<title>Sharing Functionality</title>
<para>
- BitBake allows for metadata sharing through include files and
- class files (<filename>.bbclass</filename>).
+ BitBake allows for metadata sharing through include files
+ (<filename>.inc</filename>) and class files
+ (<filename>.bbclass</filename>).
For example, suppose you have a piece of common functionality
such as a task definition that you want to share between
more than one recipe.
In this case, creating a <filename>.bbclass</filename>
- file that contains the common functionality and uses the
- <filename>inherit</filename> directive would be a common
- way to share the task.
+ file that contains the common functionality and then using
+ the <filename>inherit</filename> directive in your recipes to
+ inherit the class would be a common way to share the task.
</para>
<para>
allow you to share functionality between recipes.
Specifically, the mechanisms include <filename>include</filename>,
<filename>inherit</filename>, <filename>INHERIT</filename>, and
- <filename>require</filename> statements.
+ <filename>require</filename> directives.
</para>
<section id='locating-include-and-class-files'>
<title>Locating Include and Class Files</title>
<para>
- BitBake uses the <filename>BBPATH</filename> variable
- to locate needed class and configuration files.
+ BitBake uses the
+ <link linkend='var-BBPATH'><filename>BBPATH</filename></link>
+ variable to locate needed include and class files.
The <filename>BBPATH</filename> variable is analogous to
the environment variable <filename>PATH</filename>.
- Use of <filename>BBPATH</filename> is specific to the build
- environment (e.g. Yocto Project, OpenEmbedded, and so forth).
+ </para>
+
+ <para>
+ In order for include and class files to be found by BitBake,
+ they need to be located in a "classes" subdirectory that can
+ be found in <filename>BBPATH</filename>.
</para>
</section>
When writing a recipe or class file, you can use the
<filename>inherit</filename> directive to inherit the
functionality of a class (<filename>.bbclass</filename>).
- BitBake only supports this directive when used within these
- two types of files (i.e. <filename>.bb</filename> and
- <filename>.bbclass</filename> files).
+ BitBake only supports this directive when used within recipe
+ and class files (i.e. <filename>.bb</filename> and
+ <filename>.bbclass</filename>).
</para>
<para>
The <filename>inherit</filename> directive is a rudimentary
means of specifying what classes of functionality your
- recipe requires.
+ recipes require.
For example, you can easily abstract out the tasks involved in
building a package that uses Autoconf and Automake and put
those tasks into a class file that can be used by your package.
- As an example, an <filename>autotools.bbclass</filename> file
- could use the following directive to inherit needed
- site information:
+ </para>
+
+ <para>
+ As an example, your recipes could use the following directive
+ to inherit an <filename>autotools.bbclass</filename> file.
+ The class file would contain common functionality for using
+ Autotools that could be shared across recipes:
<literallayout class='monospaced'>
- inherit siteinfo
+ inherit autotools
</literallayout>
In this case, BitBake would search for the directory
- <filename>classes/siteinfo.bbclass</filename>
+ <filename>classes/autotools.bbclass</filename>
in <filename>BBPATH</filename>.
+ <note>
+ You can override any values and functions of the
+ inherited class within your recipe by doing so
+ after the "inherit" statement.
+ </note>
</para>
</section>
<para>
As an example, suppose you needed a recipe to include some
- self-test files:
+ self-test definitions:
<literallayout class='monospaced'>
- include test_recipe.inc
+ include test_defs.inc
</literallayout>
<note>
The <filename>include</filename> directive does not
</para>
<para>
- Similar to how BitBake uses <filename>include</filename>,
+ Similar to how BitBake uses
+ <link linkend='include-directive'><filename>include</filename></link>,
if the path specified
on the require line is a relative path, BitBake locates
the first file it can find within <filename>BBPATH</filename>.
</para>
<para>
- As an example, suppose you needed a recipe to require some
- self-test files:
+ As an example, suppose you have two versions of a recipe
+ (e.g. <filename>foo_1.2.2.bb</filename> and
+ <filename>foo_2.0.0.bb</filename>) where
+ each version contains some identical functionality that could be
+ shared.
+ You could create an include file named <filename>foo.inc</filename>
+ that contains the common definitions needed to build "foo".
+ You need to be sure <filename>foo.inc</filename> is located in the
+ same directory as your two recipe files as well.
+ Once these conditions are set up, you can share the functionality
+ using a <filename>require</filename> directive from within each
+ recipe:
<literallayout class='monospaced'>
- require test_recipe.inc
+ require foo.inc
</literallayout>
</para>
</section>
</para>
<para>
- Suppose you needed to inherit a multilib global class.
+ As an example, suppose you needed to inherit a class
+ file called <filename>abc.bbclass</filename> from a
+ configuration file as follows:
<literallayout class='monospaced'>
- INHERIT += "multilib_global"
+ INHERIT += "abc"
</literallayout>
- This configuration directive causes the named class to be inherited
- at the point of the directive during parsing.
+ This configuration directive causes the named
+ class to be inherited at the point of the directive
+ during parsing.
+ As with the <filename>inherit</filename> directive, the
+ <filename>.bbclass</filename> file must be located in a
+ "classes" subdirectory in one of the directories specified
+ in <filename>BBPATH</filename>.
+ <note>
+ Because <filename>.conf</filename> files are parsed
+ first during BitBake's execution, using
+ <filename>INHERIT</filename> to inherit a class effectively
+ inherits the class globally (i.e. for all recipes).
+ </note>
</para>
</section>
</section>
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