poky/documentation/ref-manual/ref-terms.xml

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<chapter id='ref-terms'>
<title>Yocto Project Terms</title>

    <para>
        Following is a list of terms and definitions users new to the Yocto
        Project development environment might find helpful.
        While some of these terms are universal, the list includes them
        just in case:
        <itemizedlist>
            <listitem><para>
                <emphasis>Append Files:</emphasis>
                Files that append build information to a recipe file.
                Append files are known as BitBake append files and
                <filename>.bbappend</filename> files.
                The OpenEmbedded build system expects every append file to have
                a corresponding recipe (<filename>.bb</filename>) file.
                Furthermore, the append file and corresponding recipe file
                must use the same root filename.
                The filenames can differ only in the file type suffix used
                (e.g.
                <filename>formfactor_0.0.bb</filename> and
                <filename>formfactor_0.0.bbappend</filename>).</para>

                <para>Information in append files extends or overrides the
                information in the similarly-named recipe file.
                For an example of an append file in use, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#using-bbappend-files'>Using .bbappend Files in Your Layer</ulink>"
                section in the Yocto Project Development Tasks Manual.</para>

                <para>When you name an append file, you can use the
                "<filename>%</filename>" wildcard character to allow for
                matching recipe names.
                For example, suppose you have an append file named as follows:
                <literallayout class='monospaced'>
     busybox_1.21.%.bbappend
                </literallayout>
                That append file would match any
                <filename>busybox_1.21.</filename><replaceable>x</replaceable><filename>.bb</filename>
                version of the recipe.
                So, the append file would match any of the following recipe names:
                <literallayout class='monospaced'>
     busybox_1.21.1.bb
     busybox_1.21.2.bb
     busybox_1.21.3.bb
     busybox_1.21.10.bb
     busybox_1.21.25.bb
                </literallayout>
                <note><title>Important</title>
                    The use of the "<filename>%</filename>" character
                    is limited in that it only works directly in front of the
                    <filename>.bbappend</filename> portion of the append file's
                    name.
                    You cannot use the wildcard character in any other
                    location of the name.
                </note>
                </para></listitem>
            <listitem><para id='bitbake-term'>
                <emphasis>BitBake:</emphasis>
                The task executor and scheduler used by the OpenEmbedded build
                system to build images.
                For more information on BitBake, see the
                <ulink url='&YOCTO_DOCS_BB_URL;'>BitBake User Manual</ulink>.
                </para></listitem>
            <listitem><para id='board-support-package-bsp-term'>
                <emphasis>Board Support Package (BSP):</emphasis>
                A group of drivers, definitions, and other components that
                provide support for a specific hardware configuration.
                For more information on BSPs, see the
                <ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
                </para></listitem>
            <listitem>
                <para id='build-directory'>
                <emphasis>Build Directory:</emphasis>
                This term refers to the area used by the OpenEmbedded build
                system for builds.
                The area is created when you <filename>source</filename> the
                setup environment script that is found in the Source Directory
                (i.e. <link linkend='structure-core-script'><filename>&OE_INIT_FILE;</filename></link>).
                The
                <link linkend='var-TOPDIR'><filename>TOPDIR</filename></link>
                variable points to the Build Directory.</para>

                <para>You have a lot of flexibility when creating the Build
                Directory.
                Following are some examples that show how to create the
                directory.
                The examples assume your
                <link linkend='source-directory'>Source Directory</link> is
                named <filename>poky</filename>:
                <itemizedlist>
                    <listitem><para>Create the Build Directory inside your
                        Source Directory and let the name of the Build
                        Directory default to <filename>build</filename>:
                        <literallayout class='monospaced'>
     $ cd $HOME/poky
     $ source &OE_INIT_FILE;
                        </literallayout>
                        </para></listitem>
                    <listitem><para>Create the Build Directory inside your
                        home directory and specifically name it
                        <filename>test-builds</filename>:
                        <literallayout class='monospaced'>
     $ cd $HOME
     $ source poky/&OE_INIT_FILE; test-builds
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        Provide a directory path and specifically name the
                        Build Directory.
                        Any intermediate folders in the pathname must exist.
                        This next example creates a Build Directory named
                        <filename>YP-&POKYVERSION;</filename>
                        in your home directory within the existing
                        directory <filename>mybuilds</filename>:
                        <literallayout class='monospaced'>
     $ cd $HOME
     $ source $HOME/poky/&OE_INIT_FILE; $HOME/mybuilds/YP-&POKYVERSION;
                        </literallayout>
                        </para></listitem>
                </itemizedlist>
                <note>
                    By default, the Build Directory contains
                    <link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>,
                    which is a temporary directory the build system uses for
                    its work.
                    <filename>TMPDIR</filename> cannot be under NFS.
                    Thus, by default, the Build Directory cannot be under NFS.
                    However, if you need the Build Directory to be under NFS,
                    you can set this up by setting <filename>TMPDIR</filename>
                    in your <filename>local.conf</filename> file
                    to use a local drive.
                    Doing so effectively separates <filename>TMPDIR</filename>
                    from <filename>TOPDIR</filename>, which is the Build
                    Directory.
                </note>
                </para></listitem>
            <listitem><para id='hardware-build-system-term'>
                <emphasis>Build Host:</emphasis>
                The system used to build images in a Yocto Project
                Development environment.
                The build system is sometimes referred to as the
                <firstterm>development host</firstterm>.
                </para></listitem>
            <listitem><para>
                <emphasis>Classes:</emphasis>
                Files that provide for logic encapsulation and inheritance so
                that commonly used patterns can be defined once and then
                easily used in multiple recipes.
                For reference information on the Yocto Project classes, see the
                "<link linkend='ref-classes'>Classes</link>" chapter.
                Class files end with the <filename>.bbclass</filename>
                filename extension.
                </para></listitem>
            <listitem><para>
                <emphasis>Configuration File:</emphasis>
                Files that hold global definitions of variables,
                user-defined variables, and hardware configuration
                information.
                These files tell the OpenEmbedded build system what to
                build and what to put into the image to support a
                particular platform.</para>

                <para>Configuration files end with a <filename>.conf</filename>
                filename extension.
                The <filename>conf/local.conf</filename> configuration file in
                the
                <link linkend='build-directory'>Build Directory</link>
                contains user-defined variables that affect every build.
                The <filename>meta-poky/conf/distro/poky.conf</filename>
                configuration file defines Yocto "distro" configuration
                variables used only when building with this policy.
                Machine configuration files, which
                are located throughout the
                <link linkend='source-directory'>Source Directory</link>, define
                variables for specific hardware and are only used when building
                for that target (e.g. the
                <filename>machine/beaglebone.conf</filename> configuration
                file defines variables for the Texas Instruments ARM Cortex-A8
                development board).
                </para></listitem>
            <listitem><para id='term-container-layer'>
                <emphasis>Container Layer:</emphasis>
                Layers that hold other layers.
                An example of a container layer is OpenEmbedded's
                <ulink url='https://github.com/openembedded/meta-openembedded'><filename>meta-openembedded</filename></ulink>
                layer.
                The <filename>meta-openembedded</filename> layer contains
                many <filename>meta-*</filename> layers.
                </para></listitem>
            <listitem><para id='cross-development-toolchain'>
                <emphasis>Cross-Development Toolchain:</emphasis>
                In general, a cross-development toolchain is a collection of
                software development tools and utilities that run on one
                architecture and allow you to develop software for a
                different, or targeted, architecture.
                These toolchains contain cross-compilers, linkers, and
                debuggers that are specific to the target architecture.</para>

                <para>The Yocto Project supports two different cross-development
                toolchains:
                <itemizedlist>
                    <listitem><para>
                        A toolchain only used by and within
                        BitBake when building an image for a target
                        architecture.
                        </para></listitem>
                    <listitem><para>A relocatable toolchain used outside of
                        BitBake by developers when developing applications
                        that will run on a targeted device.
                        </para></listitem>
                </itemizedlist></para>

                <para>Creation of these toolchains is simple and automated.
                For information on toolchain concepts as they apply to the
                Yocto Project, see the
                "<ulink url='&YOCTO_DOCS_OM_URL;#cross-development-toolchain-generation'>Cross-Development Toolchain Generation</ulink>"
                section in the Yocto Project Overview and Concepts Manual.
                You can also find more information on using the
                relocatable toolchain in the
                <ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Application Development and the Extensible Software Development Kit (eSDK)</ulink>
                manual.
                </para></listitem>
            <listitem><para>
                <emphasis>Extensible Software Development Kit (eSDK):</emphasis>
                A custom SDK for application developers.
                This eSDK allows developers to incorporate their library
                and programming changes back into the image to make
                their code available to other application developers.</para>

                <para>For information on the eSDK, see the
                <ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Application Development and the Extensible Software Development Kit (eSDK)</ulink>
                manual.
                </para></listitem>
            <listitem><para>
                <emphasis>Image:</emphasis>
                An image is an artifact of the BitBake build process given
                a collection of recipes and related Metadata.
                Images are the binary output that run on specific hardware or
                QEMU and are used for specific use-cases.
                For a list of the supported image types that the Yocto Project
                provides, see the
                "<link linkend='ref-images'>Images</link>"
                chapter.
                </para></listitem>
            <listitem><para>
                <emphasis>Layer:</emphasis>
                A collection of related recipes.
                Layers allow you to consolidate related metadata to
                customize your build.
                Layers also isolate information used when building
                for multiple architectures.
                Layers are hierarchical in their ability to override
                previous specifications.
                You can include any number of available layers from the
                Yocto Project and customize the build by adding your
                layers after them.
                You can search the Layer Index for layers used within
                Yocto Project.</para>

                <para>For introductory information on layers, see the
                "<ulink url='&YOCTO_DOCS_OM_URL;#the-yocto-project-layer-model'>The Yocto Project Layer Model</ulink>"
                section in the Yocto Project Overview and Concepts Manual.
                For more detailed information on layers, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>"
                section in the Yocto Project Development Tasks Manual.
                For a discussion specifically on BSP Layers, see the
                "<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-layers'>BSP Layers</ulink>"
                section in the Yocto Project Board Support Packages (BSP)
                Developer's Guide.
                </para></listitem>
            <listitem><para id='metadata'>
                <emphasis>Metadata:</emphasis>
                A key element of the Yocto Project is the Metadata that
                is used to construct a Linux distribution and is contained
                in the files that the
                <link linkend='build-system-term'>OpenEmbedded build system</link>
                parses when building an image.
                In general, Metadata includes recipes, configuration
                files, and other information that refers to the build
                instructions themselves, as well as the data used to
                control what things get built and the effects of the
                build.
                Metadata also includes commands and data used to
                indicate what versions of software are used, from
                where they are obtained, and changes or additions to the
                software itself (patches or auxiliary files) that
                are used to fix bugs or customize the software for use
                in a particular situation.
                OpenEmbedded-Core is an important set of validated
                metadata.</para>

                <para>In the context of the kernel ("kernel Metadata"), the
                term refers to the kernel config fragments and features
                contained in the
                <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/yocto-kernel-cache'><filename>yocto-kernel-cache</filename></ulink>
                Git repository.
                </para></listitem>
            <listitem><para id='oe-core'>
                <emphasis>OpenEmbedded-Core (OE-Core):</emphasis>
                OE-Core is metadata comprised of foundational recipes,
                classes, and associated files that are meant to be
                common among many different OpenEmbedded-derived systems,
                including the Yocto Project.
                OE-Core is a curated subset of an original repository
                developed by the OpenEmbedded community that has been
                pared down into a smaller, core set of continuously
                validated recipes.
                The result is a tightly controlled and an quality-assured
                core set of recipes.</para>

                <para>You can see the Metadata in the
                <filename>meta</filename> directory of the Yocto Project
                <ulink url='http://git.yoctoproject.org/cgit/cgit.cgi'>Source Repositories</ulink>.
                </para></listitem>
            <listitem><para id='build-system-term'>
                <emphasis>OpenEmbedded Build System:</emphasis>
                The build system specific to the Yocto Project.
                The OpenEmbedded build system is based on another project known
                as "Poky", which uses
                <link linkend='bitbake-term'>BitBake</link> as the task
                executor.
                Throughout the Yocto Project documentation set, the
                OpenEmbedded build system is sometimes referred to simply
                as "the build system".
                If other build systems, such as a host or target build system
                are referenced, the documentation clearly states the
                difference.
                <note>
                    For some historical information about Poky, see the
                    <link linkend='poky'>Poky</link> term.
                </note>
                </para></listitem>
            <listitem><para>
                <emphasis>Package:</emphasis>
                In the context of the Yocto Project, this term refers to a
                recipe's packaged output produced by BitBake (i.e. a
                "baked recipe").
                A package is generally the compiled binaries produced from the
                recipe's sources.
                You "bake" something by running it through BitBake.</para>

                <para>It is worth noting that the term "package" can,
                in general, have subtle meanings.
                For example, the packages referred to in the
                "<link linkend='required-packages-for-the-build-host'>Required Packages for the Build Host</link>"
                section are compiled binaries that, when installed, add
                functionality to your Linux distribution.</para>

                <para>Another point worth noting is that historically within
                the Yocto Project, recipes were referred to as packages - thus,
                the existence of several BitBake variables that are seemingly
                mis-named,
                (e.g. <link linkend='var-PR'><filename>PR</filename></link>,
                <link linkend='var-PV'><filename>PV</filename></link>, and
                <link linkend='var-PE'><filename>PE</filename></link>).
                </para></listitem>
            <listitem><para>
                <emphasis>Package Groups:</emphasis>
                Arbitrary groups of software Recipes.
                You use package groups to hold recipes that, when built,
                usually accomplish a single task.
                For example, a package group could contain the recipes for a
                company's proprietary or value-add software.
                Or, the package group could contain the recipes that enable
                graphics.
                A package group is really just another recipe.
                Because package group files are recipes, they end with the
                <filename>.bb</filename> filename extension.
                </para></listitem>
            <listitem><para id='poky'>
                <emphasis>Poky:</emphasis>
                Poky, which is pronounced <emphasis>Pock</emphasis>-ee,
                is a reference embedded distribution and a reference
                test configuration.
                Poky provides the following:
                <itemizedlist>
                    <listitem><para>
                        A base-level functional distro used to illustrate
                        how to customize a distribution.
                        </para></listitem>
                    <listitem><para>
                        A means by which to test the Yocto Project
                        components (i.e. Poky is used to validate
                        the Yocto Project).
                        </para></listitem>
                    <listitem><para>
                        A vehicle through which you can download
                        the Yocto Project.
                        </para></listitem>
                </itemizedlist>
                Poky is not a product level distro.
                Rather, it is a good starting point for customization.
                <note>
                    Poky began as an open-source
                    project initially developed by OpenedHand.
                    OpenedHand developed Poky from the existing
                    OpenEmbedded build system to create a commercially
                    supportable build system for embedded Linux.
                    After Intel Corporation acquired OpenedHand, the
                    poky project became the basis for the Yocto Project's
                    build system.
                </note>
                </para></listitem>
            <listitem><para>
                <emphasis>Recipe:</emphasis>
                A set of instructions for building packages.
                A recipe describes where you get source code, which patches
                to apply, how to configure the source, how to compile it and so on.
                Recipes also describe dependencies for libraries or for other
                recipes.
                Recipes represent the logical unit of execution, the software
                to build, the images to build, and use the
                <filename>.bb</filename> file extension.
                </para></listitem>
            <listitem><para id='reference-kit-term'>
                <emphasis>Reference Kit:</emphasis>
                A working example of a system, which includes a
                <link linkend='board-support-package-bsp-term'>BSP</link>
                as well as a
                <link linkend='hardware-build-system-term'>build host</link>
                and other components, that can work on specific hardware.
                </para></listitem>
            <listitem>
                <para id='source-directory'>
                <emphasis>Source Directory:</emphasis>
                This term refers to the directory structure created as a result
                of creating a local copy of the <filename>poky</filename> Git
                repository <filename>git://git.yoctoproject.org/poky</filename>
                or expanding a released <filename>poky</filename> tarball.
                <note>
                    Creating a local copy of the <filename>poky</filename>
                    Git repository is the recommended method for setting up
                    your Source Directory.
                </note>
                Sometimes you might hear the term "poky directory" used to refer
                to this directory structure.
                <note>
                    The OpenEmbedded build system does not support file or
                    directory names that contain spaces.
                    Be sure that the Source Directory you use does not contain
                    these types of names.
                </note></para>

                <para>The Source Directory contains BitBake, Documentation,
                Metadata and other files that all support the Yocto Project.
                Consequently, you must have the Source Directory in place on
                your development system in order to do any development using
                the Yocto Project.</para>

                <para>When you create a local copy of the Git repository, you
                can name the repository anything you like.
                Throughout much of the documentation, "poky"
                is used as the name of the top-level folder of the local copy of
                the poky Git repository.
                So, for example, cloning the <filename>poky</filename> Git
                repository results in a local Git repository whose top-level
                folder is also named "poky".</para>

                <para>While it is not recommended that you use tarball expansion
                to set up the Source Directory, if you do, the top-level
                directory name of the Source Directory is derived from the
                Yocto Project release tarball.
                For example, downloading and unpacking
                <filename>&YOCTO_POKY_TARBALL;</filename> results in a
                Source Directory whose root folder is named
                <filename>&YOCTO_POKY;</filename>.</para>

                <para>It is important to understand the differences between the
                Source Directory created by unpacking a released tarball as
                compared to cloning
                <filename>git://git.yoctoproject.org/poky</filename>.
                When you unpack a tarball, you have an exact copy of the files
                based on the time of release - a fixed release point.
                Any changes you make to your local files in the Source Directory
                are on top of the release and will remain local only.
                On the other hand, when you clone the <filename>poky</filename>
                Git repository, you have an active development repository with
                access to the upstream repository's branches and tags.
                In this case, any local changes you make to the local
                Source Directory can be later applied to active development
                branches of the upstream <filename>poky</filename> Git
                repository.</para>

                <para>For more information on concepts related to Git
                repositories, branches, and tags, see the
                "<ulink url='&YOCTO_DOCS_OM_URL;#repositories-tags-and-branches'>Repositories, Tags, and Branches</ulink>"
                section in the Yocto Project Overview and Concepts Manual.
                </para></listitem>
            <listitem><para><emphasis>Task:</emphasis>
                A unit of execution for BitBake (e.g.
                <link linkend='ref-tasks-compile'><filename>do_compile</filename></link>,
                <link linkend='ref-tasks-fetch'><filename>do_fetch</filename></link>,
                <link linkend='ref-tasks-patch'><filename>do_patch</filename></link>,
                and so forth).
                </para></listitem>
            <listitem><para id='toaster-term'><emphasis>Toaster:</emphasis>
                A web interface to the Yocto Project's
                <link linkend='build-system-term'>OpenEmbedded Build System</link>.
                The interface enables you to configure and run your builds.
                Information about builds is collected and stored in a database.
                For information on Toaster, see the
                <ulink url='&YOCTO_DOCS_TOAST_URL;'>Toaster User Manual</ulink>.
                </para></listitem>
            <listitem><para>
                <emphasis>Upstream:</emphasis>
                A reference to source code or repositories
                that are not local to the development system but located in a
                master area that is controlled by the maintainer of the source
                code.
                For example, in order for a developer to work on a particular
                piece of code, they need to first get a copy of it from an
                "upstream" source.
                </para></listitem>
        </itemizedlist>
    </para>

</chapter>
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