sphinx: initial sphinx support

This commit is autogenerated pandoc to generate an inital set
of reST files based on DocBook XML files.

A .rst file is generated for each .xml files in all manuals with this
command:

cd <manual>
for i in *.xml; do \
  pandoc -f docbook -t rst --shift-heading-level-by=-1 \
  $i -o $(basename $i .xml).rst \
done

The conversion was done with: pandoc 2.9.2.1-91 (Arch Linux).

Also created an initial top level index file for each document, and
added all 'books' to the top leve index.rst file.

The YP manuals layout is organized as:

Book
  Chapter
    Section
      Section
        Section

Sphinx uses section headers to create the document structure.
ReStructuredText defines sections headers like that:

   To break longer text up into sections, you use section headers. These
   are a single line of text (one or more words) with adornment: an
   underline alone, or an underline and an overline together, in dashes
   "-----", equals "======", tildes "~~~~~~" or any of the
   non-alphanumeric characters = - ` : ' " ~ ^ _ * + # < > that you feel
   comfortable with. An underline-only adornment is distinct from an
   overline-and-underline adornment using the same character. The
   underline/overline must be at least as long as the title text. Be
   consistent, since all sections marked with the same adornment style
   are deemed to be at the same level:

Let's define the following convention when converting from Docbook:

Book                => overline ===   (Title)
  Chapter           => overline ***   (1.)
    Section         => ====           (1.1)
      Section       => ----           (1.1.1)
        Section     => ~~~~           (1.1.1.1)
          Section   => ^^^^           (1.1.1.1.1)

During the conversion with pandoc, we used --shift-heading-level=-1 to
convert most of DocBook headings automatically. However with this
setting, the Chapter header was removed, so I added it back
manually. Without this setting all headings were off by one, which was
more difficult to manually fix.

At least with this change, we now have the same TOC with Sphinx and
DocBook.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>

1 files changed, 1728 insertions, 0 deletions

diff --git a/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst
new file mode 100644
index 000000000..790065ef6
--- /dev/null
+++ b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst
@@ -0,0 +1,1728 @@
+====================
+Syntax and Operators
+====================
+
+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.
+
+Basic Syntax
+============
+
+This section provides some basic syntax examples.
+
+Basic Variable Setting
+----------------------
+
+The following example sets ``VARIABLE`` to "value". This assignment
+occurs immediately as the statement is parsed. It is a "hard"
+assignment. VARIABLE = "value" As expected, if you include leading or
+trailing spaces as part of an assignment, the spaces are retained:
+VARIABLE = " value" VARIABLE = "value " Setting ``VARIABLE`` 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). VARIABLE = "" VARIABLE
+= " "
+
+You can use single quotes instead of double quotes when setting a
+variable's value. Doing so allows you to use values that contain the
+double quote character: VARIABLE = 'I have a " in my value'
+
+.. note::
+
+   Unlike in Bourne shells, single quotes work identically to double
+   quotes in all other ways. They do not suppress variable expansions.
+
+Modifying Existing Variables
+----------------------------
+
+Sometimes you need to modify existing variables. Following are some
+cases where you might find you want to modify an existing variable:
+
+-  Customize a recipe that uses the variable.
+
+-  Change a variable's default value used in a ``*.bbclass`` file.
+
+-  Change the variable in a ``*.bbappend`` file to override the variable
+   in the original recipe.
+
+-  Change the variable in a configuration file so that the value
+   overrides an existing configuration.
+
+Changing a variable value can sometimes depend on how the value was
+originally assigned and also on the desired intent of the change. In
+particular, when you append a value to a variable that has a default
+value, the resulting value might not be what you expect. In this case,
+the value you provide might replace the value rather than append to the
+default value.
+
+If after you have changed a variable's value and something unexplained
+occurs, you can use BitBake to check the actual value of the suspect
+variable. You can make these checks for both configuration and recipe
+level changes:
+
+-  For configuration changes, use the following: $ bitbake -e This
+   command displays variable values after the configuration files (i.e.
+   ``local.conf``, ``bblayers.conf``, ``bitbake.conf`` and so forth)
+   have been parsed.
+
+   .. note::
+
+      Variables that are exported to the environment are preceded by the
+      string "export" in the command's output.
+
+-  For recipe changes, use the following: $ bitbake recipe -e \| grep
+   VARIABLE=" This command checks to see if the variable actually makes
+   it into a specific recipe.
+
+Line Joining
+------------
+
+Outside of `functions <#functions>`__, BitBake joins any line ending in
+a backslash character ("\") with the following line before parsing
+statements. The most common use for the "\" character is to split
+variable assignments over multiple lines, as in the following example:
+FOO = "bar \\ baz \\ qaz" Both the "\" character and the newline
+character that follow it are removed when joining lines. Thus, no
+newline characters end up in the value of ``FOO``.
+
+Consider this additional example where the two assignments both assign
+"barbaz" to ``FOO``: FOO = "barbaz" FOO = "bar\\ baz"
+
+.. note::
+
+   BitBake does not interpret escape sequences like "\n" in variable
+   values. For these to have an effect, the value must be passed to some
+   utility that interprets escape sequences, such as
+   printf
+   or
+   echo -n
+   .
+
+Variable Expansion
+------------------
+
+Variables can reference the contents of other variables using a syntax
+that is similar to variable expansion in Bourne shells. The following
+assignments result in A containing "aval" and B evaluating to
+"preavalpost". A = "aval" B = "pre${A}post"
+
+.. note::
+
+   Unlike in Bourne shells, the curly braces are mandatory: Only
+   ${FOO}
+   and not
+   $FOO
+   is recognized as an expansion of
+   FOO
+   .
+
+The "=" operator does not immediately expand variable references in the
+right-hand side. Instead, expansion is deferred until the variable
+assigned to is actually used. The result depends on the current values
+of the referenced variables. The following example should clarify this
+behavior: A = "${B} baz" B = "${C} bar" C = "foo" \*At this point, ${A}
+equals "foo bar baz"\* C = "qux" \*At this point, ${A} equals "qux bar
+baz"\* B = "norf" \*At this point, ${A} equals "norf baz"\* Contrast
+this behavior with the `immediate variable
+expansion <#immediate-variable-expansion>`__ operator (i.e. ":=").
+
+If the variable expansion syntax is used on a variable that does not
+exist, the string is kept as is. For example, given the following
+assignment, ``BAR`` expands to the literal string "${FOO}" as long as
+``FOO`` does not exist. BAR = "${FOO}"
+
+Setting a default value (?=)
+----------------------------
+
+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: A ?= "aval" If ``A`` is
+set at the time this statement is parsed, the variable retains its
+value. However, if ``A`` 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.
+
+Setting a weak default value (??=)
+----------------------------------
+
+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: A ??= "somevalue"
+A ??= "someothervalue" If ``A`` is set before the above statements are
+parsed, the variable retains its value. If ``A`` is not set, the
+variable is set to "someothervalue".
+
+Again, this assignment is a "lazy" or "weak" assignment because it does
+not occur until the end of the parsing process.
+
+Immediate variable expansion (:=)
+---------------------------------
+
+The ":=" operator results in a variable's contents being expanded
+immediately, rather than when the variable is actually used: T = "123" A
+:= "test ${T}" T = "456" B := "${T} ${C}" C = "cval" C := "${C}append"
+In this example, ``A`` contains "test 123", even though the final value
+of ``T`` is "456". The variable ``B`` will end up containing "456
+cvalappend". This is because references to undefined variables are
+preserved as is during (immediate)expansion. This is in contrast to GNU
+Make, where undefined variables expand to nothing. The variable ``C``
+contains "cvalappend" since ``${C}`` immediately expands to "cval".
+
+.. _appending-and-prepending:
+
+Appending (+=) and prepending (=+) With Spaces
+----------------------------------------------
+
+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.
+
+These operators take immediate effect during parsing. Here are some
+examples: B = "bval" B += "additionaldata" C = "cval" C =+ "test" The
+variable ``B`` contains "bval additionaldata" and ``C`` contains "test
+cval".
+
+.. _appending-and-prepending-without-spaces:
+
+Appending (.=) and Prepending (=.) Without Spaces
+-------------------------------------------------
+
+If you want to append or prepend values without an inserted space, use
+the ".=" and "=." operators.
+
+These operators take immediate effect during parsing. Here are some
+examples: B = "bval" B .= "additionaldata" C = "cval" C =. "test" The
+variable ``B`` contains "bvaladditionaldata" and ``C`` contains
+"testcval".
+
+Appending and Prepending (Override Style Syntax)
+------------------------------------------------
+
+You can also append and prepend a variable's value using an override
+style syntax. When you use this syntax, no spaces are inserted.
+
+These operators differ from the ":=", ".=", "=.", "+=", and "=+"
+operators in that their effects are applied at variable expansion time
+rather than being immediately applied. Here are some examples: B =
+"bval" B_append = " additional data" C = "cval" C_prepend = "additional
+data " D = "dval" D_append = "additional data" The variable ``B``
+becomes "bval additional data" and ``C`` becomes "additional data cval".
+The variable ``D`` becomes "dvaladditional data".
+
+.. note::
+
+   You must control all spacing when you use the override syntax.
+
+It is also possible to append and prepend to shell functions and
+BitBake-style Python functions. See the "`Shell
+Functions <#shell-functions>`__" and "`BitBake-Style Python
+Functions <#bitbake-style-python-functions>`__ sections for examples.
+
+.. _removing-override-style-syntax:
+
+Removal (Override Style Syntax)
+-------------------------------
+
+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.
+
+When you use this syntax, BitBake expects one or more strings.
+Surrounding spaces and spacing are preserved. Here is an example: FOO =
+"123 456 789 123456 123 456 123 456" FOO_remove = "123" FOO_remove =
+"456" FOO2 = " abc def ghi abcdef abc def abc def def" FOO2_remove = "
+\\ def \\ abc \\ ghi \\ " The variable ``FOO`` becomes
+"  789 123456    " and ``FOO2`` becomes "     abcdef      ".
+
+Like "_append" and "_prepend", "_remove" is applied at variable
+expansion time.
+
+Override Style Operation Advantages
+-----------------------------------
+
+An advantage of the override style operations "_append", "_prepend", and
+"_remove" as compared to the "+=" and "=+" operators is that the
+override style operators provide guaranteed operations. For example,
+consider a class ``foo.bbclass`` that needs to add the value "val" to
+the variable ``FOO``, and a recipe that uses ``foo.bbclass`` as follows:
+inherit foo FOO = "initial" If ``foo.bbclass`` uses the "+=" operator,
+as follows, then the final value of ``FOO`` will be "initial", which is
+not what is desired: FOO += "val" If, on the other hand, ``foo.bbclass``
+uses the "_append" operator, then the final value of ``FOO`` will be
+"initial val", as intended: FOO_append = " val"
+
+.. note::
+
+   It is never necessary to use "+=" together with "_append". The
+   following sequence of assignments appends "barbaz" to
+   FOO
+   :
+   ::
+
+           FOO_append = "bar"
+           FOO_append = "baz"
+                          
+
+   The only effect of changing the second assignment in the previous
+   example to use "+=" would be to add a space before "baz" in the
+   appended value (due to how the "+=" operator works).
+
+Another advantage of the override style operations is that you can
+combine them with other overrides as described in the "`Conditional
+Syntax (Overrides) <#conditional-syntax-overrides>`__" section.
+
+Variable Flag Syntax
+--------------------
+
+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 "`Variable
+Flags <#variable-flags>`__" section.
+
+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. "_prepend", "_append", and
+"_remove").
+
+Here are some examples showing how to set variable flags: FOO[a] = "abc"
+FOO[b] = "123" FOO[a] += "456" The variable ``FOO`` has two flags:
+``[a]`` and ``[b]``. The flags are immediately set to "abc" and "123",
+respectively. The ``[a]`` flag becomes "abc 456".
+
+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: CACHE[doc] = "The
+directory holding the cache of the metadata."
+
+Inline Python Variable Expansion
+--------------------------------
+
+You can use inline Python variable expansion to set variables. Here is
+an example: DATE = "${@time.strftime('%Y%m%d',time.gmtime())}" This
+example results in the ``DATE`` variable being set to the current date.
+
+Probably the most common use of this feature is to extract the value of
+variables from BitBake's internal data dictionary, ``d``. The following
+lines select the values of a package name and its version number,
+respectively: 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'}"
+
+.. note::
+
+   Inline Python expressions work just like variable expansions insofar
+   as the "=" and ":=" operators are concerned. Given the following
+   assignment,
+   foo()
+   is called each time
+   FOO
+   is expanded:
+   ::
+
+           FOO = "${@foo()}"
+                          
+
+   Contrast this with the following immediate assignment, where
+   foo()
+   is only called once, while the assignment is parsed:
+   ::
+
+           FOO := "${@foo()}"
+                          
+
+For a different way to set variables with Python code during parsing,
+see the "`Anonymous Python Functions <#anonymous-python-functions>`__"
+section.
+
+Unsetting variables
+-------------------
+
+It is possible to completely remove a variable or a variable flag from
+BitBake's internal data dictionary by using the "unset" keyword. Here is
+an example: unset DATE unset do_fetch[noexec] These two statements
+remove the ``DATE`` and the ``do_fetch[noexec]`` flag.
+
+Providing Pathnames
+-------------------
+
+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.
+
+Instead, provide a fuller path as the following example illustrates:
+BBLAYERS ?= " \\ /home/scott-lenovo/LayerA \\ "
+
+Exporting Variables to the Environment
+======================================
+
+You can export variables to the environment of running tasks by using
+the ``export`` keyword. For example, in the following example, the
+``do_foo`` task prints "value from the environment" when run: export
+ENV_VARIABLE ENV_VARIABLE = "value from the environment" do_foo() {
+bbplain "$ENV_VARIABLE" }
+
+.. note::
+
+   BitBake does not expand
+   $ENV_VARIABLE
+   in this case because it lacks the obligatory
+   {}
+   . Rather,
+   $ENV_VARIABLE
+   is expanded by the shell.
+
+It does not matter whether ``export ENV_VARIABLE`` appears before or
+after assignments to ``ENV_VARIABLE``.
+
+It is also possible to combine ``export`` with setting a value for the
+variable. Here is an example: export ENV_VARIABLE = "variable-value" In
+the output of ``bitbake -e``, variables that are exported to the
+environment are preceded by "export".
+
+Among the variables commonly exported to the environment are ``CC`` and
+``CFLAGS``, which are picked up by many build systems.
+
+Conditional Syntax (Overrides)
+==============================
+
+BitBake uses ```OVERRIDES`` <#var-bb-OVERRIDES>`__ to control what
+variables are overridden after BitBake parses recipes and configuration
+files. This section describes how you can use ``OVERRIDES`` as
+conditional metadata, talks about key expansion in relationship to
+``OVERRIDES``, and provides some examples to help with understanding.
+
+Conditional Metadata
+--------------------
+
+You can use ``OVERRIDES`` 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.
+
+-  *Selecting a Variable:* The ``OVERRIDES`` 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 ``OVERRIDES``, then the
+   “arm”-specific version of the variable is used rather than the
+   non-conditional version. Here is an example: OVERRIDES =
+   "architecture:os:machine" TEST = "default" TEST_os = "osspecific"
+   TEST_nooverride = "othercondvalue" In this example, the ``OVERRIDES``
+   variable lists three overrides: "architecture", "os", and "machine".
+   The variable ``TEST`` by itself has a default value of "default". You
+   select the os-specific version of the ``TEST`` variable by appending
+   the "os" override to the variable (i.e.\ ``TEST_os``).
+
+   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 ``KBRANCH`` to a default value, then conditionally override
+   that value based on the architecture of the build: 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"
+
+-  *Appending and Prepending:* BitBake also supports append and prepend
+   operations to variable values based on whether a specific item is
+   listed in ``OVERRIDES``. Here is an example: DEPENDS = "glibc
+   ncurses" OVERRIDES = "machine:local" DEPENDS_append_machine = "
+   libmad" In this example, ``DEPENDS`` becomes "glibc ncurses libmad".
+
+   Again, using an OpenEmbedded metadata-based kernel recipe file as an
+   example, the following lines will conditionally append to the
+   ``KERNEL_FEATURES`` variable based on the architecture:
+   KERNEL_FEATURES_append = " ${KERNEL_EXTRA_FEATURES}"
+   KERNEL_FEATURES_append_qemux86=" cfg/sound.scc cfg/paravirt_kvm.scc"
+   KERNEL_FEATURES_append_qemux86-64=" cfg/sound.scc
+   cfg/paravirt_kvm.scc"
+
+-  *Setting a Variable for a Single Task:* BitBake supports setting a
+   variable just for the duration of a single task. Here is an example:
+   FOO_task-configure = "val 1" FOO_task-compile = "val 2" In the
+   previous example, ``FOO`` has the value "val 1" while the
+   ``do_configure`` task is executed, and the value "val 2" while the
+   ``do_compile`` task is executed.
+
+   Internally, this is implemented by prepending the task (e.g.
+   "task-compile:") to the value of
+   ```OVERRIDES`` <#var-bb-OVERRIDES>`__ for the local datastore of the
+   ``do_compile`` task.
+
+   You can also use this syntax with other combinations (e.g.
+   "``_prepend``") as shown in the following example:
+   EXTRA_OEMAKE_prepend_task-compile = "${PARALLEL_MAKE} "
+
+Key Expansion
+-------------
+
+Key expansion happens when the BitBake datastore is finalized. To better
+understand this, consider the following example: A${B} = "X" B = "2" A2
+= "Y" In this case, after all the parsing is complete, BitBake expands
+``${B}`` into "2". This expansion causes ``A2``, which was set to "Y"
+before the expansion, to become "X".
+
+.. _variable-interaction-worked-examples:
+
+Examples
+--------
+
+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.
+
+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: OVERRIDES = "foo" A = "Z" A_foo_append = "X" For this case,
+``A`` is unconditionally set to "Z" and "X" is unconditionally and
+immediately appended to the variable ``A_foo``. Because overrides have
+not been applied yet, ``A_foo`` is set to "X" due to the append and
+``A`` simply equals "Z".
+
+Applying overrides, however, changes things. Since "foo" is listed in
+``OVERRIDES``, the conditional variable ``A`` is replaced with the "foo"
+version, which is equal to "X". So effectively, ``A_foo`` replaces
+``A``.
+
+This next example changes the order of the override and the append:
+OVERRIDES = "foo" A = "Z" A_append_foo = "X" For this case, before
+overrides are handled, ``A`` is set to "Z" and ``A_append_foo`` is set
+to "X". Once the override for "foo" is applied, however, ``A`` gets
+appended with "X". Consequently, ``A`` becomes "ZX". Notice that spaces
+are not appended.
+
+This next example has the order of the appends and overrides reversed
+back as in the first example: OVERRIDES = "foo" A = "Y" A_foo_append =
+"Z" A_foo_append = "X" For this case, before any overrides are resolved,
+``A`` is set to "Y" using an immediate assignment. After this immediate
+assignment, ``A_foo`` is set to "Z", and then further appended with "X"
+leaving the variable set to "ZX". Finally, applying the override for
+"foo" results in the conditional variable ``A`` becoming "ZX" (i.e.
+``A`` is replaced with ``A_foo``).
+
+This final example mixes in some varying operators: A = "1" A_append =
+"2" A_append = "3" A += "4" A .= "5" For this case, the type of append
+operators are affecting the order of assignments as BitBake passes
+through the code multiple times. Initially, ``A`` is set to "1 45"
+because of the three statements that use immediate operators. After
+these assignments are made, BitBake applies the "_append" operations.
+Those operations result in ``A`` becoming "1 4523".
+
+Sharing Functionality
+=====================
+
+BitBake allows for metadata sharing through include files (``.inc``) and
+class files (``.bbclass``). 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 ``.bbclass`` file
+that contains the common functionality and then using the ``inherit``
+directive in your recipes to inherit the class would be a common way to
+share the task.
+
+This section presents the mechanisms BitBake provides to allow you to
+share functionality between recipes. Specifically, the mechanisms
+include ``include``, ``inherit``, ``INHERIT``, and ``require``
+directives.
+
+Locating Include and Class Files
+--------------------------------
+
+BitBake uses the ```BBPATH`` <#var-bb-BBPATH>`__ variable to locate
+needed include and class files. Additionally, BitBake searches the
+current directory for ``include`` and ``require`` directives.
+
+.. note::
+
+   The
+   BBPATH
+   variable is analogous to the environment variable
+   PATH
+   .
+
+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
+``BBPATH``.
+
+``inherit`` Directive
+---------------------
+
+When writing a recipe or class file, you can use the ``inherit``
+directive to inherit the functionality of a class (``.bbclass``).
+BitBake only supports this directive when used within recipe and class
+files (i.e. ``.bb`` and ``.bbclass``).
+
+The ``inherit`` directive is a rudimentary means of specifying
+functionality contained in class files that 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 and then have your recipe inherit that class file.
+
+As an example, your recipes could use the following directive to inherit
+an ``autotools.bbclass`` file. The class file would contain common
+functionality for using Autotools that could be shared across recipes:
+inherit autotools In this case, BitBake would search for the directory
+``classes/autotools.bbclass`` in ``BBPATH``.
+
+.. note::
+
+   You can override any values and functions of the inherited class
+   within your recipe by doing so after the "inherit" statement.
+
+If you want to use the directive to inherit multiple classes, separate
+them with spaces. The following example shows how to inherit both the
+``buildhistory`` and ``rm_work`` classes: inherit buildhistory rm_work
+
+An advantage with the inherit directive as compared to both the
+`include <#include-directive>`__ and `require <#require-inclusion>`__
+directives is that you can inherit class files conditionally. You can
+accomplish this by using a variable expression after the ``inherit``
+statement. Here is an example: inherit ${VARNAME} If ``VARNAME`` is
+going to be set, it needs to be set before the ``inherit`` statement is
+parsed. One way to achieve a conditional inherit in this case is to use
+overrides: VARIABLE = "" VARIABLE_someoverride = "myclass"
+
+Another method is by using anonymous Python. Here is an example: python
+() { if condition == value: d.setVar('VARIABLE', 'myclass') else:
+d.setVar('VARIABLE', '') }
+
+Alternatively, you could use an in-line Python expression in the
+following form: inherit ${@'classname' if condition else ''} inherit
+${@functionname(params)} 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.
+
+``include`` Directive
+---------------------
+
+BitBake understands the ``include`` 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 ``BBPATH``.
+
+The include directive is a more generic method of including
+functionality as compared to the `inherit <#inherit-directive>`__
+directive, which is restricted to class (i.e. ``.bbclass``) files. The
+include directive is applicable for any other kind of shared or
+encapsulated functionality or configuration that does not suit a
+``.bbclass`` file.
+
+As an example, suppose you needed a recipe to include some self-test
+definitions: include test_defs.inc
+
+.. note::
+
+   The
+   include
+   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
+   require
+   instead of
+   include
+   . Doing so makes sure that an error is produced if the file cannot be
+   found.
+
+.. _require-inclusion:
+
+``require`` Directive
+---------------------
+
+BitBake understands the ``require`` directive. This directive behaves
+just like the ``include`` 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.
+
+The require directive, like the include directive previously described,
+is a more generic method of including functionality as compared to the
+`inherit <#inherit-directive>`__ directive, which is restricted to class
+(i.e. ``.bbclass``) files. The require directive is applicable for any
+other kind of shared or encapsulated functionality or configuration that
+does not suit a ``.bbclass`` file.
+
+Similar to how BitBake handles ```include`` <#include-directive>`__, if
+the path specified on the require line is a relative path, BitBake
+locates the first file it can find within ``BBPATH``.
+
+As an example, suppose you have two versions of a recipe (e.g.
+``foo_1.2.2.bb`` and ``foo_2.0.0.bb``) where each version contains some
+identical functionality that could be shared. You could create an
+include file named ``foo.inc`` that contains the common definitions
+needed to build "foo". You need to be sure ``foo.inc`` 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 ``require``
+directive from within each recipe: require foo.inc
+
+``INHERIT`` Configuration Directive
+-----------------------------------
+
+When creating a configuration file (``.conf``), you can use the
+```INHERIT`` <#var-bb-INHERIT>`__ configuration directive to inherit a
+class. BitBake only supports this directive when used within a
+configuration file.
+
+As an example, suppose you needed to inherit a class file called
+``abc.bbclass`` from a configuration file as follows: INHERIT += "abc"
+This configuration directive causes the named class to be inherited at
+the point of the directive during parsing. As with the ``inherit``
+directive, the ``.bbclass`` file must be located in a "classes"
+subdirectory in one of the directories specified in ``BBPATH``.
+
+.. note::
+
+   Because
+   .conf
+   files are parsed first during BitBake's execution, using
+   INHERIT
+   to inherit a class effectively inherits the class globally (i.e. for
+   all recipes).
+
+If you want to use the directive to inherit multiple classes, you can
+provide them on the same line in the ``local.conf`` file. Use spaces to
+separate the classes. The following example shows how to inherit both
+the ``autotools`` and ``pkgconfig`` classes: INHERIT += "autotools
+pkgconfig"
+
+Functions
+=========
+
+As with most languages, functions are the building blocks that are used
+to build up operations into tasks. BitBake supports these types of
+functions:
+
+-  *Shell Functions:* Functions written in shell script and executed
+   either directly as functions, tasks, or both. They can also be called
+   by other shell functions.
+
+-  *BitBake-Style Python Functions:* Functions written in Python and
+   executed by BitBake or other Python functions using
+   ``bb.build.exec_func()``.
+
+-  *Python Functions:* Functions written in Python and executed by
+   Python.
+
+-  *Anonymous Python Functions:* Python functions executed automatically
+   during parsing.
+
+Regardless of the type of function, you can only define them in class
+(``.bbclass``) and recipe (``.bb`` or ``.inc``) files.
+
+Shell Functions
+---------------
+
+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: some_function
+() { echo "Hello World" } 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 ``/bin/sh``, which may not be a bash
+shell but might be something such as ``dash``. You should not use
+Bash-specific script (bashisms).
+
+Overrides and override-style operators like ``_append`` and ``_prepend``
+can also be applied to shell functions. Most commonly, this application
+would be used in a ``.bbappend`` file to modify functions in the main
+recipe. It can also be used to modify functions inherited from classes.
+
+As an example, consider the following: do_foo() { bbplain first fn }
+fn_prepend() { bbplain second } fn() { bbplain third } do_foo_append() {
+bbplain fourth } Running ``do_foo`` prints the following: recipename
+do_foo: first recipename do_foo: second recipename do_foo: third
+recipename do_foo: fourth
+
+.. note::
+
+   Overrides and override-style operators can be applied to any shell
+   function, not just
+   tasks
+   .
+
+You can use the ``bitbake -e`` recipename command to view the final
+assembled function after all overrides have been applied.
+
+BitBake-Style Python Functions
+------------------------------
+
+These functions are written in Python and executed by BitBake or other
+Python functions using ``bb.build.exec_func()``.
+
+An example BitBake function is: python some_python_function () {
+d.setVar("TEXT", "Hello World") print d.getVar("TEXT") } 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.
+
+.. note::
+
+   Variable expressions (e.g.
+   ${X}
+   ) are no longer expanded within Python functions. This behavior is
+   intentional in order to allow you to freely set variable values to
+   expandable expressions without having them expanded prematurely. If
+   you do wish to expand a variable within a Python function, use
+   d.getVar("X")
+   . Or, for more complicated expressions, use
+   d.expand()
+   .
+
+Similar to shell functions, you can also apply overrides and
+override-style operators to BitBake-style Python functions.
+
+As an example, consider the following: python do_foo_prepend() {
+bb.plain("first") } python do_foo() { bb.plain("second") } python
+do_foo_append() { bb.plain("third") } Running ``do_foo`` prints the
+following: recipename do_foo: first recipename do_foo: second recipename
+do_foo: third You can use the ``bitbake -e`` recipename command to view
+the final assembled function after all overrides have been applied.
+
+Python Functions
+----------------
+
+These functions are written in Python and are executed by other Python
+code. Examples of Python functions are utility functions that you intend
+to call from in-line Python or from within other Python functions. Here
+is an example: def get_depends(d): if d.getVar('SOMECONDITION'): return
+"dependencywithcond" else: return "dependency" SOMECONDITION = "1"
+DEPENDS = "${@get_depends(d)}" This would result in ``DEPENDS``
+containing ``dependencywithcond``.
+
+Here are some things to know about Python functions:
+
+-  Python functions can take parameters.
+
+-  The BitBake datastore is not automatically available. Consequently,
+   you must pass it in as a parameter to the function.
+
+-  The "bb" and "os" Python modules are automatically available. You do
+   not need to import them.
+
+BitBake-Style Python Functions Versus Python Functions
+------------------------------------------------------
+
+Following are some important differences between BitBake-style Python
+functions and regular Python functions defined with "def":
+
+-  Only BitBake-style Python functions can be `tasks <#tasks>`__.
+
+-  Overrides and override-style operators can only be applied to
+   BitBake-style Python functions.
+
+-  Only regular Python functions can take arguments and return values.
+
+-  `Variable flags <#variable-flags>`__ such as ``[dirs]``,
+   ``[cleandirs]``, and ``[lockfiles]`` can be used on BitBake-style
+   Python functions, but not on regular Python functions.
+
+-  BitBake-style Python functions generate a separate
+   ``${``\ ```T`` <#var-bb-T>`__\ ``}/run.``\ function-name\ ``.``\ pid
+   script that is executed to run the function, and also generate a log
+   file in ``${T}/log.``\ function-name\ ``.``\ pid if they are executed
+   as tasks.
+
+   Regular Python functions execute "inline" and do not generate any
+   files in ``${T}``.
+
+-  Regular Python functions are called with the usual Python syntax.
+   BitBake-style Python functions are usually tasks and are called
+   directly by BitBake, but can also be called manually from Python code
+   by using the ``bb.build.exec_func()`` function. Here is an example:
+   bb.build.exec_func("my_bitbake_style_function", d)
+
+   .. note::
+
+      bb.build.exec_func()
+      can also be used to run shell functions from Python code. If you
+      want to run a shell function before a Python function within the
+      same task, then you can use a parent helper Python function that
+      starts by running the shell function with
+      bb.build.exec_func()
+      and then runs the Python code.
+
+   To detect errors from functions executed with
+   ``bb.build.exec_func()``, you can catch the ``bb.build.FuncFailed``
+   exception.
+
+   .. note::
+
+      Functions in metadata (recipes and classes) should not themselves
+      raise
+      bb.build.FuncFailed
+      . Rather,
+      bb.build.FuncFailed
+      should be viewed as a general indicator that the called function
+      failed by raising an exception. For example, an exception raised
+      by
+      bb.fatal()
+      will be caught inside
+      bb.build.exec_func()
+      , and a
+      bb.build.FuncFailed
+      will be raised in response.
+
+Due to their simplicity, you should prefer regular Python functions over
+BitBake-style Python functions unless you need a feature specific to
+BitBake-style Python functions. Regular Python functions in metadata are
+a more recent invention than BitBake-style Python functions, and older
+code tends to use ``bb.build.exec_func()`` more often.
+
+Anonymous Python Functions
+--------------------------
+
+Sometimes it is useful to set variables or perform other operations
+programmatically during parsing. To do this, you can define special
+Python functions, called anonymous Python functions, that run at the end
+of parsing. For example, the following conditionally sets a variable
+based on the value of another variable: python () { if
+d.getVar('SOMEVAR') == 'value': d.setVar('ANOTHERVAR', 'value2') } An
+equivalent way to mark a function as an anonymous function is to give it
+the name "__anonymous", rather than no name.
+
+Anonymous Python functions always run at the end of parsing, regardless
+of where they are defined. If a recipe contains many anonymous
+functions, they run in the same order as they are defined within the
+recipe. As an example, consider the following snippet: python () {
+d.setVar('FOO', 'foo 2') } FOO = "foo 1" python () { d.appendVar('BAR',
+' bar 2') } BAR = "bar 1" The previous example is conceptually
+equivalent to the following snippet: FOO = "foo 1" BAR = "bar 1" FOO =
+"foo 2" BAR += "bar 2" ``FOO`` ends up with the value "foo 2", and
+``BAR`` with the value "bar 1 bar 2". Just as in the second snippet, the
+values set for the variables within the anonymous functions become
+available to tasks, which always run after parsing.
+
+Overrides and override-style operators such as "``_append``" are applied
+before anonymous functions run. In the following example, ``FOO`` ends
+up with the value "foo from anonymous": FOO = "foo" FOO_append = " from
+outside" python () { d.setVar("FOO", "foo from anonymous") } For methods
+you can use with anonymous Python functions, see the "`Functions You Can
+Call From Within Python <#functions-you-can-call-from-within-python>`__"
+section. For a different method to run Python code during parsing, see
+the "`Inline Python Variable
+Expansion <#inline-python-variable-expansion>`__" section.
+
+Flexible Inheritance for Class Functions
+----------------------------------------
+
+Through coding techniques and the use of ``EXPORT_FUNCTIONS``, 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.
+
+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. ``EXPORT_FUNCTIONS`` provides a mechanism that
+enables the recipe's version of the function to call the original
+version of the function.
+
+To make use of this technique, you need the following things in place:
+
+-  The class needs to define the function as follows:
+   classname\ ``_``\ functionname For example, if you have a class file
+   ``bar.bbclass`` and a function named ``do_foo``, the class must
+   define the function as follows: bar_do_foo
+
+-  The class needs to contain the ``EXPORT_FUNCTIONS`` statement as
+   follows: EXPORT_FUNCTIONS functionname For example, continuing with
+   the same example, the statement in the ``bar.bbclass`` would be as
+   follows: EXPORT_FUNCTIONS do_foo
+
+-  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 ``bar_do_foo``.
+   Assuming ``do_foo`` was a shell function and ``EXPORT_FUNCTIONS`` was
+   used as above, the recipe's function could conditionally call the
+   class version of the function as follows: do_foo() { if [
+   somecondition ] ; then bar_do_foo else # Do something else fi } To
+   call your modified version of the function as defined in your recipe,
+   call it as ``do_foo``.
+
+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.
+
+Tasks
+=====
+
+Tasks are BitBake execution units that make up the steps that BitBake
+can run for a given recipe. Tasks are only supported in recipes and
+classes (i.e. in ``.bb`` files and files included or inherited from
+``.bb`` files). By convention, tasks have names that start with "do_".
+
+Promoting a Function to a Task
+------------------------------
+
+Tasks are either `shell functions <#shell-functions>`__ or
+`BitBake-style Python functions <#bitbake-style-python-functions>`__
+that have been promoted to tasks by using the ``addtask`` command. The
+``addtask`` command can also optionally describe dependencies between
+the task and other tasks. Here is an example that shows how to define a
+task and declare some dependencies: python do_printdate () { import time
+print time.strftime('%Y%m%d', time.gmtime()) } addtask printdate after
+do_fetch before do_build The first argument to ``addtask`` is the name
+of the function to promote to a task. If the name does not start with
+"do_", "do_" is implicitly added, which enforces the convention that all
+task names start with "do_".
+
+In the previous example, the ``do_printdate`` task becomes a dependency
+of the ``do_build`` task, which is the default task (i.e. the task run
+by the ``bitbake`` command unless another task is specified explicitly).
+Additionally, the ``do_printdate`` task becomes dependent upon the
+``do_fetch`` task. Running the ``do_build`` task results in the
+``do_printdate`` task running first.
+
+.. note::
+
+   If you try out the previous example, you might see that the
+   do_printdate
+   task is only run the first time you build the recipe with the
+   bitbake
+   command. This is because BitBake considers the task "up-to-date"
+   after that initial run. If you want to force the task to always be
+   rerun for experimentation purposes, you can make BitBake always
+   consider the task "out-of-date" by using the
+   [
+   nostamp
+   ]
+   variable flag, as follows:
+   ::
+
+           do_printdate[nostamp] = "1"
+                          
+
+   You can also explicitly run the task and provide the
+   -f
+   option as follows:
+   ::
+
+           $ bitbake recipe -c printdate -f
+                          
+
+   When manually selecting a task to run with the
+   bitbake
+   NBSP
+   recipe
+   NBSP
+   -c
+   NBSP
+   task
+   command, you can omit the "do_" prefix as part of the task name.
+
+You might wonder about the practical effects of using ``addtask``
+without specifying any dependencies as is done in the following example:
+addtask printdate In this example, assuming dependencies have not been
+added through some other means, the only way to run the task is by
+explicitly selecting it with ``bitbake`` recipe ``-c printdate``. You
+can use the ``do_listtasks`` task to list all tasks defined in a recipe
+as shown in the following example: $ bitbake recipe -c listtasks For
+more information on task dependencies, see the
+"`Dependencies <#dependencies>`__" section.
+
+See the "`Variable Flags <#variable-flags>`__" section for information
+on variable flags you can use with tasks.
+
+Deleting a Task
+---------------
+
+As well as being able to add tasks, you can delete them. Simply use the
+``deltask`` command to delete a task. For example, to delete the example
+task used in the previous sections, you would use: deltask printdate If
+you delete a task using the ``deltask`` command and the task has
+dependencies, the dependencies are not reconnected. For example, suppose
+you have three tasks named ``do_a``, ``do_b``, and ``do_c``.
+Furthermore, ``do_c`` is dependent on ``do_b``, which in turn is
+dependent on ``do_a``. Given this scenario, if you use ``deltask`` to
+delete ``do_b``, the implicit dependency relationship between ``do_c``
+and ``do_a`` through ``do_b`` no longer exists, and ``do_c``
+dependencies are not updated to include ``do_a``. Thus, ``do_c`` is free
+to run before ``do_a``.
+
+If you want dependencies such as these to remain intact, use the
+``[noexec]`` varflag to disable the task instead of using the
+``deltask`` command to delete it: do_b[noexec] = "1"
+
+Passing Information Into the Build Task Environment
+---------------------------------------------------
+
+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
+   BB_PRESERVE_ENV
+   variable.
+
+Consequently, if you do want something to get passed into the build task
+environment, you must take these two steps:
+
+1. Tell BitBake to load what you want from the environment into the
+   datastore. You can do so through the
+   ```BB_ENV_WHITELIST`` <#var-bb-BB_ENV_WHITELIST>`__ and
+   ```BB_ENV_EXTRAWHITE`` <#var-bb-BB_ENV_EXTRAWHITE>`__ variables. For
+   example, assume you want to prevent the build system from accessing
+   your ``$HOME/.ccache`` directory. The following command "whitelists"
+   the environment variable ``CCACHE_DIR`` causing BitBake to allow that
+   variable into the datastore: export
+   BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE CCACHE_DIR"
+
+2. 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 ``local.conf`` or your distribution
+   configuration file: export CCACHE_DIR
+
+   .. 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.
+
+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
+```BB_ORIGENV`` <#var-bb-BB_ORIGENV>`__.
+
+The ``BB_ORIGENV`` variable returns a datastore object that can be
+queried using the standard datastore operators such as
+``getVar(, False)``. The datastore object is useful, for example, to
+find the original ``DISPLAY`` variable. Here is an example: origenv =
+d.getVar("BB_ORIGENV", False) bar = origenv.getVar("BAR", False) The
+previous example returns ``BAR`` from the original execution
+environment.
+
+Variable Flags
+==============
+
+Variable flags (varflags) help control a task's functionality and
+dependencies. BitBake reads and writes varflags to the datastore using
+the following command forms: variable = d.getVarFlags("variable")
+self.d.setVarFlags("FOO", {"func": True})
+
+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 "`Variable Flag
+Syntax <#variable-flag-syntax>`__" section for details.
+
+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:
+
+-  *``[cleandirs]``:* Empty directories that should be created before
+   the task runs. Directories that already exist are removed and
+   recreated to empty them.
+
+-  *``[depends]``:* Controls inter-task dependencies. See the
+   ```DEPENDS`` <#var-bb-DEPENDS>`__ variable and the "`Inter-Task
+   Dependencies <#inter-task-dependencies>`__" section for more
+   information.
+
+-  *``[deptask]``:* Controls task build-time dependencies. See the
+   ```DEPENDS`` <#var-bb-DEPENDS>`__ variable and the "`Build
+   Dependencies <#build-dependencies>`__" section for more information.
+
+-  *``[dirs]``:* Directories that should be created before the task
+   runs. Directories that already exist are left as is. The last
+   directory listed is used as the current working directory for the
+   task.
+
+-  *``[lockfiles]``:* 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.
+
+-  *``[noexec]``:* When set to "1", marks the task as being empty, with
+   no execution required. You can use the ``[noexec]`` flag to set up
+   tasks as dependency placeholders, or to disable tasks defined
+   elsewhere that are not needed in a particular recipe.
+
+-  *``[nostamp]``:* When set to "1", tells BitBake to not generate a
+   stamp file for a task, which implies the task should always be
+   executed.
+
+   .. note::
+
+      Any task that depends (possibly indirectly) on a
+      [nostamp]
+      task will always be executed as well. This can cause unnecessary
+      rebuilding if you are not careful.
+
+-  *``[number_threads]``:* Limits tasks to a specific number of
+   simultaneous threads during execution. This varflag is useful when
+   your build host has a large number of cores but certain tasks need to
+   be rate-limited due to various kinds of resource constraints (e.g. to
+   avoid network throttling). ``number_threads`` works similarly to the
+   ```BB_NUMBER_THREADS`` <#var-bb-BB_NUMBER_THREADS>`__ variable but is
+   task-specific.
+
+   Set the value globally. For example, the following makes sure the
+   ``do_fetch`` task uses no more than two simultaneous execution
+   threads: do_fetch[number_threads] = "2"
+
+   .. note::
+
+      -  Setting the varflag in individual recipes rather than globally
+         can result in unpredictable behavior.
+
+      -  Setting the varflag to a value greater than the value used in
+         the ``BB_NUMBER_THREADS`` variable causes ``number_threads`` to
+         have no effect.
+
+-  *``[postfuncs]``:* List of functions to call after the completion of
+   the task.
+
+-  *``[prefuncs]``:* List of functions to call before the task executes.
+
+-  *``[rdepends]``:* Controls inter-task runtime dependencies. See the
+   ```RDEPENDS`` <#var-bb-RDEPENDS>`__ variable, the
+   ```RRECOMMENDS`` <#var-bb-RRECOMMENDS>`__ variable, and the
+   "`Inter-Task Dependencies <#inter-task-dependencies>`__" section for
+   more information.
+
+-  *``[rdeptask]``:* Controls task runtime dependencies. See the
+   ```RDEPENDS`` <#var-bb-RDEPENDS>`__ variable, the
+   ```RRECOMMENDS`` <#var-bb-RRECOMMENDS>`__ variable, and the "`Runtime
+   Dependencies <#runtime-dependencies>`__" section for more
+   information.
+
+-  *``[recideptask]``:* When set in conjunction with ``recrdeptask``,
+   specifies a task that should be inspected for additional
+   dependencies.
+
+-  *``[recrdeptask]``:* Controls task recursive runtime dependencies.
+   See the ```RDEPENDS`` <#var-bb-RDEPENDS>`__ variable, the
+   ```RRECOMMENDS`` <#var-bb-RRECOMMENDS>`__ variable, and the
+   "`Recursive Dependencies <#recursive-dependencies>`__" section for
+   more information.
+
+-  *``[stamp-extra-info]``:* Extra stamp information to append to the
+   task's stamp. As an example, OpenEmbedded uses this flag to allow
+   machine-specific tasks.
+
+-  *``[umask]``:* The umask to run the task under.
+
+Several varflags are useful for controlling how signatures are
+calculated for variables. For more information on this process, see the
+"`Checksums (Signatures) <#checksums>`__" section.
+
+-  *``[vardeps]``:* 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.
+
+-  *``[vardepsexclude]``:* Specifies a space-separated list of variables
+   that should be excluded from a variable's dependencies for the
+   purposes of calculating its signature.
+
+-  *``[vardepvalue]``:* If set, instructs BitBake to ignore the actual
+   value of the variable and instead use the specified value when
+   calculating the variable's signature.
+
+-  *``[vardepvalueexclude]``:* Specifies a pipe-separated list of
+   strings to exclude from the variable's value when calculating the
+   variable's signature.
+
+Events
+======
+
+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 recipe (i.e. ``*.bb``), the
+start of a given task, a task failure, a task success, and so forth. The
+intent is to make it easy to do things like email notification on build
+failures.
+
+Following is an example event handler that prints the name of the event
+and the content of the ``FILE`` variable: addhandler
+myclass_eventhandler python myclass_eventhandler() { from bb.event
+import getName print("The name of the Event is %s" % getName(e))
+print("The file we run for is %s" % d.getVar('FILE')) }
+myclass_eventhandler[eventmask] = "bb.event.BuildStarted
+bb.event.BuildCompleted" In the previous example, an eventmask has been
+set so that the handler only sees the "BuildStarted" and
+"BuildCompleted" events. This event handler gets called every time an
+event matching the eventmask is triggered. A global variable "e" is
+defined, which represents the current event. With the ``getName(e)``
+method, you can get the name of the triggered event. The global
+datastore is available as "d". In legacy code, you might see "e.data"
+used to get the datastore. However, realize that "e.data" is deprecated
+and you should use "d" going forward.
+
+The context of the datastore is appropriate to the event in question.
+For example, "BuildStarted" and "BuildCompleted" events run before any
+tasks are executed so would be in the global configuration datastore
+namespace. No recipe-specific metadata exists in that namespace. The
+"BuildStarted" and "BuildCompleted" events also run in the main
+cooker/server process rather than any worker context. Thus, any changes
+made to the datastore would be seen by other cooker/server events within
+the current build but not seen outside of that build or in any worker
+context. Task events run in the actual tasks in question consequently
+have recipe-specific and task-specific contents. These events run in the
+worker context and are discarded at the end of task execution.
+
+During a standard build, the following common events might occur. The
+following events are the most common kinds of events that most metadata
+might have an interest in viewing:
+
+-  ``bb.event.ConfigParsed()``: Fired when the base configuration; which
+   consists of ``bitbake.conf``, ``base.bbclass`` and any global
+   ``INHERIT`` statements; has been parsed. You can see multiple such
+   events when each of the workers parse the base configuration or if
+   the server changes configuration and reparses. Any given datastore
+   only has one such event executed against it, however. If
+   ```BB_INVALIDCONF`` <#>`__ is set in the datastore by the event
+   handler, the configuration is reparsed and a new event triggered,
+   allowing the metadata to update configuration.
+
+-  ``bb.event.HeartbeatEvent()``: Fires at regular time intervals of one
+   second. You can configure the interval time using the
+   ``BB_HEARTBEAT_EVENT`` variable. The event's "time" attribute is the
+   ``time.time()`` value when the event is triggered. This event is
+   useful for activities such as system state monitoring.
+
+-  ``bb.event.ParseStarted()``: Fired when BitBake is about to start
+   parsing recipes. This event's "total" attribute represents the number
+   of recipes BitBake plans to parse.
+
+-  ``bb.event.ParseProgress()``: Fired as parsing progresses. This
+   event's "current" attribute is the number of recipes parsed as well
+   as the "total" attribute.
+
+-  ``bb.event.ParseCompleted()``: Fired when parsing is complete. This
+   event's "cached", "parsed", "skipped", "virtuals", "masked", and
+   "errors" attributes provide statistics for the parsing results.
+
+-  ``bb.event.BuildStarted()``: Fired when a new build starts. BitBake
+   fires multiple "BuildStarted" events (one per configuration) when
+   multiple configuration (multiconfig) is enabled.
+
+-  ``bb.build.TaskStarted()``: Fired when a task starts. This event's
+   "taskfile" attribute points to the recipe from which the task
+   originates. The "taskname" attribute, which is the task's name,
+   includes the ``do_`` prefix, and the "logfile" attribute point to
+   where the task's output is stored. Finally, the "time" attribute is
+   the task's execution start time.
+
+-  ``bb.build.TaskInvalid()``: Fired if BitBake tries to execute a task
+   that does not exist.
+
+-  ``bb.build.TaskFailedSilent()``: Fired for setscene tasks that fail
+   and should not be presented to the user verbosely.
+
+-  ``bb.build.TaskFailed()``: Fired for normal tasks that fail.
+
+-  ``bb.build.TaskSucceeded()``: Fired when a task successfully
+   completes.
+
+-  ``bb.event.BuildCompleted()``: Fired when a build finishes.
+
+-  ``bb.cooker.CookerExit()``: Fired when the BitBake server/cooker
+   shuts down. This event is usually only seen by the UIs as a sign they
+   should also shutdown.
+
+This next list of example events occur based on specific requests to the
+server. These events are often used to communicate larger pieces of
+information from the BitBake server to other parts of BitBake such as
+user interfaces:
+
+-  ``bb.event.TreeDataPreparationStarted()``
+
+-  ``bb.event.TreeDataPreparationProgress()``
+
+-  ``bb.event.TreeDataPreparationCompleted()``
+
+-  ``bb.event.DepTreeGenerated()``
+
+-  ``bb.event.CoreBaseFilesFound()``
+
+-  ``bb.event.ConfigFilePathFound()``
+
+-  ``bb.event.FilesMatchingFound()``
+
+-  ``bb.event.ConfigFilesFound()``
+
+-  ``bb.event.TargetsTreeGenerated()``
+
+.. _variants-class-extension-mechanism:
+
+Variants - Class Extension Mechanism
+====================================
+
+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
+```BBCLASSEXTEND`` <#var-bb-BBCLASSEXTEND>`__ and
+```BBVERSIONS`` <#var-bb-BBVERSIONS>`__ variables.
+
+.. note::
+
+   The mechanism for this class extension is extremely specific to the
+   implementation. Usually, the recipe's
+   PROVIDES
+   ,
+   PN
+   , and
+   DEPENDS
+   variables would need to be modified by the extension class. For
+   specific examples, see the OE-Core
+   native
+   ,
+   nativesdk
+   , and
+   multilib
+   classes.
+
+-  *``BBCLASSEXTEND``:* 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. BBCLASSEXTEND = "native"
+
+-  *``BBVERSIONS``:* 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
+   ```OVERRIDES`` <#var-bb-OVERRIDES>`__ mechanism) for a single
+   version, or an optionally named range of versions. Here is an
+   example: 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" The
+   name of the range defaults to the original version of the recipe. For
+   example, in OpenEmbedded, the recipe file ``foo_1.0.0+.bb`` creates a
+   default name range of ``1.0.0+``. 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 ``file://`` search paths
+   (```FILESPATH`` <#var-bb-FILESPATH>`__).
+
+Dependencies
+============
+
+To allow for efficient parallel processing, BitBake handles dependencies
+at the task level. Dependencies can exist both between tasks within a
+single recipe and between tasks in different recipes. Following are
+examples of each:
+
+-  For tasks within a single recipe, a recipe's ``do_configure`` task
+   might need to complete before its ``do_compile`` task can run.
+
+-  For tasks in different recipes, one recipe's ``do_configure`` task
+   might require another recipe's ``do_populate_sysroot`` task to finish
+   first such that the libraries and headers provided by the other
+   recipe are available.
+
+This section describes several ways to declare dependencies. Remember,
+even though dependencies are declared in different ways, they are all
+simply dependencies between tasks.
+
+.. _dependencies-internal-to-the-bb-file:
+
+Dependencies Internal to the ``.bb`` File
+-----------------------------------------
+
+BitBake uses the ``addtask`` directive to manage dependencies that are
+internal to a given recipe file. You can use the ``addtask`` directive
+to indicate when a task is dependent on other tasks or when other tasks
+depend on that recipe. Here is an example: addtask printdate after
+do_fetch before do_build In this example, the ``do_printdate`` task
+depends on the completion of the ``do_fetch`` task, and the ``do_build``
+task depends on the completion of the ``do_printdate`` task.
+
+.. note::
+
+   For a task to run, it must be a direct or indirect dependency of some
+   other task that is scheduled to run.
+
+   For illustration, here are some examples:
+
+   -  The directive ``addtask mytask before do_configure`` causes
+      ``do_mytask`` to run before ``do_configure`` runs. Be aware that
+      ``do_mytask`` still only runs if its `input
+      checksum <#checksums>`__ has changed since the last time it was
+      run. Changes to the input checksum of ``do_mytask`` also
+      indirectly cause ``do_configure`` to run.
+
+   -  The directive ``addtask mytask after do_configure`` by itself
+      never causes ``do_mytask`` to run. ``do_mytask`` can still be run
+      manually as follows: $ bitbake recipe -c mytask Declaring
+      ``do_mytask`` as a dependency of some other task that is scheduled
+      to run also causes it to run. Regardless, the task runs after
+      ``do_configure``.
+
+Build Dependencies
+------------------
+
+BitBake uses the ```DEPENDS`` <#var-bb-DEPENDS>`__ variable to manage
+build time dependencies. The ``[deptask]`` varflag for tasks signifies
+the task of each item listed in ``DEPENDS`` that must complete before
+that task can be executed. Here is an example: do_configure[deptask] =
+"do_populate_sysroot" In this example, the ``do_populate_sysroot`` task
+of each item in ``DEPENDS`` must complete before ``do_configure`` can
+execute.
+
+Runtime Dependencies
+--------------------
+
+BitBake uses the ```PACKAGES`` <#var-bb-PACKAGES>`__,
+```RDEPENDS`` <#var-bb-RDEPENDS>`__, and
+```RRECOMMENDS`` <#var-bb-RRECOMMENDS>`__ variables to manage runtime
+dependencies.
+
+The ``PACKAGES`` variable lists runtime packages. Each of those packages
+can have ``RDEPENDS`` and ``RRECOMMENDS`` 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. do_package_qa[rdeptask] = "do_packagedata" In the previous
+example, the ``do_packagedata`` task of each item in ``RDEPENDS`` must
+have completed before ``do_package_qa`` can execute.
+Although ``RDEPENDS`` contains entries from the
+runtime dependency namespace, BitBake knows how to map them back
+to the build-time dependency namespace, in which the tasks are defined.
+
+Recursive Dependencies
+----------------------
+
+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.
+
+The ``[recrdeptask]`` flag is most commonly used in high-level recipes
+that need to wait for some task to finish "globally". For example,
+``image.bbclass`` has the following: do_rootfs[recrdeptask] +=
+"do_packagedata" This statement says that the ``do_packagedata`` task of
+the current recipe and all recipes reachable (by way of dependencies)
+from the image recipe must run before the ``do_rootfs`` task can run.
+
+BitBake allows a task to recursively depend on itself by
+referencing itself in the task list:
+do_a[recrdeptask] = "do_a do_b"
+
+In the same way as before, this means that the ``do_a``
+and ``do_b`` tasks of the current recipe and all
+recipes reachable (by way of dependencies) from the recipe
+must run before the ``do_a`` task can run. In this
+case BitBake will ignore the current recipe's ``do_a``
+task circular dependency on itself.
+
+Inter-Task Dependencies
+-----------------------
+
+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 ``DEPENDS``. Here is an example: do_patch[depends] =
+"quilt-native:do_populate_sysroot" In this example, the
+``do_populate_sysroot`` task of the target ``quilt-native`` must have
+completed before the ``do_patch`` task can execute.
+
+The ``[rdepends]`` flag works in a similar way but takes targets in the
+runtime namespace instead of the build-time dependency namespace.
+
+Functions You Can Call From Within Python
+=========================================
+
+BitBake provides many functions you can call from within Python
+functions. This section lists the most commonly used functions, and
+mentions where to find others.
+
+Functions for Accessing Datastore Variables
+-------------------------------------------
+
+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:
+
++-----------------------------------+-----------------------------------+
+| *Operation*                       | *Description*                     |
++===================================+===================================+
+| ``d.getVar("X", expand)``         | Returns the value of variable     |
+|                                   | "X". Using "expand=True" expands  |
+|                                   | the value. Returns "None" if the  |
+|                                   | variable "X" does not exist.      |
++-----------------------------------+-----------------------------------+
+| ``d.setVar("X", "value")``        | Sets the variable "X" to "value". |
++-----------------------------------+-----------------------------------+
+| ``d.appendVar("X", "value")``     | Adds "value" to the end of the    |
+|                                   | variable "X". Acts like           |
+|                                   | ``d.setVar("X", "value")`` if the |
+|                                   | variable "X" does not exist.      |
++-----------------------------------+-----------------------------------+
+| ``d.prependVar("X", "value")``    | Adds "value" to the start of the  |
+|                                   | variable "X". Acts like           |
+|                                   | ``d.setVar("X", "value")`` if the |
+|                                   | variable "X" does not exist.      |
++-----------------------------------+-----------------------------------+
+| ``d.delVar("X")``                 | Deletes the variable "X" from the |
+|                                   | datastore. Does nothing if the    |
+|                                   | variable "X" does not exist.      |
++-----------------------------------+-----------------------------------+
+| ``d.renameVar("X", "Y")``         | Renames the variable "X" to "Y".  |
+|                                   | Does nothing if the variable "X"  |
+|                                   | does not exist.                   |
++-----------------------------------+-----------------------------------+
+| ``                                | Returns the value of variable     |
+| d.getVarFlag("X", flag, expand)`` | "X". Using "expand=True" expands  |
+|                                   | the value. Returns "None" if      |
+|                                   | either the variable "X" or the    |
+|                                   | named flag does not exist.        |
++-----------------------------------+-----------------------------------+
+| ``d                               | Sets the named flag for variable  |
+| .setVarFlag("X", flag, "value")`` | "X" to "value".                   |
++-----------------------------------+-----------------------------------+
+| ``d.ap                            | Appends "value" to the named flag |
+| pendVarFlag("X", flag, "value")`` | on the variable "X". Acts like    |
+|                                   | ``d                               |
+|                                   | .setVarFlag("X", flag, "value")`` |
+|                                   | if the named flag does not exist. |
++-----------------------------------+-----------------------------------+
+| ``d.pre                           | Prepends "value" to the named     |
+| pendVarFlag("X", flag, "value")`` | flag on the variable "X". Acts    |
+|                                   | like                              |
+|                                   | ``d                               |
+|                                   | .setVarFlag("X", flag, "value")`` |
+|                                   | if the named flag does not exist. |
++-----------------------------------+-----------------------------------+
+| ``d.delVarFlag("X", flag)``       | Deletes the named flag on the     |
+|                                   | variable "X" from the datastore.  |
++-----------------------------------+-----------------------------------+
+| ``d.setVarFlags("X", flagsdict)`` | Sets the flags specified in the   |
+|                                   | ``flagsdict()`` parameter.        |
+|                                   | ``setVarFlags`` does not clear    |
+|                                   | previous flags. Think of this     |
+|                                   | operation as ``addVarFlags``.     |
++-----------------------------------+-----------------------------------+
+| ``d.getVarFlags("X")``            | Returns a ``flagsdict`` of the    |
+|                                   | flags for the variable "X".       |
+|                                   | Returns "None" if the variable    |
+|                                   | "X" does not exist.               |
++-----------------------------------+-----------------------------------+
+| ``d.delVarFlags("X")``            | Deletes all the flags for the     |
+|                                   | variable "X". Does nothing if the |
+|                                   | variable "X" does not exist.      |
++-----------------------------------+-----------------------------------+
+| ``d.expand(expression)``          | Expands variable references in    |
+|                                   | the specified string expression.  |
+|                                   | References to variables that do   |
+|                                   | not exist are left as is. For     |
+|                                   | example, ``d.expand("foo ${X}")`` |
+|                                   | expands to the literal string     |
+|                                   | "foo ${X}" if the variable "X"    |
+|                                   | does not exist.                   |
++-----------------------------------+-----------------------------------+
+
+Other Functions
+---------------
+
+You can find many other functions that can be called from Python by
+looking at the source code of the ``bb`` module, which is in
+``bitbake/lib/bb``. For example, ``bitbake/lib/bb/utils.py`` includes
+the commonly used functions ``bb.utils.contains()`` and
+``bb.utils.mkdirhier()``, which come with docstrings.
+
+Task Checksums and Setscene
+===========================
+
+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.
+
+These checksums are stored in ```STAMP`` <#var-bb-STAMP>`__. You can
+examine the checksums using the following BitBake command: $
+bitbake-dumpsigs This command returns the signature data in a readable
+format that allows you to examine the inputs used when the OpenEmbedded
+build system generates signatures. For example, using
+``bitbake-dumpsigs`` allows you to examine the ``do_compile`` task's
+“sigdata” for a C application (e.g. ``bash``). Running the command also
+reveals that the “CC” variable is part of the inputs that are hashed.
+Any changes to this variable would invalidate the stamp and cause the
+``do_compile`` task to run.
+
+The following list describes related variables:
+
+-  ```BB_HASHCHECK_FUNCTION`` <#var-bb-BB_HASHCHECK_FUNCTION>`__:
+   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.
+
+-  ```BB_SETSCENE_DEPVALID`` <#var-bb-BB_SETSCENE_DEPVALID>`__:
+   Specifies a function BitBake calls that determines whether BitBake
+   requires a setscene dependency to be met.
+
+-  ```BB_SETSCENE_VERIFY_FUNCTION2`` <#var-bb-BB_SETSCENE_VERIFY_FUNCTION2>`__:
+   Specifies a function to call that verifies the list of planned task
+   execution before the main task execution happens.
+
+-  ```BB_STAMP_POLICY`` <#var-bb-BB_STAMP_POLICY>`__: Defines the mode
+   for comparing timestamps of stamp files.
+
+-  ```BB_STAMP_WHITELIST`` <#var-bb-BB_STAMP_WHITELIST>`__: Lists stamp
+   files that are looked at when the stamp policy is "whitelist".
+
+-  ```BB_TASKHASH`` <#var-bb-BB_TASKHASH>`__: Within an executing task,
+   this variable holds the hash of the task as returned by the currently
+   enabled signature generator.
+
+-  ```STAMP`` <#var-bb-STAMP>`__: The base path to create stamp files.
+
+-  ```STAMPCLEAN`` <#var-bb-STAMPCLEAN>`__: Again, the base path to
+   create stamp files but can use wildcards for matching a range of
+   files for clean operations.
+
+Wildcard Support in Variables
+=============================
+
+Support for wildcard use in variables varies depending on the context in
+which it is used. For example, some variables and file names allow
+limited use of wildcards through the "``%``" and "``*``" characters.
+Other variables or names support Python's
+```glob`` <https://docs.python.org/3/library/glob.html>`__ syntax,
+```fnmatch`` <https://docs.python.org/3/library/fnmatch.html#module-fnmatch>`__
+syntax, or
+```Regular Expression (re)`` <https://docs.python.org/3/library/re.html#re>`__
+syntax.
+
+For variables that have wildcard suport, the documentation describes
+which form of wildcard, its use, and its limitations.