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

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

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

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

    <para>
        BitBake handles the parsing and execution of the data files.
        The data itself is of various types:
    <itemizedlist>
        <listitem><para><emphasis>Recipes:</emphasis>  Provides details about particular
            pieces of software</para></listitem>
        <listitem><para><emphasis>Class Data:</emphasis>  An abstraction of 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, etc.
            Configuration data acts as the glue to bind everything together.</para></listitem>
    </itemizedlist>
        For more information on data, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-terms'>Yocto Project Terms</ulink>"
        section in the Yocto Project Development Manual.
    </para>

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

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

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

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

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

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

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

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

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

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

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

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

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

        <para>
            The configuration files (<filename>.conf</filename>) define various configuration variables
            that govern the OpenEmbedded build process.
            These files fall into several areas that define machine configuration options,
            distribution configuration options, compiler tuning options, general common configuration
            options and user configuration options (<filename>local.conf</filename>, which is found
            in the <ulink url='build-directory'>Build Directory</ulink>).
        </para>
    </section>
</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 don't 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 don't necessarily need rebuilt.
    </para>

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

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

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

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

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

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

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

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

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

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

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

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

        <para>
            Consider a case with inline 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 Build Directory.
            The checksum (or signature) for a particular task needs to add the hashes
            of all the tasks on which the particular task depends.
            Choosing which dependencies to add is a policy decision.
            However, the effect is to generate a master checksum that combines the basehash
            and the hashes of the task's dependencies.
        </para>

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

        <para>
            The rules for deciding which hashes of dependent tasks to include through
            dependency chains are more complex and are generally accomplished with a
            python function.
            The code in <filename>meta/lib/oe/sstatesig.py</filename> shows two examples
            of this and also illustrates how you can insert your own policy into the system
            if so desired.
            This file defines the two basic signature generators <filename>OE-Core</filename>
            uses:  "OEBasic" and "OEBasicHash".
            By default, there is a dummy "noop" signature handler enabled in BitBake.
            This means that behavior is unchanged from previous versions.
            <filename>OE-Core</filename> uses the "OEBasic" signature handler by default
            through this setting in the <filename>bitbake.conf</filename> file:
            <literallayout class='monospaced'>
  BB_SIGNATURE_HANDLER ?= "OEBasic"
            </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 metadata 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.
            Currently, this behavior is not the default behavior for <filename>OE-Core</filename>
            but is the default in <filename>poky</filename>.
        </para>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    <para>
        x32 is a new processor-specific Application Binary Interface (psABI) for x86_64.
        An ABI defines the calling conventions between functions in a processing environment.
        The interface determines what registers are used and what the sizes are for various C data types.
    </para>

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

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

        <para>
            While the x32 psABI specifications are not fully finalized, this Yocto Project
            release supports current development specifications of x32 psABI.
            As of this release of the Yocto Project, x32 psABI support exists as follows:
            <itemizedlist>
                <listitem><para>You can create packages and images in x32 psABI format on x86_64 architecture targets.
                    </para></listitem>
                <listitem><para>You can use the x32 psABI support through the <filename>meta-x32</filename>
                    layer on top of the OE-core/Yocto layer.</para></listitem>
                <listitem><para>The toolchain from the <filename>experimental/meta-x32</filename> layer
                    is used for building x32 psABI program binaries.</para></listitem>
                <listitem><para>You can successfully build many recipes with the x32 toolchain.</para></listitem>
                <listitem><para>You can create and boot <filename>core-image-minimal</filename> and
                    <filename>core-image-sato</filename> images.</para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='future-development-and-limitations'>
        <title>Future Development and Limitations</title>

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

        <para>
            Future Plans for the x32 psABI in the Yocto Project include the following:
            <itemizedlist>
                <listitem><para>Enhance and fix the few remaining recipes so they
                    work with and support x32 toolchains.</para></listitem>
                <listitem><para>Enhance RPM Package Manager (RPM) support for x32 binaries.</para></listitem>
                <listitem><para>Support larger images.</para></listitem>
                <listitem><para>Integrate x32 recipes, toolchain, and kernel changes from
                    <filename>experimental/meta-x32</filename> into OE-core.</para></listitem>
            </itemizedlist>
        </para>
    </section>

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

        <para>
            Despite the fact the x32 psABI support is in development state for this release of the
            Yocto Project, you can follow these steps to use the x32 spABI:
            <itemizedlist>
                <listitem><para>Add the <filename>experimental/meta-x32</filename> layer to your local
                    <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>.
                    You can find the <filename>experimental/meta-x32</filename> source repository at
                    <ulink url='&YOCTO_GIT_URL;'></ulink>.</para></listitem>
                <listitem><para>Edit your <filename>conf/bblayers.conf</filename> file so that it includes
                    the <filename>meta-x32</filename>.
                    Here is an example:
                    <literallayout class='monospaced'>
     BBLAYERS ?= " \
        /home/nitin/prj/poky.git/meta \
        /home/nitin/prj/poky.git/meta-yocto \
        /home/nitin/prj/poky.git/meta-yocto-bsp \
        /home/nitin/prj/meta-x32.git \
        "
     BBLAYERS_NON_REMOVABLE ?= " \
        /home/nitin/prj/poky.git/meta \
        /home/nitin/prj/poky.git/meta-yocto \
        "
                    </literallayout></para></listitem>
                <listitem><para>Enable the x32 psABI tuning file for <filename>x86_64</filename>
                    machines by editing the <filename>conf/local.conf</filename> like this:
                    <literallayout class='monospaced'>
      MACHINE = "qemux86-64"
      DEFAULTTUNE = "x86-64-x32"
      baselib = "${@d.getVar('BASE_LIB_tune-' + (d.getVar('DEFAULTTUNE', True) \
         or 'INVALID'), True) or 'lib'}"
      #MACHINE = "atom-pc"
      #DEFAULTTUNE = "core2-64-x32"
                    </literallayout></para></listitem>
                <listitem><para>As usual, use BitBake to build an image that supports the x32 psABI.
                    Here is an example:
                    <literallayout class='monospaced'>
     $ bitake core-image-sato
                    </literallayout></para></listitem>
                <listitem><para>As usual, run your image using QEMU:
                    <literallayout class='monospaced'>
     $ runqemu qemux86-64 core-image-sato
                    </literallayout></para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>

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

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

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

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

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

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

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

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

            <para>
                You can also use relative paths as shown in the following example:
                <literallayout class='monospaced'>
     LIC_FILES_CHKSUM = "file://src/ls.c;beginline=5;endline=16;\
                                         md5=bb14ed3c4cda583abc85401304b5cd4e"
     LIC_FILES_CHKSUM = "file://../license.html;md5=5c94767cedb5d6987c902ac850ded2c6"
                </literallayout>
            </para>

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

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

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

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

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

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

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

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

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

            <para>
		        The definition of 'matching' in reference to a
		        recipe's <filename>LICENSE_FLAGS</filename> setting is simple.
                However, some things exist that you should know about in order to
                correctly and effectively use it.
            </para>

            <para>
                Before a flag
                defined by a particular recipe is tested against the
                contents of the <filename>LICENSE_FLAGS_WHITELIST</filename> variable, the
                string <filename>_${PN}</filename> (with
                <link linkend='var-PN'><filename>PN</filename></link> expanded of course) is
                appended to the flag, thus automatically making each
                <filename>LICENSE_FLAGS</filename> value recipe-specific.
                That string is
                then matched against the whitelist.
                So if you specify <filename>LICENSE_FLAGS = "commercial"</filename> in recipe
		        "foo" for example, the string <filename>"commercial_foo"</filename>
                would normally be what is specified in the whitelist in order for it to
                match.
            </para>

            <para>
                You can broaden the match by
                putting any "_"-separated beginning subset of a
                <filename>LICENSE_FLAGS</filename> flag in the whitelist, which will also
                match.
                For example, simply specifying "commercial" in
                the whitelist would match any expanded <filename>LICENSE_FLAGS</filename>
                definition starting with "commercial" such as
                "commercial_foo" and "commercial_bar", which are the
                strings that would be automatically generated for
                hypothetical "foo" and "bar" recipes assuming those
                recipes had simply specified the following:
                <literallayout class='monospaced'>
     LICENSE_FLAGS = "commercial"
                </literallayout>
            </para>

            <para>
                Broadening the match allows for a range of specificity for the items
                in the whitelist, from more general to perfectly
                specific.
                So you have the choice of exhaustively
                enumerating each license flag in the whitelist to
                allow only those specific recipes into the image, or
                of using a more general string to pick up anything
                matching just the first component or components of the specified
                string.
            </para>

            <para>
                This scheme works even if the flag already
                has <filename>_${PN}</filename> appended - the extra <filename>_${PN}</filename> is
                redundant, but does not affect the outcome.
                For example, a license flag of "commercial_1.2_foo" would
                turn into "commercial_1.2_foo_foo" and would match
                both the general "commercial" and the specific
                "commercial_1.2_foo", as expected.
                The flag would also match
                "commercial_1.2_foo_foo" and "commercial_1.2", which
                does not make much sense regarding use in the whitelist.
            </para>

            <para>
                For a versioned string, you could instead specify
                "commercial_foo_1.2", which would turn into
                "commercial_foo_1.2_foo".
                And, as expected, this flag allows
                you to pick up this package along with
                anything else "commercial" when you specify "commercial"
                in the whitelist.
                Or, the flag allows you to pick up this package along with anything "commercial_foo"
                regardless of version when you use "commercial_foo" in the whitelist.
                Finally, you can be completely specific about the package and version and specify
                "commercial_foo_1.2" package and version.
            </para>
        </section>

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

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

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