path: root/bitbake/doc/user-manual/user-manual-metadata.xml

<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<chapter id="user-manual-metadata">
    <title>Metadata</title>

    <section>
        <title>Overview</title>

        <para>
            The BitBake task executor together with various types of configuration
            files form the OpenEmbedded Core.
            This section provides an overview of the task executor and the
            configuration files by describing their use and interaction.
        </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>
                    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>
                    Machine-specific settings, policy decisions,
                    and so forth.
                    Configuration data acts as the glue to bind everything
                    together.</para></listitem>
            </itemizedlist>
            The remainder of this chapter provides examples of BitBake metadata.
            Any syntax not supported in any of the previously listed areas
            is documented as such.
        </para>
    </section>

    <section id='basic-syntax'>
        <title>Basic Syntax</title>

        <para>
            This section provides some basic syntax examples.
        </para>

        <section id='basic-variable-setting'>
            <title>Basic Variable Setting</title>

            <para>
                The following example sets <filename>VARIABLE</filename> to
                "value".
                This assignment occurs immediately as the statement is parsed.
                It is a "hard" assignment.
                <literallayout class='monospaced'>
     VARIABLE = "value"
                </literallayout>
            </para>
        </section>

        <section id='variable-expansion'>
            <title>Variable Expansion</title>

            <para>
                BitBake supports variables referencing one another's
                contents using a syntax that is similar to shell scripting.
                Following is an example that results in <filename>A</filename>
                containing "aval" and <filename>B</filename> containing
                "preavalpost".
                <literallayout class='monospaced'>
     A = "aval"
     B = "pre${A}post"
                </literallayout>
            </para>
        </section>

        <section id='setting-a-default-value'>
            <title>Setting a default value (?=)</title>

            <para>
                You can use the "?=" operator to achieve a "softer" assignment
                for a variable.
                This type of assignment allows you to define a variable if it
                is undefined when the statement is parsed, but to leave the
                value alone if the variable has a value.
                Here is an example:
                <literallayout class='monospaced'>
     A ?= "aval"
                </literallayout>
                If <filename>A</filename> is set at the time this statement is parsed,
                the variable retains its value.
                However, if <filename>A</filename> is not set,
                the variable is set to "aval".
                <note>
                    This assignment is immediate.
                    Consequently, if multiple "?=" assignments
                    to a single variable exist, the first of those ends up getting
                    used.
                </note>
            </para>
        </section>

        <section id='setting-a-weak-default-value'>
            <title>Setting a weak default value (??=)</title>

            <para>
                It is possible to use a "weaker" assignment that in the
                previous section by using the "??=" operator.
                This assignment behaves identical to "?=" except that the
                assignment is made at the end of the parsing process rather
                than immediately.
                Consequently, when multiple "??=" assignments exist, the last
                one is used.
                Also, any "=" or "?=" assignment will override the value set with
                "??=".
                Here is an example:
                <literallayout class='monospaced'>
     A ??= "somevalue"
     A ??= "someothervalue"
                </literallayout>
                If <filename>A</filename> is set before the above statements are parsed,
                the variable retains its value.
                If <filename>A</filename> is not set,
                the variable is set to "someothervalue".
            </para>

            <para>
                Again, this assignment is a "lazy" or "weak" assignment
                because it does not occur until the end
                of the parsing process.
            </para>
        </section>

        <section id='immediate-variable-expansion'>
            <title>Immediate variable expansion (:=)</title>

            <para>
                The ":=" operator results in a variable's
                contents being expanded immediately,
                rather than when the variable is actually used:
                <literallayout class='monospaced'>
     T = "123"
     A := "${B} ${A} test ${T}"
     T = "456"
     B = "${T} bval"
     C = "cval"
     C := "${C}append"
                </literallayout>
                In this example, <filename>A</filename> contains
                "test 123" because <filename>${B}</filename> and
                <filename>${A}</filename> at the time of parsing are undefined,
                which leaves "test 123".
                And, the variable <filename>C</filename>
                contains "cvalappend" since <filename>${C}</filename> immediately
                expands to "cval".
            </para>
        </section>

        <section id='appending-and-prepending'>
            <title>Appending (+=) and prepending (=+) With Spaces</title>

            <para>
                Appending and prepending values is common and can be accomplished
                using the "+=" and "=+" operators.
                These operators insert a space between the current
                value and prepended or appended value.
                Here are some examples:
                <literallayout class='monospaced'>
     B = "bval"
     B += "additionaldata"
     C = "cval"
     C =+ "test"
                </literallayout>
                The variable <filename>B</filename> contains
                "bval additionaldata" and <filename>C</filename>
                contains "test cval".
            </para>
        </section>

        <section id='appending-and-prepending-without-spaces'>
            <title>Appending (.=) and Prepending (=.) Without Spaces</title>

            <para>
                If you want to append or prepend values without an
                inserted space, use the ".=" and "=." operators.
                Here are some examples:
                <literallayout class='monospaced'>
     B = "bval"
     B .= "additionaldata"
     C = "cval"
     C =. "test"
                </literallayout>
                The variable <filename>B</filename> contains
                "bvaladditionaldata" and
                <filename>C</filename> contains "testcval".
            </para>
        </section>

        <section id='appending-and-prepending-override-style-syntax'>
            <title>Appending and Prepending (Override Style Syntax)</title>

            <para>
                You can also append and prepend a variable's value
                using an override style syntax.
                When you use this syntax, no spaces are inserted.
                Here are some examples:
                <literallayout class='monospaced'>
     B = "bval"
     B_append = " additional data"
     C = "cval"
     C_prepend = "additional data "
     D = "dval"
     D_append = "additional data"
                </literallayout>
                The variable <filename>B</filename> becomes
                "bval additional data" and <filename>C</filename> becomes
                "additional data cval".
                The variable <filename>D</filename> becomes
                "dvaladditional data".
                <note>
                    You must control all spacing when you use the
                    override syntax.
                </note>
            </para>
        </section>

        <section id='removing-override-style-syntax'>
            <title>Removal (Override Style Syntax)</title>

            <para>
                You can remove values from lists using the removal
                override style syntax.
                Specifying a value for removal causes all occurrences of that
                value to be removed from the variable.
            </para>

            <para>
                When you use this syntax, BitBake expects one or more strings.
                Surrounding spaces are removed as well.
                Here is an example:
                <literallayout class='monospaced'>
     FOO = "123 456 789 123456 123 456 123 456"
     FOO_remove = "123"
     FOO_remove = "456"
     FOO2 = "abc def ghi abcdef abc def abc def"
     FOO2_remove = "abc def"
                </literallayout>
                The variable <filename>FOO</filename> becomes
                "789 123456" and <filename>FOO2</filename> becomes
                "ghi abcdef".
            </para>
        </section>

        <section id='variable-flag-syntax'>
            <title>Variable Flag Syntax</title>

            <para>
                Variable flags are BitBake's implementation of variable properties
                or attributes.
                It is a way of tagging extra information onto a variable.
                You can find more out about variable flags in general in the
                "<link linkend='variable-flags'>Variable Flags</link>"
                section.
            </para>

            <para>
                You can define, append, and prepend values to variable flags.
                All the standard syntax operations previously mentioned work
                for variable flags except for override style syntax
                (i.e. <filename>_prepend</filename>, <filename>_append</filename>,
                and <filename>_remove</filename>).
            </para>

            <para>
                Here are some examples showing how to set variable flags:
                <literallayout class='monospaced'>
     FOO[a] = "abc"
     FOO[b] = "123"
     FOO[a] += "456"
                </literallayout>
                The variable <filename>FOO</filename> has two flags:
                <filename>a</filename> and <filename>b</filename>.
                The flags are immediately set to "abc" and "123", respectively.
                The <filename>a</filename> flag becomes "abc456".
            </para>
        </section>

        <section id='inline-python-variable-expansion'>
            <title>Inline Python Variable Expansion</title>

            <para>
                You can use inline Python variable expansion to
                set variables.
                Here is an example:
                <literallayout class='monospaced'>
     DATE = "${@time.strftime('%Y%m%d',time.gmtime())}"
                </literallayout>
                This example results in the <filename>DATE</filename>
                variable becoming the current date.
            </para>
        </section>
    </section>

    <section id='conditional-syntax-overrides'>
        <title>Conditional Syntax (Overrides)</title>

        <para>
            BitBake uses
            <link linkend='var-OVERRIDES'><filename>OVERRIDES</filename></link>
            to control what variables are overridden after BitBake
            parses recipes and configuration files.
            This section describes how you can use
            <filename>OVERRIDES</filename> as conditional metadata,
            talks about key expansion in relationship to
            <filename>OVERRIDES</filename>, and provides some examples
            to help with understanding.
        </para>

        <section id='conditional-metadata'>
            <title>Conditional Metadata</title>

            <para>
                You can use <filename>OVERRIDES</filename> to conditionally select
                a specific version of a variable and to conditionally
                append or prepend the value of a variable.
                <itemizedlist>
                    <listitem><para><emphasis>Selecting a Variable:</emphasis>
                        The <filename>OVERRIDES</filename> variable is
                        a colon-character-separated list that contains items
                        for which you want to satisfy conditions.
                        Thus, if you have a variable that is conditional on “arm”, and “arm”
                        is in <filename>OVERRIDES</filename>, then the “arm”-specific
                        version of the variable is used rather than the non-conditional
                        version.
                        Here is an example:
                        <literallayout class='monospaced'>
     OVERRIDES = "architecture:os:machine"
     TEST = "default"
     TEST_os = "osspecific"
     TEST_nooverride = "othercondvalue"
                        </literallayout>
                        In this example, the <filename>OVERRIDES</filename>
                        variable lists three overrides:
                        "architecture", "os", and "machine".
                        The variable <filename>TEST</filename> by itself has a default
                        value of "default".
                        You select the os-specific version of the <filename>TEST</filename>
                        variable by appending the "os" override to the variable
                        (i.e.<filename>TEST_os</filename>).
                        </para></listitem>
                    <listitem><para><emphasis>Appending and Prepending:</emphasis>
                        BitBake also supports append and prepend operations to
                        variable values based on whether a specific item is
                        listed in <filename>OVERRIDES</filename>.
                        Here is an example:
                        <literallayout class='monospaced'>
     DEPENDS = "glibc ncurses"
     OVERRIDES = "machine:local"
     DEPENDS_append_machine = "libmad"
                        </literallayout>
                        In this example, <filename>DEPENDS</filename> becomes
                        "glibc ncurses libmad".
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='key-expansion'>
            <title>Key Expansion</title>

            <para>
                Key expansion happens when the BitBake datastore is finalized
                just before BitBake expands overrides.
                To better understand this, consider the following example:
                <literallayout class='monospaced'>
     A${B} = "X"
     B = "2"
     A2 = "Y"
                </literallayout>
                In this case, after all the parsing is complete, and
                before any overrides are handled, BitBake expands
                <filename>${B}</filename> into "2".
                This expansion causes <filename>A2</filename>, which was
                set to "Y" before the expansion, to become "X".
            </para>
        </section>

        <section id='variable-interaction-worked-examples'>
            <title>Examples</title>

            <para>
                Despite the previous explanations that show the different forms of
                variable definitions, it can be hard to work
                out exactly what happens when variable operators, conditional
                overrides, and unconditional overrides are combined.
                This section presents some common scenarios along
                with explanations for variable interactions that
                typically confuse users.
            </para>

            <para>
                There is often confusion concerning the order in which
                overrides and various "append" operators take effect.
                Recall that an append or prepend operation using "_append"
                and "_prepend" does not result in an immediate assignment
                as would "+=", ".=", "=+", or "=.".
                Consider the following example:
                <literallayout class='monospaced'>
     OVERRIDES = "foo"
     A = "Z"
     A_foo_append = "X"
                </literallayout>
                For this case, <filename>A</filename> is
                unconditionally set to "Z" and "X" is
                unconditionally and immediately appended to the variable
                <filename>A_foo</filename>.
                Because overrides have not been applied yet,
                <filename>A_foo</filename> is set to "X" due to the append
                and <filename>A</filename> simply equals "Z".
            </para>

            <para>
                Applying overrides, however, changes things.
                Since "foo" is listed in <filename>OVERRIDES</filename>,
                the conditional variable <filename>A</filename> is replaced
                with the "foo" version, which is equal to "X".
                So effectively, <filename>A_foo</filename> replaces <filename>A</filename>.
            </para>

            <para>
                This next example changes the order of the override and
                the append:
                <literallayout class='monospaced'>
     OVERRIDES = "foo"
     A = "Z"
     A_append_foo = "X"
                </literallayout>
                For this case, before overrides are handled,
                <filename>A</filename> is set to "Z" and <filename>A_append_foo</filename>
                is set to "X".
                Once the override for "foo" is applied, however,
                <filename>A</filename> gets appended with "X".
                Consequently, <filename>A</filename> becomes "ZX".
                Notice that spaces are not appended.
            </para>

            <para>
                This next example has the order of the appends and overrides reversed
                back as in the first example:
                <literallayout class='monospaced'>
     OVERRIDES = "foo"
     A = "Y"
     A_foo_append = "Z"
     A_foo_append += "X"
                </literallayout>
                For this case, before any overrides are resolved,
                <filename>A</filename> is set to "Y" using an immediate assignment.
                After this immediate assignment, <filename>A_foo</filename> is set
                to "Z", and then further appended with
                "X" leaving the variable set to "Z X".
                Finally, applying the override for "foo" results in the conditional
                variable <filename>A</filename> becoming "Z X" (i.e.
                <filename>A</filename> is replaced with <filename>A_foo</filename>).
            </para>

            <para>
                This final example mixes in some varying operators:
                <literallayout class='monospaced'>
     A = "1"
     A_append = "2"
     A_append = "3"
     A += "4"
     A .= "5"
                </literallayout>
                For this case, the type of append operators are affecting the
                order of assignments as BitBake passes through the code
                multiple times.
                Initially, <filename>A</filename> is set to "1 45" because
                of the three statements that use immediate operators.
                After these assignments are made, BitBake applies the
                <filename>_append</filename> operations.
                Those operations result in <filename>A</filename> becoming "1 4523".
            </para>
        </section>
    </section>

    <section id='sharing-functionality'>
        <title>Sharing Functionality</title>

        <para>
            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 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>
            This section presents the mechanisms BitBake provides to
            allow you to share functionality between recipes.
            Specifically, the mechanisms include <filename>include</filename>,
            <filename>inherit</filename>, <filename>INHERIT</filename>, and
            <filename>require</filename> directives.
        </para>

        <section id='locating-include-and-class-files'>
            <title>Locating Include and Class Files</title>

            <para>
                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>.
            </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>

        <section id='inherit-directive'>
            <title><filename>inherit</filename> Directive</title>

            <para>
                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 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
                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.
            </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 autotools
                </literallayout>
                In this case, BitBake would search for the directory
                <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>

        <section id='include-directive'>
            <title><filename>include</filename> Directive</title>

            <para>
                BitBake understands the <filename>include</filename>
                directive.
                This directive causes BitBake to parse whatever file you specify,
                and to insert that file at that location.
                The directive is much like Make except that if the path specified
                on the include 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 include some
                self-test definitions:
                <literallayout class='monospaced'>
     include test_defs.inc
                </literallayout>
                <note>
                    The <filename>include</filename> directive does not
                    produce an error when the file cannot be found.
                    Consequently, it is recommended that if the file you
                    are including is expected to exist, you should use
                    <link linkend='require-inclusion'><filename>require</filename></link>
                    instead of <filename>include</filename>.
                    Doing so makes sure that an error is produced if the
                    file cannot be found.
                </note>
            </para>
        </section>

        <section id='require-inclusion'>
            <title><filename>require</filename> Directive</title>

            <para>
                BitBake understands the <filename>require</filename>
                directive.
                This directive behaves just like the
                <filename>include</filename> directive with the exception that
                BitBake raises a parsing error if the file to be included cannot
                be found.
                Thus, any file you require is inserted into the file that is
                being parsed at the location of the directive.
            </para>

            <para>
                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 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 foo.inc
                </literallayout>
            </para>
        </section>

        <section id='inherit-configuration-directive'>
            <title><filename>INHERIT</filename> Configuration Directive</title>

            <para>
                When creating a configuration file (<filename>.conf</filename>),
                you can use the <filename>INHERIT</filename> directive to
                inherit a class.
                BitBake only supports this directive when used within
                a configuration file.
            </para>

            <para>
                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 += "abc"
                </literallayout>
                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>

    <section id='functions'>
        <title>Functions</title>

        <para>
            As with most languages, functions are the building blocks that
            are used to build up operations into tasks.
            BitBake supports three types of functions:
            <itemizedlist>
                <listitem><para><emphasis>Shell Functions:</emphasis>
                    Functions written in shell script and executed either
                    directly as functions, tasks, or both.
                    They can also be called by other shell functions.
                    </para></listitem>
                <listitem><para><emphasis>BitBake Style Python Functions:</emphasis>
                    Functions written in Python and executed by BitBake or other
                    Python functions using <filename>bb.build.exec_func()</filename>.
                    </para></listitem>
                <listitem><para><emphasis>Python Functions:</emphasis>
                    Functions written in Python and executed by Python.
                    </para></listitem>
            </itemizedlist>
            Regardless of the type of function, you can only
            define them in class (<filename>.bbclass</filename>)
            and recipe (<filename>.bb</filename> or <filename>.inc</filename>)
            files.
        </para>

        <section id='shell-functions'>
            <title>Shell Functions</title>

            <para>
                Functions written in shell script and executed either
                directly as functions, tasks, or both.
                They can also be called by other shell functions.
                Here is an example shell function definition:
                <literallayout class='monospaced'>
     some_function () {
         echo "Hello World"
     }
                </literallayout>
                When you create these types of functions in your recipe
                or class files, you need to follow the shell programming
                rules.
                The scripts are executed by <filename>/bin/sh</filename>,
                which may not be a bash shell but might be something
                such as <filename>dash</filename>.
                You should not use Bash-specific script (bashisms).
            </para>
        </section>

        <section id='bitbake-style-python-functions'>
            <title>BitBake Style Python Functions</title>

            <para>
                These functions are written in Python and executed by
                BitBake or other Python functions using
                <filename>bb.build.exec_func()</filename>.
            </para>

            <para>
                An example BitBake function is:
                <literallayout class='monospaced'>
     python some_python_function () {
         d.setVar("TEXT", "Hello World")
         print d.getVar("TEXT", True)
     }
                </literallayout>
                Because the Python "bb" and "os" modules are already
                imported, you do not need to import these modules.
                Also in these types of functions, the datastore ("d")
                is a global variable and is always automatically
                available.
           </para>
        </section>

        <section id='python-functions'>
            <title>Python Functions</title>

            <para>
                These functions are written in Python and are executed by
                other Python code.
                Examples of Python functions are utility functions
                that you intend to call from in-line Python or
                from within other Python functions.
                Here is an example:
                <literallayout class='monospaced'>
     def get_depends(d):
         if d.getVar('SOMECONDITION', True):
             return "dependencywithcond"
         else:
             return "dependency"
     SOMECONDITION = "1"
     DEPENDS = "${@get_depends(d)}"
                </literallayout>
                This would result in <filename>DEPENDS</filename>
                containing <filename>dependencywithcond</filename>.
            </para>

            <para>
                Here are some things to know about Python functions:
                <itemizedlist>
                    <listitem><para>Python functions can take parameters.
                        </para></listitem>
                    <listitem><para>The BitBake datastore is not
                        automatically available.
                        Consequently, you must pass it in as a
                        parameter to the function.
                        </para></listitem>
                    <listitem><para>The "bb" and "os" Python modules are
                        automatically available.
                        You do not need to import them.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>
    </section>

    <section id='tasks'>
        <title>Tasks</title>

        <para>
            Tasks are BitBake execution units that originate as
            functions and make up the steps that BitBake needs to run
            for given recipe.
            Tasks are only supported in recipe (<filename>.bb</filename>
            or <filename>.inc</filename>) and class
            (<filename>.bbclass</filename>) files.
            By convention, tasks begin with the string "do_".
        </para>

        <para>
            Here is an example of a task that prints out the date:
            <literallayout class='monospaced'>
     python do_printdate () {
         import time print
         time.strftime('%Y%m%d', time.gmtime())
     }
            </literallayout>
        </para>

        <section id='promoting-a-function-to-a-task'>
            <title>Promoting a Function to a Task</title>

            <para>
                Any function can be promoted to a task by applying the
                <filename>addtask</filename> command.
                The <filename>addtask</filename> command also describes
                inter-task dependencies.
                Here is the function from the previous section but with the
                <filename>addtask</filename> command promoting it to a task
                and defining some dependencies:
                <literallayout class='monospaced'>
     python do_printdate () {
         import time print
         time.strftime('%Y%m%d', time.gmtime())
     }
     addtask printdate after do_fetch before do_build
                </literallayout>
                In the example, the function is defined and then promoted
                as a task.
                The <filename>do_printdate</filename> task becomes a dependency of
                the <filename>do_build</filename> task, which is the default
                task.
                And, the <filename>do_printdate</filename> task is dependent upon
                the <filename>do_fetch</filename> task.
                Execution of the <filename>do_build</filename> task results
                in the <filename>do_printdate</filename> task running first.
            </para>
        </section>

        <section id='passing-information-into-the-build-task-environment'>
            <title>Passing Information Into the Build Task Environment</title>

            <para>
                When running a task, BitBake tightly controls the execution
                environment of the build tasks to make
                sure unwanted contamination from the build machine cannot
                influence the build.
                Consequently, if you do want something to get passed into the
                build task environment, you must take these two steps:
                <orderedlist>
                    <listitem><para>
                        Tell BitBake to load what you want from the environment
                        into the datastore.
                        You can do so through the
                        <link linkend='var-BB_ENV_EXTRAWHITE'><filename>BB_ENV_EXTRAWHITE</filename></link>
                        variable.
                        For example, assume you want to prevent the build system from
                        accessing your <filename>$HOME/.ccache</filename>
                        directory.
                        The following command tells BitBake to load
                        <filename>CCACHE_DIR</filename> from the environment into
                        the datastore:
                        <literallayout class='monospaced'>
     export BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE CCACHE_DIR"
                        </literallayout></para></listitem>
                    <listitem><para>
                        Tell BitBake to export what you have loaded into the
                        datastore to the task environment of every running task.
                        Loading something from the environment into the datastore
                        (previous step) only makes it available in the datastore.
                        To export it to the task environment of every running task,
                        use a command similar to the following in your local configuration
                        file <filename>local.conf</filename> or your
                        distribution configuration file:
                        <literallayout class='monospaced'>
     export CCACHE_DIR
                        </literallayout>
                        <note>
                            A side effect of the previous steps is that BitBake
                            records the variable as a dependency of the build process
                            in things like the setscene checksums.
                            If doing so results in unnecessary rebuilds of tasks, you can
                            whitelist the variable so that the setscene code
                            ignores the dependency when it creates checksums.
                        </note></para></listitem>
                </orderedlist>
            </para>
        </section>
    </section>

    <section id='variable-flags'>
        <title>Variable Flags</title>

        <para>
            Variable flags (varflags) help control a task's functionality
            and dependencies.
            BitBake reads and writes varflags to the datastore using the following
            command forms:
            <literallayout class='monospaced'>
     &lt;variable&gt; = d.getVarFlags("&lt;variable&gt;")
     self.d.setVarFlags("FOO", {"func": True})
            </literallayout>
        </para>

        <para>
            When working with varflags, the same syntax, with the exception of
            overrides, applies.
            In other words, you can set, append, and prepend varflags just like
            variables.
            See the
            "<link linkend='variable-flag-syntax'>Variable Flag Syntax</link>"
            section for details.
        </para>

        <para>
            BitBake has a defined set of varflags available for recipes and
            classes.
            You can discover the complete set by using <filename>grep</filename>
            within a shell and search on the string "VarFlags".
        </para>

        <para>
            Tasks support a number of these flags which control various
            functionality of the task:
            <itemizedlist>
                <listitem><para><emphasis>dirs:</emphasis>
                    Directories that should be created before the task runs.
                    </para></listitem>
                <listitem><para><emphasis>cleandirs:</emphasis>
                    Empty directories that should created before the task runs.
                    </para></listitem>
                <listitem><para><emphasis>noexec:</emphasis>
                    Marks the tasks as being empty and no execution required.
                    These flags are used as dependency placeholders or used when
                    added tasks need to be subsequently disabled.
                    </para></listitem>
                <listitem><para><emphasis>nostamp:</emphasis>
                    Tells BitBake to not generate a stamp file for a task,
                    which implies the task is always executed.
                    </para></listitem>
                <listitem><para><emphasis>fakeroot:</emphasis>
                    Causes a task to be run in a fakeroot environment,
                    obtained by adding the variables in
                    <link linkend='var-FAKEROOTENV'><filename>FAKEROOTENV</filename></link>
                    to the environment.
                    </para></listitem>
                <listitem><para><emphasis>umask:</emphasis>
                    The umask to run the task under.
                    </para></listitem>
                <listitem><para><emphasis>deptask:</emphasis>
                    Controls task build-time dependencies.
                    See the
                    <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                    variable and the
                    "<link linkend='build-dependencies'>Build Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>rdeptask:</emphasis>
                    Controls task runtime dependencies.
                    See the
                    <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>
                    variable, the
                    <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                    variable, and the
                    "<link linkend='runtime-dependencies'>Runtime Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>recrdeptask:</emphasis>
                    Controls task recursive runtime dependencies.
                    See the
                    <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>
                    variable, the
                    <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                    variable, and the
                    "<link linkend='recursive-dependencies'>Recursive Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>depends:</emphasis>
                    Controls inter-task dependencies.
                    See the
                    <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                    variable and the
                    "<link linkend='inter-task-dependencies'>Inter-Task Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>rdepends:</emphasis>
                    Controls inter-task runtime dependencies.
                    See the
                    <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>
                    variable, the
                    <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                    variable, and the
                    "<link linkend='inter-task-dependencies'>Inter-Task Dependencies</link>"
                    section for more information.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='events'>
        <title>Events</title>

        <para>
            BitBake allows installation of event handlers within
            recipe and class files.
            Events are triggered at certain points during operation,
            such as the beginning of operation against a given
            <filename>.bb</filename>, the start of a given task,
            task failure, task success, and so forth.
            The intent is to make it easy to do things like email
            notification on build failure.
        </para>

        <para>
            Following is an example event handler that
            prints the name of the event and the content of
            the <filename>FILE</filename> variable:
            <literallayout class='monospaced'>
     addhandler myclass_eventhandler
     python myclass_eventhandler() {
         from bb.event import getName
         from bb import data
         print("The name of the Event is %s" % getName(e))
         print("The file we run for is %s" % data.getVar('FILE', e.data, True))
     }
            </literallayout>
            This event handler gets called every time an event is
            triggered.
            A global variable "<filename>e</filename>" is defined and
            "<filename>e.data</filename>" contains an instance of
            "<filename>bb.data</filename>".
            With the <filename>getName(e)</filename> method, one can get
            the name of the triggered event.
        </para>

        <para>
            During all builds, the following common events occur:
            <itemizedlist>
                <listitem><para>
                    <filename>bb.event.ConfigParsed()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ParseStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ParseProgress()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ParseCompleted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.BuildStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskInvalid()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskFailedSilent()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskFailed()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskSucceeded()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.BuildCompleted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.cooker.CookerExit()</filename>
                    </para></listitem>
            </itemizedlist>
            Here is a list of other events that occur based on specific requests
            to the server:
            <itemizedlist>
                <listitem><para>
                    <filename>bb.event.TreeDataPreparationStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.TreeDataPreparationProgress</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.TreeDataPreparationCompleted</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.DepTreeGenerated</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.CoreBaseFilesFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ConfigFilePathFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.FilesMatchingFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ConfigFilesFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.TargetsTreeGenerated</filename>
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='variants-class-extension-mechanism'>
        <title>Variants - Class Extension Mechanism</title>

        <para>
            BitBake supports two features that facilitate creating
            from a single recipe file multiple incarnations of that
            recipe file where all incarnations are buildable.
            These features are enabled through the
            <link linkend='var-BBCLASSEXTEND'><filename>BBCLASSEXTEND</filename></link>
            and
            <link linkend='var-BBVERSIONS'><filename>BBVERSIONS</filename></link>
            variables:
            <itemizedlist>
                <listitem><para><emphasis><filename>BBCLASSEXTEND</filename>:</emphasis>
                    This variable is a space separated list of classes used to "extend" the
                    recipe for each variant.
                    Here is an example that results in a second incarnation of the current
                    recipe being available.
                    This second incarnation will have the "native" class inherited.
                    <literallayout class='monospaced'>
     BBCLASSEXTEND = "native"
                    </literallayout></para></listitem>
                <listitem><para><emphasis><filename>BBVERSIONS</filename>:</emphasis>
                    This variable allows a single recipe to build multiple versions of a
                    project from a single recipe file.
                    You can also specify conditional metadata
                    (using the
                    <link linkend='var-OVERRIDES'><filename>OVERRIDES</filename></link>
                    mechanism) for a single version, or an optionally named range of versions.
                    Here is an example:
                    <literallayout class='monospaced'>
     BBVERSIONS = "1.0 2.0 git"
     SRC_URI_git = "git://someurl/somepath.git"

     BBVERSIONS = "1.0.[0-6]:1.0.0+ \ 1.0.[7-9]:1.0.7+"
     SRC_URI_append_1.0.7+ = "file://some_patch_which_the_new_versions_need.patch;patch=1"
                    </literallayout>
                    The name of the range defaults to the original version of the
                    recipe.
                    For example, in OpenEmbedded, the recipe file
                    <filename>foo_1.0.0+.bb</filename> creates a default name range
                    of <filename>1.0.0+</filename>.
                    This is useful because the range name is not only placed
                    into overrides, but it is also made available for the metadata to use
                    in the variable that defines the base recipe versions for use in
                    <filename>file://</filename> search paths
                    (<link linkend='var-FILESPATH'><filename>FILESPATH</filename></link>).
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='dependencies'>
        <title>Dependencies</title>

        <para>
            To allow for efficient operation given multiple processes
            executing in parallel, BitBake handles dependencies at
            the task level.
            BitBake supports a robust method to handle these dependencies.
        </para>

        <para>
            This section describes several types of dependency mechanisms.
        </para>

        <section id='dependencies-internal-to-the-bb-file'>
            <title>Dependencies Internal to the <filename>.bb</filename> File</title>

            <para>
                BitBake uses the <filename>addtask</filename> directive
                to manage dependencies that are internal to a given recipe
                file.
                You can use the <filename>addtask</filename> directive to
                indicate when a task is dependent on other tasks or when
                other tasks depend on that recipe.
                Here is an example:
                <literallayout class='monospaced'>
     addtask printdate after do_fetch before do_build
                </literallayout>
                In this example, the <filename>printdate</filename> task is
                depends on the completion of the <filename>do_fetch</filename>
                task.
                And, the <filename>do_build</filename> depends on the completion
                of the <filename>printdate</filename> task.
            </para>
        </section>

        <section id='build-dependencies'>
            <title>Build Dependencies</title>

            <para>
                BitBake uses the
                <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                variable to manage build time dependencies.
                The "deptask" varflag for tasks signifies the task of each
                item listed in <filename>DEPENDS</filename> that must
                complete before that task can be executed.
                Here is an example:
                <literallayout class='monospaced'>
     do_configure[deptask] = "do_populate_staging"
                </literallayout>
                In this example, the <filename>do_populate_staging</filename>
                task of each item in <filename>DEPENDS</filename> must complete before
                <filename>do_configure</filename> can execute.
            </para>
        </section>

        <section id='runtime-dependencies'>
            <title>Runtime Dependencies</title>

            <para>
                BitBake uses the
                <link linkend='var-PACKAGES'><filename>PACKAGES</filename></link>,
                <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>, and
                <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                variables to manage runtime dependencies.
            </para>

            <para>
                The <filename>PACKAGES</filename> variable lists runtime
                packages.
                Each of those packages can have <filename>RDEPENDS</filename> and
                <filename>RRECOMMENDS</filename> runtime dependencies.
                The "rdeptask" flag for tasks is used to signify the task of each
                item runtime dependency which must have completed before that
                task can be executed.
                <literallayout class='monospaced'>
     do_package_write[rdeptask] = "do_package"
                </literallayout>
                In the previous example, the <filename>do_package</filename>
                task of each item in <filename>RDEPENDS</filename> must have
                completed before <filename>do_package_write</filename> can execute.
            </para>
        </section>

        <section id='recursive-dependencies'>
            <title>Recursive Dependencies</title>

            <para>
                BitBake uses the "recrdeptask" flag to manage
                recursive task dependencies.
                BitBake looks through the build-time and runtime
                dependencies of the current recipe, looks through
                the task's inter-task
                dependencies, and then adds dependencies for the
                listed task.
                Once BitBake has accomplished this, it recursively works through
                the dependencies of those tasks.
                Iterative passes continue until all dependencies are discovered
                and added.
            </para>

            <para>
                You might want to not only have BitBake look for
                dependencies of those tasks, but also have BitBake look
                for build-time and runtime dependencies of the dependent
                tasks as well.
                If that is the case, you need to reference the task name
                itself in the task list:
                <literallayout class='monospaced'>
     do_a[recrdeptask] = "do_a do_b"
                </literallayout>
            </para>
        </section>

        <section id='inter-task-dependencies'>
            <title>Inter-Task Dependencies</title>

            <para>
                BitBake uses the "depends" flag in a more generic form
                to manage inter-task dependencies.
                This more generic form allows for inter-dependency
                checks for specific tasks rather than checks for
                the data in <filename>DEPENDS</filename>.
                Here is an example:
                <literallayout class='monospaced'>
     do_patch[depends] = "quilt-native:do_populate_staging"
                </literallayout>
                In this example, the <filename>do_populate_staging</filename>
                task of the target <filename>quilt-native</filename>
                must have completed before the
                <filename>do_patch</filename> task can execute.
            </para>

            <para>
                The "rdepends" flag works in a similar way but takes targets
                in the runtime namespace instead of the build-time dependency
                namespace.
            </para>
        </section>
    </section>

    <section id='accessing-datastore-variables-using-python'>
        <title>Accessing Datastore Variables Using Python</title>

        <para>
            It is often necessary to access variables in the
            BitBake datastore using Python functions.
            The Bitbake datastore has an API that allows you this
            access.
            Here is a list of available operations:
        </para>

        <para>
            <informaltable frame='none'>
                <tgroup cols='2' align='left' colsep='1' rowsep='1'>
                    <colspec colname='c1' colwidth='1*'/>
                    <colspec colname='c2' colwidth='1*'/>
                    <thead>
                        <row>
                            <entry align="left"><emphasis>Operation</emphasis></entry>
                            <entry align="left"><emphasis>Description</emphasis></entry>
                        </row>
                    </thead>
                    <tbody>
                        <row>
                            <entry align="left"><filename>d.getVar("X", expand=False)</filename></entry>
                            <entry align="left">Returns the value of variable "X".
                                Using "expand=True" expands the value.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVar("X", value)</filename></entry>
                            <entry align="left">Sets the variable "X" to "value".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.appendVar("X", value)</filename></entry>
                            <entry align="left">Adds "value" to the end of the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.prependVar("X", value)</filename></entry>
                            <entry align="left">Adds "value" to the start of the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVar("X")</filename></entry>
                            <entry align="left">Deletes the variable "X" from the datastore.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.renameVar("X", "Y")</filename></entry>
                            <entry align="left">Renames the variable "X" to "Y".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.getVarFlag("X", flag, expand=False)</filename></entry>
                            <entry align="left">Gets "flag" from the variable "X".
                                Using "expand=True" expands the flag.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVarFlag("X", flag, value)</filename></entry>
                            <entry align="left">Sets "flag" for variable "X" to "value".
                                <filename>setVarFlags</filename> does not clear previous flags.
                                Think of this operation as <filename>addVarFlags</filename>.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.appendVarFlag("X", flag, value)</filename></entry>
                            <entry align="left">Need description.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.prependVarFlag("X", flag, value)</filename></entry>
                            <entry align="left">Need description.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVarFlag("X", flag)</filename></entry>
                            <entry align="left">Need description.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVarFlags("X", flagsdict)</filename></entry>
                            <entry align="left">Sets the flags specified in
                                the <filename>dict()</filename> parameter.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.getVarFlags("X")</filename></entry>
                            <entry align="left">Returns a <filename>dict</filename> of the flags for
                                the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVarFlags</filename></entry>
                            <entry align="left">Deletes all the flags for a variable.</entry>
                        </row>
                    </tbody>
                </tgroup>
            </informaltable>
        </para>
    </section>

    <section id='task-checksums-and-setscene'>
        <title>Task Checksums and Setscene</title>

        <para>
            BitBake uses checksums (or signatures) along with the setscene
            to determine if a task needs to be run.
            This section describes the process.
            To help understand how BitBake does this, the section assumes an
            OpenEmbedded metadata-based example.
        </para>

        <para>
            This list is a place holder of content existed from previous work
            on the manual.
            Some or all of it probably needs integrated into the subsections
            that make up this section.
            For now, I have just provided a short glossary-like description
            for each variable.
            Ultimately, this list goes away.
            <itemizedlist>
                <listitem><para><filename>STAMP</filename>:
                    The base path to create stamp files.</para></listitem>
                <listitem><para><filename>STAMPCLEAN</filename>
                    Again, the base path to create stamp files but can use wildcards
                    for matching a range of files for clean operations.
                    </para></listitem>
                <listitem><para><filename>BB_STAMP_WHITELIST</filename>
                    Lists stamp files that are looked at when the stamp policy
                    is "whitelist".
                    </para></listitem>
                <listitem><para><filename>BB_STAMP_POLICY</filename>
                    Defines the mode for comparing timestamps of stamp files.
                    </para></listitem>
                <listitem><para><filename>BB_HASHCHECK_FUNCTION</filename>
                    Specifies the name of the function to call during
                    the "setscene" part of the task's execution in order
                    to validate the list of task hashes.
                    </para></listitem>
                <listitem><para><filename>BB_SETSCENE_VERIFY_FUNCTION</filename>
                    Specifies a function to call that verifies the list of
                    planned task execution before the main task execution
                    happens.
                    </para></listitem>
                <listitem><para><filename>BB_SETSCENE_DEPVALID</filename>
                    Specifies a function BitBake calls that determines
                    whether BitBake requires a setscene dependency to
                    be met.
                    </para></listitem>
                <listitem><para><filename>BB_TASKHASH</filename>
                    Within an executing task, this variable holds the hash
                    of the task as returned by the currently enabled
                    signature generator.
                    </para></listitem>
            </itemizedlist>
        </para>

        <section id='setscene'>
            <title>Setscene</title>

            <para>
                This section needs to get the concept of the setscene across.
                The reader needs to know what it is and what it is used for during
                the build process.
            </para>
        </section>

        <section id='checksums'>
            <title>Checksums (Signatures)</title>

            <para>
                A checksum is a unique signature of a task's inputs.
                The setscene code uses a checksum to determine if a task needs
                to be run.
                Because it is a change in a task's inputs that triggers running
                the task, the process needs to detect all the inputs to a given task.
                For shell tasks, this turns out to be fairly easy because
                BitBake generates a "run" shell script for each task and
                it is possible to create a checksum that gives you a good idea of when
                the task's data changes.
            </para>

            <para>
                To complicate the problem, some things should not be included in
                the checksum.
                First, there is the actual specific build path of a given task -
                the working directory.
                It does not matter if the working directory changes because it should not
                affect the output for target packages.
                The simplistic approach for excluding the working directory is to set
                it to some fixed value and create the checksum for the "run" script.
            </para>

            <para>
                Another problem results from the "run" scripts containing functions that
                might or might not get called.
                The incremental build solution contains code that figures out dependencies
                between shell functions.
                This code is used to prune the "run" scripts down to the minimum set,
                thereby alleviating this problem and making the "run" scripts much more
                readable as a bonus.
            </para>

            <para>
                So far we have solutions for shell scripts.
                What about Python tasks?
                The same approach applies even though these tasks are more difficult.
                The process needs to figure out what variables a Python function accesses
                and what functions it calls.
                Again, the incremental build solution contains code that first figures out
                the variable and function dependencies, and then creates a checksum for the data
                used as the input to the task.
            </para>

            <para>
                Like the working directory case, situations exist where dependencies
                should be ignored.
                For these cases, you can instruct the build process to ignore a dependency
                by using a line like the following:
                <literallayout class='monospaced'>
     PACKAGE_ARCHS[vardepsexclude] = "MACHINE"
                </literallayout>
                This example ensures that the <filename>PACKAGE_ARCHS</filename> variable does not
                depend on the value of <filename>MACHINE</filename>, even if it does reference it.
            </para>

            <para>
                Equally, there are cases where we need to add dependencies BitBake
                is not able to find.
                You can accomplish this by using a line like the following:
                <literallayout class='monospaced'>
      PACKAGE_ARCHS[vardeps] = "MACHINE"
                </literallayout>
                This example explicitly adds the <filename>MACHINE</filename> variable as a
                dependency for <filename>PACKAGE_ARCHS</filename>.
            </para>

            <para>
                Consider a case with in-line Python, for example, where BitBake is not
                able to figure out dependencies.
                When running in debug mode (i.e. using <filename>-DDD</filename>), BitBake
                produces output when it discovers something for which it cannot figure out
                dependencies.
            </para>

            <para>
                Thus far, this section has limited discussion to the direct inputs into a task.
                Information based on direct inputs is referred to as the "basehash" in the
                code.
                However, there is still the question of a task's indirect inputs - the
                things that were already built and present in the build directory.
                The checksum (or signature) for a particular task needs to add the hashes
                of all the tasks on which the particular task depends.
                Choosing which dependencies to add is a policy decision.
                However, the effect is to generate a master checksum that combines the basehash
                and the hashes of the task's dependencies.
            </para>

            <para>
                At the code level, there are a variety of ways both the basehash and the
                dependent task hashes can be influenced.
                Within the BitBake configuration file, we can give BitBake some extra information
                to help it construct the basehash.
                The following statement effectively results in a list of global variable
                dependency excludes - variables never included in any checksum.
                This example uses variables from OpenEmbedded to help illustrate
                the concept:
                <literallayout class='monospaced'>
     BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH DL_DIR \
         SSTATE_DIR THISDIR FILESEXTRAPATHS FILE_DIRNAME HOME LOGNAME SHELL TERM \
         USER FILESPATH STAGING_DIR_HOST STAGING_DIR_TARGET COREBASE PRSERV_HOST \
         PRSERV_DUMPDIR PRSERV_DUMPFILE PRSERV_LOCKDOWN PARALLEL_MAKE \
         CCACHE_DIR EXTERNAL_TOOLCHAIN CCACHE CCACHE_DISABLE LICENSE_PATH SDKPKGSUFFIX"
                </literallayout>
                The previous example excludes the work directory, which is part of
                <filename>TMPDIR</filename>.
            </para>

            <para>
                The rules for deciding which hashes of dependent tasks to include through
                dependency chains are more complex and are generally accomplished with a
                Python function.
                The code in <filename>meta/lib/oe/sstatesig.py</filename> shows two examples
                of this and also illustrates how you can insert your own policy into the system
                if so desired.
                This file defines the two basic signature generators OpenEmbedded Core
                uses:  "OEBasic" and "OEBasicHash".
                By default, there is a dummy "noop" signature handler enabled in BitBake.
                This means that behavior is unchanged from previous versions.
                <filename>OE-Core</filename> uses the "OEBasicHash" signature handler by default
                through this setting in the <filename>bitbake.conf</filename> file:
                <literallayout class='monospaced'>
     BB_SIGNATURE_HANDLER ?= "OEBasicHash"
                </literallayout>
                The "OEBasicHash" <filename>BB_SIGNATURE_HANDLER</filename> is the same as the
                "OEBasic" version but adds the task hash to the stamp files.
                This results in any metadata change that changes the task hash, automatically
                causing the task to be run again.
                This removes the need to bump
                <link linkend='var-PR'><filename>PR</filename></link>
                values, and changes to metadata automatically ripple across the build.
            </para>

            <para>
                It is also worth noting that the end result of these signature generators is to
                make some dependency and hash information available to the build.
                This information includes:
                <itemizedlist>
                    <listitem><para><filename>BB_BASEHASH_task-&lt;taskname&gt;</filename>:
                        The base hashes for each task in the recipe.
                        </para></listitem>
                    <listitem><para><filename>BB_BASEHASH_&lt;filename:taskname&gt;</filename>:
                        The base hashes for each dependent task.
                        </para></listitem>
                    <listitem><para><filename>BBHASHDEPS_&lt;filename:taskname&gt;</filename>:
                        The task dependencies for each task.
                        </para></listitem>
                    <listitem><para><filename>BB_TASKHASH</filename>:
                        The hash of the currently running task.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>
    </section>
</chapter>