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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
[<!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.
</para>
<para>
Specifically, this chapter addresses open source philosophy, workflows,
Git, source repositories, licensing, recipe syntax, and development
syntax.
</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 Tasks 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 Tasks 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 Tasks 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 Tasks 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 Tasks 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 Tasks 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 Tasks 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 Tasks 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 Tasks 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 in the Yocto Project Development Tasks Manual.
</para>
</section>
<section id='recipe-syntax'>
<title>Recipe Syntax</title>
<para>
Understanding recipe file syntax is important for
writing recipes.
The following list overviews the basic items that make up a
BitBake recipe file.
For more complete BitBake syntax descriptions, see the
"<ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual-metadata'>Syntax and Operators</ulink>"
chapter of the BitBake User Manual.
<itemizedlist>
<listitem><para><emphasis>Variable Assignments and Manipulations:</emphasis>
Variable assignments allow a value to be assigned to a
variable.
The assignment can be static text or might include
the contents of other variables.
In addition to the assignment, appending and prepending
operations are also supported.</para>
<para>The following example shows some of the ways
you can use variables in recipes:
<literallayout class='monospaced'>
S = "${WORKDIR}/postfix-${PV}"
CFLAGS += "-DNO_ASM"
SRC_URI_append = " file://fixup.patch"
</literallayout>
</para></listitem>
<listitem><para><emphasis>Functions:</emphasis>
Functions provide a series of actions to be performed.
You usually use functions to override the default
implementation of a task function or to complement
a default function (i.e. append or prepend to an
existing function).
Standard functions use <filename>sh</filename> shell
syntax, although access to OpenEmbedded variables and
internal methods are also available.</para>
<para>The following is an example function from the
<filename>sed</filename> recipe:
<literallayout class='monospaced'>
do_install () {
autotools_do_install
install -d ${D}${base_bindir}
mv ${D}${bindir}/sed ${D}${base_bindir}/sed
rmdir ${D}${bindir}/
}
</literallayout>
It is also possible to implement new functions that
are called between existing tasks as long as the
new functions are not replacing or complementing the
default functions.
You can implement functions in Python
instead of shell.
Both of these options are not seen in the majority of
recipes.</para></listitem>
<listitem><para><emphasis>Keywords:</emphasis>
BitBake recipes use only a few keywords.
You use keywords to include common
functions (<filename>inherit</filename>), load parts
of a recipe from other files
(<filename>include</filename> and
<filename>require</filename>) and export variables
to the environment (<filename>export</filename>).</para>
<para>The following example shows the use of some of
these keywords:
<literallayout class='monospaced'>
export POSTCONF = "${STAGING_BINDIR}/postconf"
inherit autoconf
require otherfile.inc
</literallayout>
</para></listitem>
<listitem><para><emphasis>Comments:</emphasis>
Any lines that begin with the hash character
(<filename>#</filename>) are treated as comment lines
and are ignored:
<literallayout class='monospaced'>
# This is a comment
</literallayout>
</para></listitem>
</itemizedlist>
</para>
<para>
This next list summarizes the most important and most commonly
used parts of the recipe syntax.
For more information on these parts of the syntax, you can
reference the
<ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual-metadata'>Syntax and Operators</ulink>
chapter in the BitBake User Manual.
<itemizedlist>
<listitem><para><emphasis>Line Continuation: <filename>\</filename></emphasis> -
Use the backward slash (<filename>\</filename>)
character to split a statement over multiple lines.
Place the slash character at the end of the line that
is to be continued on the next line:
<literallayout class='monospaced'>
VAR = "A really long \
line"
</literallayout>
<note>
You cannot have any characters including spaces
or tabs after the slash character.
</note>
</para></listitem>
<listitem><para>
<emphasis>Using Variables: <filename>${...}</filename></emphasis> -
Use the <filename>${<replaceable>VARNAME</replaceable>}</filename> syntax to
access the contents of a variable:
<literallayout class='monospaced'>
SRC_URI = "${SOURCEFORGE_MIRROR}/libpng/zlib-${PV}.tar.gz"
</literallayout>
<note>
It is important to understand that the value of a
variable expressed in this form does not get
substituted automatically.
The expansion of these expressions happens
on-demand later (e.g. usually when a function that
makes reference to the variable executes).
This behavior ensures that the values are most
appropriate for the context in which they are
finally used.
On the rare occasion that you do need the variable
expression to be expanded immediately, you can use
the <filename>:=</filename> operator instead of
<filename>=</filename> when you make the
assignment, but this is not generally needed.
</note>
</para></listitem>
<listitem><para><emphasis>Quote All Assignments: <filename>"<replaceable>value</replaceable>"</filename></emphasis> -
Use double quotes around the value in all variable
assignments.
<literallayout class='monospaced'>
VAR1 = "${OTHERVAR}"
VAR2 = "The version is ${PV}"
</literallayout>
</para></listitem>
<listitem><para><emphasis>Conditional Assignment: <filename>?=</filename></emphasis> -
Conditional assignment is used to assign a value to
a variable, but only when the variable is currently
unset.
Use the question mark followed by the equal sign
(<filename>?=</filename>) to make a "soft" assignment
used for conditional assignment.
Typically, "soft" assignments are used in the
<filename>local.conf</filename> file for variables
that are allowed to come through from the external
environment.
</para>
<para>Here is an example where
<filename>VAR1</filename> is set to "New value" if
it is currently empty.
However, if <filename>VAR1</filename> has already been
set, it remains unchanged:
<literallayout class='monospaced'>
VAR1 ?= "New value"
</literallayout>
In this next example, <filename>VAR1</filename>
is left with the value "Original value":
<literallayout class='monospaced'>
VAR1 = "Original value"
VAR1 ?= "New value"
</literallayout>
</para></listitem>
<listitem><para><emphasis>Appending: <filename>+=</filename></emphasis> -
Use the plus character followed by the equals sign
(<filename>+=</filename>) to append values to existing
variables.
<note>
This operator adds a space between the existing
content of the variable and the new content.
</note></para>
<para>Here is an example:
<literallayout class='monospaced'>
SRC_URI += "file://fix-makefile.patch"
</literallayout>
</para></listitem>
<listitem><para><emphasis>Prepending: <filename>=+</filename></emphasis> -
Use the equals sign followed by the plus character
(<filename>=+</filename>) to prepend values to existing
variables.
<note>
This operator adds a space between the new content
and the existing content of the variable.
</note></para>
<para>Here is an example:
<literallayout class='monospaced'>
VAR =+ "Starts"
</literallayout>
</para></listitem>
<listitem><para><emphasis>Appending: <filename>_append</filename></emphasis> -
Use the <filename>_append</filename> operator to
append values to existing variables.
This operator does not add any additional space.
Also, the operator is applied after all the
<filename>+=</filename>, and
<filename>=+</filename> operators have been applied and
after all <filename>=</filename> assignments have
occurred.
</para>
<para>The following example shows the space being
explicitly added to the start to ensure the appended
value is not merged with the existing value:
<literallayout class='monospaced'>
SRC_URI_append = " file://fix-makefile.patch"
</literallayout>
You can also use the <filename>_append</filename>
operator with overrides, which results in the actions
only being performed for the specified target or
machine:
<literallayout class='monospaced'>
SRC_URI_append_sh4 = " file://fix-makefile.patch"
</literallayout>
</para></listitem>
<listitem><para><emphasis>Prepending: <filename>_prepend</filename></emphasis> -
Use the <filename>_prepend</filename> operator to
prepend values to existing variables.
This operator does not add any additional space.
Also, the operator is applied after all the
<filename>+=</filename>, and
<filename>=+</filename> operators have been applied and
after all <filename>=</filename> assignments have
occurred.
</para>
<para>The following example shows the space being
explicitly added to the end to ensure the prepended
value is not merged with the existing value:
<literallayout class='monospaced'>
CFLAGS_prepend = "-I${S}/myincludes "
</literallayout>
You can also use the <filename>_prepend</filename>
operator with overrides, which results in the actions
only being performed for the specified target or
machine:
<literallayout class='monospaced'>
CFLAGS_prepend_sh4 = "-I${S}/myincludes "
</literallayout>
</para></listitem>
<listitem><para><emphasis>Overrides:</emphasis> -
You can use overrides to set a value conditionally,
typically based on how the recipe is being built.
For example, to set the
<link linkend='var-KBRANCH'><filename>KBRANCH</filename></link>
variable's value to "standard/base" for any target
<link linkend='var-MACHINE'><filename>MACHINE</filename></link>,
except for qemuarm where it should be set to
"standard/arm-versatile-926ejs", you would do the
following:
<literallayout class='monospaced'>
KBRANCH = "standard/base"
KBRANCH_qemuarm = "standard/arm-versatile-926ejs"
</literallayout>
Overrides are also used to separate alternate values
of a variable in other situations.
For example, when setting variables such as
<link linkend='var-FILES'><filename>FILES</filename></link>
and
<link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>
that are specific to individual packages produced by
a recipe, you should always use an override that
specifies the name of the package.
</para></listitem>
<listitem><para><emphasis>Indentation:</emphasis>
Use spaces for indentation rather than than tabs.
For shell functions, both currently work.
However, it is a policy decision of the Yocto Project
to use tabs in shell functions.
Realize that some layers have a policy to use spaces
for all indentation.
</para></listitem>
<listitem><para><emphasis>Using Python for Complex Operations: <filename>${@<replaceable>python_code</replaceable>}</filename></emphasis> -
For more advanced processing, it is possible to use
Python code during variable assignments (e.g.
search and replacement on a variable).</para>
<para>You indicate Python code using the
<filename>${@<replaceable>python_code</replaceable>}</filename>
syntax for the variable assignment:
<literallayout class='monospaced'>
SRC_URI = "ftp://ftp.info-zip.org/pub/infozip/src/zip${@d.getVar('PV',1).replace('.', '')}.tgz
</literallayout>
</para></listitem>
<listitem><para><emphasis>Shell Function Syntax:</emphasis>
Write shell functions as if you were writing a shell
script when you describe a list of actions to take.
You should ensure that your script works with a generic
<filename>sh</filename> and that it does not require
any <filename>bash</filename> or other shell-specific
functionality.
The same considerations apply to various system
utilities (e.g. <filename>sed</filename>,
<filename>grep</filename>, <filename>awk</filename>,
and so forth) that you might wish to use.
If in doubt, you should check with multiple
implementations - including those from BusyBox.
</para></listitem>
</itemizedlist>
</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
<link linkend='metadata'>Metadata</link>) 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
<link linkend='source-directory'>Source Directory</link>.
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>).
</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> 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 Tasks 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 Tasks 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
<link linkend='ref-tasks-image'><filename>do_image</filename></link>
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 OpenEmbedded build system dynamically creates
<filename>do_image_*</filename> tasks as needed, based
on the image types specified in the
<link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
variable.
The process turns everything into an image file or a set of
image files and compresses the root filesystem image to reduce
the overall size of the image.
The formats used for the root filesystem depend on the
<filename>IMAGE_FSTYPES</filename> variable.
</para>
<para>
As an example, a dynamically created task when creating a
particular image <replaceable>type</replaceable> would take the
following form:
<literallayout class='monospaced'>
do_image_<replaceable>type</replaceable>[depends]
</literallayout>
So, if the <replaceable>type</replaceable> as specified by the
<filename>IMAGE_FSTYPES</filename> were
<filename>ext4</filename>, the dynamically generated task
would be as follows:
<literallayout class='monospaced'>
do_image_ext4[depends]
</literallayout>
</para>
<para>
The final task involved in image creation is the
<link linkend='ref-tasks-image-complete'><filename>do_image_complete</filename></link>
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 Application Development and the
Extensible Software Development Kit (SDK) manual.
</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 Application Development and the Extensible Software Development Kit (eSDK)</ulink>
manual.
</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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