path: root/documentation/ref-manual/ref-development-environment.xml

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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<chapter id='ref-development-environment'>
<title>The Yocto Project Development Environment</title>

<para>
    This chapter takes a look at the Yocto Project development
    environment and also provides a detailed look at what goes on during
    development in that environment.
    The chapter provides Yocto Project Development environment concepts that
    help you understand how work is accomplished in an open source environment,
    which is very different as compared to work accomplished in a closed,
    proprietary environment.
    This chapter specifically addresses open source philosophy, using the
    Yocto Project in a team environment, source repositories, Yocto Project
    terms, licensing, the open source distributed version control system Git,
    workflows, bug tracking, and how to submit changes.
</para>

<section id='open-source-philosophy'>
    <title>Open Source Philosophy</title>

    <para>
        Open source philosophy is characterized by software development
        directed by peer production and collaboration through an active
        community of developers.
        Contrast this to the more standard centralized development models
        used by commercial software companies where a finite set of developers
        produces a product for sale using a defined set of procedures that
        ultimately result in an end product whose architecture and source
        material are closed to the public.
    </para>

    <para>
        Open source projects conceptually have differing concurrent agendas,
        approaches, and production.
        These facets of the development process can come from anyone in the
        public (community) that has a stake in the software project.
        The open source environment contains new copyright, licensing, domain,
        and consumer issues that differ from the more traditional development
        environment.
        In an open source environment, the end product, source material,
        and documentation are all available to the public at no cost.
    </para>

    <para>
        A benchmark example of an open source project is the Linux kernel,
        which was initially conceived and created by Finnish computer science
        student Linus Torvalds in 1991.
        Conversely, a good example of a non-open source project is the
        <trademark class='registered'>Windows</trademark> family of operating
        systems developed by
        <trademark class='registered'>Microsoft</trademark> Corporation.
    </para>

    <para>
        Wikipedia has a good historical description of the Open Source
        Philosophy
        <ulink url='http://en.wikipedia.org/wiki/Open_source'>here</ulink>.
        You can also find helpful information on how to participate in the
        Linux Community
        <ulink url='http://ldn.linuxfoundation.org/book/how-participate-linux-community'>here</ulink>.
    </para>
</section>

<section id='workflows'>
    <title>Workflows</title>

    <para>
        This section provides workflow concepts using the Yocto Project and
        Git.
        In particular, the information covers basic practices that describe
        roles and actions in a collaborative development environment.
        <note>
            If you are familiar with this type of development environment, you
            might not want to read this section.
        </note>
    </para>

    <para>
        The Yocto Project files are maintained using Git in "master"
        branches whose Git histories track every change and whose structures
        provides branches for all diverging functionality.
        Although there is no need to use Git, many open source projects do so.
    <para>

    </para>
        For the Yocto Project, a key individual called the "maintainer" is
        responsible for the "master" branch of a given Git repository.
        The "master" branch is the “upstream” repository from which final or
        most recent builds of the project occur.
        The maintainer is responsible for accepting changes from other
        developers and for organizing the underlying branch structure to
        reflect release strategies and so forth.
        <note>For information on finding out who is responsible for (maintains)
            a particular area of code, see the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>Submitting a Change to the Yocto Project</ulink>"
            section of the Yocto Project Development Manual.
        </note>
    </para>

    <para>
        The Yocto Project <filename>poky</filename> Git repository also has an
        upstream contribution Git repository named
        <filename>poky-contrib</filename>.
        You can see all the branches in this repository using the web interface
        of the
        <ulink url='&YOCTO_GIT_URL;'>Source Repositories</ulink> organized
        within the "Poky Support" area.
        These branches temporarily hold changes to the project that have been
        submitted or committed by the Yocto Project development team and by
        community members who contribute to the project.
        The maintainer determines if the changes are qualified to be moved
        from the "contrib" branches into the "master" branch of the Git
        repository.
    </para>

    <para>
        Developers (including contributing community members) create and
        maintain cloned repositories of the upstream "master" branch.
        The cloned repositories are local to their development platforms and
        are used to develop changes.
        When a developer is satisfied with a particular feature or change,
        they "push" the changes to the appropriate "contrib" repository.
    </para>

    <para>
        Developers are responsible for keeping their local repository
        up-to-date with "master".
        They are also responsible for straightening out any conflicts that
        might arise within files that are being worked on simultaneously by
        more than one person.
        All this work is done locally on the developer’s machine before
        anything is pushed to a "contrib" area and examined at the maintainer’s
        level.
    </para>

    <para>
        A somewhat formal method exists by which developers commit changes
        and push them into the "contrib" area and subsequently request that
        the maintainer include them into "master".
        This process is called “submitting a patch” or "submitting a change."
        For information on submitting patches and changes, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>Submitting a Change to the Yocto Project</ulink>"
        section in the Yocto Project Development Manual.
    </para>

    <para>
        To summarize the development workflow:  a single point of entry
        exists for changes into the project’s "master" branch of the
        Git repository, which is controlled by the project’s maintainer.
        And, a set of developers exist who independently develop, test, and
        submit changes to "contrib" areas for the maintainer to examine.
        The maintainer then chooses which changes are going to become a
        permanent part of the project.
    </para>

    <para>
        <imagedata fileref="figures/git-workflow.png" width="6in" depth="3in" align="left" scalefit="1" />
    </para>

    <para>
        While each development environment is unique, there are some best
        practices or methods that help development run smoothly.
        The following list describes some of these practices.
        For more information about Git workflows, see the workflow topics in
        the
        <ulink url='http://book.git-scm.com'>Git Community Book</ulink>.
        <itemizedlist>
            <listitem><para>
                <emphasis>Make Small Changes:</emphasis>
                It is best to keep the changes you commit small as compared to
                bundling many disparate changes into a single commit.
                This practice not only keeps things manageable but also allows
                the maintainer to more easily include or refuse changes.</para>

                <para>It is also good practice to leave the repository in a
                state that allows you to still successfully build your project.
                In other words, do not commit half of a feature,
                then add the other half as a separate, later commit.
                Each commit should take you from one buildable project state
                to another buildable state.
                </para></listitem>
            <listitem><para>
                <emphasis>Use Branches Liberally:</emphasis>
                It is very easy to create, use, and delete local branches in
                your working Git repository.
                You can name these branches anything you like.
                It is helpful to give them names associated with the particular
                feature or change on which you are working.
                Once you are done with a feature or change and have merged it
                into your local master branch, simply discard the temporary
                branch.
                </para></listitem>
            <listitem><para>
                <emphasis>Merge Changes:</emphasis>
                The <filename>git merge</filename> command allows you to take
                the changes from one branch and fold them into another branch.
                This process is especially helpful when more than a single
                developer might be working on different parts of the same
                feature.
                Merging changes also automatically identifies any collisions
                or "conflicts" that might happen as a result of the same lines
                of code being altered by two different developers.
                </para></listitem>
            <listitem><para>
                <emphasis>Manage Branches:</emphasis>
                Because branches are easy to use, you should use a system
                where branches indicate varying levels of code readiness.
                For example, you can have a "work" branch to develop in, a
                "test" branch where the code or change is tested, a "stage"
                branch where changes are ready to be committed, and so forth.
                As your project develops, you can merge code across the
                branches to reflect ever-increasing stable states of the
                development.
                </para></listitem>
            <listitem><para>
                <emphasis>Use Push and Pull:</emphasis>
                The push-pull workflow is based on the concept of developers
                "pushing" local commits to a remote repository, which is
                usually a contribution repository.
                This workflow is also based on developers "pulling" known
                states of the project down into their local development
                repositories.
                The workflow easily allows you to pull changes submitted by
                other developers from the upstream repository into your
                work area ensuring that you have the most recent software
                on which to develop.
                The Yocto Project has two scripts named
                <filename>create-pull-request</filename> and
                <filename>send-pull-request</filename> that ship with the
                release to facilitate this workflow.
                You can find these scripts in the <filename>scripts</filename>
                folder of the
                <link linkend='source-directory'>Source Directory</link>.
                For information on how to use these scripts, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#pushing-a-change-upstream'>Using Scripts to Push a Change Upstream and Request a Pull</ulink>"
                section in the Yocto Project Development Manual.
                </para></listitem>
            <listitem><para>
                <emphasis>Patch Workflow:</emphasis>
                This workflow allows you to notify the maintainer through an
                email that you have a change (or patch) you would like
                considered for the "master" branch of the Git repository.
                To send this type of change, you format the patch and then
                send the email using the Git commands
                <filename>git format-patch</filename> and
                <filename>git send-email</filename>.
                For information on how to use these scripts, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>Submitting a Change to the Yocto Project</ulink>"
                section in the Yocto Project Development Manual.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='git'>
    <title>Git</title>

    <para>
        The Yocto Project makes extensive use of Git, which is a
        free, open source distributed version control system.
        Git supports distributed development, non-linear development,
        and can handle large projects.
        It is best that you have some fundamental understanding
        of how Git tracks projects and how to work with Git if
        you are going to use the Yocto Project for development.
        This section provides a quick overview of how Git works and
        provides you with a summary of some essential Git commands.
        <note><title>Notes</title>
            <itemizedlist>
                <listitem><para>
                    For more information on Git, see
                    <ulink url='http://git-scm.com/documentation'></ulink>.
                    </para></listitem>
                <listitem><para>
                    If you need to download Git, it is recommended that you add
                    Git to your system through your distribution's "software
                    store" (e.g. for Ubuntu, use the Ubuntu Software feature).
                    For the Git download page, see
                    <ulink url='http://git-scm.com/download'></ulink>.
                    </para></listitem>
                <listitem><para>
                    For examples beyond the limited few in this section on how
                    to use Git with the Yocto Project, see the
                    "<ulink url='&YOCTO_DOCS_DEV_URL;#working-with-yocto-project-source-files'>Working With Yocto Project Source Files</ulink>"
                    section in the Yocto Project Development Manual.
                    </para></listitem>
            </itemizedlist>
        </note>
    </para>

    <section id='repositories-tags-and-branches'>
        <title>Repositories, Tags, and Branches</title>

        <para>
            As mentioned briefly in the previous section and also in the
            "<link linkend='workflows'>Workflows</link>" section,
            the Yocto Project maintains source repositories at
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi'></ulink>.
            If you look at this web-interface of the repositories, each item
            is a separate Git repository.
        </para>

        <para>
            Git repositories use branching techniques that track content
            change (not files) within a project (e.g. a new feature or updated
            documentation).
            Creating a tree-like structure based on project divergence allows
            for excellent historical information over the life of a project.
            This methodology also allows for an environment from which you can
            do lots of local experimentation on projects as you develop
            changes or new features.
        </para>

        <para>
            A Git repository represents all development efforts for a given
            project.
            For example, the Git repository <filename>poky</filename> contains
            all changes and developments for Poky over the course of its
            entire life.
            That means that all changes that make up all releases are captured.
            The repository maintains a complete history of changes.
        </para>

        <para>
            You can create a local copy of any repository by "cloning" it
            with the <filename>git clone</filename> command.
            When you clone a Git repository, you end up with an identical
            copy of the repository on your development system.
            Once you have a local copy of a repository, you can take steps to
            develop locally.
            For examples on how to clone Git repositories, see the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#working-with-yocto-project-source-files'>Working With Yocto Project Source Files</ulink>"
            section in the Yocto Project Development Manual.
        </para>

        <para>
            It is important to understand that Git tracks content change and
            not files.
            Git uses "branches" to organize different development efforts.
            For example, the <filename>poky</filename> repository has
            several branches that include the current "&DISTRO_NAME_NO_CAP;"
            branch, the "master" branch, and many branches for past
            Yocto Project releases.
            You can see all the branches by going to
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/'></ulink> and
            clicking on the
            <filename><ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/refs/heads'>[...]</ulink></filename>
            link beneath the "Branch" heading.
        </para>

        <para>
            Each of these branches represents a specific area of development.
            The "master" branch represents the current or most recent
            development.
            All other branches represent offshoots of the "master" branch.
        </para>

        <para>
            When you create a local copy of a Git repository, the copy has
            the same set of branches as the original.
            This means you can use Git to create a local working area
            (also called a branch) that tracks a specific development branch
            from the upstream source Git repository.
            in other words, you can define your local Git environment to
            work on any development branch in the repository.
            To help illustrate, consider the following example Git commands:
            <literallayout class='monospaced'>
     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git checkout -b &DISTRO_NAME_NO_CAP; origin/&DISTRO_NAME_NO_CAP;
            </literallayout>
            In the previous example after moving to the home directory, the
            <filename>git clone</filename> command creates a
            local copy of the upstream <filename>poky</filename> Git repository.
            By default, Git checks out the "master" branch for your work.
            After changing the working directory to the new local repository
            (i.e. <filename>poky</filename>), the
            <filename>git checkout</filename> command creates
            and checks out a local branch named "&DISTRO_NAME_NO_CAP;", which
            tracks the upstream "origin/&DISTRO_NAME_NO_CAP;" branch.
            Changes you make while in this branch would ultimately affect
            the upstream "&DISTRO_NAME_NO_CAP;" branch of the
            <filename>poky</filename> repository.
        </para>

        <para>
            It is important to understand that when you create and checkout a
            local working branch based on a branch name,
            your local environment matches the "tip" of that particular
            development branch at the time you created your local branch,
            which could be different from the files in the "master" branch
            of the upstream repository.
            In other words, creating and checking out a local branch based on
            the "&DISTRO_NAME_NO_CAP;" branch name is not the same as
            cloning and checking out the "master" branch if the repository.
            Keep reading to see how you create a local snapshot of a Yocto
            Project Release.
        </para>

        <para>
            Git uses "tags" to mark specific changes in a repository.
            Typically, a tag is used to mark a special point such as the final
            change before a project is released.
            You can see the tags used with the <filename>poky</filename> Git
            repository by going to
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/'></ulink> and
            clicking on the
            <filename><ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/refs/tags'>[...]</ulink></filename>
            link beneath the "Tag" heading.
        </para>

        <para>
            Some key tags for the <filename>poky</filename> are
            <filename>jethro-14.0.3</filename>,
            <filename>morty-16.0.1</filename>,
            <filename>pyro-17.0.0</filename>, and
            <filename>&DISTRO_NAME_NO_CAP;-&POKYVERSION;</filename>.
            These tags represent Yocto Project releases.
        </para>

        <para>
            When you create a local copy of the Git repository, you also
            have access to all the tags in the upstream repository.
            Similar to branches, you can create and checkout a local working
            Git branch based on a tag name.
            When you do this, you get a snapshot of the Git repository that
            reflects the state of the files when the change was made associated
            with that tag.
            The most common use is to checkout a working branch that matches
            a specific Yocto Project release.
            Here is an example:
            <literallayout class='monospaced'>
     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git fetch --all --tags --prune
     $ git checkout tags/pyro-17.0.0 -b my-pyro-17.0.0
            </literallayout>
            In this example, the name of the top-level directory of your
            local Yocto Project repository is <filename>poky</filename>.
            After moving to the <filename>poky</filename> directory, the
            <filename>git fetch</filename> command makes all the upstream
            tags available locally in your repository.
            Finally, the <filename>git checkout</filename> command
            creates and checks out a branch named "my-pyro-17.0.0" that is
            based on the specific change upstream in the repository
            associated with the "pyro-17.0.0" tag.
            The files in your repository now exactly match that particular
            Yocto Project release as it is tagged in the upstream Git
            repository.
            It is important to understand that when you create and
            checkout a local working branch based on a tag, your environment
            matches a specific point in time and not the entire development
            branch (i.e. the "tip" of the branch).
        </para>
    </section>

    <section id='basic-commands'>
        <title>Basic Commands</title>

        <para>
            Git has an extensive set of commands that lets you manage changes
            and perform collaboration over the life of a project.
            Conveniently though, you can manage with a small set of basic
            operations and workflows once you understand the basic
            philosophy behind Git.
            You do not have to be an expert in Git to be functional.
            A good place to look for instruction on a minimal set of Git
            commands is
            <ulink url='http://git-scm.com/documentation'>here</ulink>.
        </para>

        <para>
            If you do not know much about Git, you should educate
            yourself by visiting the links previously mentioned.
        </para>

        <para>
            The following list of Git commands briefly describes some basic
            Git operations as a way to get started.
            As with any set of commands, this list (in most cases) simply shows
            the base command and omits the many arguments they support.
            See the Git documentation for complete descriptions and strategies
            on how to use these commands:
            <itemizedlist>
                <listitem><para>
                    <emphasis><filename>git init</filename>:</emphasis>
                    Initializes an empty Git repository.
                    You cannot use Git commands unless you have a
                    <filename>.git</filename> repository.
                    </para></listitem>
                <listitem><para id='git-commands-clone'>
                    <emphasis><filename>git clone</filename>:</emphasis>
                    Creates a local clone of a Git repository that is on
                    equal footing with a fellow developer’s Git repository
                    or an upstream repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git add</filename>:</emphasis>
                    Locally stages updated file contents to the index that
                    Git uses to track changes.
                    You must stage all files that have changed before you
                    can commit them.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git commit</filename>:</emphasis>
                    Creates a local "commit" that documents the changes you
                    made.
                    Only changes that have been staged can be committed.
                    Commits are used for historical purposes, for determining
                    if a maintainer of a project will allow the change,
                    and for ultimately pushing the change from your local
                    Git repository into the project’s upstream repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git status</filename>:</emphasis>
                    Reports any modified files that possibly need to be
                    staged and gives you a status of where you stand regarding
                    local commits as compared to the upstream repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git checkout</filename> <replaceable>branch-name</replaceable>:</emphasis>
                    Changes your working branch.
                    This command is analogous to "cd".
                    </para></listitem>
                <listitem><para><emphasis><filename>git checkout –b</filename> <replaceable>working-branch</replaceable>:</emphasis>
                    Creates and checks out a working branch on your local
                    machine that you can use to isolate your work.
                    It is a good idea to use local branches when adding
                    specific features or changes.
                    Using isolated branches facilitates easy removal of
                    changes if they do not work out.
                    </para></listitem>
                <listitem><para><emphasis><filename>git branch</filename>:</emphasis>
                    Displays the existing local branches associated with your
                    local repository.
                    The branch that you have currently checked out is noted
                    with an asterisk character.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git branch -D</filename> <replaceable>branch-name</replaceable>:</emphasis>
                    Deletes an existing local branch.
                    You need to be in a local branch other than the one you
                    are deleting in order to delete
                    <replaceable>branch-name</replaceable>.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git pull</filename>:</emphasis>
                    Retrieves information from an upstream Git repository
                    and places it in your local Git repository.
                    You use this command to make sure you are synchronized with
                    the repository from which you are basing changes
                    (.e.g. the "master" branch).
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git push</filename>:</emphasis>
                    Sends all your committed local changes to the upstream Git
                    repository that your local repository is tracking
                    (e.g. a contribution repository).
                    The maintainer of the project draws from these repositories
                    to merge changes (commits) into the appropriate branch
                    of project's upstream repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git merge</filename>:</emphasis>
                    Combines or adds changes from one
                    local branch of your repository with another branch.
                    When you create a local Git repository, the default branch
                    is named "master".
                    A typical workflow is to create a temporary branch that is
                    based off "master" that you would use for isolated work.
                    You would make your changes in that isolated branch,
                    stage and commit them locally, switch to the "master"
                    branch, and then use the <filename>git merge</filename>
                    command to apply the changes from your isolated branch
                    into the currently checked out branch (e.g. "master").
                    After the merge is complete and if you are done with
                    working in that isolated branch, you can safely delete
                    the isolated branch.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git cherry-pick</filename>:</emphasis>
                    Choose and apply specific commits from one branch
                    into another branch.
                    There are times when you might not be able to merge
                    all the changes in one branch with
                    another but need to pick out certain ones.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>gitk</filename>:</emphasis>
                    Provides a GUI view of the branches and changes in your
                    local Git repository.
                    This command is a good way to graphically see where things
                    have diverged in your local repository.
                    <note>
                        You need to install the <filename>gitk</filename>
                        package on your development system to use this
                        command.
                    </note>
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git log</filename>:</emphasis>
                    Reports a history of your commits to the repository.
                    This report lists all commits regardless of whether you
                    have pushed them upstream or not.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git diff</filename>:</emphasis>
                    Displays line-by-line differences between a local
                    working file and the same file as understood by Git.
                    This command is useful to see what you have changed
                    in any given file.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>

<section id='yocto-project-repositories'>
    <title>Yocto Project Source Repositories</title>

    <para>
        The Yocto Project team maintains complete source repositories for all
        Yocto Project files at
        <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi'></ulink>.
        This web-based source code browser is organized into categories by
        function such as IDE Plugins, Matchbox, Poky, Yocto Linux Kernel, and
        so forth.
        From the interface, you can click on any particular item in the "Name"
        column and see the URL at the bottom of the page that you need to clone
        a Git repository for that particular item.
        Having a local Git repository of the
        <link linkend='source-directory'>Source Directory</link>, which is
        usually named "poky", allows
        you to make changes, contribute to the history, and ultimately enhance
        the Yocto Project's tools, Board Support Packages, and so forth.
    </para>

    <para>
        For any supported release of Yocto Project, you can also go to the
        <ulink url='&YOCTO_HOME_URL;'>Yocto Project Website</ulink> and
        select the "Downloads" tab and get a released tarball of the
        <filename>poky</filename> repository or any supported BSP tarballs.
        Unpacking these tarballs gives you a snapshot of the released
        files.
        <note><title>Notes</title>
            <itemizedlist>
                <listitem><para>
                    The recommended method for setting up the Yocto Project
                    <link linkend='source-directory'>Source Directory</link>
                    and the files for supported BSPs
                    (e.g., <filename>meta-intel</filename>) is to use
                    <link linkend='git'>Git</link> to create a local copy of
                    the upstream repositories.
                    </para></listitem>
                <listitem><para>
                    Be sure to always work in matching branches for both
                    the selected BSP repository and the
                    <link linkend='source-directory'>Source Directory</link>
                    (i.e. <filename>poky</filename>) repository.
                    For example, if you have checked out the "master" branch
                    of <filename>poky</filename> and you are going to use
                    <filename>meta-intel</filename>, be sure to checkout the
                    "master" branch of <filename>meta-intel</filename>.
                    </para></listitem>
            </itemizedlist>
        </note>
    </para>

    <para>
        In summary, here is where you can get the project files needed for
        development:
        <itemizedlist>
            <listitem><para id='source-repositories'>
                <emphasis>
                <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi'>Source Repositories:</ulink>
                </emphasis>
                This area contains IDE Plugins, Matchbox, Poky, Poky Support,
                Tools, Yocto Linux Kernel, and Yocto Metadata Layers.
                You can create local copies of Git repositories for each of
                these areas.</para>

                <para>
                <imagedata fileref="figures/source-repos.png" align="center" width="6in" depth="4in" />
                For steps on how to view and access these upstream Git
                repositories, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#accessing-source-repositories'>Accessing Source Repositories</ulink>"
                Section in the Yocto Project Development Manual.
                </para></listitem>
            <listitem><para><anchor id='index-downloads' />
                <emphasis>
                <ulink url='&YOCTO_DL_URL;/releases/'>Index of /releases:</ulink>
                </emphasis>
                This is an index of releases such as
                the <trademark class='trade'>Eclipse</trademark>
                Yocto Plug-in, miscellaneous support, Poky, Pseudo, installers
                for cross-development toolchains, and all released versions of
                Yocto Project in the form of images or tarballs.
                Downloading and extracting these files does not produce a local
                copy of the Git repository but rather a snapshot of a
                particular release or image.</para>

                <para>
                <imagedata fileref="figures/index-downloads.png" align="center" width="6in" depth="3.5in" />
                For steps on how to view and access these files, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#accessing-index-of-releases'>Accessing Index of Releases</ulink>"
                section in the Yocto Project Development Manual.
                </para></listitem>
            <listitem><para id='downloads-page'>
                <emphasis>"Downloads" page for the
                <ulink url='&YOCTO_HOME_URL;'>Yocto Project Website</ulink>:
                </emphasis></para>

                <para role="writernotes">This section will change due to
                reworking of the YP Website.</para>

                <para>The Yocto Project website includes a "Downloads" tab
                that allows you to download any Yocto Project
                release and Board Support Package (BSP) in tarball form.
                The tarballs are similar to those found in the
                <ulink url='&YOCTO_DL_URL;/releases/'>Index of /releases:</ulink> area.</para>

                <para>
                <imagedata fileref="figures/yp-download.png" align="center" width="6in" depth="4in" />
                For steps on how to use the "Downloads" page, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#using-the-downloads-page'>Using the Downloads Page</ulink>"
                section in the Yocto Project Development Manual.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='licensing'>
    <title>Licensing</title>

    <para>
        Because open source projects are open to the public, they have
        different licensing structures in place.
        License evolution for both Open Source and Free Software has an
        interesting history.
        If you are interested in this history, you can find basic information
        here:
        <itemizedlist>
            <listitem><para>
                <ulink url='http://en.wikipedia.org/wiki/Open-source_license'>Open source license history</ulink>
                </para></listitem>
            <listitem><para>
                <ulink url='http://en.wikipedia.org/wiki/Free_software_license'>Free software license history</ulink>
                </para></listitem>
        </itemizedlist>
    </para>

    <para>
        In general, the Yocto Project is broadly licensed under the
        Massachusetts Institute of Technology (MIT) License.
        MIT licensing permits the reuse of software within proprietary
        software as long as the license is distributed with that software.
        MIT is also compatible with the GNU General Public License (GPL).
        Patches to the Yocto Project follow the upstream licensing scheme.
        You can find information on the MIT license
        <ulink url='http://www.opensource.org/licenses/mit-license.php'>here</ulink>.
        You can find information on the GNU GPL
        <ulink url='http://www.opensource.org/licenses/LGPL-3.0'>here</ulink>.
    </para>

    <para>
        When you build an image using the Yocto Project, the build process
        uses a known list of licenses to ensure compliance.
        You can find this list in the
        <link linkend='source-directory'>Source Directory</link> at
        <filename>meta/files/common-licenses</filename>.
        Once the build completes, the list of all licenses found and used
        during that build are kept in the
        <link linkend='build-directory'>Build Directory</link>
        at <filename>tmp/deploy/licenses</filename>.
    </para>

    <para>
        If a module requires a license that is not in the base list, the
        build process generates a warning during the build.
        These tools make it easier for a developer to be certain of the
        licenses with which their shipped products must comply.
        However, even with these tools it is still up to the developer to
        resolve potential licensing issues.
    </para>

    <para>
        The base list of licenses used by the build process is a combination
        of the Software Package Data Exchange (SPDX) list and the Open
        Source Initiative (OSI) projects.
        <ulink url='http://spdx.org'>SPDX Group</ulink> is a working group of
        the Linux Foundation that maintains a specification for a standard
        format for communicating the components, licenses, and copyrights
        associated with a software package.
        <ulink url='http://opensource.org'>OSI</ulink> is a corporation
        dedicated to the Open Source Definition and the effort for reviewing
        and approving licenses that conform to the Open Source Definition
        (OSD).
    </para>

    <para>
        You can find a list of the combined SPDX and OSI licenses that the
        Yocto Project uses in the
        <filename>meta/files/common-licenses</filename> directory in your
        <link linkend='source-directory'>Source Directory</link>.
    </para>

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

<section id="development-concepts">
    <title>Development Concepts</title>

    <para>
        This section takes a more detailed look inside the development
        process.
        The following diagram represents development at a high level.
        The remainder of this chapter expands on the fundamental input, output,
        process, and
        <ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>) blocks
        that make up development in the Yocto Project environment.
    </para>

    <para id='general-yocto-environment-figure'>
        <imagedata fileref="figures/yocto-environment-ref.png" align="center" width="8in" depth="4.25in" />
    </para>

    <para>
        In general, development consists of several functional areas:
        <itemizedlist>
            <listitem><para><emphasis>User Configuration:</emphasis>
                Metadata you can use to control the build process.
                </para></listitem>
            <listitem><para><emphasis>Metadata Layers:</emphasis>
                Various layers that provide software, machine, and
                distro Metadata.</para></listitem>
            <listitem><para><emphasis>Source Files:</emphasis>
                Upstream releases, local projects, and SCMs.</para></listitem>
            <listitem><para><emphasis>Build System:</emphasis>
                Processes under the control of
                <link linkend='bitbake-term'>BitBake</link>.
                This block expands on how BitBake fetches source, applies
                patches, completes compilation, analyzes output for package
                generation, creates and tests packages, generates images, and
                generates cross-development tools.</para></listitem>
            <listitem><para><emphasis>Package Feeds:</emphasis>
                Directories containing output packages (RPM, DEB or IPK),
                which are subsequently used in the construction of an image or
                SDK, produced by the build system.
                These feeds can also be copied and shared using a web server or
                other means to facilitate extending or updating existing
                images on devices at runtime if runtime package management is
                enabled.</para></listitem>
            <listitem><para><emphasis>Images:</emphasis>
                Images produced by the development process.
                </para></listitem>
            <listitem><para><emphasis>Application Development SDK:</emphasis>
                Cross-development tools that are produced along with an image
                or separately with BitBake.</para></listitem>
        </itemizedlist>
    </para>

    <section id="user-configuration">
        <title>User Configuration</title>

        <para>
            User configuration helps define the build.
            Through user configuration, you can tell BitBake the
            target architecture for which you are building the image,
            where to store downloaded source, and other build properties.
        </para>

        <para>
            The following figure shows an expanded representation of the
            "User Configuration" box of the
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>:
        </para>

        <para>
            <imagedata fileref="figures/user-configuration.png" align="center" />
        </para>

        <para>
            BitBake needs some basic configuration files in order to complete
            a build.
            These files are <filename>*.conf</filename> files.
            The minimally necessary ones reside as example files in the
            <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>.
            For simplicity, this section refers to the Source Directory as
            the "Poky Directory."
        </para>

        <para>
            When you clone the <filename>poky</filename> Git repository or you
            download and unpack a Yocto Project release, you can set up the
            Source Directory to be named anything you want.
            For this discussion, the cloned repository uses the default
            name <filename>poky</filename>.
            <note>
                The Poky repository is primarily an aggregation of existing
                repositories.
                It is not a canonical upstream source.
            </note>
        </para>

        <para>
            The <filename>meta-poky</filename> layer inside Poky contains
            a <filename>conf</filename> directory that has example
            configuration files.
            These example files are used as a basis for creating actual
            configuration files when you source the build environment
            script
            (i.e.
            <link linkend='structure-core-script'><filename>&OE_INIT_FILE;</filename></link>
            or
            <link linkend='structure-memres-core-script'><filename>oe-init-build-env-memres</filename></link>).
        </para>

        <para>
            Sourcing the build environment script creates a
            <link linkend='build-directory'>Build Directory</link>
            if one does not already exist.
            BitBake uses the Build Directory for all its work during builds.
            The Build Directory has a <filename>conf</filename> directory that
            contains default versions of your <filename>local.conf</filename>
            and <filename>bblayers.conf</filename> configuration files.
            These default configuration files are created only if versions
            do not already exist in the Build Directory at the time you
            source the build environment setup script.
        </para>

        <para>
            Because the Poky repository is fundamentally an aggregation of
            existing repositories, some users might be familiar with running
            the <filename>&OE_INIT_FILE;</filename> or
            <filename>oe-init-build-env-memres</filename> script in the context
            of separate OpenEmbedded-Core and BitBake repositories rather than a
            single Poky repository.
            This discussion assumes the script is executed from within a cloned
            or unpacked version of Poky.
        </para>

        <para>
            Depending on where the script is sourced, different sub-scripts
            are called to set up the Build Directory (Yocto or OpenEmbedded).
            Specifically, the script
            <filename>scripts/oe-setup-builddir</filename> inside the
            poky directory sets up the Build Directory and seeds the directory
            (if necessary) with configuration files appropriate for the
            Yocto Project development environment.
            <note>
                The <filename>scripts/oe-setup-builddir</filename> script
                uses the <filename>$TEMPLATECONF</filename> variable to
                determine which sample configuration files to locate.
            </note>
        </para>

        <para>
            The <filename>local.conf</filename> file provides many
            basic variables that define a build environment.
            Here is a list of a few.
            To see the default configurations in a <filename>local.conf</filename>
            file created by the build environment script, see the
            <filename>local.conf.sample</filename> in the
            <filename>meta-poky</filename> layer:
            <itemizedlist>
                <listitem><para><emphasis>Parallelism Options:</emphasis>
                    Controlled by the
                    <link linkend='var-BB_NUMBER_THREADS'><filename>BB_NUMBER_THREADS</filename></link>,
                    <link linkend='var-PARALLEL_MAKE'><filename>PARALLEL_MAKE</filename></link>,
                    and
                    <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_NUMBER_PARSE_THREADS'><filename>BB_NUMBER_PARSE_THREADS</filename></ulink>
                    variables.</para></listitem>
                <listitem><para><emphasis>Target Machine Selection:</emphasis>
                    Controlled by the
                    <link linkend='var-MACHINE'><filename>MACHINE</filename></link>
                    variable.</para></listitem>
                <listitem><para><emphasis>Download Directory:</emphasis>
                    Controlled by the
                    <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
                    variable.</para></listitem>
                <listitem><para><emphasis>Shared State Directory:</emphasis>
                    Controlled by the
                    <link linkend='var-SSTATE_DIR'><filename>SSTATE_DIR</filename></link>
                    variable.</para></listitem>
                <listitem><para><emphasis>Build Output:</emphasis>
                    Controlled by the
                    <link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>
                    variable.</para></listitem>
            </itemizedlist>
            <note>
                Configurations set in the <filename>conf/local.conf</filename>
                file can also be set in the
                <filename>conf/site.conf</filename> and
                <filename>conf/auto.conf</filename> configuration files.
            </note>
        </para>

        <para>
            The <filename>bblayers.conf</filename> file tells BitBake what
            layers you want considered during the build.
            By default, the layers listed in this file include layers
            minimally needed by the build system.
            However, you must manually add any custom layers you have created.
            You can find more information on working with the
            <filename>bblayers.conf</filename> file in the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#enabling-your-layer'>Enabling Your Layer</ulink>"
            section in the Yocto Project Development Manual.
        </para>

        <para>
            The files <filename>site.conf</filename> and
            <filename>auto.conf</filename> are not created by the environment
            initialization script.
            If you want the <filename>site.conf</filename> file, you need to
            create that yourself.
            The <filename>auto.conf</filename> file is typically created by
            an autobuilder:
            <itemizedlist>
                <listitem><para><emphasis><filename>site.conf</filename>:</emphasis>
                    You can use the <filename>conf/site.conf</filename>
                    configuration file to configure multiple build directories.
                    For example, suppose you had several build environments and
                    they shared some common features.
                    You can set these default build properties here.
                    A good example is perhaps the packaging format to use
                    through the
                    <link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>
                    variable.</para>
                    <para>One useful scenario for using the
                    <filename>conf/site.conf</filename> file is to extend your
                    <link linkend='var-BBPATH'><filename>BBPATH</filename></link>
                    variable to include the path to a
                    <filename>conf/site.conf</filename>.
                    Then, when BitBake looks for Metadata using
                    <filename>BBPATH</filename>, it finds the
                    <filename>conf/site.conf</filename> file and applies your
                    common configurations found in the file.
                    To override configurations in a particular build directory,
                    alter the similar configurations within that build
                    directory's <filename>conf/local.conf</filename> file.
                    </para></listitem>
                <listitem><para><emphasis><filename>auto.conf</filename>:</emphasis>
                    The file is usually created and written to by
                    an autobuilder.
                    The settings put into the file are typically the same as
                    you would find in the <filename>conf/local.conf</filename>
                    or the <filename>conf/site.conf</filename> files.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            You can edit all configuration files to further define
            any particular build environment.
            This process is represented by the "User Configuration Edits"
            box in the figure.
        </para>

        <para>
            When you launch your build with the
            <filename>bitbake <replaceable>target</replaceable></filename>
            command, BitBake sorts out the configurations to ultimately
            define your build environment.
            It is important to understand that the OpenEmbedded build system
            reads the configuration files in a specific order:
            <filename>site.conf</filename>, <filename>auto.conf</filename>,
            and <filename>local.conf</filename>.
            And, the build system applies the normal assignment statement
            rules.
            Because the files are parsed in a specific order, variable
            assignments for the same variable could be affected.
            For example, if the <filename>auto.conf</filename> file and
            the <filename>local.conf</filename> set
            <replaceable>variable1</replaceable> to different values, because
            the build system parses <filename>local.conf</filename> after
            <filename>auto.conf</filename>,
            <replaceable>variable1</replaceable> is assigned the value from
            the <filename>local.conf</filename> file.
        </para>
    </section>

    <section id="metadata-machine-configuration-and-policy-configuration">
        <title>Metadata, Machine Configuration, and Policy Configuration</title>

        <para>
            The previous section described the user configurations that
            define BitBake's global behavior.
            This section takes a closer look at the layers the build system
            uses to further control the build.
            These layers provide Metadata for the software, machine, and
            policy.
        </para>

        <para>
            In general, three types of layer input exist:
            <itemizedlist>
                <listitem><para><emphasis>Policy Configuration:</emphasis>
                    Distribution Layers provide top-level or general
                    policies for the image or SDK being built.
                    For example, this layer would dictate whether BitBake
                    produces RPM or IPK packages.</para></listitem>
                <listitem><para><emphasis>Machine Configuration:</emphasis>
                    Board Support Package (BSP) layers provide machine
                    configurations.
                    This type of information is specific to a particular
                    target architecture.</para></listitem>
                <listitem><para><emphasis>Metadata:</emphasis>
                    Software layers contain user-supplied recipe files,
                    patches, and append files.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            The following figure shows an expanded representation of the
            Metadata, Machine Configuration, and Policy Configuration input
            (layers) boxes of the
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>:
        </para>

        <para>
            <imagedata fileref="figures/layer-input.png" align="center" width="8in" depth="7.5in" />
        </para>

        <para>
            In general, all layers have a similar structure.
            They all contain a licensing file
            (e.g. <filename>COPYING</filename>) if the layer is to be
            distributed, a <filename>README</filename> file as good practice
            and especially if the layer is to be distributed, a
            configuration directory, and recipe directories.
        </para>

        <para>
            The Yocto Project has many layers that can be used.
            You can see a web-interface listing of them on the
            <ulink url="http://git.yoctoproject.org/">Source Repositories</ulink>
            page.
            The layers are shown at the bottom categorized under
            "Yocto Metadata Layers."
            These layers are fundamentally a subset of the
            <ulink url="http://layers.openembedded.org/layerindex/layers/">OpenEmbedded Metadata Index</ulink>,
            which lists all layers provided by the OpenEmbedded community.
            <note>
                Layers exist in the Yocto Project Source Repositories that
                cannot be found in the OpenEmbedded Metadata Index.
                These layers are either deprecated or experimental in nature.
            </note>
        </para>

        <para>
            BitBake uses the <filename>conf/bblayers.conf</filename> file,
            which is part of the user configuration, to find what layers it
            should be using as part of the build.
        </para>

        <para>
            For more 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 Manual.
        </para>

        <section id="distro-layer">
            <title>Distro Layer</title>

            <para>
                The distribution layer provides policy configurations for your
                distribution.
                Best practices dictate that you isolate these types of
                configurations into their own layer.
                Settings you provide in
                <filename>conf/distro/<replaceable>distro</replaceable>.conf</filename> override
                similar
                settings that BitBake finds in your
                <filename>conf/local.conf</filename> file in the Build
                Directory.
            </para>

            <para>
                The following list provides some explanation and references
                for what you typically find in the distribution layer:
                <itemizedlist>
                    <listitem><para><emphasis>classes:</emphasis>
                        Class files (<filename>.bbclass</filename>) hold
                        common functionality that can be shared among
                        recipes in the distribution.
                        When your recipes inherit a class, they take on the
                        settings and functions for that class.
                        You can read more about class files in the
                        "<link linkend='ref-classes'>Classes</link>" section.
                        </para></listitem>
                    <listitem><para><emphasis>conf:</emphasis>
                        This area holds configuration files for the
                        layer (<filename>conf/layer.conf</filename>),
                        the distribution
                        (<filename>conf/distro/<replaceable>distro</replaceable>.conf</filename>),
                        and any distribution-wide include files.
                        </para></listitem>
                    <listitem><para><emphasis>recipes-*:</emphasis>
                        Recipes and append files that affect common
                        functionality across the distribution.
                        This area could include recipes and append files
                        to add distribution-specific configuration,
                        initialization scripts, custom image recipes,
                        and so forth.</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id="bsp-layer">
            <title>BSP Layer</title>

            <para>
                The BSP Layer provides machine configurations.
                Everything in this layer is specific to the machine for which
                you are building the image or the SDK.
                A common structure or form is defined for BSP layers.
                You can learn more about this structure in the
                <ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
                <note>
                    In order for a BSP layer to be considered compliant with the
                    Yocto Project, it must meet some structural requirements.
                </note>
            </para>

            <para>
                The BSP Layer's configuration directory contains
                configuration files for the machine
                (<filename>conf/machine/<replaceable>machine</replaceable>.conf</filename>) and,
                of course, the layer (<filename>conf/layer.conf</filename>).
            </para>

            <para>
                The remainder of the layer is dedicated to specific recipes
                by function: <filename>recipes-bsp</filename>,
                <filename>recipes-core</filename>,
                <filename>recipes-graphics</filename>, and
                <filename>recipes-kernel</filename>.
                Metadata can exist for multiple formfactors, graphics
                support systems, and so forth.
                <note>
                    While the figure shows several <filename>recipes-*</filename>
                    directories, not all these directories appear in all
                    BSP layers.
                </note>
            </para>
        </section>

        <section id="software-layer">
            <title>Software Layer</title>

            <para>
                The software layer provides the Metadata for additional
                software packages used during the build.
                This layer does not include Metadata that is specific to the
                distribution or the machine, which are found in their
                respective layers.
            </para>

            <para>
                This layer contains any new recipes that your project needs
                in the form of recipe files.
            </para>
        </section>
    </section>

    <section id="sources-dev-environment">
        <title>Sources</title>

        <para>
            In order for the OpenEmbedded build system to create an image or
            any target, it must be able to access source files.
            The
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>
            represents source files using the "Upstream Project Releases",
            "Local Projects", and "SCMs (optional)" boxes.
            The figure represents mirrors, which also play a role in locating
            source files, with the "Source Mirror(s)" box.
        </para>

        <para>
            The method by which source files are ultimately organized is
            a function of the project.
            For example, for released software, projects tend to use tarballs
            or other archived files that can capture the state of a release
            guaranteeing that it is statically represented.
            On the other hand, for a project that is more dynamic or
            experimental in nature, a project might keep source files in a
            repository controlled by a Source Control Manager (SCM) such as
            Git.
            Pulling source from a repository allows you to control
            the point in the repository (the revision) from which you want to
            build software.
            Finally, a combination of the two might exist, which would give the
            consumer a choice when deciding where to get source files.
        </para>

        <para>
            BitBake uses the
            <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
            variable to point to source files regardless of their location.
            Each recipe must have a <filename>SRC_URI</filename> variable
            that points to the source.
        </para>

        <para>
            Another area that plays a significant role in where source files
            come from is pointed to by the
            <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
            variable.
            This area is a cache that can hold previously downloaded source.
            You can also instruct the OpenEmbedded build system to create
            tarballs from Git repositories, which is not the default behavior,
            and store them in the <filename>DL_DIR</filename> by using the
            <link linkend='var-BB_GENERATE_MIRROR_TARBALLS'><filename>BB_GENERATE_MIRROR_TARBALLS</filename></link>
            variable.
        </para>

        <para>
            Judicious use of a <filename>DL_DIR</filename> directory can
            save the build system a trip across the Internet when looking
            for files.
            A good method for using a download directory is to have
            <filename>DL_DIR</filename> point to an area outside of your
            Build Directory.
            Doing so allows you to safely delete the Build Directory
            if needed without fear of removing any downloaded source file.
        </para>

        <para>
            The remainder of this section provides a deeper look into the
            source files and the mirrors.
            Here is a more detailed look at the source file area of the
            base figure:
            <imagedata fileref="figures/source-input.png" align="center" width="7in" depth="7.5in" />
        </para>

        <section id='upstream-project-releases'>
            <title>Upstream Project Releases</title>

            <para>
                Upstream project releases exist anywhere in the form of an
                archived file (e.g. tarball or zip file).
                These files correspond to individual recipes.
                For example, the figure uses specific releases each for
                BusyBox, Qt, and Dbus.
                An archive file can be for any released product that can be
                built using a recipe.
            </para>
        </section>

        <section id='local-projects'>
            <title>Local Projects</title>

            <para>
                Local projects are custom bits of software the user provides.
                These bits reside somewhere local to a project - perhaps
                a directory into which the user checks in items (e.g.
                a local directory containing a development source tree
                used by the group).
            </para>

            <para>
                The canonical method through which to include a local project
                is to use the
                <link linkend='ref-classes-externalsrc'><filename>externalsrc</filename></link>
                class to include that local project.
                You use either the <filename>local.conf</filename> or a
                recipe's append file to override or set the
                recipe to point to the local directory on your disk to pull
                in the whole source tree.
            </para>

            <para>
                For information on how to use the
                <filename>externalsrc</filename> class, see the
                "<link linkend='ref-classes-externalsrc'><filename>externalsrc.bbclass</filename></link>"
                section.
            </para>
        </section>

        <section id='scms'>
            <title>Source Control Managers (Optional)</title>

            <para>
                Another place the build system can get source files from is
                through an SCM such as Git or Subversion.
                In this case, a repository is cloned or checked out.
                The
                <link linkend='ref-tasks-fetch'><filename>do_fetch</filename></link>
                task inside BitBake uses
                the <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
                variable and the argument's prefix to determine the correct
                fetcher module.
            </para>

            <note>
                For information on how to have the OpenEmbedded build system
                generate tarballs for Git repositories and place them in the
                <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
                directory, see the
                <link linkend='var-BB_GENERATE_MIRROR_TARBALLS'><filename>BB_GENERATE_MIRROR_TARBALLS</filename></link>
                variable.
            </note>

            <para>
                When fetching a repository, BitBake uses the
                <link linkend='var-SRCREV'><filename>SRCREV</filename></link>
                variable to determine the specific revision from which to
                build.
            </para>
        </section>

        <section id='source-mirrors'>
            <title>Source Mirror(s)</title>

            <para>
                Two kinds of mirrors exist: pre-mirrors and regular mirrors.
                The <link linkend='var-PREMIRRORS'><filename>PREMIRRORS</filename></link>
                and
                <link linkend='var-MIRRORS'><filename>MIRRORS</filename></link>
                variables point to these, respectively.
                BitBake checks pre-mirrors before looking upstream for any
                source files.
                Pre-mirrors are appropriate when you have a shared directory
                that is not a directory defined by the
                <link linkend='var-DL_DIR'><filename>DL_DIR</filename></link>
                variable.
                A Pre-mirror typically points to a shared directory that is
                local to your organization.
            </para>

            <para>
                Regular mirrors can be any site across the Internet that is
                used as an alternative location for source code should the
                primary site not be functioning for some reason or another.
            </para>
        </section>
    </section>

    <section id="package-feeds-dev-environment">
        <title>Package Feeds</title>

        <para>
            When the OpenEmbedded build system generates an image or an SDK,
            it gets the packages from a package feed area located in the
            <link linkend='build-directory'>Build Directory</link>.
            The
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>
            shows this package feeds area in the upper-right corner.
        </para>

        <para>
            This section looks a little closer into the package feeds area used
            by the build system.
            Here is a more detailed look at the area:
            <imagedata fileref="figures/package-feeds.png" align="center" width="7in" depth="6in" />
        </para>

        <para>
            Package feeds are an intermediary step in the build process.
            The OpenEmbedded build system provides classes to generate
            different package types, and you specify which classes to enable
            through the
            <link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>
            variable.
            Before placing the packages into package feeds,
            the build process validates them with generated output quality
            assurance checks through the
            <link linkend='ref-classes-insane'><filename>insane</filename></link>
            class.
        </para>

        <para>
            The package feed area resides in the Build Directory.
            The directory the build system uses to temporarily store packages
            is determined by a combination of variables and the particular
            package manager in use.
            See the "Package Feeds" box in the illustration and note the
            information to the right of that area.
            In particular, the following defines where package files are
            kept:
            <itemizedlist>
                <listitem><para><link linkend='var-DEPLOY_DIR'><filename>DEPLOY_DIR</filename></link>:
                    Defined as <filename>tmp/deploy</filename> in the Build
                    Directory.
                    </para></listitem>
                <listitem><para><filename>DEPLOY_DIR_*</filename>:
                    Depending on the package manager used, the package type
                    sub-folder.
                    Given RPM, IPK, or DEB packaging and tarball creation, the
                    <link linkend='var-DEPLOY_DIR_RPM'><filename>DEPLOY_DIR_RPM</filename></link>,
                    <link linkend='var-DEPLOY_DIR_IPK'><filename>DEPLOY_DIR_IPK</filename></link>,
                    <link linkend='var-DEPLOY_DIR_DEB'><filename>DEPLOY_DIR_DEB</filename></link>,
                    or
                    <link linkend='var-DEPLOY_DIR_TAR'><filename>DEPLOY_DIR_TAR</filename></link>,
                    variables are used, respectively.
                    </para></listitem>
                <listitem><para><link linkend='var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></link>:
                    Defines architecture-specific sub-folders.
                    For example, packages could exist for the i586 or qemux86
                    architectures.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            BitBake uses the <filename>do_package_write_*</filename> tasks to
            generate packages and place them into the package holding area (e.g.
            <filename>do_package_write_ipk</filename> for IPK packages).
            See the
            "<link linkend='ref-tasks-package_write_deb'><filename>do_package_write_deb</filename></link>",
            "<link linkend='ref-tasks-package_write_ipk'><filename>do_package_write_ipk</filename></link>",
            "<link linkend='ref-tasks-package_write_rpm'><filename>do_package_write_rpm</filename></link>",
            and
            "<link linkend='ref-tasks-package_write_tar'><filename>do_package_write_tar</filename></link>"
            sections for additional information.
            As an example, consider a scenario where an IPK packaging manager
            is being used and package architecture support for both i586
            and qemux86 exist.
            Packages for the i586 architecture are placed in
            <filename>build/tmp/deploy/ipk/i586</filename>, while packages for
            the qemux86 architecture are placed in
            <filename>build/tmp/deploy/ipk/qemux86</filename>.
        </para>
    </section>

    <section id='bitbake-dev-environment'>
        <title>BitBake</title>

        <para>
            The OpenEmbedded build system uses
            <link linkend='bitbake-term'>BitBake</link>
            to produce images.
            You can see from the
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>,
            the BitBake area consists of several functional areas.
            This section takes a closer look at each of those areas.
        </para>

        <para>
            Separate documentation exists for the BitBake tool.
            See the
            <ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual'>BitBake User Manual</ulink>
            for reference material on BitBake.
        </para>

        <section id='source-fetching-dev-environment'>
            <title>Source Fetching</title>

            <para>
                The first stages of building a recipe are to fetch and unpack
                the source code:
                <imagedata fileref="figures/source-fetching.png" align="center" width="6.5in" depth="5in" />
            </para>

            <para>
                The
                <link linkend='ref-tasks-fetch'><filename>do_fetch</filename></link>
                and
                <link linkend='ref-tasks-unpack'><filename>do_unpack</filename></link>
                tasks fetch the source files and unpack them into the work
                directory.
                <note>
                    For every local file (e.g. <filename>file://</filename>)
                    that is part of a recipe's
                    <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
                    statement, the OpenEmbedded build system takes a checksum
                    of the file for the recipe and inserts the checksum into
                    the signature for the <filename>do_fetch</filename>.
                    If any local file has been modified, the
                    <filename>do_fetch</filename> task and all tasks that
                    depend on it are re-executed.
                </note>
                By default, everything is accomplished in the
                <link linkend='build-directory'>Build Directory</link>,
                which has a defined structure.
                For additional general information on the Build Directory,
                see the
                "<link linkend='structure-core-build'><filename>build/</filename></link>"
                section.
            </para>

            <para>
                Unpacked source files are pointed to by the
                <link linkend='var-S'><filename>S</filename></link> variable.
                Each recipe has an area in the Build Directory where the
                unpacked source code resides.
                The name of that directory for any given recipe is defined from
                several different variables.
                You can see the variables that define these directories
                by looking at the figure:
                <itemizedlist>
                    <listitem><para><link linkend='var-TMPDIR'><filename>TMPDIR</filename></link> -
                        The base directory where the OpenEmbedded build system
                        performs all its work during the build.
                        </para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_ARCH'><filename>PACKAGE_ARCH</filename></link> -
                        The architecture of the built package or packages.
                        </para></listitem>
                    <listitem><para><link linkend='var-TARGET_OS'><filename>TARGET_OS</filename></link> -
                        The operating system of the target device.
                        </para></listitem>
                    <listitem><para><link linkend='var-PN'><filename>PN</filename></link> -
                        The name of the built package.
                        </para></listitem>
                    <listitem><para><link linkend='var-PV'><filename>PV</filename></link> -
                        The version of the recipe used to build the package.
                        </para></listitem>
                    <listitem><para><link linkend='var-PR'><filename>PR</filename></link> -
                        The revision of the recipe used to build the package.
                        </para></listitem>
                    <listitem><para><link linkend='var-WORKDIR'><filename>WORKDIR</filename></link> -
                        The location within <filename>TMPDIR</filename> where
                        a specific package is built.
                        </para></listitem>
                    <listitem><para><link linkend='var-S'><filename>S</filename></link> -
                        Contains the unpacked source files for a given recipe.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='patching-dev-environment'>
            <title>Patching</title>

            <para>
                Once source code is fetched and unpacked, BitBake locates
                patch files and applies them to the source files:
                <imagedata fileref="figures/patching.png" align="center" width="6in" depth="5in" />
            </para>

            <para>
                The
                <link linkend='ref-tasks-patch'><filename>do_patch</filename></link>
                task processes recipes by
                using the
                <link linkend='var-SRC_URI'><filename>SRC_URI</filename></link>
                variable to locate applicable patch files, which by default
                are <filename>*.patch</filename> or
                <filename>*.diff</filename> files, or any file if
                "apply=yes" is specified for the file in
                <filename>SRC_URI</filename>.
            </para>

            <para>
                BitBake finds and applies multiple patches for a single recipe
                in the order in which it finds the patches.
                Patches are applied to the recipe's source files located in the
                <link linkend='var-S'><filename>S</filename></link> directory.
            </para>

            <para>
                For more information on how the source directories are
                created, see the
                "<link linkend='source-fetching-dev-environment'>Source Fetching</link>"
                section.
            </para>
        </section>

        <section id='configuration-and-compilation-dev-environment'>
            <title>Configuration and Compilation</title>

            <para>
                After source code is patched, BitBake executes tasks that
                configure and compile the source code:
                <imagedata fileref="figures/configuration-compile-autoreconf.png" align="center" width="7in" depth="5in" />
            </para>

            <para>
                This step in the build process consists of three tasks:
                <itemizedlist>
                    <listitem><para>
                        <emphasis><link linkend='ref-tasks-prepare_recipe_sysroot'><filename>do_prepare_recipe_sysroot</filename></link>:</emphasis>
                        This task sets up the two sysroots in
                        <filename>${</filename><link linkend='var-WORKDIR'><filename>WORKDIR</filename></link><filename>}</filename>
                        (i.e. <filename>recipe-sysroot</filename> and
                        <filename>recipe-sysroot-native</filename>) so that
                        the sysroots contain the contents of the
                        <link linkend='ref-tasks-populate_sysroot'><filename>do_populate_sysroot</filename></link>
                        tasks of the recipes on which the recipe
                        containing the tasks depends.
                        A sysroot exists for both the target and for the native
                        binaries, which run on the host system.
                        </para></listitem>
                    <listitem><para><emphasis><filename>do_configure</filename>:</emphasis>
                        This task configures the source by enabling and
                        disabling any build-time and configuration options for
                        the software being built.
                        Configurations can come from the recipe itself as well
                        as from an inherited class.
                        Additionally, the software itself might configure itself
                        depending on the target for which it is being built.
                        </para>

                        <para>The configurations handled by the
                        <link linkend='ref-tasks-configure'><filename>do_configure</filename></link>
                        task are specific
                        to source code configuration for the source code
                        being built by the recipe.</para>

                        <para>If you are using the
                        <link linkend='ref-classes-autotools'><filename>autotools</filename></link>
                        class,
                        you can add additional configuration options by using
                        the <link linkend='var-EXTRA_OECONF'><filename>EXTRA_OECONF</filename></link>
                        or
                        <link linkend='var-PACKAGECONFIG_CONFARGS'><filename>PACKAGECONFIG_CONFARGS</filename></link>
                        variables.
                        For information on how this variable works within
                        that class, see the
                        <filename>meta/classes/autotools.bbclass</filename> file.
                        </para></listitem>
                    <listitem><para><emphasis><filename>do_compile</filename>:</emphasis>
                        Once a configuration task has been satisfied, BitBake
                        compiles the source using the
                        <link linkend='ref-tasks-compile'><filename>do_compile</filename></link>
                        task.
                        Compilation occurs in the directory pointed to by the
                        <link linkend='var-B'><filename>B</filename></link>
                        variable.
                        Realize that the <filename>B</filename> directory is, by
                        default, the same as the
                        <link linkend='var-S'><filename>S</filename></link>
                        directory.</para></listitem>
                    <listitem><para><emphasis><filename>do_install</filename>:</emphasis>
                        Once compilation is done, BitBake executes the
                        <link linkend='ref-tasks-install'><filename>do_install</filename></link>
                        task.
                        This task copies files from the <filename>B</filename>
                        directory and places them in a holding area pointed to
                        by the
                        <link linkend='var-D'><filename>D</filename></link>
                        variable.</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='package-splitting-dev-environment'>
            <title>Package Splitting</title>

            <para>
                After source code is configured and compiled, the
                OpenEmbedded build system analyzes
                the results and splits the output into packages:
                <imagedata fileref="figures/analysis-for-package-splitting.png" align="center" width="7in" depth="7in" />
            </para>

            <para>
                The
                <link linkend='ref-tasks-package'><filename>do_package</filename></link>
                and
                <link linkend='ref-tasks-packagedata'><filename>do_packagedata</filename></link>
                tasks combine to analyze
                the files found in the
                <link linkend='var-D'><filename>D</filename></link> directory
                and split them into subsets based on available packages and
                files.
                The analyzing process involves the following as well as other
                items: splitting out debugging symbols,
                looking at shared library dependencies between packages,
                and looking at package relationships.
                The <filename>do_packagedata</filename> task creates package
                metadata based on the analysis such that the
                OpenEmbedded build system can generate the final packages.
                Working, staged, and intermediate results of the analysis
                and package splitting process use these areas:
                <itemizedlist>
                    <listitem><para><link linkend='var-PKGD'><filename>PKGD</filename></link> -
                        The destination directory for packages before they are
                        split.
                        </para></listitem>
                    <listitem><para><link linkend='var-PKGDATA_DIR'><filename>PKGDATA_DIR</filename></link> -
                        A shared, global-state directory that holds data
                        generated during the packaging process.
                        </para></listitem>
                    <listitem><para><link linkend='var-PKGDESTWORK'><filename>PKGDESTWORK</filename></link> -
                        A temporary work area used by the
                        <filename>do_package</filename> task.
                        </para></listitem>
                    <listitem><para><link linkend='var-PKGDEST'><filename>PKGDEST</filename></link> -
                        The parent directory for packages after they have
                        been split.
                        </para></listitem>
                </itemizedlist>
                The <link linkend='var-FILES'><filename>FILES</filename></link>
                variable defines the files that go into each package in
                <link linkend='var-PACKAGES'><filename>PACKAGES</filename></link>.
                If you want details on how this is accomplished, you can
                look at the
                <link linkend='ref-classes-package'><filename>package</filename></link>
                class.
            </para>

            <para>
                Depending on the type of packages being created (RPM, DEB, or
                IPK), the <filename>do_package_write_*</filename> task
                creates the actual packages and places them in the
                Package Feed area, which is
                <filename>${TMPDIR}/deploy</filename>.
                You can see the
                "<link linkend='package-feeds-dev-environment'>Package Feeds</link>"
                section for more detail on that part of the build process.
                <note>
                    Support for creating feeds directly from the
                    <filename>deploy/*</filename> directories does not exist.
                    Creating such feeds usually requires some kind of feed
                    maintenance mechanism that would upload the new packages
                    into an official package feed (e.g. the
                    Ångström distribution).
                    This functionality is highly distribution-specific
                    and thus is not provided out of the box.
                </note>
            </para>
        </section>

        <section id='image-generation-dev-environment'>
            <title>Image Generation</title>

            <para>
                Once packages are split and stored in the Package Feeds area,
                the OpenEmbedded build system uses BitBake to generate the
                root filesystem image:
                <imagedata fileref="figures/image-generation.png" align="center" width="6in" depth="7in" />
            </para>

            <para>
                The image generation process consists of several stages and
                depends on several tasks and variables.
                The
                <link linkend='ref-tasks-rootfs'><filename>do_rootfs</filename></link>
                task creates the root filesystem (file and directory structure)
                for an image.
                This task uses several key variables to help create the list
                of packages to actually install:
                <itemizedlist>
                    <listitem><para><link linkend='var-IMAGE_INSTALL'><filename>IMAGE_INSTALL</filename></link>:
                        Lists out the base set of packages to install from
                        the Package Feeds area.</para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_EXCLUDE'><filename>PACKAGE_EXCLUDE</filename></link>:
                        Specifies packages that should not be installed.
                        </para></listitem>
                    <listitem><para><link linkend='var-IMAGE_FEATURES'><filename>IMAGE_FEATURES</filename></link>:
                        Specifies features to include in the image.
                        Most of these features map to additional packages for
                        installation.</para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>:
                        Specifies the package backend to use and consequently
                        helps determine where to locate packages within the
                        Package Feeds area.</para></listitem>
                    <listitem><para><link linkend='var-IMAGE_LINGUAS'><filename>IMAGE_LINGUAS</filename></link>:
                        Determines the language(s) for which additional
                        language support packages are installed.
                        </para></listitem>
                    <listitem><para><link linkend='var-PACKAGE_INSTALL'><filename>PACKAGE_INSTALL</filename></link>:
                        The final list of packages passed to the package manager
                        for installation into the image.
                        </para></listitem>
                </itemizedlist>
            </para>

            <para>
                With
                <link linkend='var-IMAGE_ROOTFS'><filename>IMAGE_ROOTFS</filename></link>
                pointing to the location of the filesystem under construction and
                the <filename>PACKAGE_INSTALL</filename> variable providing the
                final list of packages to install, the root file system is
                created.
            </para>

            <para>
                Package installation is under control of the package manager
                (e.g. dnf/rpm, opkg, or apt/dpkg) regardless of whether or
                not package management is enabled for the target.
                At the end of the process, if package management is not
                enabled for the target, the package manager's data files
                are deleted from the root filesystem.
                As part of the final stage of package installation, postinstall
                scripts that are part of the packages are run.
                Any scripts that fail to run
                on the build host are run on the target when the target system
                is first booted.
                If you are using a
                <ulink url='&YOCTO_DOCS_DEV_URL;#creating-a-read-only-root-filesystem'>read-only root filesystem</ulink>,
                all the post installation scripts must succeed during the
                package installation phase since the root filesystem is
                read-only.
            </para>

            <para>
                The final stages of the <filename>do_rootfs</filename> task
                handle post processing.
                Post processing includes creation of a manifest file and
                optimizations.
            </para>

            <para>
                The manifest file (<filename>.manifest</filename>) resides
                in the same directory as the root filesystem image.
                This file lists out, line-by-line, the installed packages.
                The manifest file is useful for the
                <link linkend='ref-classes-testimage*'><filename>testimage</filename></link>
                class, for example, to determine whether or not to run
                specific tests.
                See the
                <link linkend='var-IMAGE_MANIFEST'><filename>IMAGE_MANIFEST</filename></link>
                variable for additional information.
            </para>

            <para>
                Optimizing processes run across the image include
                <filename>mklibs</filename>, <filename>prelink</filename>,
                and any other post-processing commands as defined by the
                <link linkend='var-ROOTFS_POSTPROCESS_COMMAND'><filename>ROOTFS_POSTPROCESS_COMMAND</filename></link>
                variable.
                The <filename>mklibs</filename> process optimizes the size
                of the libraries, while the
                <filename>prelink</filename> process optimizes the dynamic
                linking of shared libraries to reduce start up time of
                executables.
            </para>

            <para>
                After the root filesystem is built, processing begins on
                the image through the <filename>do_image</filename> task.
                The build system runs any pre-processing commands as defined
                by the
                <link linkend='var-IMAGE_PREPROCESS_COMMAND'><filename>IMAGE_PREPROCESS_COMMAND</filename></link>
                variable.
                This variable specifies a list of functions to call before
                the OpenEmbedded build system creates the final image output
                files.
            </para>

            <para>
                The <filename>do_image</filename> task dynamically creates
                other <filename>do_image_*</filename> tasks as needed, which
                include compressing the root filesystem image to reduce the
                overall size of the image.
                The process turns everything into an image file or a set of
                image files.
                The formats used for the root filesystem depend on the
                <link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
                variable.
            </para>

            <para>
                The final task involved in image creation is the
                <filename>do_image_complete</filename> task.
                This task completes the image by applying any image
                post processing as defined through the
                <link linkend='var-IMAGE_POSTPROCESS_COMMAND'><filename>IMAGE_POSTPROCESS_COMMAND</filename></link>
                variable.
                The variable specifies a list of functions to call once the
                OpenEmbedded build system has created the final image output
                files.
            </para>

            <note>
                The entire image generation process is run under Pseudo.
                Running under Pseudo ensures that the files in the root
                filesystem have correct ownership.
            </note>
        </section>

        <section id='sdk-generation-dev-environment'>
            <title>SDK Generation</title>

            <para>
                The OpenEmbedded build system uses BitBake to generate the
                Software Development Kit (SDK) installer script for both the
                standard and extensible SDKs:
                <imagedata fileref="figures/sdk-generation.png" align="center" />
            </para>

            <note>
                For more information on the cross-development toolchain
                generation, see the
                "<link linkend='cross-development-toolchain-generation'>Cross-Development Toolchain Generation</link>"
                section.
                For information on advantages gained when building a
                cross-development toolchain using the
                <link linkend='ref-tasks-populate_sdk'><filename>do_populate_sdk</filename></link>
                task, see the
                "<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-building-an-sdk-installer'>Building an SDK Installer</ulink>"
                section in the Yocto Project Software Development Kit (SDK)
                Developer's Guide.
            </note>

            <para>
                Like image generation, the SDK script process consists of
                several stages and depends on many variables.
                The <filename>do_populate_sdk</filename> and
                <filename>do_populate_sdk_ext</filename> tasks use these
                key variables to help create the list of packages to actually
                install.
                For information on the variables listed in the figure, see the
                "<link linkend='sdk-dev-environment'>Application Development SDK</link>"
                section.
            </para>

            <para>
                The <filename>do_populate_sdk</filename> task helps create
                the standard SDK and handles two parts: a target part and a
                host part.
                The target part is the part built for the target hardware and
                includes libraries and headers.
                The host part is the part of the SDK that runs on the
                <link linkend='var-SDKMACHINE'><filename>SDKMACHINE</filename></link>.
            </para>

            <para>
                The <filename>do_populate_sdk_ext</filename> task helps create
                the extensible SDK and handles host and target parts
                differently than its counter part does for the standard SDK.
                For the extensible SDK, the task encapsulates the build system,
                which includes everything needed (host and target) for the SDK.
            </para>

            <para>
                Regardless of the type of SDK being constructed, the
                tasks perform some cleanup after which a cross-development
                environment setup script and any needed configuration files
                are created.
                The final output is the Cross-development
                toolchain installation script (<filename>.sh</filename> file),
                which includes the environment setup script.
            </para>
        </section>

        <section id='stamp-files-and-the-rerunning-of-tasks'>
            <title>Stamp Files and the Rerunning of Tasks</title>

            <para>
                For each task that completes successfully, BitBake writes a
                stamp file into the
                <link linkend='var-STAMPS_DIR'><filename>STAMPS_DIR</filename></link>
                directory.
                The beginning of the stamp file's filename is determined by the
                <link linkend='var-STAMP'><filename>STAMP</filename></link>
                variable, and the end of the name consists of the task's name
                and current
                <ulink url='&YOCTO_DOCS_BB_URL;#checksums'>input checksum</ulink>.
                <note>
                    This naming scheme assumes that
                    <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_SIGNATURE_HANDLER'><filename>BB_SIGNATURE_HANDLER</filename></ulink>
                    is "OEBasicHash", which is almost always the case in
                    current OpenEmbedded.
                </note>
                To determine if a task needs to be rerun, BitBake checks if a
                stamp file with a matching input checksum exists for the task.
                If such a stamp file exists, the task's output is assumed to
                exist and still be valid.
                If the file does not exist, the task is rerun.
                <note>
                    <para>The stamp mechanism is more general than the shared
                    state (sstate) cache mechanism described in the
                    "<link linkend='setscene-tasks-and-shared-state'>Setscene Tasks and Shared State</link>"
                    section.
                    BitBake avoids rerunning any task that has a valid
                    stamp file, not just tasks that can be accelerated through
                    the sstate cache.</para>
                    <para>However, you should realize that stamp files only
                    serve as a marker that some work has been done and that
                    these files do not record task output.
                    The actual task output would usually be somewhere in
                    <link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>
                    (e.g. in some recipe's
                    <link linkend='var-WORKDIR'><filename>WORKDIR</filename></link>.)
                    What the sstate cache mechanism adds is a way to cache task
                    output that can then be shared between build machines.
                    </para>
                </note>
                Since <filename>STAMPS_DIR</filename> is usually a subdirectory
                of <filename>TMPDIR</filename>, removing
                <filename>TMPDIR</filename> will also remove
                <filename>STAMPS_DIR</filename>, which means tasks will
                properly be rerun to repopulate <filename>TMPDIR</filename>.
            </para>

            <para>
                If you want some task to always be considered "out of date",
                you can mark it with the
                <ulink url='&YOCTO_DOCS_BB_URL;#variable-flags'><filename>nostamp</filename></ulink>
                varflag.
                If some other task depends on such a task, then that task will
                also always be considered out of date, which might not be what
                you want.
            </para>

            <para>
                For details on how to view information about a task's
                signature, see the
                "<link linkend='usingpoky-viewing-task-variable-dependencies'>Viewing Task Variable Dependencies</link>"
                section.
            </para>
        </section>

        <section id='setscene-tasks-and-shared-state'>
            <title>Setscene Tasks and Shared State</title>

            <para>
                The description of tasks so far assumes that BitBake needs to
                build everything and there are no prebuilt objects available.
                BitBake does support skipping tasks if prebuilt objects are
                available.
                These objects are usually made available in the form of a
                shared state (sstate) cache.
                <note>
                    For information on variables affecting sstate, see the
                    <link linkend='var-SSTATE_DIR'><filename>SSTATE_DIR</filename></link>
                    and
                    <link linkend='var-SSTATE_MIRRORS'><filename>SSTATE_MIRRORS</filename></link>
                    variables.
                </note>
            </para>

            <para>
                The idea of a setscene task (i.e
                <filename>do_</filename><replaceable>taskname</replaceable><filename>_setscene</filename>)
                is a version of the task where
                instead of building something, BitBake can skip to the end
                result and simply place a set of files into specific locations
                as needed.
                In some cases, it makes sense to have a setscene task variant
                (e.g. generating package files in the
                <filename>do_package_write_*</filename> task).
                In other cases, it does not make sense, (e.g. a
                <link linkend='ref-tasks-patch'><filename>do_patch</filename></link>
                task or
                <link linkend='ref-tasks-unpack'><filename>do_unpack</filename></link>
                task) since the work involved would be equal to or greater than
                the underlying task.
            </para>

            <para>
                In the OpenEmbedded build system, the common tasks that have
                setscene variants are <link linkend='ref-tasks-package'><filename>do_package</filename></link>,
                <filename>do_package_write_*</filename>,
                <link linkend='ref-tasks-deploy'><filename>do_deploy</filename></link>,
                <link linkend='ref-tasks-packagedata'><filename>do_packagedata</filename></link>,
                and
                <link linkend='ref-tasks-populate_sysroot'><filename>do_populate_sysroot</filename></link>.
                Notice that these are most of the tasks whose output is an
                end result.
            </para>

            <para>
                The OpenEmbedded build system has knowledge of the relationship
                between these tasks and other tasks that precede them.
                For example, if BitBake runs
                <filename>do_populate_sysroot_setscene</filename> for
                something, there is little point in running any of the
                <filename>do_fetch</filename>, <filename>do_unpack</filename>,
                <filename>do_patch</filename>,
                <filename>do_configure</filename>,
                <filename>do_compile</filename>, and
                <filename>do_install</filename> tasks.
                However, if <filename>do_package</filename> needs to be run,
                BitBake would need to run those other tasks.
            </para>

            <para>
                It becomes more complicated if everything can come from an
                sstate cache because some objects are simply not required at
                all.
                For example, you do not need a compiler or native tools, such
                as quilt, if there is nothing to compile or patch.
                If the <filename>do_package_write_*</filename> packages are
                available from sstate, BitBake does not need the
                <filename>do_package</filename> task data.
            </para>

            <para>
                To handle all these complexities, BitBake runs in two phases.
                The first is the "setscene" stage.
                During this stage, BitBake first checks the sstate cache for
                any targets it is planning to build.
                BitBake does a fast check to see if the object exists rather
                than a complete download.
                If nothing exists, the second phase, which is the setscene
                stage, completes and the main build proceeds.
            </para>

            <para>
                If objects are found in the sstate cache, the OpenEmbedded
                build system works backwards from the end targets specified
                by the user.
                For example, if an image is being built, the OpenEmbedded build
                system first looks for the packages needed for that image and
                the tools needed to construct an image.
                If those are available, the compiler is not needed.
                Thus, the compiler is not even downloaded.
                If something was found to be unavailable, or the download or
                setscene task fails, the OpenEmbedded build system then tries
                to install dependencies, such as the compiler, from the cache.
            </para>

            <para>
                The availability of objects in the sstate cache is handled by
                the function specified by the
                <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_HASHCHECK_FUNCTION'><filename>BB_HASHCHECK_FUNCTION</filename></ulink>
                variable and returns a list of the objects that are available.
                The function specified by the
                <ulink url='&YOCTO_DOCS_BB_URL;#var-BB_SETSCENE_DEPVALID'><filename>BB_SETSCENE_DEPVALID</filename></ulink>
                variable is the function that determines whether a given
                dependency needs to be followed, and whether for any given
                relationship the function needs to be passed.
                The function returns a True or False value.
            </para>
        </section>
    </section>

    <section id='images-dev-environment'>
        <title>Images</title>

        <para>
            The images produced by the OpenEmbedded build system
            are compressed forms of the
            root filesystem that are ready to boot on a target device.
            You can see from the
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>
            that BitBake output, in part, consists of images.
            This section is going to look more closely at this output:
            <imagedata fileref="figures/images.png" align="center" width="5.5in" depth="5.5in" />
        </para>

        <para>
            For a list of example images that the Yocto Project provides,
            see the
            "<link linkend='ref-images'>Images</link>" chapter.
        </para>

        <para>
            Images are written out to the
            <link linkend='build-directory'>Build Directory</link>
            inside the <filename>tmp/deploy/images/<replaceable>machine</replaceable>/</filename>
            folder as shown in the figure.
            This folder contains any files expected to be loaded on the
            target device.
            The
            <link linkend='var-DEPLOY_DIR'><filename>DEPLOY_DIR</filename></link>
            variable points to the <filename>deploy</filename> directory,
            while the
            <link linkend='var-DEPLOY_DIR_IMAGE'><filename>DEPLOY_DIR_IMAGE</filename></link>
            variable points to the appropriate directory containing images for
            the current configuration.
            <itemizedlist>
                <listitem><para><filename><replaceable>kernel-image</replaceable></filename>:
                    A kernel binary file.
                    The <link linkend='var-KERNEL_IMAGETYPE'><filename>KERNEL_IMAGETYPE</filename></link>
                    variable setting determines the naming scheme for the
                    kernel image file.
                    Depending on that variable, the file could begin with
                    a variety of naming strings.
                    The <filename>deploy/images/<replaceable>machine</replaceable></filename>
                    directory can contain multiple image files for the
                    machine.</para></listitem>
                <listitem><para><filename><replaceable>root-filesystem-image</replaceable></filename>:
                    Root filesystems for the target device (e.g.
                    <filename>*.ext3</filename> or <filename>*.bz2</filename>
                    files).
                    The <link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
                    variable setting determines the root filesystem image
                    type.
                    The <filename>deploy/images/<replaceable>machine</replaceable></filename>
                    directory can contain multiple root filesystems for the
                    machine.</para></listitem>
                <listitem><para><filename><replaceable>kernel-modules</replaceable></filename>:
                    Tarballs that contain all the modules built for the kernel.
                    Kernel module tarballs exist for legacy purposes and
                    can be suppressed by setting the
                    <link linkend='var-MODULE_TARBALL_DEPLOY'><filename>MODULE_TARBALL_DEPLOY</filename></link>
                    variable to "0".
                    The <filename>deploy/images/<replaceable>machine</replaceable></filename>
                    directory can contain multiple kernel module tarballs
                    for the machine.</para></listitem>
                <listitem><para><filename><replaceable>bootloaders</replaceable></filename>:
                    Bootloaders supporting the image, if applicable to the
                    target machine.
                    The <filename>deploy/images/<replaceable>machine</replaceable></filename>
                    directory can contain multiple bootloaders for the
                    machine.</para></listitem>
                <listitem><para><filename><replaceable>symlinks</replaceable></filename>:
                    The <filename>deploy/images/<replaceable>machine</replaceable></filename>
                    folder contains
                    a symbolic link that points to the most recently built file
                    for each machine.
                    These links might be useful for external scripts that
                    need to obtain the latest version of each file.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='sdk-dev-environment'>
        <title>Application Development SDK</title>

        <para>
            In the
            <link linkend='general-yocto-environment-figure'>general Yocto Project Development Environment figure</link>,
            the output labeled "Application Development SDK" represents an
            SDK.
            The SDK generation process differs depending on whether you build
            a standard SDK
            (e.g. <filename>bitbake -c populate_sdk</filename> <replaceable>imagename</replaceable>)
            or an extensible SDK
            (e.g. <filename>bitbake -c populate_sdk_ext</filename> <replaceable>imagename</replaceable>).
            This section is going to take a closer look at this output:
            <imagedata fileref="figures/sdk.png" align="center" width="9in" depth="7.25in" />
        </para>

        <para>
            The specific form of this output is a self-extracting
            SDK installer (<filename>*.sh</filename>) that, when run,
            installs the SDK, which consists of a cross-development
            toolchain, a set of libraries and headers, and an SDK
            environment setup script.
            Running this installer essentially sets up your
            cross-development environment.
            You can think of the cross-toolchain as the "host"
            part because it runs on the SDK machine.
            You can think of the libraries and headers as the "target"
            part because they are built for the target hardware.
            The environment setup script is added so that you can initialize
            the environment before using the tools.
        </para>

        <note>
            <para>
                The Yocto Project supports several methods by which you can
                set up this cross-development environment.
                These methods include downloading pre-built SDK installers
                or building and installing your own SDK installer.
            </para>

            <para>
                For background information on cross-development toolchains
                in the Yocto Project development environment, see the
                "<link linkend='cross-development-toolchain-generation'>Cross-Development Toolchain Generation</link>"
                section.
                For information on setting up a cross-development
                environment, see the
                <ulink url='&YOCTO_DOCS_SDK_URL;#sdk-manual'>Yocto Project Software Development Kit (SDK) Developer's Guide</ulink>.
            </para>
        </note>

        <para>
            Once built, the SDK installers are written out to the
            <filename>deploy/sdk</filename> folder inside the
            <link linkend='build-directory'>Build Directory</link>
            as shown in the figure at the beginning of this section.
            Depending on the type of SDK, several variables exist that help
            configure these files.
            The following list shows the variables associated with a standard
            SDK:
            <itemizedlist>
                <listitem><para><link linkend='var-DEPLOY_DIR'><filename>DEPLOY_DIR</filename></link>:
                    Points to the <filename>deploy</filename>
                    directory.</para></listitem>
                <listitem><para><link linkend='var-SDKMACHINE'><filename>SDKMACHINE</filename></link>:
                    Specifies the architecture of the machine
                    on which the cross-development tools are run to
                    create packages for the target hardware.
                    </para></listitem>
                <listitem><para><link linkend='var-SDKIMAGE_FEATURES'><filename>SDKIMAGE_FEATURES</filename></link>:
                    Lists the features to include in the "target" part
                    of the SDK.
                    </para></listitem>
                <listitem><para><link linkend='var-TOOLCHAIN_HOST_TASK'><filename>TOOLCHAIN_HOST_TASK</filename></link>:
                    Lists packages that make up the host
                    part of the SDK (i.e. the part that runs on
                    the <filename>SDKMACHINE</filename>).
                    When you use
                    <filename>bitbake -c populate_sdk <replaceable>imagename</replaceable></filename>
                    to create the SDK, a set of default packages
                    apply.
                    This variable allows you to add more packages.
                    </para></listitem>
                <listitem><para><link linkend='var-TOOLCHAIN_TARGET_TASK'><filename>TOOLCHAIN_TARGET_TASK</filename></link>:
                    Lists packages that make up the target part
                    of the SDK (i.e. the part built for the
                    target hardware).
                    </para></listitem>
                <listitem><para><link linkend='var-SDKPATH'><filename>SDKPATH</filename></link>:
                    Defines the default SDK installation path offered by the
                    installation script.
                    </para></listitem>
            </itemizedlist>
            This next list, shows the variables associated with an extensible
            SDK:
            <itemizedlist>
                <listitem><para><link linkend='var-DEPLOY_DIR'><filename>DEPLOY_DIR</filename></link>:
                    Points to the <filename>deploy</filename> directory.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_EXT_TYPE'><filename>SDK_EXT_TYPE</filename></link>:
                    Controls whether or not shared state artifacts are copied
                    into the extensible SDK.
                    By default, all required shared state artifacts are copied
                    into the SDK.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_INCLUDE_PKGDATA'><filename>SDK_INCLUDE_PKGDATA</filename></link>:
                    Specifies whether or not packagedata will be included in
                    the extensible SDK for all recipes in the "world" target.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_INCLUDE_TOOLCHAIN'><filename>SDK_INCLUDE_TOOLCHAIN</filename></link>:
                    Specifies whether or not the toolchain will be included
                    when building the extensible SDK.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_LOCAL_CONF_WHITELIST'><filename>SDK_LOCAL_CONF_WHITELIST</filename></link>:
                    A list of variables allowed through from the build system
                    configuration into the extensible SDK configuration.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_LOCAL_CONF_BLACKLIST'><filename>SDK_LOCAL_CONF_BLACKLIST</filename></link>:
                    A list of variables not allowed through from the build
                    system configuration into the extensible SDK configuration.
                    </para></listitem>
                <listitem><para><link linkend='var-SDK_INHERIT_BLACKLIST'><filename>SDK_INHERIT_BLACKLIST</filename></link>:
                    A list of classes to remove from the
                    <link linkend='var-INHERIT'><filename>INHERIT</filename></link>
                    value globally within the extensible SDK configuration.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>

</chapter>
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