# Execution

The primary purpose for running BitBake is to produce some kind of output such as a single installable package, a kernel, a software development kit, or even a full, board-specific bootable Linux image, complete with bootloader, kernel, and root filesystem. Of course, you can execute the `bitbake` command with options that cause it to execute single tasks, compile single recipe files, capture or clear data, or simply return information about the execution environment.

This chapter describes BitBake's execution process from start to finish when you use it to create an image. The execution process is launched using the following command form:

         $ bitbake target

For information on the BitBake command and its options, see "The BitBake Command" section.

Prior to executing BitBake, you should take advantage of available parallel thread execution on your build host by setting the `BB_NUMBER_THREADS` variable in your project's `local.conf` configuration file.

A common method to determine this value for your build host is to run the following:

         $ grep processor /proc/cpuinfo

This command returns the number of processors, which takes into account hyper-threading. Thus, a quad-core build host with hyper-threading most likely shows eight processors, which is the value you would then assign to `BB_NUMBER_THREADS`.

A possibly simpler solution is that some Linux distributions (e.g. Debian and Ubuntu) provide the `ncpus` command.

## Parsing the Base Configuration Metadata

The first thing BitBake does is parse base configuration metadata. Base configuration metadata consists of your project's `bblayers.conf` file to determine what layers BitBake needs to recognize, all necessary `layer.conf` files (one from each layer), and `bitbake.conf`. 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 `layer.conf` files are used to construct key variables such as `BBPATH` and `BBFILES`. `BBPATH` is used to search for configuration and class files under the `conf` and `classes` directories, respectively. `BBFILES` is used to locate both recipe and recipe append files (`.bb` and `.bbappend`). If there is no `bblayers.conf` file, it is assumed the user has set the `BBPATH` and `BBFILES` directly in the environment.

Next, the `bitbake.conf` file is located using the `BBPATH` variable that was just constructed. The `bitbake.conf` file may also include other configuration files using the `include` or `require` directives.

Prior to parsing configuration files, Bitbake looks at certain variables, including:

- `BB_ENV_WHITELIST`

- `BB_ENV_EXTRAWHITE`

- `BB_PRESERVE_ENV`

- `BB_ORIGENV`

- `BITBAKE_UI`

The first four variables in this list relate to how BitBake treats shell environment variables during task execution. By default, BitBake cleans the environment variables and provides tight control over the shell execution environment. However, through the use of these first four variables, you can apply your control regarding the environment variables allowed to be used by BitBake in the shell during execution of tasks. See the "Passing Information Into the Build Task Environment" section and the information about these variables in the variable glossary for more information on how they work and on how to use them.

The base configuration metadata is global and therefore affects all recipes and tasks that are executed.

BitBake first searches 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. Recall that if BitBake cannot find a `bblayers.conf` file, then it is assumed the user has set the `BBPATH` and `BBFILES` variables directly in the environment.

For each directory (layer) in this list, a `conf/layer.conf` file is located and parsed with the `LAYERDIR` variable being set to the directory where the layer was found. The idea is these files automatically set up `BBPATH` and other variables correctly for a given build directory.

BitBake then expects to find the `conf/bitbake.conf` file somewhere in the user-specified `BBPATH`. That configuration file generally has include directives to pull in any other metadata such as files specific to the architecture, the machine, the local environment, and so forth.

Only variable definitions and include directives are allowed in BitBake `.conf` files. Some variables directly influence BitBake's behavior. These variables might have been set from the environment depending on the environment variables previously mentioned or set in the configuration files. The "Variables Glossary" chapter presents a full list of variables.

After parsing configuration files, BitBake uses its rudimentary inheritance mechanism, which is through class files, to inherit some standard classes. BitBake parses a class when the inherit directive responsible for getting that class is encountered.

The `base.bbclass` file is always included. Other classes that are specified in the configuration using the `INHERIT` variable are also included. BitBake searches for class files in a `classes` subdirectory under the paths in `BBPATH` in the same way as configuration files.

A good way to get an idea of the configuration files and the class files used in your execution environment is to run the following BitBake command:

         $ bitbake -e > mybb.log

Examining the top of the `mybb.log` shows you the many configuration files and class files used in your execution environment.

You need to be aware of how BitBake parses curly braces. If a recipe uses a closing curly brace within the function and the character has no leading spaces, BitBake produces a parsing error. If you use a pair of curly braces in a shell function, the closing curly brace must not be located at the start of the line without leading spaces.

Here is an example that causes BitBake to produce a parsing error:

         fakeroot create_shar() {
             cat << "EOF" > ${SDK_DEPLOY}/${TOOLCHAIN_OUTPUTNAME}.sh
         usage()
         {
           echo "test"
           ###### The following "}" at the start of the line causes a parsing error ######
         }
         EOF
         }

Writing the recipe this way avoids the error:

         fakeroot create_shar() {
             cat << "EOF" > ${SDK_DEPLOY}/${TOOLCHAIN_OUTPUTNAME}.sh
         usage()
         {
           echo "test"
           ######The following "}" with a leading space at the start of the line avoids the error ######
          }
         EOF
         }

## Locating and Parsing Recipes

During the configuration phase, BitBake will have set `BBFILES`. BitBake now uses it to construct a list of recipes to parse, along with any append files (`.bbappend`) to apply. `BBFILES` is a space-separated list of available files and supports wildcards. An example would be:

         BBFILES = "/path/to/bbfiles/*.bb /path/to/appends/*.bbappend"

BitBake parses each recipe and append file located with `BBFILES` and stores the values of various variables into the datastore.

Append files are applied in the order they are encountered in `BBFILES`.

For each file, a fresh copy of the base configuration is made, then the recipe is parsed line by line. Any inherit statements cause BitBake to find and then parse class files (`.bbclass`) using `BBPATH` as the search path. Finally, BitBake parses in order any append files found in `BBFILES`.

One common convention is to use the recipe filename to define pieces of metadata. For example, in `bitbake.conf` the recipe name and version are used to set the variables `PN` and `PV`:

         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'}"

In this example, a recipe called "something_1.2.3.bb" would set `PN` to "something" and `PV` to "1.2.3".

By the time parsing is complete for a recipe, BitBake has a list of tasks that the recipe defines and a set of data consisting of keys and values as well as dependency information about the tasks.

BitBake does not need all of this information. It only needs a small subset of the information to make decisions about the recipe. Consequently, BitBake caches the values in which it is interested and does not store the rest of the information. Experience has shown it is faster to re-parse the metadata than to try and write it out to the disk and then reload it.

Where possible, subsequent BitBake commands reuse this cache of recipe information. The validity of this cache is determined by first computing a checksum of the base configuration data (see `BB_HASHCONFIG_WHITELIST`) and then checking if the checksum matches. If that checksum matches what is in the cache and the recipe and class files have not changed, Bitbake is able to use the cache. BitBake then reloads the cached information about the recipe instead of reparsing it from scratch.

Recipe file collections exist to allow the user to have multiple repositories of `.bb` files that contain the same exact package. For example, one could easily use them to make one's own local copy of an upstream repository, but with custom modifications that one does not want upstream. Here is an example:

        BBFILES = "/stuff/openembedded/*/*.bb /stuff/openembedded.modified/*/*.bb"
        BBFILE_COLLECTIONS = "upstream local"
        BBFILE_PATTERN_upstream = "^/stuff/openembedded/"
        BBFILE_PATTERN_local = "^/stuff/openembedded.modified/"
        BBFILE_PRIORITY_upstream = "5"
        BBFILE_PRIORITY_local = "10"

The layers mechanism is now the preferred method of collecting code. While the collections code remains, its main use is to set layer priorities and to deal with overlap (conflicts) between layers.

## Providers

Assuming BitBake has been instructed to execute a target and that all the recipe files have been parsed, BitBake starts to figure out how to build the target. BitBake looks through the `PROVIDES` list for each of the recipes. A `PROVIDES` list is the list of names by which the recipe can be known. Each recipe's `PROVIDES` list is created implicitly through the recipe's `PN` variable and explicitly through the recipe's `PROVIDES` variable, which is optional.

When a recipe uses `PROVIDES`, that recipe's functionality can be found under an alternative name or names other than the implicit `PN` name. As an example, suppose a recipe named `keyboard_1.0.bb` contained the following:

         PROVIDES += "fullkeyboard"

The `PROVIDES` list for this recipe becomes "keyboard", which is implicit, and "fullkeyboard", which is explicit. Consequently, the functionality found in `keyboard_1.0.bb` can be found under two different names.

## Preferences

The `PROVIDES` list is only part of the solution for figuring out a target's recipes. Because targets might have multiple providers, BitBake needs to prioritize providers by determining provider preferences.

A common example in which a target has multiple providers is "virtual/kernel", which is on the `PROVIDES` list for each kernel recipe. Each machine often selects the best kernel provider by using a line similar to the following in the machine configuration file:

         PREFERRED_PROVIDER_virtual/kernel = "linux-yocto"

The default `PREFERRED_PROVIDER` is the provider with the same name as the target. Bitbake iterates through each target it needs to build and resolves them and their dependencies using this process.

Understanding how providers are chosen is made complicated by the fact that multiple versions might exist for a given provider. BitBake defaults to the highest version of a provider. Version comparisons are made using the same method as Debian. You can use the `PREFERRED_VERSION` variable to specify a particular version. You can influence the order by using the `DEFAULT_PREFERENCE` variable.

By default, files have a preference of "0". Setting `DEFAULT_PREFERENCE` to "-1" makes the recipe unlikely to be used unless it is explicitly referenced. Setting `DEFAULT_PREFERENCE` to "1" makes it likely the recipe is used. `PREFERRED_VERSION` overrides any `DEFAULT_PREFERENCE` setting. `DEFAULT_PREFERENCE` is often used to mark newer and more experimental recipe versions until they have undergone sufficient testing to be considered stable.

When there are multiple “versions” of a given recipe, BitBake defaults to selecting the most recent version, unless otherwise specified. If the recipe in question has a `DEFAULT_PREFERENCE` set lower than the other recipes (default is 0), then it will not be selected. This allows the person or persons maintaining the repository of recipe files to specify their preference for the default selected version. Additionally, the user can specify their preferred version.

If the first recipe is named `a_1.1.bb`, then the `PN` variable will be set to “a”, and the `PV` variable will be set to 1.1.

Thus, if a recipe named `a_1.2.bb` exists, BitBake will choose 1.2 by default. However, if you define the following variable in a `.conf` file that BitBake parses, you can change that preference:

         PREFERRED_VERSION_a = "1.1"

It is common for a recipe to provide two versions -- a stable, numbered (and preferred) version, and a version that is automatically checked out from a source code repository that is considered more "bleeding edge" but can be selected only explicitly.

For example, in the OpenEmbedded codebase, there is a standard, versioned recipe file for BusyBox, `busybox_1.22.1.bb`, but there is also a Git-based version, `busybox_git.bb`, which explicitly contains the line

        DEFAULT_PREFERENCE = "-1"

to ensure that the numbered, stable version is always preferred unless the developer selects otherwise.

## Dependencies

Each target BitBake builds consists of multiple tasks such as `fetch`, `unpack`, `patch`, `configure`, and `compile`. For best performance on multi-core systems, BitBake considers each task as an independent entity with its own set of dependencies.

Dependencies are defined through several variables. You can find information about variables BitBake uses in the Variables Glossary near the end of this manual. At a basic level, it is sufficient to know that BitBake uses the `DEPENDS` and `RDEPENDS` variables when calculating dependencies.

For more information on how BitBake handles dependencies, see the "Dependencies" section.

## The Task List

Based on the generated list of providers and the dependency information, BitBake can now calculate exactly what tasks it needs to run and in what order it needs to run them. The "Executing Tasks" section has more information on how BitBake chooses which task to execute next.

The build now starts with BitBake forking off threads up to the limit set in the `BB_NUMBER_THREADS` variable. BitBake continues to fork threads as long as there are tasks ready to run, those tasks have all their dependencies met, and the thread threshold has not been exceeded.

It is worth noting that you can greatly speed up the build time by properly setting the `BB_NUMBER_THREADS` variable.

As each task completes, a timestamp is written to the directory specified by the `STAMP` variable. On subsequent runs, BitBake looks in the build directory within `tmp/stamps` and does not rerun tasks that are already completed unless a timestamp is found to be invalid. Currently, invalid timestamps are only considered on a per recipe file basis. So, for example, if the configure stamp has a timestamp greater than the compile timestamp for a given target, then the compile task would rerun. Running the compile task again, however, has no effect on other providers that depend on that target.

The exact format of the stamps is partly configurable. In modern versions of BitBake, a hash is appended to the stamp so that if the configuration changes, the stamp becomes invalid and the task is automatically rerun. This hash, or signature used, is governed by the signature policy that is configured (see the "Checksums (Signatures)" section for information). It is also possible to append extra metadata to the stamp using the `[stamp-extra-info]` task flag. For example, OpenEmbedded uses this flag to make some tasks machine-specific.

Some tasks are marked as "nostamp" tasks. No timestamp file is created when these tasks are run. Consequently, "nostamp" tasks are always rerun.

For more information on tasks, see the "Tasks" section.

## Executing Tasks

Tasks can be either a shell task or a Python task. For shell tasks, BitBake writes a shell script to `${` `T` `}/run.do_taskname.pid` and then executes the script. The generated shell script contains all the exported variables, and the shell functions with all variables expanded. Output from the shell script goes to the file `${T}/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.

The order in which BitBake runs the tasks is controlled by its task scheduler. It is possible to configure the scheduler and define custom implementations for specific use cases. For more information, see these variables that control the behavior:

- `BB_SCHEDULER`

- `BB_SCHEDULERS`

It is possible to have functions run before and after a task's main function. This is done using the `[prefuncs]` and `[postfuncs]` flags of the task that lists the functions to run.

## Checksums (Signatures)

A checksum is a unique signature of a task's inputs. The signature of a task can be used to determine if a task needs to be run. Because it is a change in a task's inputs that triggers running the task, BitBake 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 working directory changes because it should not affect the output for target packages. The simplistic approach for excluding the working directory is to set it to some fixed value and create the checksum for the "run" script. BitBake goes one step better and uses the `BB_HASHBASE_WHITELIST` variable to define a list of variables that should never be included when generating the signatures.

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.

It is worth noting that BitBake's "-S" option lets you debug Bitbake's processing of signatures. The options passed to -S allow different debugging modes to be used, either using BitBake's own debug functions or possibly those defined in the metadata/signature handler itself. The simplest parameter to pass is "none", which causes a set of signature information to be written out into `STAMPS_DIR` corresponding to the targets specified. The other currently available parameter is "printdiff", which causes BitBake to try to establish the closest signature match it can (e.g. in the sstate cache) and then run `bitbake-diffsigs` over the matches to determine the stamps and delta where these two stamp trees diverge.

It is likely that future versions of BitBake will provide other signature handlers triggered through additional "-S" parameters.

You can find more information on checksum metadata in the "Task Checksums and Setscene" section.

## Setscene

The setscene process enables BitBake to handle "pre-built" artifacts. The ability to handle and reuse these artifacts allows BitBake the luxury of not having to build something from scratch every time. Instead, BitBake can use, when possible, existing build artifacts.

BitBake needs to have reliable data indicating whether or not an artifact is compatible. Signatures, described in the previous section, provide an ideal way of representing whether an artifact is compatible. If a signature is the same, an object can be reused.

If an object can be reused, the problem then becomes how to replace a given task or set of tasks with the pre-built artifact. BitBake solves the problem with the "setscene" process.

When BitBake is asked to build a given target, before building anything, it first asks whether cached information is available for any of the targets it's building, or any of the intermediate targets. If cached information is available, BitBake uses this information instead of running the main tasks.

BitBake first calls the function defined by the `BB_HASHCHECK_FUNCTION` variable with a list of tasks and corresponding hashes it wants to build. This function is designed to be fast and returns a list of the tasks for which it believes in can obtain artifacts.

Next, for each of the tasks that were returned as possibilities, BitBake executes a setscene version of the task that the possible artifact covers. Setscene versions of a task have the string "_setscene" appended to the task name. So, for example, the task with the name `xxx` has a setscene task named `xxx_setscene`. The setscene version of the task executes and provides the necessary artifacts returning either success or failure.

As previously mentioned, an artifact can cover more than one task. For example, it is pointless to obtain a compiler if you already have the compiled binary. To handle this, BitBake calls the `BB_SETSCENE_DEPVALID` function for each successful setscene task to know whether or not it needs to obtain the dependencies of that task.

Finally, after all the setscene tasks have executed, BitBake calls the function listed in `BB_SETSCENE_VERIFY_FUNCTION2` with the list of tasks BitBake thinks has been "covered". The metadata can then ensure that this list is correct and can inform BitBake that it wants specific tasks to be run regardless of the setscene result.

You can find more information on setscene metadata in the "Task Checksums and Setscene" section.