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<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<appendix id='ref-bitbake'>
<title>Reference: Bitbake</title>
<para>
Bitbake a program written in Python which interprets the metadata
that makes up Poky. At some point, people wonder what actually happens
when you type <command>bitbake poky-image-sato</command>. This section
aims to give an overview of what happens behind the scenes from a
BitBake perspective.
</para>
<para>
It is worth noting that bitbake aims to be a generic "task" executor
capable of handling complex dependency relationships. As such it has no
real knowledge of what the tasks its executing actually do. It just
considers a list of tasks with dependencies and handles metadata
consisting of variables in a certain format which get passed to the
tasks.
</para>
<section id='ref-bitbake-parsing'>
<title>Parsing</title>
<para>
The first thing BitBake does is work out its configuration by
looking for a file called <filename>bitbake.conf</filename>.
Bitbake searches through the <varname>BBPATH</varname> environment
variable looking for a <filename class="directory">conf/</filename>
directory containing a <filename>bitbake.conf</filename> file and
adds the first <filename>bitbake.conf</filename> file found in
<varname>BBPATH</varname> (similar to the PATH environment variable).
For Poky, <filename>bitbake.conf</filename> is found in <filename
class="directory">meta/conf/</filename>.
</para>
<para>
In Poky, <filename>bitbake.conf</filename> lists other configuration
files to include from a <filename class="directory">conf/</filename>
directory below the directories listed in <varname>BBPATH</varname>.
In general the most important configuration file from a user's perspective
is <filename>local.conf</filename>, which contains a users customized
settings for Poky. Other notable configuration files are the distribution
configuration file (set by the <glossterm><link linkend='var-DISTRO'>
DISTRO</link></glossterm> variable) and the machine configuration file
(set by the <glossterm><link linkend='var-MACHINE'>MACHINE</link>
</glossterm> variable). The <glossterm><link linkend='var-DISTRO'>
DISTRO</link></glossterm> and <glossterm><link linkend='var-MACHINE'>
MACHINE</link></glossterm> environment variables are both usually set in
the <filename>local.conf</filename> file. Valid distribution
configuration files are available in the <filename class="directory">
meta/conf/distro/</filename> directory and valid machine configuration
files in the <filename class="directory">meta/conf/machine/</filename>
directory. Within the <filename class="directory">
meta/conf/machine/include/</filename> directory are various <filename>
tune-*.inc</filename> configuration files which provide common
"tuning" settings specific to and shared between particular
architectures and machines.
</para>
<para>
After the parsing of the configuration files some standard classes
are included. In particular, <filename>base.bbclass</filename> is
always included, as will any other classes
specified in the configuration using the <glossterm><link
linkend='var-INHERIT'>INHERIT</link></glossterm>
variable. Class files are searched for in a classes subdirectory
under the paths in <varname>BBPATH</varname> in the same way as
configuration files.
</para>
<para>
After the parsing of the configuration files is complete, the
variable <glossterm><link linkend='var-BBFILES'>BBFILES</link></glossterm>
is set, usually in
<filename>local.conf</filename>, and defines the list of places to search for
<filename class="extension">.bb</filename> files. By
default this specifies the <filename class="directory">meta/packages/
</filename> directory within Poky, but other directories such as
<filename class="directory">meta-extras/</filename> can be included
too. If multiple directories are specified a system referred to as
<link linkend='usingpoky-changes-collections'>"collections"</link> is used to
determine which files have priority.
</para>
<para>
Bitbake parses each <filename class="extension">.bb</filename> file in
<glossterm><link linkend='var-BBFILES'>BBFILES</link></glossterm> and
stores the values of various variables. In summary, for each
<filename class="extension">.bb</filename>
file the configuration + base class of variables are set, followed
by the data in the <filename class="extension">.bb</filename> file
itself, followed by any inherit commands that
<filename class="extension">.bb</filename> file might contain.
</para>
<para>
Parsing <filename class="extension">.bb</filename> files is a time
consuming process, so a cache is kept to speed up subsequent parsing.
This cache is invalid if the timestamp of the <filename class="extension">.bb</filename>
file itself has changed, or if the timestamps of any of the include,
configuration or class files the <filename class="extension">.bb</filename>
file depends on have changed.
</para>
</section>
<section id='ref-bitbake-providers'>
<title>Preferences and Providers</title>
<para>
Once all the <filename class="extension">.bb</filename> files have been
parsed, BitBake will proceed to build "poky-image-sato" (or whatever was
specified on the commandline) and looks for providers of that target.
Once a provider is selected, BitBake resolves all the dependencies for
the target. In the case of "poky-image-sato", it would lead to
<filename>task-oh.bb</filename> and <filename>task-base.bb</filename>
which in turn would lead to packages like <application>Contacts</application>,
<application>Dates</application>, <application>BusyBox</application>
and these in turn depend on glibc and the toolchain.
</para>
<para>
Sometimes a target might have multiple providers and a common example
is "virtual/kernel" that is provided by each kernel package. Each machine
will often elect the best provider of its kernel with a line like the
following in the machine configuration file:
</para>
<programlisting><glossterm><link linkend='var-PREFERRED_PROVIDER'>PREFERRED_PROVIDER</link></glossterm>_virtual/kernel = "linux-rp"</programlisting>
<para>
The default <glossterm><link linkend='var-PREFERRED_PROVIDER'>
PREFERRED_PROVIDER</link></glossterm> is the provider with the same name as
the target.
</para>
<para>
Understanding how providers are chosen is complicated by the fact
multiple versions might be present. Bitbake defaults to the highest
version of a provider by default. Version comparisons are made using
the same method as Debian. The <glossterm><link
linkend='var-PREFERRED_VERSION'>PREFERRED_VERSION</link></glossterm>
variable can be used to specify a particular version
(usually in the distro configuration) but the order can
also be influenced by the <glossterm><link
linkend='var-DEFAULT_PREFERENCE'>DEFAULT_PREFERENCE</link></glossterm>
variable. By default files
have a preference of "0". Setting the
<glossterm><link
linkend='var-DEFAULT_PREFERENCE'>DEFAULT_PREFERENCE</link></glossterm> to "-1" will
make a package unlikely to be used unless it was explicitly referenced and
"1" makes it likely the package will be used.
<glossterm><link
linkend='var-PREFERRED_VERSION'>PREFERRED_VERSION</link></glossterm> overrides
any default preference. <glossterm><link
linkend='var-DEFAULT_PREFERENCE'>DEFAULT_PREFERENCE</link></glossterm>
is often used to mark more
experimental new versions of packages until they've undergone sufficient
testing to be considered stable.
</para>
<para>
The end result is that internally, BitBake has now built a list of
providers for each target it needs in order of priority.
</para>
</section>
<section id='ref-bitbake-dependencies'>
<title>Dependencies</title>
<para>
Each target BitBake builds consists of multiple tasks (e.g. fetch,
unpack, patch, configure, compile etc.). For best performance on
multi-core systems, BitBake considers each task as an independent
entity with a set of dependencies. There are many variables that
are used to signify these dependencies and more information can be found
found about these in the <ulink url='http://bitbake.berlios.de/manual/'>
BitBake manual</ulink>. At a basic level it is sufficient to know
that BitBake uses the <glossterm><link
linkend='var-DEPENDS'>DEPENDS</link></glossterm> and
<glossterm><link linkend='var-RDEPENDS'>RDEPENDS</link></glossterm> variables when
calculating dependencies and descriptions of these variables are
available through the links.
</para>
</section>
<section id='ref-bitbake-tasklist'>
<title>The Task List</title>
<para>
Based on the generated list of providers and the dependency information,
BitBake can now calculate exactly which tasks it needs to run and in what
order. The build now starts with BitBake forking off threads up to
the limit set in the <glossterm><link
linkend='var-BB_NUMBER_THREADS'>BB_NUMBER_THREADS</link></glossterm> variable
as long there are tasks ready to run, i.e. tasks with all their
dependencies met.
</para>
<para>
As each task completes, a timestamp is written to the directory
specified by the <glossterm><link
linkend='var-STAMPS'>STAMPS</link></glossterm> variable (usually
<filename class="directory">build/tmp/stamps/*/</filename>). On
subsequent runs, BitBake looks at the <glossterm><link
linkend='var-STAMPS'>STAMPS</link></glossterm>
directory and will not rerun
tasks its already completed unless a timestamp is found to be invalid.
Currently, invalid timestamps are only considered on a per <filename
class="extension">.bb</filename> file basis so if for example the configure stamp has a timestamp greater than the
compile timestamp for a given target the compile task would rerun but this
has no effect on other providers depending on that target. This could
change or become configurable in future versions of BitBake. Some tasks
are marked as "nostamp" tasks which means no timestamp file will be written
and the task will always rerun.
</para>
<para>Once all the tasks have been completed BitBake exits.</para>
</section>
<section id='ref-bitbake-runtask'>
<title>Running a Task</title>
<para>
It's worth noting what BitBake does to run a task. A task can either
be a shell task or a python task. For shell tasks, BitBake writes a
shell script to <filename>${WORKDIR}/temp/run.do_taskname.pid</filename>
and then executes the script. The generated
shell script contains all the exported variables, and the shell functions
with all variables expanded. Output from the shell script is
sent to the file <filename>${WORKDIR}/temp/log.do_taskname.pid</filename>.
Looking at the
expanded shell functions in the run file and the output in the log files
is a useful debugging technique.
</para>
<para>
Python functions are executed internally to BitBake itself and
logging goes to the controlling terminal. Future versions of BitBake will
write the functions to files in a similar way to shell functions and
logging will also go to the log files in a similar way.
</para>
</section>
<section id='ref-bitbake-commandline'>
<title>Commandline</title>
<para>
To quote from "bitbake --help":
</para>
<screen>Usage: bitbake [options] [package ...]
Executes the specified task (default is 'build') for a given set of BitBake files.
It expects that BBFILES is defined, which is a space separated list of files to
be executed. BBFILES does support wildcards.
Default BBFILES are the .bb files in the current directory.
Options:
--version show program's version number and exit
-h, --help show this help message and exit
-b BUILDFILE, --buildfile=BUILDFILE
execute the task against this .bb file, rather than a
package from BBFILES.
-k, --continue continue as much as possible after an error. While the
target that failed, and those that depend on it,
cannot be remade, the other dependencies of these
targets can be processed all the same.
-f, --force force run of specified cmd, regardless of stamp status
-i, --interactive drop into the interactive mode also called the BitBake
shell.
-c CMD, --cmd=CMD Specify task to execute. Note that this only executes
the specified task for the providee and the packages
it depends on, i.e. 'compile' does not implicitly call
stage for the dependencies (IOW: use only if you know
what you are doing). Depending on the base.bbclass a
listtasks tasks is defined and will show available
tasks
-r FILE, --read=FILE read the specified file before bitbake.conf
-v, --verbose output more chit-chat to the terminal
-D, --debug Increase the debug level. You can specify this more
than once.
-n, --dry-run don't execute, just go through the motions
-p, --parse-only quit after parsing the BB files (developers only)
-d, --disable-psyco disable using the psyco just-in-time compiler (not
recommended)
-s, --show-versions show current and preferred versions of all packages
-e, --environment show the global or per-package environment (this is
what used to be bbread)
-g, --graphviz emit the dependency trees of the specified packages in
the dot syntax
-I IGNORED_DOT_DEPS, --ignore-deps=IGNORED_DOT_DEPS
Stop processing at the given list of dependencies when
generating dependency graphs. This can help to make
the graph more appealing
-l DEBUG_DOMAINS, --log-domains=DEBUG_DOMAINS
Show debug logging for the specified logging domains
-P, --profile profile the command and print a report</screen>
</section>
<section id='ref-bitbake-fetchers'>
<title>Fetchers</title>
<para>
As well as the containing the parsing and task/dependency handling
code, bitbake also contains a set of "fetcher" modules which allow
fetching of source code from various types of sources. Example
sources might be from disk with the metadata, from websites, from
remote shell accounts or from SCM systems like cvs/subversion/git.
</para>
<para>
The fetchers are usually triggered by entries in
<glossterm><link linkend='var-SRC_URI'>SRC_URI</link></glossterm>. Information about the
options and formats of entries for specific fetchers can be found in the
<ulink url='http://bitbake.berlios.de/manual/'>BitBake manual</ulink>.
</para>
<para>
One useful feature for certain SCM fetchers is the ability to
"auto-update" when the upstream SCM changes version. Since this
requires certain functionality from the SCM only certain systems
support it, currently Subversion, Bazaar and to a limited extent, Git. It
works using the <glossterm><link linkend='var-SRCREV'>SRCREV</link>
</glossterm> variable. See the <link linkend='platdev-appdev-srcrev'>
developing with an external SCM based project</link> section for more
information.
</para>
</section>
</appendix>
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