATA_DIR</filename> is queried to
                see if some package (likely from a different recipe) contains
                the shared library.
                If such a package is found, a runtime dependency is added from
                the package that depends on the shared library to the package
                that contains the library.</para>

                <para>The automatically added runtime dependency also includes
                a version restriction.
                This version restriction specifies that at least the current
                version of the package that provides the shared library must be
                used, as if
                "<replaceable>package</replaceable> (>= <replaceable>version</replaceable>)"
                had been added to
                <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>.
                This forces an upgrade of the package containing the shared
                library when installing the package that depends on the
                library, if needed.</para>

                <para>If you want to avoid a package being registered as
                providing a particular shared library (e.g. because the library
                is for internal use only), then add the library to
                <link linkend='var-PRIVATE_LIBS'><filename>PRIVATE_LIBS</filename></link>
                inside the package's recipe.
                </para></listitem>
            <listitem><para>
                <filename>pcdeps</filename>:
                During the
                <link linkend='ref-tasks-package'><filename>do_package</filename></link>
                task of each recipe, all pkg-config modules
                (<filename>*.pc</filename> files) installed by the recipe are
                located.
                For each module, the package that contains the module is
                registered as providing the module.
                The resulting module-to-package mapping is saved globally in
                <link linkend='var-PKGDATA_DIR'><filename>PKGDATA_DIR</filename></link>
                by the
                <link linkend='ref-tasks-packagedata'><filename>do_packagedata</filename></link>
                task.</para>

                <para>Simultaneously, all pkg-config modules installed by the
                recipe are inspected to see what other pkg-config modules they
                depend on.
                A module is seen as depending on another module if it contains
                a "Requires:" line that specifies the other module.
                For each module dependency,
                <filename>PKGDATA_DIR</filename> is queried to see if some
                package contains the module.
                If such a package is found, a runtime dependency is added from
                the package that depends on the module to the package that
                contains the module.
                <note>
                    The <filename>pcdeps</filename> mechanism most often infers
                    dependencies between <filename>-dev</filename> packages.
                </note>
                </para></listitem>
            <listitem><para>
                <filename>depchains</filename>:
                If a package <filename>foo</filename> depends on a package
                <filename>bar</filename>, then <filename>foo-dev</filename>
                and <filename>foo-dbg</filename> are also made to depend on
                <filename>bar-dev</filename> and <filename>bar-dbg</filename>,
                respectively.
                Taking the <filename>-dev</filename> packages as an example,
                the <filename>bar-dev</filename> package might provide
                headers and shared library symlinks needed by
                <filename>foo-dev</filename>, which shows the need
                for a dependency between the packages.</para>

                <para>The dependencies added by <filename>depchains</filename>
                are in the form of
                <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>.
                <note>
                    By default, <filename>foo-dev</filename> also has an
                    <filename>RDEPENDS</filename>-style dependency on
                    <filename>foo</filename>, because the default value of
                    <filename>RDEPENDS_${PN}-dev</filename> (set in
                    <filename>bitbake.conf</filename>) includes
                    "${PN}".
                </note></para>

                <para>To ensure that the dependency chain is never broken,
                <filename>-dev</filename> and <filename>-dbg</filename>
                packages are always generated by default, even if the packages
                turn out to be empty.
                See the
                <link linkend='var-ALLOW_EMPTY'><filename>ALLOW_EMPTY</filename></link>
                variable for more information.
                </para></listitem>
        </itemizedlist>
    </para>

    <para>
        The <filename>do_package</filename> task depends on the
        <link linkend='ref-tasks-packagedata'><filename>do_packagedata</filename></link>
        task of each recipe in
        <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
        through use of a
        <filename>[</filename><ulink url='&YOCTO_DOCS_BB_URL;#variable-flags'><filename>deptask</filename></ulink><filename>]</filename>
        declaration, which guarantees that the required
        shared-library/module-to-package mapping information will be available
        when needed as long as <filename>DEPENDS</filename> has been
        correctly set.
    </para>
</section>

<section id='fakeroot-and-pseudo'>
    <title>Fakeroot and Pseudo</title>

    <para>
        Some tasks are easier to implement when allowed to perform certain
        operations that are normally reserved for the root user.
        For example, the
        <link linkend='ref-tasks-install'><filename>do_install</filename></link>
        task benefits from being able to set the UID and GID of installed files
        to arbitrary values.
    </para>

    <para>
        One approach to allowing tasks to perform root-only operations
        would be to require BitBake to run as root.
        However, this method is cumbersome and has security issues.
        The approach that is actually used is to run tasks that benefit from
        root privileges in a "fake" root environment.
        Within this environment, the task and its child processes believe that
        they are running as the root user, and see an internally consistent
        view of the filesystem.
        As long as generating the final output (e.g. a package or an image)
        does not require root privileges, the fact that some earlier steps ran
        in a fake root environment does not cause problems.
    </para>

    <para>
        The capability to run tasks in a fake root environment is known as
        "fakeroot", which is derived from the BitBake keyword/variable
        flag that requests a fake root environment for a task.
        In current versions of the OpenEmbedded build system,
        the program that implements fakeroot is known as Pseudo.
    </para>

    <para>
        Pseudo overrides system calls through the
        <filename>LD_PRELOAD</filename> mechanism to give the
        illusion of running as root.
        To keep track of "fake" file ownership and permissions resulting from
        operations that require root permissions, an sqlite3
        database is used.
        This database is stored in
        <filename>${</filename><link linkend='var-WORKDIR'><filename>WORKDIR</filename></link><filename>}/pseudo/files.db</filename>
        for individual recipes.
        Storing the database in a file as opposed to in memory
        gives persistence between tasks, and even between builds.
        <note><title>Caution</title>
            If you add your own task that manipulates the same files or
            directories as a fakeroot task, then that task should also run
            under fakeroot.
            Otherwise, the task will not be able to run root-only operations,
            and will not see the fake file ownership and permissions set by the
            other task.
            You should also add a dependency on
            <filename>virtual/fakeroot-native:do_populate_sysroot</filename>,
            giving the following:
            <literallayout class='monospaced'>
       fakeroot do_mytask () {
           ...
       }
       do_mytask[depends] += "virtual/fakeroot-native:do_populate_sysroot"
            </literallayout>
        </note>
        For more information, see the
        <ulink url='&YOCTO_DOCS_BB_URL;#var-FAKEROOT'><filename>FAKEROOT*</filename></ulink>
        variables in the BitBake User Manual.
        You can also reference this
        <ulink url='http://www.ibm.com/developerworks/opensource/library/os-aapseudo1/index.html'>Pseudo</ulink>
        article.
    </para>
</section>

<section id='wic-plug-ins-interface'>
    <title>Wic Plug-Ins Interface</title>

    <para>
        You can extend and specialize Wic functionality by using
        Wic plug-ins.
        This section explains the Wic plug-in interface.
        For information on using Wic in general, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#creating-partitioned-images-using-wic'>Creating Partitioned Images Using Wic</ulink>"
        section in the Yocto Project Development Tasks Manual.
        <note>
            Wic plug-ins consist of "source" and "imager" plug-ins.
            Imager plug-ins are beyond the scope of this section.
        </note>
    </para>

    <para>
        Source plug-ins provide a mechanism to customize partition
        content during the Wic image generation process.
        You can use source plug-ins to map values that you specify
        using <filename>--source</filename> commands in kickstart
        files (i.e. <filename>*.wks</filename>) to a plug-in
        implementation used to populate a given partition.
        <note>
            If you use plug-ins that have build-time dependencies
            (e.g. native tools, bootloaders, and so forth)
            when building a Wic image, you need to specify those
            dependencies using the
            <link linkend='var-WKS_FILE_DEPENDS'><filename>WKS_FILE_DEPENDS</filename></link>
            variable.
        </note>
    </para>

    <para>
        Source plug-ins are subclasses defined in plug-in files.
        As shipped, the Yocto Project provides several plug-in
        files.
        You can see the source plug-in files that ship with the
        Yocto Project
        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/scripts/lib/wic/plugins/source'>here</ulink>.
        Each of these plug-in files contain source plug-ins that
        are designed to populate a specific Wic image partition.
    </para>

    <para>
        Source plug-ins are subclasses of the
        <filename>SourcePlugin</filename> class, which is
        defined in the
        <filename>poky/scripts/lib/wic/pluginbase.py</filename>
        file.
        For example, the <filename>BootimgEFIPlugin</filename>
        source plug-in found in the
        <filename>bootimg-efi.py</filename> file is a subclass of
        the <filename>SourcePlugin</filename> class, which is found
        in the <filename>pluginbase.py</filename> file.
    </para>

    <para>
        You can also implement source plug-ins in a layer outside
        of the Source Repositories (external layer).
        To do so, be sure that your plug-in files are located in
        a directory whose path is
        <filename>scripts/lib/wic/plugins/source/</filename>
        within your external layer.
        When the plug-in files are located there, the source
        plug-ins they contain are made available to Wic.
    </para>

    <para>
        When the Wic implementation needs to invoke a
        partition-specific implementation, it looks for the plug-in
        with the same name as the <filename>--source</filename>
        parameter used in the kickstart file given to that
        partition.
        For example, if the partition is set up using the following
        command in a kickstart file:
        <literallayout class='monospaced'>
     part /boot --source bootimg-pcbios --ondisk sda --label boot --active --align 1024
        </literallayout>
        The methods defined as class members of the matching
        source plug-in (i.e. <filename>bootimg-pcbios</filename>)
        in the <filename>bootimg-pcbios.py</filename> plug-in file
        are used.
    </para>

    <para>
        To be more concrete, here is the corresponding plug-in
        definition from the <filename>bootimg-pcbios.py</filename>
        file for the previous command along with an example
        method called by the Wic implementation when it needs to
        prepare a partition using an implementation-specific
        function:
        <literallayout class='monospaced'>
     bootimg-pcbios.py
                  .
                  .
                  .
        class BootimgPcbiosPlugin(SourcePlugin):
        """
        Create MBR boot partition and install syslinux on it.
        """

        name = 'bootimg-pcbios'
                  .
                  .
                  .
        @classmethod
        def do_prepare_partition(cls, part, source_params, creator, cr_workdir,
                                 oe_builddir, bootimg_dir, kernel_dir,
                                 rootfs_dir, native_sysroot):
            """
            Called to do the actual content population for a partition i.e. it
            'prepares' the partition to be incorporated into the image.
            In this case, prepare content for legacy bios boot partition.
            """
                  .
                  .
                  .
        </literallayout>
        If a subclass (plug-in) itself does not implement a
        particular function, Wic locates and uses the default
        version in the superclass.
        It is for this reason that all source plug-ins are derived
        from the <filename>SourcePlugin</filename> class.
    </para>

    <para>
        The <filename>SourcePlugin</filename> class defined in
        the <filename>pluginbase.py</filename> file defines
        a set of methods that source plug-ins can implement or
        override.
        Any plug-ins (subclass of
        <filename>SourcePlugin</filename>) that do not implement
        a particular method inherit the implementation of the
        method from the <filename>SourcePlugin</filename> class.
        For more information, see the
        <filename>SourcePlugin</filename> class in the
        <filename>pluginbase.py</filename> file for details:
    </para>

    <para>
        The following list describes the methods implemented in the
        <filename>SourcePlugin</filename> class:
        <itemizedlist>
            <listitem><para>
                <emphasis><filename>do_prepare_partition()</filename>:</emphasis>
                Called to populate a partition with actual content.
                In other words, the method prepares the final
                partition image that is incorporated into the
                disk image.
                </para></listitem>
            <listitem><para>
                <emphasis><filename>do_configure_partition()</filename>:</emphasis>
                Called before
                <filename>do_prepare_partition()</filename> to
                create custom configuration files for a partition
                (e.g. syslinux or grub configuration files).
                </para></listitem>
            <listitem><para>
                <emphasis><filename>do_install_disk()</filename>:</emphasis>
                Called after all partitions have been prepared and
                assembled into a disk image.
                This method provides a hook to allow finalization
                of a disk image (e.g. writing an MBR).
                </para></listitem>
            <listitem><para>
                <emphasis><filename>do_stage_partition()</filename>:</emphasis>
                Special content-staging hook called before
                <filename>do_prepare_partition()</filename>.
                This method is normally empty.</para>

                <para>Typically, a partition just uses the passed-in
                parameters (e.g. the unmodified value of
                <filename>bootimg_dir</filename>).
                However, in some cases, things might need to be
                more tailored.
                As an example, certain files might additionally
                need to be taken from
                <filename>bootimg_dir + /boot</filename>.
                This hook allows those files to be staged in a
                customized fashion.
                <note>
                    <filename>get_bitbake_var()</filename>
                    allows you to access non-standard variables
                    that you might want to use for this
                    behavior.
                </note>
                </para></listitem>
        </itemizedlist>
    </para>

    <para>
        You can extend the source plug-in mechanism.
        To add more hooks, create more source plug-in methods
        within <filename>SourcePlugin</filename> and the
        corresponding derived subclasses.
        The code that calls the plug-in methods uses the
        <filename>plugin.get_source_plugin_methods()</filename>
        function to find the method or methods needed by the call.
        Retrieval of those methods is accomplished by filling up
        a dict with keys that contain the method names of interest.
        On success, these will be filled in with the actual
        methods.
        See the Wic implementation for examples and details.
    </para>
</section>

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

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

    <para>
        The Yocto Project provides the Wayland protocol libraries and the
        reference
        <ulink url='http://en.wikipedia.org/wiki/Wayland_(display_server_protocol)#Weston'>Weston</ulink>
        compositor as part of its release.
        This section describes what you need to do to implement Wayland and
        use the compositor when building an image for a supporting target.
    </para>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

            <para>
                The <filename>LIC_FILES_CHKSUM</filename>
                variable contains checksums of the license text in the source
                code for the recipe.
                Following is an example of how to specify
                <filename>LIC_FILES_CHKSUM</filename>:
                <literallayout class='monospaced'>
     LIC_FILES_CHKSUM = "file://COPYING;md5=xxxx \
                         file://licfile1.txt;beginline=5;endline=29;md5=yyyy \
                         file://licfile2.txt;endline=50;md5=zzzz \
                         ..."
                </literallayout>
                <note><title>Notes</title>
                    <itemizedlist>
                        <listitem><para>
                            When using "beginline" and "endline", realize that
                            line numbering begins with one and not zero.
                            Also, the included lines are inclusive (i.e. lines
                            five through and including 29 in the previous
                            example for <filename>licfile1.txt</filename>).
                            </para></listitem>
                        <listitem><para>
                            When a license check fails, the selected license
                            text is included as part of the QA message.
                            Using this output, you can determine the exact
                            start and finish for the needed license text.
                            </para></listitem>
                    </itemizedlist>
                </note>
            </para>

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

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

            <para>
                The first line locates a file in
                <filename>${S}/src/ls.c</filename> and isolates lines five
                through 16 as license text.
                The second line refers to a file in
                <filename><link linkend='var-WORKDIR'>WORKDIR</link></filename>.
            </para>
            <para>
                Note that <filename>LIC_FILES_CHKSUM</filename> variable is
                mandatory for all recipes, unless the
                <filename>LICENSE</filename> variable is set to "CLOSED".
            </para>
        </section>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

            <para>
                Specifying audio and video plug-ins as part of the
                <filename>COMMERCIAL_AUDIO_PLUGINS</filename> and
                <filename>COMMERCIAL_VIDEO_PLUGINS</filename> statements
                (along with the enabling
                <filename>LICENSE_FLAGS_WHITELIST</filename>) includes the
                plug-ins or components into built images, thus adding
                support for media formats or components.
            </para>
        </section>
    </section>
</section>
</chapter>
<!--
vim: expandtab tw=80 ts=4
-->