import-layers/yocto-poky/bitbake/doc/bitbake-user-manual/bitbake-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="bitbake-user-manual-metadata">
    <title>Syntax and Operators</title>

    <para>
        Bitbake files have their own syntax.
        The syntax has similarities to several
        other languages but also has some unique features.
        This section describes the available syntax and operators
        as well as provides examples.
    </para>

    <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>
                As expected, if you include leading or trailing spaces as part of
                an assignment, the spaces are retained:
                <literallayout class='monospaced'>
     VARIABLE = " value"
     VARIABLE = "value "
                </literallayout>
                Setting <filename>VARIABLE</filename> to "" sets it to an empty string,
                while setting the variable to " " sets it to a blank space
                (i.e. these are not the same values).
                <literallayout class='monospaced'>
     VARIABLE = ""
     VARIABLE = " "
                </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> evaluating to
                "preavalpost" based on that current value of
                <filename>A</filename>.
                <literallayout class='monospaced'>
     A = "aval"
     B = "pre${A}post"
                </literallayout>
                You should realize that whenever <filename>B</filename> is
                referenced, its evaluation will depend on the state of
                <filename>A</filename> at that time.
                Thus, later evaluations of <filename>B</filename> in the
                previous example could result in different values
                depending on the value of <filename>A</filename>.
            </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 than 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.
            </para>

            <para>
                These operators take immediate effect during parsing.
                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.
            </para>

            <para>
                These operators take immediate effect during parsing.
                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.
            </para>

            <para>
                These operators differ from the ":=", ".=", "=.", "+=", and "=+"
                operators in that their effects are deferred
                until after parsing completes rather than being immediately
                applied.
                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 "abc 456".
            </para>

            <para>
                No need exists to pre-define variable flags.
                You can simply start using them.
                One extremely common application
                is to attach some brief documentation to a BitBake variable as
                follows:
                <literallayout class='monospaced'>
     CACHE[doc] = "The directory holding the cache of the metadata."
                </literallayout>
            </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 being set to the current date.
            </para>

            <para>
                Probably the most common use of this feature is to extract
                the value of variables from BitBake's internal data dictionary,
                <filename>d</filename>.
                The following lines select the values of a package name
                and its version number, respectively:
                <literallayout class='monospaced'>
     PN = "${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE', False),d)[0] or 'defaultpkgname'}"
     PV = "${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE', False),d)[1] or '1.0'}"
                </literallayout>
            </para>
        </section>

        <section id='providing-pathnames'>
            <title>Providing Pathnames</title>

            <para>
                When specifying pathnames for use with BitBake,
                do not use the tilde ("~") character as a shortcut
                for your home directory.
                Doing so might cause BitBake to not recognize the
                path since BitBake does not expand this character in
                the same way a shell would.
            </para>

            <para>
                Instead, provide a fuller path as the following
                example illustrates:
                <literallayout class='monospaced'>
     BBLAYERS ?= " \
       /home/scott-lenovo/LayerA \
       "
                </literallayout>
            </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.
                <note>
                    Overrides can only use lower-case characters.
                    Additionally, underscores are not permitted in override names
                    as they are used to separate overrides from each other and
                    from the variable name.
                </note>
                <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>

                        <para>
                            To better understand this, consider a practical example
                            that assumes an OpenEmbedded metadata-based Linux
                            kernel recipe file.
                            The following lines from the recipe file first set
                            the kernel branch variable <filename>KBRANCH</filename>
                            to a default value, then conditionally override that
                            value based on the architecture of the build:
                        <literallayout class='monospaced'>
     KBRANCH = "standard/base"
     KBRANCH_qemuarm  = "standard/arm-versatile-926ejs"
     KBRANCH_qemumips = "standard/mti-malta32"
     KBRANCH_qemuppc  = "standard/qemuppc"
     KBRANCH_qemux86  = "standard/common-pc/base"
     KBRANCH_qemux86-64  = "standard/common-pc-64/base"
     KBRANCH_qemumips64 = "standard/mti-malta64"
                        </literallayout>
                        </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>

                        <para>
                            Again, using an OpenEmbedded metadata-based
                            kernel recipe file as an example, the
                            following lines will conditionally append to the
                            <filename>KERNEL_FEATURES</filename> variable based
                            on the architecture:
                        <literallayout class='monospaced'>
     KERNEL_FEATURES_append = " ${KERNEL_EXTRA_FEATURES}"
     KERNEL_FEATURES_append_qemux86=" cfg/sound.scc cfg/paravirt_kvm.scc"
     KERNEL_FEATURES_append_qemux86-64=" cfg/sound.scc cfg/paravirt_kvm.scc"
                        </literallayout>
                        </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.
                Additionally, BitBake searches the current directory for
                <filename>include</filename> and <filename>require</filename>
                directives.
                <note>
                    The <filename>BBPATH</filename> variable is analogous to
                    the environment variable <filename>PATH</filename>.
                </note>
            </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 recipe.
            </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>

            <para>
                If necessary, it is possible to inherit a class
                conditionally by using
                a variable expression after the <filename>inherit</filename>
                statement.
                Here is an example:
                <literallayout class='monospaced'>
     inherit ${VARNAME}
                </literallayout>
                If <filename>VARNAME</filename> is going to be set, it needs
                to be set before the <filename>inherit</filename> statement
                is parsed.
                One way to achieve a conditional inherit in this case is to use
                overrides:
                <literallayout class='monospaced'>
     VARIABLE = ""
     VARIABLE_someoverride = "myclass"
                </literallayout>
            </para>

            <para>
                Another method is by using anonymous Python.
                Here is an example:
                <literallayout class='monospaced'>
     python () {
         if condition == value:
             d.setVar('VARIABLE', 'myclass')
         else:
             d.setVar('VARIABLE', '')
     }
                </literallayout>
            </para>

            <para>
                Alternatively, you could use an in-line Python expression
                in the following form:
                <literallayout class='monospaced'>
     inherit ${@'classname' if condition else ''}
     inherit ${@functionname(params)}
                </literallayout>
                In all cases, if the expression evaluates to an empty
                string, the statement does not trigger a syntax error
                because it becomes a no-op.
            </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 its equivalent in 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 handles
                <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>
                If you want to use the directive to inherit
                multiple classes, you can provide them on the same line in the
                <filename>local.conf</filename> file.
                Use spaces to separate the classes.
                The following example shows how to inherit both the
                <filename>autotools</filename> and <filename>pkgconfig</filename>
                classes:
                <literallayout class='monospaced'>
     inherit autotools pkgconfig
                </literallayout>
            </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 these 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>
                <listitem><para><emphasis>Anonymous Python Functions:</emphasis>
                    Python functions executed automatically during
                    parsing.
                    </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>

           <note>
                Variable expressions (e.g. <filename>${X}</filename>) are no
                longer expanded within Python functions.
                This behavior is intentional in order to allow you to freely
                set variable values to expandable expressions without having
                them expanded prematurely.
                If you do wish to expand a variable within a Python function,
                use <filename>d.getVar("X", True)</filename>.
                Or, for more complicated expressions, use
                <filename>d.expand()</filename>.
           </note>
        </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 id='anonymous-python-functions'>
            <title>Anonymous Python Functions</title>

            <para>
                Sometimes it is useful to run some code during
                parsing to set variables or to perform other operations
                programmatically.
                To do this, you can define an anonymous Python function.
                Here is an example that conditionally sets a
                variable based on the value of another variable:
                <literallayout class='monospaced'>
     python __anonymous () {
         if d.getVar('SOMEVAR', True) == 'value':
             d.setVar('ANOTHERVAR', 'value2')
     }
                </literallayout>
                The "__anonymous" function name is optional, so the
                following example is functionally equivalent to the above:
                <literallayout class='monospaced'>
     python () {
         if d.getVar('SOMEVAR', True) == 'value':
             d.setVar('ANOTHERVAR', 'value2')
     }
                </literallayout>
                Because unlike other Python functions anonymous
                Python functions are executed during parsing, the
                "d" variable within an anonymous Python function represents
                the datastore for the entire recipe.
                Consequently, you can set variable values here and
                those values can be picked up by other functions.
            </para>
        </section>

        <section id='flexible-inheritance-for-class-functions'>
            <title>Flexible Inheritance for Class Functions</title>

            <para>
                Through coding techniques and the use of
                <filename>EXPORT_FUNCTIONS</filename>, BitBake supports
                exporting a function from a class such that the
                class function appears as the default implementation
                of the function, but can still be called if a recipe
                inheriting the class needs to define its own version of
                the function.
            </para>

            <para>
                To understand the benefits of this feature, consider
                the basic scenario where a class defines a task function
                and your recipe inherits the class.
                In this basic scenario, your recipe inherits the task
                function as defined in the class.
                If desired, your recipe can add to the start and end of the
                function by using the "_prepend" or "_append" operations
                respectively, or it can redefine the function completely.
                However, if it redefines the function, there is
                no means  for it to call the class version of the function.
                <filename>EXPORT_FUNCTIONS</filename> provides a mechanism
                that enables the recipe's version of the function to call
                the original version of the function.
            </para>

            <para>
                To make use of this technique, you need the following
                things in place:
                <itemizedlist>
                    <listitem><para>
                        The class needs to define the function as follows:
                        <literallayout class='monospaced'>
     <replaceable>classname</replaceable><filename>_</filename><replaceable>functionname</replaceable>
                        </literallayout>
                        For example, if you have a class file
                        <filename>bar.bbclass</filename> and a function named
                        <filename>do_foo</filename>, the class must define the function
                        as follows:
                        <literallayout class='monospaced'>
     bar_do_foo
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        The class needs to contain the <filename>EXPORT_FUNCTIONS</filename>
                        statement as follows:
                        <literallayout class='monospaced'>
     EXPORT_FUNCTIONS <replaceable>functionname</replaceable>
                        </literallayout>
                        For example, continuing with the same example, the
                        statement in the <filename>bar.bbclass</filename> would be
                        as follows:
                        <literallayout class='monospaced'>
     EXPORT_FUNCTIONS do_foo
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        You need to call the function appropriately from within your
                        recipe.
                        Continuing with the same example, if your recipe
                        needs to call the class version of the function,
                        it should call <filename>bar_do_foo</filename>.
                        Assuming <filename>do_foo</filename> was a shell function
                        and <filename>EXPORT_FUNCTIONS</filename> was used as above,
                        the recipe's function could conditionally call the
                        class version of the function as follows:
                        <literallayout class='monospaced'>
     do_foo() {
             if [ somecondition ] ; then
                     bar_do_foo
             else
                     # Do something else
             fi
     }
                        </literallayout>
                        To call your modified version of the function as defined
                        in your recipe, call it as <filename>do_foo</filename>.
                        </para></listitem>
                </itemizedlist>
                With these conditions met, your single recipe
                can freely choose between the original function
                as defined in the class file and the modified function in your recipe.
                If you do not set up these conditions, you are limited to using one function
                or the other.
            </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, task names 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())
     }
     addtask printdate after do_fetch before do_build
            </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='deleting-a-task'>
            <title>Deleting a Task</title>

            <para>
                As well as being able to add tasks, you can delete them.
                Simply use the <filename>deltask</filename> command to
                delete a task.
                For example, to delete the example task used in the previous
                sections, you would use:
                <literallayout class='monospaced'>
     deltask printdate
                </literallayout>
                If you delete a task using the <filename>deltask</filename>
                command and the task has dependencies, the dependencies are
                not reconnected.
                For example, suppose you have three tasks named
                <filename>do_a</filename>, <filename>do_b</filename>, and
                <filename>do_c</filename>.
                Furthermore, <filename>do_c</filename> is dependent on
                <filename>do_b</filename>, which in turn is dependent on
                <filename>do_a</filename>.
                Given this scenario, if you use <filename>deltask</filename>
                to delete <filename>do_b</filename>, the implicit dependency
                relationship between <filename>do_c</filename> and
                <filename>do_a</filename> through <filename>do_b</filename>
                no longer exists, and <filename>do_c</filename> dependencies
                are not updated to include <filename>do_a</filename>.
                Thus, <filename>do_c</filename> is free to run before
                <filename>do_a</filename>.
            </para>

            <para>
                If you want dependencies such as these to remain intact, use
                the <filename>noexec</filename> varflag to disable the task
                instead of using the <filename>deltask</filename> command to
                delete it:
                <literallayout class='monospaced'>
     do_b[noexec] = "1"
                </literallayout>
            </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 shell execution
                environment of the build tasks to make
                sure unwanted contamination from the build machine cannot
                influence the build.
                <note>
                    By default, BitBake cleans the environment to include only those
                    things exported or listed in its whitelist to ensure that the build
                    environment is reproducible and consistent.
                    You can prevent this "cleaning" by setting the
                    <link linkend='var-BB_PRESERVE_ENV'><filename>BB_PRESERVE_ENV</filename></link>
                    variable.
                </note>
                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_WHITELIST'><filename>BB_ENV_WHITELIST</filename></link>
                        and
                        <link linkend='var-BB_ENV_EXTRAWHITE'><filename>BB_ENV_EXTRAWHITE</filename></link>
                        variables.
                        For example, assume you want to prevent the build system from
                        accessing your <filename>$HOME/.ccache</filename>
                        directory.
                        The following command "whitelists" the environment variable
                        <filename>CCACHE_DIR</filename> causing BitBack to allow that
                        variable into the datastore:
                        <literallayout class='monospaced'>
     export BB_ENV_EXTRAWHITE="$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>

            <para>
                Sometimes, it is useful to be able to obtain information
                from the original execution environment.
                Bitbake saves a copy of the original environment into
                a special variable named
                <link linkend='var-BB_ORIGENV'><filename>BB_ORIGENV</filename></link>.
            </para>

            <para>
                The <filename>BB_ORIGENV</filename> variable returns a datastore
                object that can be queried using the standard datastore operators
                such as <filename>getVar(, False)</filename>.
                The datastore object is useful, for example, to find the original
                <filename>DISPLAY</filename> variable.
                Here is an example:
                <literallayout class='monospaced'>
     origenv = d.getVar("BB_ORIGENV", False)
     bar = origenv.getVar("BAR", False)
                </literallayout>
                The previous example returns <filename>BAR</filename> from the original
                execution environment.
            </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'>
     <replaceable>variable</replaceable> = d.getVarFlags("<replaceable>variable</replaceable>")
     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.
            Tasks support a number of these flags which control various
            functionality of the task:
            <itemizedlist>
                <listitem><para><emphasis>cleandirs:</emphasis>
                    Empty directories that should created before the task runs.
                    </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>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>dirs:</emphasis>
                    Directories that should be created before the task runs.
                    The last directory listed will be used as the work directory
                    for the task.
                    </para></listitem>
                <listitem><para><emphasis>lockfiles:</emphasis>
                     Specifies one or more lockfiles to lock while the task
                     executes.
                     Only one task may hold a lockfile, and any task that
                     attempts to lock an already locked file will block until
                     the lock is released.
                     You can use this variable flag to accomplish mutual
                     exclusion.
                    </para></listitem>
                <listitem><para><emphasis>noexec:</emphasis>
                    Marks the tasks as being empty and no execution required.
                    The <filename>noexec</filename> flag can be used to set up
                    tasks as dependency placeholders, or to disable tasks defined
                    elsewhere that are not needed in a particular recipe.
                    </para></listitem>
                <listitem><para><emphasis>nostamp:</emphasis>
                    Tells BitBake to not generate a stamp file for a task,
                    which implies the task should always be executed.
                    </para></listitem>
                <listitem><para><emphasis>postfuncs:</emphasis>
                    List of functions to call after the completion of the task.
                    </para></listitem>
                <listitem><para><emphasis>prefuncs:</emphasis>
                    List of functions to call before the task executes.
                    </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>
                <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>recideptask:</emphasis>
                    When set in conjunction with
                    <filename>recrdeptask</filename>, specifies a task that
                    should be inspected for additional dependencies.
                    </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>stamp-extra-info:</emphasis>
                    Extra stamp information to append to the task's stamp.
                    As an example, OpenEmbedded uses this flag to allow
                    machine-specific tasks.
                    </para></listitem>
                <listitem><para><emphasis>umask:</emphasis>
                    The umask to run the task under.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            Several varflags are useful for controlling how signatures are
            calculated for variables.
            For more information on this process, see the
            "<link linkend='checksums'>Checksums (Signatures)</link>"
            section.
            <itemizedlist>
                <listitem><para><emphasis>vardeps:</emphasis>
                    Specifies a space-separated list of additional
                    variables to add to a variable's dependencies
                    for the purposes of calculating its signature.
                    Adding variables to this list is useful, for example, when
                    a function refers to a variable in a manner that
                    does not allow BitBake to automatically determine
                    that the variable is referred to.
                    </para></listitem>
                <listitem><para><emphasis>vardepsexclude:</emphasis>
                    Specifies a space-separated list of variables
                    that should be excluded from a variable's dependencies
                    for the purposes of calculating its signature.
                    </para></listitem>
                <listitem><para><emphasis>vardepvalue:</emphasis>
                    If set, instructs BitBake to ignore the actual
                    value of the variable and instead use the specified
                    value when calculating the variable's signature.
                    </para></listitem>
                <listitem><para><emphasis>vardepvalueexclude:</emphasis>
                    Specifies a pipe-separated list of strings to exclude
                    from the variable's value when calculating the
                    variable's signature.
                    </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 an operation against a given recipe
            (<filename>*.bb</filename> file), 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>
            Because you probably are only interested in a subset of events,
            you would likely use the <filename>[eventmask]</filename> flag
            for your event handler to be sure that only certain events
            trigger the handler.
            Given the previous example, suppose you only wanted the
            <filename>bb.build.TaskFailed</filename> event to trigger that
            event handler.
            Use the flag as follows:
            <literallayout class='monospaced'>
     addhandler myclass_eventhandler
     myclass_eventhandler[eventmask] = "bb.build.TaskFailed"
     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>
        </para>

        <para>
            During a standard build, the following common events might 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.
            <note>
                The mechanism for this class extension is extremely
                specific to the implementation.
                Usually, the recipe's
                <link linkend='var-PROVIDES'><filename>PROVIDES</filename></link>,
                <link linkend='var-PN'><filename>PN</filename></link>, and
                <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                variables would need to be modified by the extension class.
                For specific examples, see the OE-Core
                <filename>native</filename>, <filename>nativesdk</filename>,
                and <filename>multilib</filename> classes.
            </note>
            <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_sysroot"
                </literallayout>
                In this example, the <filename>do_populate_sysroot</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_qa[rdeptask] = "do_packagedata"
                </literallayout>
                In the previous example, the <filename>do_packagedata</filename>
                task of each item in <filename>RDEPENDS</filename> must have
                completed before <filename>do_package_qa</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_sysroot"
                </literallayout>
                In this example, the <filename>do_populate_sysroot</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 then named flag from the variable "X".
                                Using "expand=True" expands the named flag.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVarFlag("X", flag, "value")</filename></entry>
                            <entry align="left">Sets the named flag for variable "X" to "value".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.appendVarFlag("X", flag, "value")</filename></entry>
                            <entry align="left">Appends "value" to the named flag on the
                            variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.prependVarFlag("X", flag, "value")</filename></entry>
                            <entry align="left">Prepends "value" to the named flag on
                               the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVarFlag("X", flag)</filename></entry>
                            <entry align="left">Deletes the named flag on the variable
                                "X" from the datastore.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVarFlags("X", flagsdict)</filename></entry>
                            <entry align="left">Sets the flags specified in
                                the <filename>flagsdict()</filename> parameter.
                                <filename>setVarFlags</filename> does not clear previous flags.
                                Think of this operation as <filename>addVarFlags</filename>.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.getVarFlags("X")</filename></entry>
                            <entry align="left">Returns a <filename>flagsdict</filename> of the flags for
                                the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVarFlags("X")</filename></entry>
                            <entry align="left">Deletes all the flags for the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.expand(expression)</filename></entry>
                            <entry align="left">Expands variable references in the specified string expression.</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>
</chapter>