path: root/documentation/ref-manual/ref-classes.xml

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<chapter id='ref-classes'>
<title>Classes</title>

<para>
    Class files are used to abstract common functionality and share it amongst multiple
    <filename>.bb</filename> files.
    Any <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink> usually
    found in a <filename>.bb</filename> file can also be placed in a class
    file.
    Class files are identified by the extension <filename>.bbclass</filename> and are usually placed
    in a <filename>classes/</filename> directory beneath the
    <filename>meta*/</filename> directory found in the
    <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>.
    Class files can also be pointed to by
    <link linkend='var-BUILDDIR'><filename>BUILDDIR</filename></link>
    (e.g. <filename>build/</filename>)in the same way as
    <filename>.conf</filename> files in the <filename>conf</filename> directory.
    Class files are searched for in <link linkend='var-BBPATH'><filename>BBPATH</filename></link>
    using the same method by which <filename>.conf</filename> files are searched.
</para>

<para>
    In most cases inheriting the class is enough to enable its features, although
    for some classes you might need to set variables or override some of the
    default behavior.
</para>

<para>
    This chapter discusses only the most useful and important classes.
        Other classes do exist within the <filename>meta/classes</filename>
        directory in the
        <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>.
        You can reference the <filename>.bbclass</filename> files directly
        for more information.
</para>

<section id='ref-classes-base'>
    <title>The base Class - <filename>base.bbclass</filename></title>

    <para>
        The base class is special in that every <filename>.bb</filename>
        file inherits it automatically.
        This class contains definitions for standard basic
        tasks such as fetching, unpacking, configuring (empty by default), compiling
        (runs any <filename>Makefile</filename> present), installing (empty by default) and packaging
        (empty by default).
        These classes are often overridden or extended by other classes
        such as <filename>autotools.bbclass</filename> or <filename>package.bbclass</filename>.
        The class also contains some commonly used functions such as <filename>oe_runmake</filename>.
    </para>
</section>

<section id='ref-classes-autotools'>
    <title>Autotooled Packages - <filename>autotools.bbclass</filename></title>

    <para>
        Autotools (<filename>autoconf</filename>, <filename>automake</filename>,
        and <filename>libtool</filename>) bring standardization.
        This class defines a set of tasks (configure, compile etc.) that
        work for all Autotooled packages.
        It should usually be enough to define a few standard variables
        and then simply <filename>inherit autotools</filename>.
        This class can also work with software that emulates Autotools.
        For more information, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#usingpoky-extend-addpkg-autotools'>Autotooled Package</ulink>"
        section in the Yocto Project Development Manual.
    </para>

    <para>
        It's useful to have some idea of how the tasks defined by this class work
        and what they do behind the scenes.
        <itemizedlist>
            <listitem><para><filename>do_configure</filename> &dash; Regenerates the
                configure script (using <filename>autoreconf</filename>) and then launches it
                with a standard set of arguments used during cross-compilation.
                You can pass additional parameters to <filename>configure</filename> through the
                <filename><link linkend='var-EXTRA_OECONF'>EXTRA_OECONF</link></filename> variable.
                </para></listitem>
            <listitem><para><filename>do_compile</filename> &dash; Runs <filename>make</filename> with
                arguments that specify the compiler and linker.
                You can pass additional arguments through
                the <filename><link linkend='var-EXTRA_OEMAKE'>EXTRA_OEMAKE</link></filename> variable.
                </para></listitem>
            <listitem><para><filename>do_install</filename> &dash; Runs <filename>make install</filename>
                and passes a destination directory option, which takes its value
                from the standard
                <filename><link linkend='var-DESTDIR'>DESTDIR</link></filename> variable.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='ref-classes-update-alternatives'>
    <title>Alternatives - <filename>update-alternatives.bbclass</filename></title>

    <para>
        This class helps the alternatives system when multiple sources provide
        the same command.
        This situation occurs when several programs that have the same or
        similar function are installed with the same name.
        For example, the <filename>ar</filename> command is available from the
        <filename>busybox</filename>, <filename>binutils</filename> and
        <filename>elfutils</filename> packages.
        The <filename>update-alternatives.bbclass</filename> class handles
        renaming the binaries so that multiple packages can be installed
        without conflicts.
        The <filename>ar</filename> command still works regardless of which
        packages are installed or subsequently removed.
        The class renames the conflicting binary in each package and symlinks
        the highest priority binary during installation or removal of packages.
    </para>

    <para>
        To use this class, you need to define a number of variables.
        These variables list alternative commands needed by a package,
        provide pathnames for links, default links for targets, and
        so forth.
        For details on how to use this class, see the comments in the
        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta/classes/update-alternatives.bbclass'><filename>update-alternatives.bbclass</filename></ulink>.
    </para>

    <note>
        You can use the <filename>update-alternatives</filename> command
        directly in your recipes.
        However, this class simplifies things in most cases.
    </note>
</section>

<section id='ref-classes-update-rc.d'>
    <title>Initscripts - <filename>update-rc.d.bbclass</filename></title>

    <para>
        This class uses <filename>update-rc.d</filename> to safely install an
        initialization script on behalf of the package.
        The OpenEmbedded build system takes care of details such as making sure the script is stopped before
        a package is removed and started when the package is installed.
        Three variables control this class:
        <filename><link linkend='var-INITSCRIPT_PACKAGES'>INITSCRIPT_PACKAGES</link></filename>,
        <filename><link linkend='var-INITSCRIPT_NAME'>INITSCRIPT_NAME</link></filename> and
        <filename><link linkend='var-INITSCRIPT_PARAMS'>INITSCRIPT_PARAMS</link></filename>.
        See the variable links for details.
    </para>
</section>

<section id='ref-classes-binconfig'>
    <title>Binary Configuration Scripts - <filename>binconfig.bbclass</filename></title>

    <para>
        Before <filename>pkg-config</filename> had become widespread, libraries shipped shell
        scripts to give information about the libraries and include paths needed
        to build software (usually named <filename>LIBNAME-config</filename>).
        This class assists any recipe using such scripts.
    </para>

    <para>
        During staging, BitBake installs such scripts into the
        <filename>sysroots/</filename> directory.
        BitBake also changes all paths to point into the <filename>sysroots/</filename>
        directory so all builds that use the script will use the correct
        directories for the cross compiling layout.
    </para>
</section>

<section id='ref-classes-debian'>
    <title>Debian Renaming - <filename>debian.bbclass</filename></title>

    <para>
        This class renames packages so that they follow the Debian naming
        policy (i.e. <filename>eglibc</filename> becomes <filename>libc6</filename>
        and <filename>eglibc-devel</filename> becomes <filename>libc6-dev</filename>.
    </para>
</section>

<section id='ref-classes-pkgconfig'>
    <title>Pkg-config - <filename>pkgconfig.bbclass</filename></title>

    <para>
        <filename>pkg-config</filename> brought standardization and this class
        aims to smooth integration of <filename>pkg-config</filename>
        into libraries that use it.
    </para>

    <para>
        During staging, BitBake installs <filename>pkg-config</filename> data into the
        <filename>sysroots/</filename> directory.
        By making use of sysroot functionality within <filename>pkg-config</filename>,
        this class no longer has to manipulate the files.
    </para>
</section>

<section id='ref-classes-src-distribute'>
    <title>Distribution of Sources - <filename>src_distribute_local.bbclass</filename></title>

    <para>
        Many software licenses require that source files be provided along with the binaries.
        To simplify this process, two classes were created:
        <filename>src_distribute.bbclass</filename> and
        <filename>src_distribute_local.bbclass</filename>.
    </para>

    <para>
        The results of these classes are <filename>tmp/deploy/source/</filename>
        subdirectories with sources sorted by
        <filename><link linkend='var-LICENSE'>LICENSE</link></filename> field.
        If recipes list few licenses (or have entries like "Bitstream Vera"),
        the source archive is placed in each license directory.
    </para>

    <para>
        This class operates using three modes:
        <itemizedlist>
            <listitem><para><emphasis>copy:</emphasis> Copies the files to the
                distribution directory.</para></listitem>
            <listitem><para><emphasis>symlink:</emphasis> Creates symbolic
                links for the files to the distribution directory.
                </para></listitem>
            <listitem><para><emphasis>move+symlink:</emphasis> Moves the files
                into the distribution directory and then creates symbolic
                links back to where they originated.</para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='ref-classes-perl'>
    <title>Perl Modules - <filename>cpan.bbclass</filename></title>

    <para>
        Recipes for Perl modules are simple.
        These recipes usually only need to point to the source's archive and then inherit the
        proper <filename>.bbclass</filename> file.
        Building is split into two methods depending on which method the module authors used.
        <itemizedlist>
            <listitem><para>Modules that use old
                <filename>Makefile.PL</filename>-based build system require
                <filename>cpan.bbclass</filename> in their recipes.
                </para></listitem>
            <listitem><para>Modules that use
                <filename>Build.PL</filename>-based build system require
                using <filename>cpan_build.bbclass</filename> in their recipes.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='ref-classes-distutils'>
    <title>Python Extensions - <filename>distutils.bbclass</filename></title>

    <para>
        Recipes for Python extensions are simple.
        These recipes usually only need to point to the source's archive and then inherit
        the proper <filename>.bbclass</filename> file.
        Building is split into two methods depending on which method the module authors used.
        <itemizedlist>
            <listitem><para>Extensions that use an Autotools-based build system
                require Autotools and
                <filename>distutils</filename>-based
                <filename>.bbclasse</filename> files in their recipes.
                </para></listitem>
            <listitem><para>Extensions that use
                <filename>distutils</filename>-based build systems require
                <filename>distutils.bbclass</filename> in their recipes.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='ref-classes-devshell'>
    <title>Developer Shell - <filename>devshell.bbclass</filename></title>

    <para>
        This class adds the <filename>devshell</filename> task.
        Distribution policy dictates whether to include this class.
        See the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#platdev-appdev-devshell'>Using a Development Shell</ulink>" section
        in the Yocto Project Development Manual for more information about using <filename>devshell</filename>.
    </para>
</section>

<section id='ref-classes-packagegroup'>
    <title>Package Groups - <filename>packagegroup.bbclass</filename></title>

    <para>
        This class sets default values appropriate for package group recipes (e.g.
        <filename><link linkend='var-PACKAGES'>PACKAGES</link></filename>,
        <filename><link linkend='var-PACKAGE_ARCH'>PACKAGE_ARCH</link></filename>,
        <filename><link linkend='var-ALLOW_EMPTY'>ALLOW_EMPTY</link></filename>,
        and so forth).
        It is highly recommended that all package group recipes inherit this class.
    </para>
    <para>
        For information on how to use this class, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#usingpoky-extend-customimage-customtasks'>Customizing Images Using Custom Package Groups</ulink>"
        section in the Yocto Project Development Manual.
    </para>
    <para>
        Previously, this class was named <filename>task.bbclass</filename>.
    </para>
</section>


<section id='ref-classes-package'>
    <title>Packaging - <filename>package*.bbclass</filename></title>

    <para>
        The packaging classes add support for generating packages from a build's
        output.
        The core generic functionality is in <filename>package.bbclass</filename>.
        The code specific to particular package types is contained in various sub-classes such as
        <filename>package_deb.bbclass</filename>, <filename>package_ipk.bbclass</filename>,
        and <filename>package_rpm.bbclass</filename>.
        Most users will want one or more of these classes.
    </para>

    <para>
        You can control the list of resulting package formats by using the
        <filename><link linkend='var-PACKAGE_CLASSES'>PACKAGE_CLASSES</link></filename>
        variable defined in the <filename>local.conf</filename> configuration file,
        which is located in the <filename>conf</filename> folder of the
        <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>.
        When defining the variable, you can specify one or more package types.
        Since images are generated from packages, a packaging class is
        needed to enable image generation.
        The first class listed in this variable is used for image generation.
    </para>

    <para>
        The package class you choose can affect build-time performance and has space
        ramifications.
        In general, building a package with IPK takes about thirty percent less
        time as compared to using RPM to build the same or similar package.
        This comparison takes into account a complete build of the package with 
        all dependencies previously built.
        The reason for this discrepancy is because the RPM package manager 
        creates and processes more 
        <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink> than the 
        IPK package manager.
        Consequently, you might consider setting <filename>PACKAGE_CLASSES</filename>
        to "package_ipk" if you are building smaller systems.
    </para>

    <para>
        Before making your decision on package manager, however, you should 
        consider some further things about using RPM:
        <itemizedlist>
            <listitem><para>
                RPM starts to provide more abilities than IPK due to
                the fact that it processes more metadata.
                For example, this information includes individual file types, 
                file checksum generation and evaluation on install, sparse file 
                support, conflict detection and resolution for Multilib systems, 
                ACID style upgrade, and repackaging abilities for rollbacks.
                </para></listitem>
            <listitem><para>
                For smaller systems, the extra space used for the Berkley 
                Database and the amount of metadata when using RPM can affect 
                your ability to perform on-device upgrades.
                </para></listitem>
            <listitem><para>
                If you take the steps to set up a repository (package feed)    
                on the development host that can be used by Smart, you can
                install packages from the feed while you are running the image
                on the target (i.e. runtime installation of packages).
                For information on how to set up this repository, see the 
                "<ulink url='&YOCTO_DOCS_DEV_URL;#setting-up-runtime-package-management'>Setting Up Runtime Package Management</ulink>"
                 in the Yocto Project Development Manual.
            </para></listitem>
        </itemizedlist> 
    </para>

    <para>
        You can find additional information on the effects of the package 
        class at these two Yocto Project mailing list links:
        <itemizedlist>
            <listitem><para><ulink url='&YOCTO_LISTS_URL;/pipermail/poky/2011-May/006362.html'>
                https://lists.yoctoproject.org/pipermail/poky/2011-May/006362.html</ulink></para></listitem>
            <listitem><para><ulink url='&YOCTO_LISTS_URL;/pipermail/poky/2011-May/006363.html'>
                https://lists.yoctoproject.org/pipermail/poky/2011-May/006363.html</ulink></para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='ref-classes-kernel'>
    <title>Building Kernels - <filename>kernel.bbclass</filename></title>

    <para>
        This class handles building Linux kernels.
        The class contains code to build all kernel trees.
        All needed headers are staged into the
        <filename><link linkend='var-STAGING_KERNEL_DIR'>STAGING_KERNEL_DIR</link></filename>
        directory to allow out-of-tree module builds using <filename>module.bbclass</filename>.
    </para>

    <para>
        This means that each built kernel module is packaged separately and inter-module
        dependencies are created by parsing the <filename>modinfo</filename> output.
        If all modules are required, then installing the <filename>kernel-modules</filename>
        package installs all packages with modules and various other kernel packages
        such as <filename>kernel-vmlinux</filename>.
    </para>

    <para>
        Various other classes are used by the kernel and module classes internally including
        <filename>kernel-arch.bbclass</filename>, <filename>module_strip.bbclass</filename>,
        <filename>module-base.bbclass</filename>, and <filename>linux-kernel-base.bbclass</filename>.
    </para>
</section>

<section id='ref-classes-image'>
    <title>Creating Images - <filename>image.bbclass</filename> and <filename>rootfs*.bbclass</filename></title>

    <para>
        These classes add support for creating images in several formats.
        First, the root filesystem is created from packages using
        one of the <filename>rootfs_*.bbclass</filename>
        files (depending on the package format used) and then the image is created.
        <itemizedlist>
            <listitem><para>The
                <filename><link linkend='var-IMAGE_FSTYPES'>IMAGE_FSTYPES</link></filename>
                variable controls the types of images to generate.
                </para></listitem>
            <listitem><para>The
                <filename><link linkend='var-IMAGE_INSTALL'>IMAGE_INSTALL</link></filename>
                variable controls the list of packages to install into the
                image.</para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='ref-classes-sanity'>
    <title>Host System Sanity Checks - <filename>sanity.bbclass</filename></title>

    <para>
        This class checks to see if prerequisite software is present so that
        users can be notified of potential problems that might affect their build.
        The class also performs basic user configuration checks from
        the <filename>local.conf</filename> configuration file to
        prevent common mistakes that cause build failures.
        Distribution policy usually determines whether to include this class.
    </para>
</section>

<section id='ref-classes-insane'>
<title>Generated Output Quality Assurance Checks - <filename>insane.bbclass</filename></title>

    <para>
        This class adds a step to the package generation process that sanity checks the
        packages generated by the OpenEmbedded build system.
        A range of checks are performed that check the build's output
        for common problems that show up during runtime.
        Distribution policy usually dictates whether to include this class.
    </para>

    <para>
        You can configure the sanity checks so that specific test failures either raise a warning or
        an error message.
        Typically, failures for new tests generate a warning.
        Subsequent failures for the same test would then generate an error message
        once the metadata is in a known and good condition.
        You use the <filename>WARN_QA</filename> variable to specify tests for which you
        want to generate a warning message on failure.
        You use the <filename>ERROR_QA</filename> variable to specify tests for which you
        want to generate an error message on failure.
    </para>

    <para>
        The following list shows the tests you can list with the <filename>WARN_QA</filename>
        and <filename>ERROR_QA</filename> variables:
        <itemizedlist>
            <listitem><para><emphasis><filename>ldflags:</filename></emphasis>
                Ensures that the binaries were linked with the
                <filename>LDFLAGS</filename> options provided by the build system.
                If this test fails, check that the <filename>LDFLAGS</filename> variable
                is being passed to the linker command.</para></listitem>
            <listitem><para><emphasis><filename>useless-rpaths:</filename></emphasis>
                Checks for dynamic library load paths (rpaths) in the binaries that
                by default on a standard system are searched by the linker (e.g.
                <filename>/lib</filename> and <filename>/usr/lib</filename>).
                While these paths will not cause any breakage, they do waste space and
                are unnecessary.</para></listitem>
            <listitem><para><emphasis><filename>rpaths:</filename></emphasis>
                Checks for rpaths in the binaries that contain build system paths such
                as <filename>TMPDIR</filename>.
                If this test fails, bad <filename>-rpath</filename> options are being
                passed to the linker commands and your binaries have potential security
                issues.</para></listitem>
            <listitem><para><emphasis><filename>dev-so:</filename></emphasis>
                Checks that the <filename>.so</filename> symbolic links are in the
                <filename>-dev</filename> package and not in any of the other packages.
                In general, these symlinks are only useful for development purposes.
                Thus, the <filename>-dev</filename> package is the correct location for
                them.
                Some very rare cases do exist for dynamically loaded modules where
                these symlinks are needed instead in the main package.
                </para></listitem>
            <listitem><para><emphasis><filename>debug-files:</filename></emphasis>
                Checks for <filename>.debug</filename> directories in anything but the
                <filename>-dbg</filename> package.
                The debug files should all be in the <filename>-dbg</filename> package.
                Thus, anything packaged elsewhere is incorrect packaging.</para></listitem>
            <listitem><para><emphasis><filename>arch:</filename></emphasis>
                Checks the Executable and Linkable Format (ELF) type, bit size, and endianness
                of any binaries to ensure they match the target architecture.
                This test fails if any binaries don't match the type since there would be an
                incompatibility.
                Sometimes software, like bootloaders, might need to bypass this check.
                </para></listitem>
            <listitem><para><emphasis><filename>debug-deps:</filename></emphasis>
                Checks that <filename>-dbg</filename> packages only depend on other
                <filename>-dbg</filename> packages and not on any other types of packages,
                which would cause a packaging bug.</para></listitem>
            <listitem><para><emphasis><filename>dev-deps:</filename></emphasis>
                Checks that <filename>-dev</filename> packages only depend on other
                <filename>-dev</filename> packages and not on any other types of packages,
                which would be a packaging bug.</para></listitem>
            <listitem><para><emphasis><filename>pkgconfig:</filename></emphasis>
                Checks <filename>.pc</filename> files for any
                <link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>/<link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>
                paths.
                Any <filename>.pc</filename> file containing these paths is incorrect
                since <filename>pkg-config</filename> itself adds the correct sysroot prefix
                when the files are accessed.</para></listitem>
            <listitem><para><emphasis><filename>la:</filename></emphasis>
                Checks <filename>.la</filename> files for any <filename>TMPDIR</filename>
                paths.
                Any <filename>.la</filename> file containing these paths is incorrect since
                <filename>libtool</filename> adds the correct sysroot prefix when using the
                files automatically itself.</para></listitem>
            <listitem><para><emphasis><filename>desktop:</filename></emphasis>
                Runs the <filename>desktop-file-validate</filename> program against any
                <filename>.desktop</filename> files to validate their contents against
                the specification for <filename>.desktop</filename> files.</para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='ref-classes-siteinfo'>
    <title>Autotools Configuration Data Cache - <filename>siteinfo.bbclass</filename></title>

    <para>
        Autotools can require tests that must execute on the target hardware.
        Since this is not possible in general when cross compiling, site information is
        used to provide cached test results so these tests can be skipped over but
        still make the correct values available.
        The <filename><link linkend='structure-meta-site'>meta/site directory</link></filename>
        contains test results sorted into different categories such as architecture, endianness, and
        the <filename>libc</filename> used.
        Site information provides a list of files containing data relevant to
        the current build in the
        <filename><link linkend='var-CONFIG_SITE'>CONFIG_SITE</link></filename> variable
        that Autotools automatically picks up.
    </para>

    <para>
        The class also provides variables like
        <filename><link linkend='var-SITEINFO_ENDIANNESS'>SITEINFO_ENDIANNESS</link></filename>
        and <filename><link linkend='var-SITEINFO_BITS'>SITEINFO_BITS</link></filename>
        that can be used elsewhere in the metadata.
    </para>

    <para>
        Because this class is included from <filename>base.bbclass</filename>, it is always active.
    </para>
</section>

<section id='ref-classes-useradd'>
    <title>Adding Users - <filename>useradd.bbclass</filename></title>

    <para>
        If you have packages that install files that are owned by custom users or groups,
        you can use this class to specify those packages and associate the users and groups
        with those packages.
        The <filename>meta-skeleton/recipes-skeleton/useradd/useradd-example.bb</filename>
        recipe in the <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
        provides a simple example that shows how to add three
        users and groups to two packages.
        See the <filename>useradd-example.bb</filename> for more information on how to
        use this class.
    </para>
</section>

<section id='ref-classes-externalsrc'>
    <title>Using External Source - <filename>externalsrc.bbclass</filename></title>

    <para>
        You can use this class to build software from source code that is external to the
        OpenEmbedded build system.
        In other words, your source code resides in an external tree outside of the Yocto Project.
        Building software from an external source tree means that the normal fetch, unpack, and
        patch process is not used.
    </para>

    <para>
        To use the class, you need to define the
        <link linkend='var-S'><filename>S</filename></link> variable to point to the directory that contains the source files.
        You also need to have your recipe inherit the <filename>externalsrc.bbclass</filename> class.
    </para>

    <para>
        This class expects the source code to support recipe builds that use the
        <link linkend='var-B'><filename>B</filename></link> variable to point to the directory in
        which the OpenEmbedded build system places the generated objects built from the recipes.
        By default, the <filename>B</filename> directory is set to the following, which is separate from the
        Source Directory (<filename>S</filename>):
        <literallayout class='monospaced'>
     ${WORKDIR}/${BPN}/{PV}/
        </literallayout>
        See the glossary entries for the
        <link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>,
        <link linkend='var-BPN'><filename>BPN</filename></link>,
        <link linkend='var-PV'><filename>PV</filename></link>,
        <link linkend='var-S'><filename>S</filename></link>, and
        <link linkend='var-B'><filename>B</filename></link> for more information.
    </para>

    <para>
        You can build object files in the external tree by setting the
        <filename>B</filename> variable equal to <filename>"${S}"</filename>.
        However, this practice does not work well if you use the source for more than one variant
        (i.e., "natives" such as <filename>quilt-native</filename>,
        or "crosses" such as <filename>gcc-cross</filename>).
        So, be sure there are no "native", "cross", or "multilib" variants of the recipe.
    </para>

    <para>
        If you do want to build different variants of a recipe, you can use the
        <link linkend='var-BBCLASSEXTEND'><filename>BBCLASSEXTEND</filename></link> variable.
        When you do, the <link linkend='var-B'><filename>B</filename></link> variable must support the
        recipe's ability to build variants in different working directories.
        Most Autotools-based recipes support separating these directories.
        The OpenEmbedded build system defaults to using separate directories for <filename>gcc</filename>
        and some kernel recipes.
        Alternatively, you can make sure that separate recipes exist that each
        use the <filename>BBCLASSEXTEND</filename> variable to build each variant.
        The separate recipes can inherit a single target recipe.
    </para>

    <para>
        For information on how to use this class, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#building-software-from-an-external-source'>Building
        Software from an External Source</ulink>" section in the Yocto Project Development Manual.
    </para>
</section>

<section id='ref-classes-others'>
    <title>Other Classes</title>

    <para>
        Thus far, this chapter has discussed only the most useful and important
        classes.
        However, other classes exist within the <filename>meta/classes</filename> directory
        in the <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>.
        You can examine the <filename>.bbclass</filename> files directly for more
        information.
    </para>
</section>

<!-- Undocumented classes are:
        allarch.bbclass
        archive*.bbclass
        binconfig.bbclass
        blacklist.bbclass
        bootimg.bbclass
        boot-directdisk.bbclass
        bugzilla.bbclass
        buildhistory.bbclass
        buildstats.bbclass
        ccache.bbclass
        chrpath.bbclass
        cmake.bbclass
        cml1.bbclass
        copyleft_compliance.bbclass
        core-image.bbclass
        cross.bbclass
        cross-canadian.bbclass
        crosssdk.bbclass
        deploy.bbclass
        distrodata.bbclass
        dummy.bbclass
        gconf.bbclass
        gettext.bbclass
        gnomebase.bbclass
        gnome.bbclass
        gtk-doc.bbclass
        gtk-icon-cache.bbclass
        gzipnative.bbclass
        icecc.bbclass
        image-empty.bbclass
        image-live.bbclass
        image-vmdk.bbclass
        image-mklibs.bbclass
        image-prelink.bbclass
        image-swab.bbclass
        imagetest-dummy.bbclass
        imagetest-qemu.bbclass
        image_types.bbclass
        image_types_uboot.bbclass
        insserv.bbclass
        kernel-arch.bbclass
        kernel-yocto.bbclass
        lib_package.bbclass
        linux-kernel-base.bbclass
        license.bbclass
        logging.bbclass
        meta.bbclass
        metadata_scm.bbclass
        mime.bbclass
        mirrors.bbclass
        multilib*.bbclass
        native.bbclass
        nativesdk.bbclass
        oelint.bbclass
        own-mirrors.bbclass
        packagedata.bbclass
        packageinfo.bbclass
        patch.bbclass
        perlnative.bbclass
        pkg_distribute.bbclass
        pkg_metainfo.bbclass
        populate_sdk*.bbclass
        prexport.bbclass
        primport.bbclass
        prserv.bbclass
        python-dir.bbclass
        pythonnative.bbclass
        qemu.bbclass
        qmake*.bbclass
        qt4*.bbclass
        recipe_sanity.bbclass
        relocatable.bbclass
        rm_work.bbclass
        scons.bbclass
        sdl.bbclass
        setuptools.bbclass
        sip.bbclass
        siteconfig.bbclass
        sourcepkg.bbclass
        sstate.bbclass
        staging.bbclass
        syslinux.bbclass
        terminal.bbclass
        tinderclient.bbclass
        toolchain-scripts.bbclass
        typecheck.bbclass
        utility-tasks.bbclass
        utils.bbclass
-->


</chapter>
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