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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">
<chapter id='technical-details'>
<title>Technical Details</title>
<para>
This chapter provides technical details for various parts of the Yocto Project.
Currently, topics include Yocto Project components and shared state (sstate) cache.
</para>
<section id='usingpoky-components'>
<title>Yocto Project Components</title>
<para>
The BitBake task executor together with various types of configuration files form the
Yocto Project core.
This section overviews the BitBake task executor and the
configuration files by describing what they are used for and how they interact.
</para>
<para>
BitBake handles the parsing and execution of the data files.
The data itself is of various types:
<itemizedlist>
<listitem><para><emphasis>Recipes:</emphasis> Provides details about particular
pieces of software</para></listitem>
<listitem><para><emphasis>Class Data:</emphasis> An abstraction of common build
information (e.g. how to build a Linux kernel).</para></listitem>
<listitem><para><emphasis>Configuration Data:</emphasis> Defines machine-specific settings,
policy decisions, etc.
Configuration data acts as the glue to bind everything together.</para></listitem>
</itemizedlist>
For more information on data, see the
<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#yocto-project-terms'>
Yocto Project Terms</ulink> section in
<ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html'>
The Yocto Project Development Manual</ulink>.
</para>
<para>
BitBake knows how to combine multiple data sources together and refers to each data source
as a <link linkend='usingpoky-changes-layers'>'layer'</link>.
</para>
<para>
Following are some brief details on these core components.
For more detailed information on these components see the
<link linkend='ref-structure'>'Reference: Directory Structure'</link>
appendix.
</para>
<section id='usingpoky-components-bitbake'>
<title>BitBake</title>
<para>
BitBake is the tool at the heart of the Yocto Project and is responsible
for parsing the metadata, generating a list of tasks from it,
and then executing those tasks.
To see a list of the options BitBake supports, use the following help command:
<literallayout class='monospaced'>
$ bitbake --help
</literallayout>
</para>
<para>
The most common usage for BitBake is <filename>bitbake <packagename></filename>, where
<filename>packagename</filename> is the name of the package you want to build
(referred to as the "target" in this manual).
The target often equates to the first part of a <filename>.bb</filename> filename.
So, to run the <filename>matchbox-desktop_1.2.3.bb</filename> file, you
might type the following:
<literallayout class='monospaced'>
$ bitbake matchbox-desktop
</literallayout>
Several different versions of <filename>matchbox-desktop</filename> might exist.
BitBake chooses the one selected by the distribution configuration.
You can get more details about how BitBake chooses between different
target versions and providers in the
<link linkend='ref-bitbake-providers'>Preferences and Providers</link> section.
</para>
<para>
BitBake also tries to execute any dependent tasks first.
So for example, before building <filename>matchbox-desktop</filename>, BitBake
would build a cross compiler and <filename>eglibc</filename> if they had not already
been built.
<note>This release of the Yocto Project does not support the <filename>glibc</filename>
GNU version of the Unix standard C library. By default, the Yocto Project builds with
<filename>eglibc</filename>.</note>
</para>
<para>
A useful BitBake option to consider is the <filename>-k</filename> or
<filename>--continue</filename> option.
This option instructs BitBake to try and continue processing the job as much
as possible even after encountering an error.
When an error occurs, the target that
failed and those that depend on it cannot be remade.
However, when you use this option other dependencies can still be processed.
</para>
</section>
<section id='usingpoky-components-metadata'>
<title>Metadata (Recipes)</title>
<para>
The <filename>.bb</filename> files are usually referred to as "recipes."
In general, a recipe contains information about a single piece of software.
The information includes the location from which to download the source patches
(if any are needed), which special configuration options to apply,
how to compile the source files, and how to package the compiled output.
</para>
<para>
The term "package" can also be used to describe recipes.
However, since the same word is used for the packaged output from the Yocto
Project (i.e. <filename>.ipk</filename> or <filename>.deb</filename> files),
this document avoids using the term "package" to refer to recipes.
</para>
</section>
<section id='usingpoky-components-classes'>
<title>Classes</title>
<para>
Class files (<filename>.bbclass</filename>) contain information that is useful to share
between metadata files.
An example is the Autotools class, which contains
common settings for any application that Autotools uses.
The <link linkend='ref-classes'>Reference: Classes</link> appendix provides details
about common classes and how to use them.
</para>
</section>
<section id='usingpoky-components-configuration'>
<title>Configuration</title>
<para>
The configuration files (<filename>.conf</filename>) define various configuration variables
that govern the Yocto Project build process.
These files fall into several areas that define machine configuration options,
distribution configuration options, compiler tuning options, general common configuration
options and user configuration options (<filename>local.conf</filename>, which is found
in the Yocto Project files build directory).
</para>
</section>
</section>
<section id="considering-shared-state-cache">
<title>Considering Shared State Cache</title>
<para>
By design, the Yocto Project builds everything from scratch unless it can determine that
a given task's inputs have not changed.
While building from scratch ensures that everything is current, it does also
mean that a lot of time could be spent rebuiding things that don't necessarily need built.
</para>
<para>
The Yocto Project build process uses a shared state caching scheme to avoid having to
rebuild software when it is not necessary.
Because the build time for a Yocto image can be significant, it is helpful to try and
determine what really needs built and what can be skipped given a particular project's
build process.
</para>
<para>
The scheme that the Yocto Project uses involves checksum generation and comparison for
a task's inputs.
The scheme also employs an area of memory called the shared state cache that is
pointed to by the <filename>SSTATE_DIR</filename> variable.
This area contains task output generated from a previous build.
If a given task's checksum matches the checksum of a previous build for the same
task, the build process uses the state of the cache rather than rerunning that
task.
</para>
<para>
The previous paragraph is a simplistic explanation of how the build process
uses checksums and shared state memory cache to avoide building tasks that
don't need built.
If you want a bit more explanation on the topic,
see "<ulink url='https://lists.yoctoproject.org/pipermail/yocto/2011-March/003366.html'>Shared
State - What does it mean and why should I care?</ulink>" from the Yocto
Project discussion archives.
</para>
<para>
As with all schemes, this one has some drawbacks.
It is possible that you could make implicit changes that are not factored into the checksum
calculation, but do affect a task's output.
A good example is perhaps when a tool changes its output.
Let's say that the output of <filename>rpmdeps</filename> needed to change.
The result of the change should be that all the "package", "package_write_rpm",
and "package_deploy-rpm" sstate-cache items would become invalid.
But, because this is a change that is external to the code and therefore implicit,
the associated sstate-cache items do not become invalidated.
In this case, the build process would use the cache items rather than running the
task again.
Obviously, these types of implicit changes can cause problems.
</para>
<para>
To avoid these problems during the build, you need to understand the effects of any
change you make.
Note that any changes you make directly to a function automatically are factored into
the checksum calculation and thus, will invalidate the associated area of sstate cache.
You need to be aware of any implicit changes that are not obvious changes to the
code and could affect the output of a given task.
Once you are aware of such a change, you can take steps to invalidate the cache
and force the task to run.
The step to take is as simple as changing a function's comments in the source code.
For example, to invalidate package sstate files, change the comment statments
of <filename>do_package</filename> or one of the functions it calls.
The change is purely cosmetic, but it causes the checksum to be recalculated and
forces the task to be run again.
</para>
<note>
For an example of a commit that makes a cosmetic change to invalidate an sstate,
see this
<ulink url='http://git.yoctoproject.org/cgit.cgi/poky/commit/meta/classes/package.bbclass?id=737f8bbb4f27b4837047cb9b4fbfe01dfde36d54'>commit</ulink>.
</note>
</section>
</chapter>
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