# 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 = " "

### Variable Expansion

BitBake supports variables referencing one another's contents using a syntax that is similar to shell scripting. Following is an example that results in `A` containing "aval" and `B` evaluating to "preavalpost" based on that current value of `A`.

         A = "aval"
         B = "pre${A}post"

You should realize that whenever `B` is referenced, its evaluation will depend on the state of `A` at that time. Thus, later evaluations of `B` in the previous example could result in different values depending on the value of `A`.

### 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".
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 := "${B} ${A} test ${T}"
         T = "456"
         B = "${T} bval"
         C = "cval"
         C := "${C}append"

In this example, `A` contains "test 123" because `${B}` and `${A}` at the time of parsing are undefined, which leaves "test 123". And, the variable `C` contains "cvalappend" since `${C}` immediately expands to "cval".

### 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

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 deferred until after parsing completes 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".
You must control all spacing when you use the override 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 are removed as well. 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"
         FOO2_remove = "abc def"

The variable `FOO` becomes "789 123456" and `FOO2` becomes "ghi abcdef".

### 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."

### 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 = "${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE'),d)[0] or 'defaultpkgname'}"
         PV = "${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE'),d)[1] or '1.0'}"

### 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 \
           "

## 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.

### 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.

- 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"

### Key Expansion

Key expansion happens when the BitBake datastore is finalized just before BitBake expands overrides. To better understand this, consider the following example:

         A${B} = "X"
         B = "2"
         A2 = "Y"

In this case, after all the parsing is complete, and before any overrides are handled, BitBake expands `${B}` into "2". This expansion causes `A2`, which was set to "Y" before the expansion, to become "X".

### 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 "Z X". Finally, applying the override for "foo" results in the conditional variable `A` becoming "Z X" (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` variable to locate needed include and class files. 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 what classes of functionality your recipes require. For example, you can easily abstract out the tasks involved in building a package that uses Autoconf and Automake and put those tasks into a class file that can be used by your recipe.

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`.
You can override any values and functions of the inherited class within your recipe by doing so after the "inherit" statement.

### `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`.

As an example, suppose you needed a recipe to include some self-test definitions:

         include test_defs.inc

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` 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.

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

### `INHERIT` Configuration Directive

When creating a configuration file (`.conf`), you can use the `INHERIT` 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`.
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).

## 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).

### 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", True)
         }

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.

### 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', True):
                 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.

### Anonymous Python Functions

Sometimes it is useful to run some code during parsing to set variables or to perform other operations programmatically. To do this, you can define an anonymous Python function. Here is an example that conditionally sets a variable based on the value of another variable:

         python __anonymous () {
             if d.getVar('SOMEVAR', True) == 'value':
                 d.setVar('ANOTHERVAR', 'value2')
         }

The "__anonymous" function name is optional, so the following example is functionally equivalent to the above:

         python () {
             if d.getVar('SOMEVAR', True) == 'value':
                 d.setVar('ANOTHERVAR', 'value2')
         }

Because unlike other Python functions anonymous Python functions are executed during parsing, the "d" variable within an anonymous Python function represents the datastore for the entire recipe. Consequently, you can set variable values here and those values can be picked up by other functions.

### 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 originate as functions and make up the steps that BitBake needs to run for given recipe. Tasks are only supported in recipe (`.bb` or `.inc`) and class (`.bbclass`) files. By convention, task names begin with the string "do_".

Here is an example of a task that prints out the date:

         python do_printdate () {
             import time
             print time.strftime('%Y%m%d', time.gmtime())
         }
         addtask printdate after do_fetch before do_build

### Promoting a Function to a Task

Any function can be promoted to a task by applying the `addtask` command. The `addtask` command also describes inter-task dependencies. Here is the function from the previous section but with the `addtask` command promoting it to a task and defining some dependencies:

         python do_printdate () {
             import time
             print time.strftime('%Y%m%d', time.gmtime())
         }
         addtask printdate after do_fetch before do_build

In the example, the function is defined and then promoted as a task. The `do_printdate` task becomes a dependency of the `do_build` task, which is the default task. And, the `do_printdate` task is dependent upon the `do_fetch` task. Execution of the `do_build` task results in the `do_printdate` task running first.

### Deleting a Task

As well as being able to add tasks, tasks can also be deleted. This is done simply with `deltask` command. For example, to delete the example task used in the previous sections, you would use:

         deltask printdate

### Passing Information Into the Build Task Environment

When running a task, BitBake tightly controls the execution environment of the build tasks to make sure unwanted contamination from the build machine cannot influence the build. Consequently, if you do want something to get passed into the build task environment, you must take these two steps:

1. Tell BitBake to load what you want from the environment into the datastore. You can do so through the `BB_ENV_EXTRAWHITE` variable. For example, assume you want to prevent the build system from accessing your `$HOME/.ccache` directory. The following command tells BitBake to load `CCACHE_DIR` from the environment 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

   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()`. 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.

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.

## 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:

- dirs: Directories that should be created before the task runs.

- cleandirs: Empty directories that should created before the task runs.

- noexec: Marks the tasks as being empty and no execution required. The `noexec` flag can be used to set up tasks as dependency placeholders, or to disable tasks defined elsewhere that are not needed in a particular recipe.

- nostamp: Tells BitBake to not generate a stamp file for a task, which implies the task should always be executed.

- umask: The umask to run the task under.

- deptask: Controls task build-time dependencies. See the `DEPENDS` variable and the "Build 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.

- recrdeptask: Controls task recursive runtime dependencies. See the `RDEPENDS` variable, the `RRECOMMENDS` variable, and the "Recursive Dependencies" section for more information.

- depends: Controls inter-task dependencies. See the `DEPENDS` variable and the "Inter-Task Dependencies" section for more information.

- rdepends: Controls inter-task runtime dependencies. See the `RDEPENDS` variable, the `RRECOMMENDS` variable, and the "Inter-Task Dependencies" section for more information.

- postfuncs: List of functions to call after the completion of the task.

- prefuncs: List of functions to call before the task executes.

- 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.

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.

- 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.

- vardepsexclude: Specifies a space-separated list of variables that should be excluded from a variable's dependencies for the purposes of calculating its 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 an operation against a given recipe (`*.bb` file), the start of a given task, task failure, task success, and so forth. The intent is to make it easy to do things like email notification on build failure.

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
             from bb import data
             print("The name of the Event is %s" % getName(e))
             print("The file we run for is %s" % data.getVar('FILE', e.data, True))
         }

This event handler gets called every time an event is triggered. A global variable "`e`" is defined and "`e.data`" contains an instance of "`bb.data`". With the `getName(e)` method, one can get the name of the triggered event.

Because you probably are only interested in a subset of events, you would likely use the `[eventmask]` flag for your event handler to be sure that only certain events trigger the handler. Given the previous example, suppose you only wanted the `bb.build.TaskFailed` event to trigger that event handler. Use the flag as follows:

         addhandler myclass_eventhandler
         myclass_eventhandler[eventmask] = "bb.build.TaskFailed"
         python myclass_eventhandler() {
             from bb.event import getName
             from bb import data
             print("The name of the Event is %s" % getName(e))
             print("The file we run for is %s" % data.getVar('FILE', e.data, True))
         }

During a standard build, the following common events might occur:

- `bb.event.ConfigParsed()`

- `bb.event.ParseStarted()`

- `bb.event.ParseProgress()`

- `bb.event.ParseCompleted()`

- `bb.event.BuildStarted()`

- `bb.build.TaskStarted()`

- `bb.build.TaskInvalid()`

- `bb.build.TaskFailedSilent()`

- `bb.build.TaskFailed()`

- `bb.build.TaskSucceeded()`

- `bb.event.BuildCompleted()`

- `bb.cooker.CookerExit()`

Here is a list of other events that occur based on specific requests to the server:

- `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

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.

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`).

## Dependencies

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

This section describes several types of dependency mechanisms.

### 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 `printdate` task is depends on the completion of the `do_fetch` task. And, the `do_build` depends on the completion of the `printdate` task.

### 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.

### 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.

### 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.

You might want to not only have BitBake look for dependencies of those tasks, but also have BitBake look for build-time and runtime dependencies of the dependent tasks as well. If that is the case, you need to reference the task name itself in the task list:

         do_a[recrdeptask] = "do_a do_b"

### 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.

## Accessing Datastore Variables Using Python

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=False)` | Returns the value of variable "X". Using "expand=True" expands the value.
`d.setVar("X", "value")` | Sets the variable "X" to "value".
`d.appendVar("X", "value")` | Adds "value" to the end of the variable "X".
`d.prependVar("X", "value")` | Adds "value" to the start of the variable "X".
`d.delVar("X")` | Deletes the variable "X" from the datastore.
`d.renameVar("X", "Y")` | Renames the variable "X" to "Y".
`d.getVarFlag("X", flag, expand=False)` | Gets then named flag from the variable "X". Using "expand=True" expands the named flag.
`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".
`d.prependVarFlag("X", flag, "value")` | Prepends "value" to the named flag on the variable "X".
`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".
`d.delVarFlags("X")` | Deletes all the flags for the variable "X".

## 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.

This list is a place holder of content existed from previous work on the manual. Some or all of it probably needs integrated into the subsections that make up this section. For now, I have just provided a short glossary-like description for each variable. Ultimately, this list goes away.

- `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.

- `BB_STAMP_WHITELIST` Lists stamp files that are looked at when the stamp policy is "whitelist".

- `BB_STAMP_POLICY` Defines the mode for comparing timestamps of stamp files.

- `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_VERIFY_FUNCTION` Specifies a function to call that verifies the list of planned task execution before the main task execution happens.

- `BB_SETSCENE_DEPVALID` Specifies a function BitBake calls that determines whether BitBake requires a setscene dependency to be met.

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