The BitBake task executor together with various types of configuration files form the OpenEmbedded Core. This section provides an overview of the task executor and the configuration files by describing their use and interaction. BitBake handles the parsing and execution of the data files. The data itself is of various types: Recipes: Details about particular pieces of software. Class Data: An abstraction of common build information (e.g. how to build a Linux kernel). Configuration Data: Machine-specific settings, policy decisions, and so forth. Configuration data acts as the glue to bind everything together. The remainder of this chapter provides examples of BitBake metadata. Any syntax not supported in any of the previously listed areas is documented as such.

This section provides some basic syntax examples.

The following example sets VARIABLE to "value". This assignment occurs immediately as the statement is parsed. It is a "hard" assignment. VARIABLE = "value"

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 containing "preavalpost". A = "aval" B = "pre${A}post"

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.

It is possible to use a "weaker" assignment that in the previous section by using the "??=" operator. This assignment behaves identical to "?=" except that the assignment is made at the end of the parsing process rather than immediately. Consequently, when multiple "??=" assignments exist, the last one is used. Also, any "=" or "?=" assignment will override the value set with "??=". Here is an example: 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.

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 values is common and can be accomplished using the "+=" and "=+" operators. These operators insert a space between the current value and prepended or appended value. Here are some examples: B = "bval" B += "additionaldata" C = "cval" C =+ "test" The variable B contains "bval additionaldata" and C contains "test cval".

If you want to append or prepend values without an inserted space, use the ".=" and "=." operators. Here are some examples: B = "bval" B .= "additionaldata" C = "cval" C =. "test" The variable B contains "bvaladditionaldata" and C contains "testcval".

You can also append and prepend a variable's value using an override style syntax. When you use this syntax, no spaces are inserted. Here are some examples: 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.

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 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 "abc456".

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 becoming the current date.

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.

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). 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".

Key expansion happens when the BitBake data store 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".

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".

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.

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.

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 package. 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.

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 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.

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 uses 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

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).

As with most languages, functions are the building blocks that define operations. BitBake supports three types of functions: Shell Functions: Functions written in a shell language and executed by the shell. BitBake Functions: Functions written in Python but executed by BitBake using bb.build.exec_func(). Python Functions: Functions written in Python and executed by Python. Regardless of the type of function, you can only define them in class (.bbclass) and recipe (.bb) files.

These functions are written using a shell language and executed by the shell. 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.

These functions are written in Python and are executed 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.

These functions are written in Python but are executed by Python. 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 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.

This is only supported in .bb and .bbclass files. A shell or Python function executable through the exec_func can be promoted to become a task. Tasks are the execution unit Bitbake uses and each step that needs to be run for a given .bb is known as a task. There is an addtask command to add new tasks and promote functions which by convention must start with “do_”. The addtask command is also used to describe intertask dependencies. python do_printdate () { import time print time.strftime('%Y%m%d', time.gmtime()) } addtask printdate after do_fetch before do_build The above example defined a Python function, then adds it as a task which is now a dependency of do_build, the default task and states it has to happen after do_fetch. If anyone executes the do_build task, that will result in do_printdate being run first.

Tasks can either be a shell task or a Python task. For shell tasks, BitBake writes a shell script to ${WORKDIR}/temp/run.do_taskname.pid and then executes the script. The generated shell script contains all the exported variables, and the shell functions with all variables expanded. Output from the shell script goes to the file ${WORKDIR}/temp/log.do_taskname.pid. Looking at the expanded shell functions in the run file and the output in the log files is a useful debugging technique. For Python tasks, BitBake executes the task internally and logs information to the controlling terminal. Future versions of BitBake will write the functions to files similar to the way shell tasks are handled. Logging will be handled in a way similar to shell tasks as well. Once all the tasks have been completed BitBake exits. 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's environment, you must take a few steps: Tell BitBake to load what you want from the environment into the data store. You can do so through the BB_ENV_EXTRAWHITE variable. For example, assume you want to prevent the build system from accessing your $HOME/.ccache directory. The following command tells BitBake to load CCACHE_DIR from the environment into the data store: export BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE CCACHE_DIR" Tell BitBake to export what you have loaded into the environment store to the task environment of every running task. Loading something from the environment into the data store (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.conf or 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 shared state checksums. If doing so results in unnecessary rebuilds of tasks, you can whitelist the variable so that the shared state code ignores the dependency when it creates checksums.

This section describes variable flags.

Tasks support a number of flags which control various functionality of the task. These are as follows: dirs: Directories which should be created before the task runs. cleandirs: Directories which should created before the task runs but should be empty. noexec: Marks the tasks as being empty and no execution required. These are used as dependency placeholders or used when added tasks need to be subsequently disabled. nostamp: Do not generate a stamp file for a task. This means the task is always executed. fakeroot: This task needs to be run in a fakeroot environment, obtained by adding the variables in FAKEROOTENV to the environment. umask: The umask to run the task under. For the 'deptask', 'rdeptask', 'depends', 'rdepends'and 'recrdeptask' flags, please see the dependencies section.

BitBake parses configuration files, classes, and .bb files. The first thing BitBake does is look for the bitbake.conf file. This file resides in the within the conf/ directory. BitBake finds it by examining its BBPATH environment variable and looking for the conf/ directory. The bitbake.conf file lists other configuration files to include from a conf/ directory below the directories listed in BBPATH. In general, the most important configuration file from a user's perspective is local.conf, which contains a user's customized settings for the build environment. Other notable configuration files are the distribution configuration file (set by the DISTRO variable) and the machine configuration file (set by the MACHINE variable). The DISTRO and MACHINE BitBake environment variables are both usually set in the local.conf file. Valid distribution configuration files are available in the conf/distro/ directory and valid machine configuration files in the meta/conf/machine/ directory. Within the conf/machine/include/ directory are various tune-*.inc configuration files that provide common "tuning" settings specific to and shared between particular architectures and machines. After parsing of the configuration files, some standard classes are included. The base.bbclass file is always included. Other classes that are specified in the configuration using the INHERIT variable are also included. Class files are searched for in a classes subdirectory under the paths in BBPATH in the same way as configuration files. After classes are included, the variable BBFILES is set, usually in local.conf, and defines the list of places to search for .bb files. Adding extra content to BBFILES is best achieved through the use of BitBake layers as described in the Layers section below. BitBake parses each .bb file in BBFILES and stores the values of various variables. In summary, for each .bb file the configuration plus the base class of variables are set, followed by the data in the .bb file itself, followed by any inherit commands that .bb file might contain. Because parsing .bb files is a time consuming process, a cache is kept to speed up subsequent parsing. This cache is invalid if the timestamp of the .bb file itself changes, or if the timestamps of any of the include, configuration files or class files on which the .bb file depends change.

Prior to parsing configuration files, Bitbake looks at certain variables, including: BB-ENV-WHITELIST BB_PRESERVE-ENV BB_ENV_EXTRAWHITE BB_ORIG_ENV PREFERRED_VERSIONS PREFERRED_PROVIDERS The first kind of metadata in BitBake is configuration metadata. This metadata is global, and therefore affects all packages and tasks that are executed. BitBake will first search the current working directory for an optional conf/bblayers.conf configuration file. This file is expected to contain a BBLAYERS variable that is a space delimited list of 'layer' directories. For each directory in this list, a conf/layer.conf file will be searched for and parsed with the LAYERDIR variable being set to the directory where the layer was found. The idea is these files will setup BBPATH and other variables correctly for a given build directory automatically for the user. BitBake will then expect to find conf/bitbake.conf file somewhere in the user specified BBPATH. That configuration file generally has include directives to pull in any other metadata (generally files specific to architecture, machine, local and so on). Only variable definitions and include directives are allowed in .conf files. The following variables include: BITBAKE_UI BBDEBUG MULTI_PROVIDER_WHITELIST BB_NUMBER_PARSE_THREADS BBPKGS BB_DEFAULT_TASK TOPDIR BB_VERBOSE_LOGS BB_NICE_LEVEL BBFILE_COLLECTIONS ASSUME_PROVIDED BB_DANGLINGAPPENDS_WARNONLY BBINCLUDED BBFILE_PRIORITY BUILDNAME BBMASK

Layers allow you to isolate different types of customizations from each other. You might find it tempting to keep everything in one layer when working on a single project. However, the more modular you organize your Metadata, the easier it is to cope with future changes. To illustrate how layers are used to keep things modular, consider machine customizations. These types of customizations typically reside in a special layer, rather than a general layer, called a Board Specific Package (BSP) Layer. Furthermore, the machine customizations should be isolated from recipes and Metadata that support a new GUI environment, for example. This situation gives you a couple of layers: one for the machine configurations, and one for the GUI environment. It is important to understand, however, that the BSP layer can still make machine-specific additions to recipes within the GUI environment layer without polluting the GUI layer itself with those machine-specific changes. You can accomplish this through a recipe that is a BitBake append (.bbappend) file, which is described later in this section. There are certain variable specific to layers, including: LAYERDEPENDS LAYERVERSION

There are variables specific to scheduling functionality including: BB_SCHEDULER BB_SCHEDULERS

BitBake classes are our rudimentary inheritance mechanism. As briefly mentioned in the metadata introduction, they're parsed when an inherit directive is encountered, and they are located in the classes/ directory relative to the directories in BBPATH.

A BitBake (.bb) file is a logical unit of tasks to be executed. Normally this is a package to be built. Inter-.bb dependencies are obeyed. The files themselves are located via the BBFILES variable, which is set to a space separated list of .bb files, and does handle wildcards.

This is only supported in .bb and .bbclass files. BitBake allows installation of event handlers. Events are triggered at certain points during operation, such as the beginning of operation against a given .bb, 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. 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. "e.data" contains an instance of "bb.data". With the getName(e) method one can get the name of the triggered event. The above event handler prints the name of the event and the content of the FILE variable. During a Build, the following common events 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() 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

Two BitBake features exist to facilitate the creation of multiple buildable incarnations from a single recipe file. The first is 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" The second feature is BBVERSIONS. This variable allows a single recipe to build multiple versions of a project from a single recipe file, and allows you to specify conditional metadata (using the OVERRIDES mechanism) for a single version, or an optionally named range of versions: 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 will default to the original version of the recipe, so given OE, a recipe file of foo_1.0.0+.bb will default the name of its versions to 1.0.0+. This is useful, as the range name is not only placed into overrides; it's also made available for the metadata to use in the form of the BPV variable, for use in file:// search paths (FILESPATH).

BitBake handles dependencies at the task level since to allow for efficient operation with multiple processes executing in parallel, a robust method of specifying task dependencies is needed.

Where the dependencies are internal to a given .bb file, the dependencies are handled by the previously detailed addtask directive.

DEPENDS lists build time dependencies. The 'deptask' flag for tasks is used to signify the task of each item listed in DEPENDS which must have completed before that task can be executed. do_configure[deptask] = "do_populate_staging" In the previous example, the do_populate_staging task of each item in DEPENDS must have completed before do_configure can execute.

The PACKAGES variable lists runtime packages and each of these 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_write[rdeptask] = "do_package" In the previous example, the do_package task of each item in RDEPENDS must have completed before do_package_write can execute.

These are specified with the 'recrdeptask' flag which is used to signify the task(s) of dependencies which must have completed before that task can be executed. It works by looking though the build and runtime dependencies of the current recipe as well as any inter-task dependencies the task has, then adding a dependency on the listed task. It will then recurse through the dependencies of those tasks and so on. It may be desirable to recurse not just through the dependencies of those tasks but through the build and runtime dependencies of dependent tasks too. If that is the case, the taskname itself should be referenced in the task list (e.g. do_a[recrdeptask] = "do_a do_b").

The 'depends' flag for tasks is a more generic form which allows an inter-dependency on specific tasks rather than specifying the data in DEPENDS. do_patch[depends] = "quilt-native:do_populate_staging" In the previous example, the do_populate_staging 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.

It is often necessary to manipulate variables within python functions and the Bitbake data store has an API which allows this. The operations available are: d.getVar("X", expand=False) returns the value of variable "X", expanding the value if specified. d.setVar("X", value) sets the value of "X" to "value". d.appendVar("X", value) adds "value" to the end of variable X. d.prependVar("X", value) adds "value" to the start of variable X. d.delVar("X") deletes the variable X from the data store. d.renameVar("X", "Y") renames variable X to Y. d.getVarFlag("X", flag, expand=False) gets given flag from variable X but does not expand it. d.setVarFlag("X", flag, value) sets given flag for variable X to value. setVarFlags will not clear previous flags. Think of this method as addVarFlags. d.appendVarFlag("X", flag, value) Need description. d.prependVarFlag("X", flag, value) Need description. d.delVarFlag("X", flag) Need description. d.setVarFlags("X", flagsdict) sets the flags specified in the dict() parameter. d.getVarFlags("X") returns a dict of the flags for X. d.delVarFlags deletes all the flags for a variable.

This list is a place holder of content that needs explanation here. Items should be moved to appropriate sections below as completed. STAMP STAMPCLEAN BB_STAMP_WHITELIST BB_STAMP_POLICY BB_HASHCHECK_FUNCTION BB_SETSCENE_VERIFY_FUNCTION BB_SETSCENE_DEPVALID BB_TASKHASH

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. The setscene code uses a checksum, which is a unique signature of a task's inputs, to determine if a task needs to be run again. Because it is a change in a task's inputs that triggers a rerun, the process needs to detect all the inputs to a given task. For shell tasks, this turns out to be fairly easy because BitBake generates a "run" shell script for each task and it is possible to create a checksum that gives you a good idea of when the task's data changes. To complicate the problem, some things should not be included in the checksum. First, there is the actual specific build path of a given task - the working directory. It does not matter if the work directory changes because it should not affect the output for target packages. The simplistic approach for excluding the work directory is to set it to some fixed value and create the checksum for the "run" script. Another problem results from the "run" scripts containing functions that might or might not get called. The incremental build solution contains code that figures out dependencies between shell functions. This code is used to prune the "run" scripts down to the minimum set, thereby alleviating this problem and making the "run" scripts much more readable as a bonus. So far we have solutions for shell scripts. What about Python tasks? The same approach applies even though these tasks are more difficult. The process needs to figure out what variables a Python function accesses and what functions it calls. Again, the incremental build solution contains code that first figures out the variable and function dependencies, and then creates a checksum for the data used as the input to the task. Like the working directory case, situations exist where dependencies should be ignored. For these cases, you can instruct the build process to ignore a dependency by using a line like the following: PACKAGE_ARCHS[vardepsexclude] = "MACHINE" This example ensures that the PACKAGE_ARCHS variable does not depend on the value of MACHINE, even if it does reference it. Equally, there are cases where we need to add dependencies BitBake is not able to find. You can accomplish this by using a line like the following: PACKAGE_ARCHS[vardeps] = "MACHINE" This example explicitly adds the MACHINE variable as a dependency for PACKAGE_ARCHS. Consider a case with in-line Python, for example, where BitBake is not able to figure out dependencies. When running in debug mode (i.e. using -DDD), BitBake produces output when it discovers something for which it cannot figure out dependencies. Thus far, this section has limited discussion to the direct inputs into a task. Information based on direct inputs is referred to as the "basehash" in the code. However, there is still the question of a task's indirect inputs - the things that were already built and present in the build directory. The checksum (or signature) for a particular task needs to add the hashes of all the tasks on which the particular task depends. Choosing which dependencies to add is a policy decision. However, the effect is to generate a master checksum that combines the basehash and the hashes of the task's dependencies. At the code level, there are a variety of ways both the basehash and the dependent task hashes can be influenced. Within the BitBake configuration file, we can give BitBake some extra information to help it construct the basehash. The following statement effectively results in a list of global variable dependency excludes - variables never included in any checksum. This example uses variables from OpenEmbedded to help illustrate the concept: BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH DL_DIR \ SSTATE_DIR THISDIR FILESEXTRAPATHS FILE_DIRNAME HOME LOGNAME SHELL TERM \ USER FILESPATH STAGING_DIR_HOST STAGING_DIR_TARGET COREBASE PRSERV_HOST \ PRSERV_DUMPDIR PRSERV_DUMPFILE PRSERV_LOCKDOWN PARALLEL_MAKE \ CCACHE_DIR EXTERNAL_TOOLCHAIN CCACHE CCACHE_DISABLE LICENSE_PATH SDKPKGSUFFIX" The previous example excludes the work directory, which is part of TMPDIR. The rules for deciding which hashes of dependent tasks to include through dependency chains are more complex and are generally accomplished with a Python function. The code in meta/lib/oe/sstatesig.py shows two examples of this and also illustrates how you can insert your own policy into the system if so desired. This file defines the two basic signature generators OpenEmbedded Core uses: "OEBasic" and "OEBasicHash". By default, there is a dummy "noop" signature handler enabled in BitBake. This means that behavior is unchanged from previous versions. OE-Core uses the "OEBasicHash" signature handler by default through this setting in the bitbake.conf file: BB_SIGNATURE_HANDLER ?= "OEBasicHash" The "OEBasicHash" BB_SIGNATURE_HANDLER is the same as the "OEBasic" version but adds the task hash to the stamp files. This results in any metadata change that changes the task hash, automatically causing the task to be run again. This removes the need to bump PR values, and changes to metadata automatically ripple across the build. It is also worth noting that the end result of these signature generators is to make some dependency and hash information available to the build. This information includes: BB_BASEHASH_task-<taskname>: The base hashes for each task in the recipe. BB_BASEHASH_<filename:taskname>: The base hashes for each dependent task. BBHASHDEPS_<filename:taskname>: The task dependencies for each task. BB_TASKHASH: The hash of the currently running task.