3. 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.


3.1. Basic Syntax
=================

This section provides some basic syntax examples.


3.1.1. 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.


3.1.2. 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.


3.1.3. Line Joining
-------------------

Outside of 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".


3.1.4. 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 (:=)
operator.

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}"


3.1.5. 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.


3.1.6. 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.


3.1.7. 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”.


3.1.8. 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”.


3.1.9. 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”.


3.1.10. 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” and
“BitBake-Style Python Functions” sections for examples.


3.1.11. 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.


3.1.12. 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)” section.


3.1.13. 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” 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."


3.1.14. 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” section.


3.1.15. 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.


3.1.16. 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 \
   "


3.2. 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.


3.3. Conditional Syntax (Overrides)
===================================

BitBake uses *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.


3.3.1. 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* 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} "


3.3.2. 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”.


3.3.3. 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”.


3.4. 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.


3.4.1. Locating Include and Class Files
---------------------------------------

BitBake uses the *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".


3.4.2. "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 and require 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.


3.4.3. "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 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.


3.4.4. "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 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, 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


3.4.5. "INHERIT" Configuration Directive
----------------------------------------

When creating a configuration file (".conf"), you can use the
*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"


3.5. 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.


3.5.1. 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.


3.5.2. 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.


3.5.3. 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.


3.5.4. 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.

* 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 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 "${"*T*"}/run
  ."function-name"."pid script that is executed to run the function,
  and also generate a log file in "${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.


3.5.5. 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” section. For a different
method to run Python code during parsing, see the “Inline Python
Variable Expansion” section.


3.5.6. 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.


3.6. 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_”.


3.6.1. Promoting a Function to a Task
-------------------------------------

Tasks are either shell functions or 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 "recipe -c
  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”
section.

See the “Variable Flags” section for information on variable flags you
can use with tasks.

Note:

  While it’s infrequent, it’s possible to define multiple tasks as
  dependencies when calling "addtask". For example, here’s a snippet
  from the OpenEmbedded class file "package_tar.bbclass":

     addtask package_write_tar before do_build after do_packagedata do_package

  Note how the "package_write_tar" task has to wait until both of
  "do_packagedata" and "do_package" complete.


3.6.2. 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"


3.6.3. 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* and
   *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*.

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.


3.7. 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” 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*
  variable and the “Inter-Task Dependencies” section for more
  information.

* "[deptask]": Controls task build-time dependencies. See the
  *DEPENDS* variable and the “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.

  Caution:

    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* 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”

  Warning:

    * 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* variable, the *RRECOMMENDS* variable, and the “Inter-Task
  Dependencies” section for more information.

* "[rdeptask]": Controls task runtime dependencies. See the *RDEPENDS*
  variable, the *RRECOMMENDS* variable, and the “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* variable, the *RRECOMMENDS* variable, and the
  “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)” 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.


3.8. 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()"


3.9. 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* and *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* 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*).


3.10. 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.


3.10.1. 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 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".


3.10.2. Build Dependencies
--------------------------

BitBake uses the *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.


3.10.3. Runtime Dependencies
----------------------------

BitBake uses the *PACKAGES*, *RDEPENDS*, and *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.


3.10.4. 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.


3.10.5. 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.


3.11. 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.


3.11.1. 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.                   |
+----------------------------------------------------+----------------------------------------------------+
| "d.getVarFlag("X", flag, expand)"                  | Returns the value of variable “X”. Using           |
|                                                    | “expand=True” expands the value. Returns “None” if |
|                                                    | either the variable “X” or the named flag does not |
|                                                    | exist.                                             |
+----------------------------------------------------+----------------------------------------------------+
| "d.setVarFlag("X", flag, "value")"                 | Sets the named flag for variable “X” to “value”.   |
+----------------------------------------------------+----------------------------------------------------+
| "d.appendVarFlag("X", flag, "value")"              | Appends “value” to the named flag on the variable  |
|                                                    | “X”. Acts like "d.setVarFlag("X", flag, "value")"  |
|                                                    | if the named flag does not exist.                  |
+----------------------------------------------------+----------------------------------------------------+
| "d.prependVarFlag("X", flag, "value")"             | Prepends “value” to the named 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.                                             |
+----------------------------------------------------+----------------------------------------------------+


3.11.2. 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.


3.12. 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*. 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*: 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*: Specifies a function BitBake calls that
  determines whether BitBake requires a setscene dependency to be met.

* *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*: Defines the mode for comparing timestamps of
  stamp files.

* *BB_STAMP_WHITELIST*: Lists stamp files that are looked at when the
  stamp policy is “whitelist”.

* *BB_TASKHASH*: Within an executing task, this variable holds the
  hash of the task as returned by the currently enabled signature
  generator.

* *STAMP*: The base path to create stamp files.

* *STAMPCLEAN*: Again, the base path to create stamp files but can use
  wildcards for matching a range of files for clean operations.


3.13. 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 syntax, fnmatch syntax, or
Regular Expression (re) syntax.

For variables that have wildcard suport, the documentation describes
which form of wildcard, its use, and its limitations.
