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

<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<appendix id='ref-classes'>
<title>Reference: Classes</title>

<para>
    Class files are used to abstract common functionality and share it amongst multiple 
    <filename>.bb</filename> files. 
    Any metadata 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 Yocto Project file's area
    Class files can also be pointed to by BUILDDIR (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 <filename>BBPATH</filename>
    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 behaviour.
</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 as documented in the 
        <link linkend='usingpoky-extend-addpkg-autotools'>Autotooled Package</link> section 
        and then simply <filename>inherit autotools</filename>. 
        This class can also work with software that emulates Autotools.
    </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 DESTDIR 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>
        Several programs can fulfill the same or similar function and be 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>
        Four variables control this class:
        <itemizedlist>
            <listitem><para><filename>ALTERNATIVE_NAME</filename> &dash; The name of the 
                binary that is replaced (<filename>ar</filename> in this example).</para></listitem>
            <listitem><para><filename>ALTERNATIVE_LINK</filename> &dash; The path to 
                the resulting binary (<filename>/bin/ar</filename> in this example).</para></listitem>
            <listitem><para><filename>ALTERNATIVE_PATH</filename> &dash; The path to the 
                real binary (<filename>/usr/bin/ar.binutils</filename> in this example).</para></listitem>
            <listitem><para><filename>ALTERNATIVE_PRIORITY</filename> &dash; The priority of 
                the binary. 
                The version with the most features should have the highest priority.</para></listitem>
        </itemizedlist>
    </para>

    <para>
	Currently, the Yocto Project supports only one binary per package.
    </para>
</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 Yocto Project 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 config 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 make its
        integration smooth for all libraries that make use of 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> 
        subdirs 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 
                distribute directory.</para></listitem>
            <listitem><para><emphasis>symlink:</emphasis> Symlinks the files to the 
                distribute directory.</para></listitem>
            <listitem><para><emphasis>move+symlink:</emphasis> Moves the files into 
                the distribute directory and then symlinks them back.</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.
    </para>
    
    <para>
        Modules that use old <filename>Makefile.PL</filename>-based build system require
        <filename>cpan.bbclass</filename> in their recipes.
    </para>

    <para>
        Modules that use <filename>Build.PL</filename>-based build system require
        using <filename>cpan_build.bbclass</filename> in their recipes.
    </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 dependling on which method the module authors used.
    </para>

    <para>
        Extensions that use an Autotools-based build system require Autotools and 
        <filename>distutils</filename>-based <filename>.bbclasse</filename> files in their recipes.
    </para>

    <para>
        Extensions that use <filename>distutils</filename>-based build systems require 
        <filename>distutils.bbclass</filename> in their recipes.
    </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 as the Yocto Project does.
        See the <link 
        linkend='platdev-appdev-devshell'>Development Within a Development Shell</link> section 
        for more information about using devshell.
    </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 
        found in the Yocto Project file's <filename>conf</filename> directory. 
        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 RPM takes about thirty percent more time as 
        compared to using IPK 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 metadata 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>
        Keep in mind, however, that 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>

    <para>
        Another consideration for packages built using the RPM package manager is space.
        For smaller systems, the extra space used for the Berkley Database and the amount 
        of metadata can affect your ability to do on-device upgrades.
    </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='https://lists.yoctoproject.org/pipermail/poky/2011-May/006362.html'>
                https://lists.yoctoproject.org/pipermail/poky/2011-May/006362.html</ulink></para></listitem>
            <listitem><para><ulink url='https://lists.yoctoproject.org/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.
    </para>

    <para>
        The <filename><link linkend='var-IMAGE_FSTYPES'>IMAGE_FSTYPES</link></filename>
        variable controls the types of images to generate.
    </para>

    <para>        
        The <filename><link linkend='var-IMAGE_INSTALL'>IMAGE_INSTALL</link></filename>
        variable controls the list of packages to install into the image.
    </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 whether to include this class as the Yocto Project does.
    </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 Yocto Project.
        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 as the Yocto Project does.
    </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 it matches 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 
                <filename>TMPDIR/WORKDIR</filename> 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 continaing 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 Yocto Project Files provides a simple exmample 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-others'>
    <title>Other Classes</title>

    <para>
        Thus far, this appendix has discussed only the most useful and important 
        classes.
        However, other classes exist within the <filename>meta/classes</filename> directory 
        in the Yocto Project file's directory structure.
        You can examine the <filename>.bbclass</filename> files directly for more 
        information. 
    </para>
</section>

<!-- Undocumented classes are:
        allarch.bbclass
        binconfig.bbclass
        bootimg.bbclass
        buildstats.bbclass
        ccache.inc
        cmake.bbclass
        cml1.bbclass
        cross.bbclass
        cross-canadian.bbclass
        deploy.bbclass
        distrodata.bbclass
        gconf.bbclass
        gettext.bbclass
        gnome.bbclass
        gtk-doc.bbclass
        gtk-icon-cache.bbclass
        icecc.bbclass
        image-mklibs.bbclass
        image-prelink.bbclass
        image-swab.bbclass
        imagetest-dummy.bbclass
        imagetest-qemu.bbclass
        insserv.bbclass
        lib_package.bbclass
        license.bbclass
        logging.bbclass
        meta.bbclass
        metadata_scm.bbclass
        mirrors.bbclass
        multilib*.bbclass
        native.bbclass
        nativesdk.bbclass
        oelint.bbclass
        own-mirrors.bbclass
        packagedata.bbclass
        packagehistory.bbclass
        patch.bbclass
        perlnative.bbclass
        pkg_distribute.bbclass
        pkg_metainfo.bbclass
        populate_sdk*.bbclass
        prserv.bbclass
        python-dir.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
        task.bbclass
        terminal.bbclass
        tinderclient.bbclass
        toolchain-scripts.bbclass
        typecheck.bbclass
        utility-tasks.bbclass
        utils.bbclass
-->


</appendix>
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