%poky; ] > This chapter provides technical details for various parts of the Yocto Project. Currently, topics include Yocto Project components, shared state (sstate) cache, x32, and Licenses.

The BitBake task executor together with various types of configuration files form the OpenEmbedded Core. This section overviews these by describing what they are used for and how they interact. BitBake handles the parsing and execution of the data files. The data itself is of various types: Recipes: Provides details about particular pieces of software. Class Data: Abstracts common build information (e.g. how to build a Linux kernel). Configuration Data: Defines machine-specific settings, policy decisions, and so forth. Configuration data acts as the glue to bind everything together. For more information on data, see the "Yocto Project Terms" section in the Yocto Project Development Manual. BitBake knows how to combine multiple data sources together and refers to each data source as a layer. For information on layers, see the "Understanding and Creating Layers" section of the Yocto Project Development Manual. Following are some brief details on these core components. For more detailed information on these components, see the "Source Directory Structure" chapter.

BitBake is the tool at the heart of the OpenEmbedded build system and is responsible for parsing the Metadata, generating a list of tasks from it, and then executing those tasks. To see a list of the options BitBake supports, use the following help command: $ bitbake --help The most common usage for BitBake is bitbake <packagename>, where packagename is the name of the package you want to build (referred to as the "target" in this manual). The target often equates to the first part of a .bb filename. So, to run the matchbox-desktop_1.2.3.bb file, you might type the following: $ bitbake matchbox-desktop Several different versions of matchbox-desktop might exist. BitBake chooses the one selected by the distribution configuration. You can get more details about how BitBake chooses between different target versions and providers in the "Preferences and Providers" section. BitBake also tries to execute any dependent tasks first. So for example, before building matchbox-desktop, BitBake would build a cross compiler and eglibc if they had not already been built. This release of the Yocto Project does not support the glibc GNU version of the Unix standard C library. By default, the OpenEmbedded build system builds with eglibc. A useful BitBake option to consider is the -k or --continue option. This option instructs BitBake to try and continue processing the job as much as possible even after encountering an error. When an error occurs, the target that failed and those that depend on it cannot be remade. However, when you use this option other dependencies can still be processed.

The .bb files are usually referred to as "recipes." In general, a recipe contains information about a single piece of software. The information includes the location from which to download the source patches (if any are needed), which special configuration options to apply, how to compile the source files, and how to package the compiled output. The term "package" can also be used to describe recipes. However, since the same word is used for the packaged output from the OpenEmbedded build system (i.e. .ipk or .deb files), this document avoids using the term "package" when referring to recipes.

Class files (.bbclass) contain information that is useful to share between Metadata files. An example is the Autotools class, which contains common settings for any application that Autotools uses. The "Classes" chapter provides details about common classes and how to use them.

The configuration files (.conf) define various configuration variables that govern the OpenEmbedded build process. These files fall into several areas that define machine configuration options, distribution configuration options, compiler tuning options, general common configuration options, and user configuration options in local.conf, which is found in the Build Directory.

This section takes a more detailed look at the Yocto Project development environment. The following diagram represents the development environment at a high level. The remainder of this section expands on the fundamental input, output, process, and Metadata) blocks in the Yocto Project development environment. The generalized Yocto Project Development Environment consists of several functional areas: User Configuration: Metadata you can use to control the build process. Metadata Layers: Various layers that provide software, machine, and distro Metadata. Source Files: Upstream releases, local projects, and SCMs. Build System: Processes under the control of BitBake. This block expands on how BitBake fetches source, applies patches, completes compilation, analyzes output for package generation, creates and tests packages, generates images, and generates cross-development tools. Package Feeds: Directories containing output packages (rpm, deb or ipk), which are subsequently used in the construction of an image or SDK, produced by the build system. These feeds can also be copied and shared using a web server or other means to facilitate extending or updating existing images on devices at runtime if runtime package management is enabled. Images: Images produced by the development process. Where do they go? Can you mess with them (i.e. freely delete them or move them?). Application Development SDK: Cross-development tools that are produced along with an image or separately with BitBake.

User configuration helps define the build. Through user configuration, you can tell BitBake the target architecture for which you are building the image, where to store downloaded source, and other build properties. The following figure shows an expanded representation of the user configuration box of the Yocto Project development environment: BitBake needs some basic configuration files in order to complete a build. These files are *.conf files. The minimally necessary ones reside as example files in the Source Directory. For simplicity, this section refers to the Source Directory as the "Poky Directory." When you clone the poky Git repository or you download and unpack a Yocto Project release, you can set up the Source Directory to be named anything you want. For this discussion, the cloned repository uses the default name poky. Within the figure, layers appear inside the Source Directory using a bold typeface. The Poky repository is primarily an aggregation of existing repositories. It is not a canonical upstream source. The meta-yocto layer inside Poky contains a conf directory that has example configuration files. These example files are used as a basis for creating actual configuration files when you source the build environment script oe-init-build-env. Sourcing the build environment script creates a Build Directory if one does not already exist. BitBake uses the Build Directory for all its work during builds. The Build Directory has a conf directory that contains default versions of your local.conf and bblayers.conf configuration files. These default configuration files are created only if versions do not already exist in the Build Directory at the time you source the oe-init-build-env script. Because the Poky repository is fundamentally an aggregation of existing repositories, some users might be familiar with running the oe-init-build-env script in the context of separate OpenEmbedded-Core and BitBake repositories rather than a single Poky repository. This discussion assumes the script is executed from within a cloned or unpacked version of Poky. Depending on where the script is sourced, different sub-scripts are called to set up the Build Directory (Yocto or OpenEmbedded). Specifically, the script scripts/oe-setup-builddir inside the poky directory sets up the Build Directory and seeds the directory (if necessary) with configuration files appropriate for the Yocto Project development environment. The scripts/oe-setup-builddir script uses the $TEMPLATECONF variable to determine which sample configuration files to locate. The local.conf file provides many basic variables that define a build environment. Here is a list of a few. To see the default configurations in a local.conf file created by the build environment script, see the local.conf.sample in the meta-yocto layer: Parallelism Options: Controlled by the BB_NUMBER_THREADS and PARALLEL_MAKE variables. Target Machine Selection: Controlled by the MACHINE variable. Download Directory: Controlled by the DL_DIR variable. Shared State Directory: Controlled by the SSTATE_DIR variable. Build Output: Controlled by the TMPDIR variable. Configurations set in the conf/local.conf file can also be set in the conf/site.conf and conf/auto.conf configuration files. The bblayers.conf file tells BitBake what layers you want considered during the build. By default, the layers listed in this file include layers minimally needed by the build system. However, you must manually add any custom layers you have created. You can find more information on working with the bblayers.conf file in the "Enabling Your Layer" section in the Yocto Project Development Manual. The files site.conf and auto.conf are not created by the environment initialization script. If you want these configuration files, you must create them yourself: site.conf: You can use the conf/site.conf configuration file to configure multiple build directories. For example, suppose you had several build environments and they shared some common features. You can set these default build properties here. A good example is perhaps the level of parallelism you want to use through the BB_NUMBER_THREADS and PARALLEL_MAKE variables. One useful scenario for using the conf/site.conf file is to extend your BBPATH variable to include the path to a conf/site.conf. Then, when BitBake looks for Metadata using BBPATH, it finds the conf/site.conf file and applies your common configurations found in the file. To override configurations in a particular build directory, alter the similar configurations within that build directory's conf/local.conf file. auto.conf: This file is not hand-created. Rather, the file is usually created and written to by an autobuilder. The settings put into the file are typically the same as you would find in the conf/local.conf or the conf/site.conf files. You can edit all configuration files to further define any particular build environment. This process is represented by the "User Configuration Edits" box in the figure. When you launch your build with the bitbake <target> command, BitBake sorts out the configurations to ultimately define your build environment.

The previous section described the user configurations that define the BitBake's global behavior. This section takes a closer look at the layers the build system uses to further control the build. These layers provide Metadata for the software, machine, and policy. In general, three types of layer input exist: Policy Configuration: Distribution Layers provide top-level or general policies for the image or SDK being built. For example, this layer would dictate whether BitBake produces RPM or IPK packages. Machine Configuration: Board Support Package (BSP) layers provide machine configurations. This type of information is specific to a particular target architecture. Metadata: Software layers contain user-supplied recipe files, patches, and append files. The following figure shows an expanded representation of the Metadata, Machine Configuration, and Policy Configuration input (layers) boxes of the Yocto Project development environment: In general, all layers have a similar structure. They all contain a licensing file (e.g. COPYING) if the layer is to be distributed, a README file as good practice and especially if the layer is to be distributed, a configuration directory, and recipe directories. The Yocto Project has many layers that can be used. You can see a web-interface listing of them on the Source Repositories page. The layers are shown at the bottom categorized under "Yocto Metadata Layers." These layers are fundamentally a subset of the OpenEmbedded Metadata Index, which lists all layers provided by the OpenEmbedded community. Layers exist in the Yocto Project Source Repositories that cannot be found in the OpenEmbedded Metadata Index. These layers are either deprecated or experimental in nature. BitBake uses the conf/bblayers.conf file, which is part of the user configuration, to find what layers it should be using as part of the build. For more information on layers, see the "Understanding and Creating Layers" section in the Yocto Project Development Manual.

The distribution layer provides policy configurations for your distribution. Best practices dictate that you isolate these types of configurations into their own layer. Settings you provide in conf/<distro>.conf override similar settings that BitBake finds in your conf/local.conf file in the Build Directory. The following list provides some explanation and references for what you typically find in the distribution layer: classes: Class files (.bbclass) holds common functionality that can be shared among recipes in the distribution. When your recipes inherit a class, they take on the settings and functions for that class. You can read more about class files in the "Classes" section. conf: This area holds configuration files for the layer (conf/layer.conf), the distribution (conf/distro/<distro>.conf), and any distribution-wide include files. recipes-*: Recipes and append files that affect common functionality across the distribution. This area could include recipes and append files to to add distribution-specific configuration, initialization scripts, custom image recipes, and so forth.

The BSP Layer provides machine configurations. Everything in this layer is specific to the machine for which you are building the image or the SDK. A common structure or form is defined for BSP layers. You can learn more about this structure in the Yocto Project Board Support Package (BSP) Developer's Guide. In order for a BSP layer to be considered compliant with the Yocto Project, it must meet some structural requirements. The BSP Layer's configuration directory contains configuration files for the machine (conf/machine/<machine>.conf) and, of course, the layer (conf/layer.conf). The remainder of the layer is dedicated to specific recipes by function: recipes-bsp, recipes-core, recipes-graphics, and recipes-kernel. Metadata can exist for multiple formfactors, graphics support systems, and so forth. While the figure shows several recipe-* directories, not all these directories appear in all BSP layers.

The software layer provides the Metadata for additional software packages used during the build. This layer does not include Metadata that is specific to the distribution or the machine, which are found in their respective layers. This layer contains any new recipes that your project needs in the form of recipe files.

In order for the OpenEmbedded build system to create an image or any target, it must be able to access source files. The main Yocto Project Development Environment figure represents source files using the "Upstream Project Releases", "Local Projects", and "SCMs (optional)" boxes. The figure represents mirrors, which also play a role in locating source files, with the "Source Mirror(s)" box. The method by which source files are ultimately organized is a function of the project. For example, for released software, projects tend to use tarballs or other archived files that can capture the state of a release guaranteeing that it is statically represented. On the other hand, for a project that is more dynamic or experimental in nature, a project might keep source files in a repository controlled by a Source Control Manager (SCM) such as Git. Pulling source from a repository allows you to control the point in the repository (the revision) from which you want to build software. Finally, a combination of the two might exist, which would give the consumer a choice when deciding where to get source files. BitBake uses the SRC_URI variable to point to source files regardless of their location. Each recipe must have a SRC_URI variable that points to the source. Another area that plays a significant role in where source files comes from is pointed to by the DL_DIR variable. This area is a cache that can hold previously downloaded source. Judicious use of a DL_DIR directory can save the build system a trip across the Internet when looking for files. A good method for using a download directory is to have DL_DIR point to an area outside of your Build Directory. Doing so allows you to safely delete the Build Directory if needed without fear of removing any downloaded source file. The remainder of this section provides a deeper look into the source files and the mirrors. Here is a more detailed look at the source file area of the base figure:

Upstream project releases exist anywhere in the form of an archived file (e.g. tarball or zip file). These files correspond to individual recipes. For example, the figure uses specific releases each for BusyBox, Qt, and Dbus. An archive file can be for any released product that can be built using a recipe.

Local projects are custom bits of software the user provides. These bits reside somewhere local to a project - perhaps a directory into which the user checks in items (e.g. a local directory containing a development source tree used by the group). The canonical method through which to include a local project is to use the externalsrc.bbclass class to include local project. You use either the local.conf or a recipe's append file to override or set the recipe to point to the local directory on your disk to pull in the whole source tree. For information on how to use the externalsrc.bbclass, see the "Using External Source - externalsrc.bbclass" section.

Another place the build system can get source files from is through an SCM such as Git or Subversion. In this case, a repository is cloned or checked out. The do_fetch task inside BitBake uses the SRC_URI variable and the argument's prefix to determine the correct fetcher module. When fetching a repository, BitBake uses the SRCREV variable to determine the specific revision from which to build.

Two kinds of mirrors exist: pre-mirrors and regular mirrors. The PREMIRRORS and MIRRORS variables point to these, respectively. BitBake checks pre-mirrors before looking upstream for any source files. Pre-mirrors are appropriate when you have a shared directory that is not a directory defined by the DL_DIR variable. A Pre-mirror typically points to a shared directory that is local to your organization. Regular mirrors can be any site across the Internet that is used as an alternative location for source code should the primary site not be functioning for some reason or another.

When the OpenEmbedded build system generates an image or an SDK, it gets the packages from a package feed area located in the Build Directory. The main Yocto Project Development Environment figure shows this package feeds area in the upper-right corner. This section looks a little closer into the package feeds area used by the build system. Here is a more detailed look at the area: Package feeds are an intermediary step in the build process. BitBake generates packages whose type is defined by the PACKAGE_CLASSES variable. Before placing the packages into package feeds, the build process validates them with generated output quality assurance checks through the insane.bbclass class. The package feed area resides in tmp/deploy of the Build Directory. Folders are created that correspond to the package type (IPK, DEB, or RPM) created. Further organization is derived through the value of the PACKAGE_ARCH variable for each package. For example, packages can exist for the i586 or qemux86 architectures. The package files themselves reside within the appropriate architecture folder. BitBake uses the do_package_write_* task to place generated packages into the package holding area (e.g. do_package_write_ipk for IPK packages).

The images produced by the OpenEmbedded build system are compressed forms of the root filesystems that are ready to boot on a target device. You can see from the main Yocto Project Development Environment figure that BitBake output in part consists of images. This section is going to look more closely at this output: For a list of example images that the Yocto Project provides, the "Images" chapter. Images are written out to the Build Directory inside the deploy/images folder as shown in the figure. This folder contains any files expected to be loaded on the target device. The DEPLOY_DIR variable points to the deploy directory. <kernel-image>: A kernel binary file. The KERNEL_IMAGETYPE variable setting determines the naming scheme for the kernel image file. Depending on that variable, the file could begin with a variety of naming strings. The deploy/images directory can contain multiple image files. <root-filesystem-image>: Root filesystems for the target device (e.g. *.ext3 or *.bz2 files). The IMAGE_FSTYPES variable setting determines the root filesystem image type. The deploy/images directory can contain multiple root filesystems. <kernel-modules>: Tarballs that contain all the modules used by the kernel. Kernel module tarballs exist for legacy purposes and can be suppressed by setting the MODULE_TARBALL_DEPLOY variable to "0". The deploy/images directory can contain multiple kernel module tarballs. <bootloaders>: Bootloaders supporting the image, if applicable to the target machine. The deploy/images directory can contain multiple bootloaders. <symlinks>: The images/deploy folder contains a symbolic link that points to the most recently built file for each machine. These links might be useful for external scripts that need to obtain the latest version of each file.

In the overview figure of the Yocto Project Development Environment the output labeled "Application Development SDK" represents an SDK. This section is going to take a closer look at this output: The specific form of this output is a self-extracting SDK installer (*.sh) that, when run, installs the SDK image, which consists of a cross-development toolchain, a set of libraries and headers, and an SDK environment setup script. Running this installer essentially sets up your cross-development environment. You can think of the cross-toolchains as the "host" part because they run on the SDK machine. You can think of the libraries and headers as the "target" part because they are built for the target hardware. The setup script is added so that you can initialize the environment before using the tools. The Yocto Project supports several methods by which you can set up this cross-development environment. These methods include downloading pre-built SDK installers, building and installing your own SDK installer, or running an Application Development Toolkit (ADT) installer to install not just cross-development toolchains but also additional tools to help in this type of development. For background information on cross-development toolchains in the Yocto Project development environment, see the "Cross-Development Toolchain Generation" section. For information on setting up a cross-development environment, see the "Installing the ADT and Toolchains" section in the Yocto Project Application Developer's Guide. Once built, the SDK installers are written out to the deploy/sdk folder inside the Build Directory as shown in the figure at the beginning of this section. Several variables exist that help configure these files: DEPLOY_DIR: Points to the deploy directory. SDKMACHINE: Specifies the architecture of the machine on which the cross-development tools are run to create packages for the target hardware. SDKIMAGE_FEATURES: Lists the features to include in the "target" part of the SDK. TOOLCHAIN_HOST_TASK: Lists packages that make up the host part of the SDK installer (i.e. the part that runs on the SDKMACHINE). When you use bitbake -c populate_sdk <imagename> to create the SDK installer, a set of default packages apply. This variable allows you to add more packages. TOOLCHAIN_TARGET_TASK: Lists packages that make up the target part of the SDK installer (i.e. the part built for the target hardware).

The Yocto Project does most of the work for you when it comes to creating cross-development toolchains. This section provides some technical background information on how cross-development toolchains are created and used. For more information on these toolchain, you can also see the the Yocto Project Application Developer's Guide. In the Yocto Project development environment, cross-development toolchains are used to build the image and applications that run on the target hardware. With just a few commands, the OpenEmbedded build system creates these necessary toolchains for you. The following figure shows a high-level build environment regarding toolchain construction and use. Most of the work occurs on the Build Host. This is the machine used to build images and generally work within the the Yocto Project environment. When you run BitBake to create an image, the OpenEmbedded build system uses the host gcc compiler to bootstrap a cross-compiler named gcc-cross. The gcc-cross compiler is what BitBake uses to compile source files when creating the target image. You can think of gcc-cross simply as an automatically generated cross-compiler that is used internally within BitBake only. The chain of events that occurs when gcc-cross is bootstrapped is as follows: gcc -> binutils-cross -> gcc-cross-initial -> linux_libc-headers -> eglibc-initial -> eglibc -> gcc-cross -> gcc-runtime gcc: The build host's GNU Compiler Collection (GCC). binutils-cross: The bare minimum binary utilities needed in order to run the gcc-cross-initial phase of the bootstrap operation. gcc-cross-initial: An early stage of the bootstrap process for creating the cross-compiler. This stage builds enough of the gcc-cross, the C library, and other pieces needed to finish building the final cross-compiler in later stages. This tool is a "native" package (i.e. it is designed to run on the build host). linux_libc-headers: Headers needed for the cross-compiler. eglibc-initial: An initial version of the Embedded GLIBC needed to bootstrap eglibc. gcc-cross: The final stage of the bootstrap process for the cross-compiler. This stage results in the actual cross-compiler that BitBake uses when it builds an image for a targeted device. If you are replacing this cross compiler toolchain with a custom version, you must replace gcc-cross. This tool is also a "native" package (i.e. it is designed to run on the build host). gcc-runtime: Runtime libraries resulting from the toolchain bootstrapping process. This tool produces a binary that consists of the runtime libraries need for the targeted device. You can use the OpenEmbedded build system to build an installer for the relocatable SDK used to develop applications. When you run the installer, it installs the toolchain, which contains the development tools (e.g., the gcc-cross-canadian), binutils-cross-canadian, and other nativesdk-* tools you need to cross-compile and test your software. The figure shows the commands you use to easily build out this toolchain. This cross-development toolchain is built to execute on the SDKMACHINE, which might or might not be the same machine as the Build Host. If your target architecture is supported by the Yocto Project, you can take advantage of pre-built images that ship with the Yocto Project and already contain cross-development toolchain installers. Here is the bootstrap process for the relocatable toolchain: gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux_libc-headers -> eglibc-initial -> nativesdk-eglibc -> gcc-crosssdk -> gcc-cross-canadian gcc: The build host's GNU Compiler Collection (GCC). binutils-crosssdk: The bare minimum binary utilities needed in order to run the gcc-crosssdk-initial phase of the bootstrap operation. gcc-crosssdk-initial: An early stage of the bootstrap process for creating the cross-compiler. This stage builds enough of the gcc-crosssdk and supporting pieces so that the final stage of the bootstrap process can produce the finished cross-compiler. This tool is a "native" binary that runs on the build host. linux_libc-headers: Headers needed for the cross-compiler. eglibc-initial: An initial version of the Embedded GLIBC needed to bootstrap nativesdk-eglibc. nativesdk-eglibc: The Embedded GLIBC needed to bootstrap the gcc-crosssdk. gcc-crosssdk: The final stage of the bootstrap process for the relocatable cross-compiler. The gcc-crosssdk is a transitory compiler and never leaves the build host. Its purpose is to help in the bootstrap process to create the eventual relocatable gcc-cross-canadian compiler, which is relocatable. This tool is also a "native" package (i.e. it is designed to run on the build host). gcc-cross-canadian: The final relocatable cross-compiler. When run on the SDKMACHINE, this tool produces executable code that runs on the target device.

By design, the OpenEmbedded build system builds everything from scratch unless BitBake can determine that parts do not need to be rebuilt. Fundamentally, building from scratch is attractive as it means all parts are built fresh and there is no possibility of stale data causing problems. When developers hit problems, they typically default back to building from scratch so they know the state of things from the start. Building an image from scratch is both an advantage and a disadvantage to the process. As mentioned in the previous paragraph, building from scratch ensures that everything is current and starts from a known state. However, building from scratch also takes much longer as it generally means rebuilding things that do not necessarily need rebuilt. The Yocto Project implements shared state code that supports incremental builds. The implementation of the shared state code answers the following questions that were fundamental roadblocks within the OpenEmbedded incremental build support system: What pieces of the system have changed and what pieces have not changed? How are changed pieces of software removed and replaced? How are pre-built components that do not need to be rebuilt from scratch used when they are available? For the first question, the build system detects changes in the "inputs" to a given task by creating a checksum (or signature) of the task's inputs. If the checksum changes, the system assumes the inputs have changed and the task needs to be rerun. For the second question, the shared state (sstate) code tracks which tasks add which output to the build process. This means the output from a given task can be removed, upgraded or otherwise manipulated. The third question is partly addressed by the solution for the second question assuming the build system can fetch the sstate objects from remote locations and install them if they are deemed to be valid. The OpenEmbedded build system does not maintain PR information as part of the shared state packages. Consequently, considerations exist that affect maintaining shared state feeds. For information on how the OpenEmbedded works with packages and can track incrementing PR information, see the "Incrementing a Package Revision Number" section. The rest of this section goes into detail about the overall incremental build architecture, the checksums (signatures), shared state, and some tips and tricks.

When determining what parts of the system need to be built, BitBake uses a per-task basis and does not use a per-recipe basis. You might wonder why using a per-task basis is preferred over a per-recipe basis. To help explain, consider having the IPK packaging backend enabled and then switching to DEB. In this case, do_install and do_package output are still valid. However, with a per-recipe approach, the build would not include the .deb files. Consequently, you would have to invalidate the whole build and rerun it. Rerunning everything is not the best situation. Also in this case, the core must be "taught" much about specific tasks. This methodology does not scale well and does not allow users to easily add new tasks in layers or as external recipes without touching the packaged-staging core.

The shared state 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 the build process 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, there are things that should not be included in the checksum. First, there is the actual specific build path of a given task - the WORKDIR. It does not matter if the working directory changes because it should not affect the output for target packages. Also, the build process has the objective of making native or cross packages relocatable. The checksum therefore needs to exclude WORKDIR. The simplistic approach for excluding the working directory is to set WORKDIR 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 WORKDIR 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. The Yocto Project team has currently not managed to cover those dependencies in detail and is aware of the need to fix this situation. 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 statements effectively result in a list of global variable dependency excludes - variables never included in any checksum: BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH" BB_HASHBASE_WHITELIST += "DL_DIR SSTATE_DIR THISDIR FILESEXTRAPATHS" BB_HASHBASE_WHITELIST += "FILE_DIRNAME HOME LOGNAME SHELL TERM USER" BB_HASHBASE_WHITELIST += "FILESPATH USERNAME STAGING_DIR_HOST STAGING_DIR_TARGET" The previous example actually excludes WORKDIR since it is actually constructed as a path within TMPDIR, which is on the whitelist. 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 OE-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

Checksums and dependencies, as discussed in the previous section, solve half the problem. The other part of the problem is being able to use checksum information during the build and being able to reuse or rebuild specific components. The shared state class (sstate.bbclass) is a relatively generic implementation of how to "capture" a snapshot of a given task. The idea is that the build process does not care about the source of a task's output. Output could be freshly built or it could be downloaded and unpacked from somewhere - the build process does not need to worry about its source. There are two types of output, one is just about creating a directory in WORKDIR. A good example is the output of either do_install or do_package. The other type of output occurs when a set of data is merged into a shared directory tree such as the sysroot. The Yocto Project team has tried to keep the details of the implementation hidden in sstate.bbclass. From a user's perspective, adding shared state wrapping to a task is as simple as this do_deploy example taken from do_deploy.bbclass: DEPLOYDIR = "${WORKDIR}/deploy-${PN}" SSTATETASKS += "do_deploy" do_deploy[sstate-name] = "deploy" do_deploy[sstate-inputdirs] = "${DEPLOYDIR}" do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}" python do_deploy_setscene () { sstate_setscene(d) } addtask do_deploy_setscene In the example, we add some extra flags to the task, a name field ("deploy"), an input directory where the task sends data, and the output directory where the data from the task should eventually be copied. We also add a _setscene variant of the task and add the task name to the SSTATETASKS list. If you have a directory whose contents you need to preserve, you can do this with a line like the following: do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}" This method, as well as the following example, also works for multiple directories. do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}" do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}" do_package[sstate-lockfile] = "${PACKAGELOCK}" These methods also include the ability to take a lockfile when manipulating shared state directory structures since some cases are sensitive to file additions or removals. Behind the scenes, the shared state code works by looking in SSTATE_DIR and SSTATE_MIRRORS for shared state files. Here is an example: SSTATE_MIRRORS ?= "\ file://.* http://someserver.tld/share/sstate/PATH \n \ file://.* file:///some/local/dir/sstate/PATH" The shared state directory (SSTATE_DIR) is organized into two-character subdirectories, where the subdirectory names are based on the first two characters of the hash. If the shared state directory structure for a mirror has the same structure as SSTATE_DIR, you must specify "PATH" as part of the URI to enable the build system to map to the appropriate subdirectory. The shared state package validity can be detected just by looking at the filename since the filename contains the task checksum (or signature) as described earlier in this section. If a valid shared state package is found, the build process downloads it and uses it to accelerate the task. The build processes use the *_setscene tasks for the task acceleration phase. BitBake goes through this phase before the main execution code and tries to accelerate any tasks for which it can find shared state packages. If a shared state package for a task is available, the shared state package is used. This means the task and any tasks on which it is dependent are not executed. As a real world example, the aim is when building an IPK-based image, only the do_package_write_ipk tasks would have their shared state packages fetched and extracted. Since the sysroot is not used, it would never get extracted. This is another reason why a task-based approach is preferred over a recipe-based approach, which would have to install the output from every task.

The code in the build system that supports incremental builds is not simple code. This section presents some tips and tricks that help you work around issues related to shared state code.

When things go wrong, debugging needs to be straightforward. Because of this, the Yocto Project team included strong debugging tools: Whenever a shared state package is written out, so is a corresponding .siginfo file. This practice results in a pickled Python database of all the metadata that went into creating the hash for a given shared state package. If you run BitBake with the --dump-signatures (or -S) option, BitBake dumps out .siginfo files in the stamp directory for every task it would have executed instead of building the specified target package. There is a bitbake-diffsigs command that can process .siginfo files. If you specify one of these files, BitBake dumps out the dependency information in the file. If you specify two files, BitBake compares the two files and dumps out the differences between the two. This more easily helps answer the question of "What changed between X and Y?"

The shared state code uses checksums and shared state cache to avoid unnecessarily rebuilding tasks. As with all schemes, this one has some drawbacks. It is possible that you could make implicit changes that are not factored into the checksum calculation, but do affect a task's output. A good example is perhaps when a tool changes its output. Assume that the output of rpmdeps needed to change. The result of the change should be that all the package, package_write_rpm, and package_deploy-rpm shared state cache items would become invalid. But, because this is a change that is external to the code and therefore implicit, the associated shared state cache items do not become invalidated. In this case, the build process uses the cached items rather than running the task again. Obviously, these types of implicit changes can cause problems. To avoid these problems during the build, you need to understand the effects of any change you make. Note that any changes you make directly to a function automatically are factored into the checksum calculation and thus, will invalidate the associated area of sstate cache. You need to be aware of any implicit changes that are not obvious changes to the code and could affect the output of a given task. Once you are aware of such changes, you can take steps to invalidate the cache and force the tasks to run. The steps to take are as simple as changing function's comments in the source code. For example, to invalidate package shared state files, change the comment statements of do_package or the comments of one of the functions it calls. The change is purely cosmetic, but it causes the checksum to be recalculated and forces the task to be run again. For an example of a commit that makes a cosmetic change to invalidate a shared state, see this commit.

x32 is a processor-specific Application Binary Interface (psABI) for x86_64. An ABI defines the calling conventions between functions in a processing environment. The interface determines what registers are used and what the sizes are for various C data types. Some processing environments prefer using 32-bit applications even when running on Intel 64-bit platforms. Consider the i386 psABI, which is a very old 32-bit ABI for Intel 64-bit platforms. The i386 psABI does not provide efficient use and access of the Intel 64-bit processor resources, leaving the system underutilized. Now consider the x86_64 psABI. This ABI is newer and uses 64-bits for data sizes and program pointers. The extra bits increase the footprint size of the programs, libraries, and also increases the memory and file system size requirements. Executing under the x32 psABI enables user programs to utilize CPU and system resources more efficiently while keeping the memory footprint of the applications low. Extra bits are used for registers but not for addressing mechanisms.

While the x32 psABI specifications are not fully finalized, this Yocto Project release supports current development specifications of x32 psABI. As of this release of the Yocto Project, x32 psABI support exists as follows: You can create packages and images in x32 psABI format on x86_64 architecture targets. You can successfully build many recipes with the x32 toolchain. You can create and boot core-image-minimal and core-image-sato images.

As of this Yocto Project release, the x32 psABI kernel and library interfaces specifications are not finalized. Future Plans for the x32 psABI in the Yocto Project include the following: Enhance and fix the few remaining recipes so they work with and support x32 toolchains. Enhance RPM Package Manager (RPM) support for x32 binaries. Support larger images.

Follow these steps to use the x32 spABI: Enable the x32 psABI tuning file for x86_64 machines by editing the conf/local.conf like this: MACHINE = "qemux86-64" DEFAULTTUNE = "x86-64-x32" baselib = "${@d.getVar('BASE_LIB_tune-' + (d.getVar('DEFAULTTUNE', True) \ or 'INVALID'), True) or 'lib'}" #MACHINE = "atom-pc" #DEFAULTTUNE = "core2-64-x32" As usual, use BitBake to build an image that supports the x32 psABI. Here is an example: $ bitbake core-image-sato As usual, run your image using QEMU: $ runqemu qemux86-64 core-image-sato

Wayland is a computer display server protocol that when implemented provides a method for compositing window managers to communicate directly with applications and video hardware and expects them to communicate with input hardware using other libraries. Using Wayland with supporting targets can result in better control over graphics frame rendering than an application might otherwise achieve. The Yocto Project provides the Wayland protocol libraries and the reference Weston compositor as part of it release. This section describes what you need to do to implement Wayland and use the compositor when building an image for a supporting target.

The Wayland protocol libraries and the reference Weston compositor ship as integrated packages in the meta layer of the Source Directory. Specifically, you can find the recipes that build both Wayland and Weston at meta/recipes-graphics/wayland. You can build both the Wayland and Weston packages for use only with targets that accept the Mesa 3D and Direct Rendering Infrastructure, which is also known as Mesa DRI. This implies that you cannot build and use the packages if your target uses, for example, the Intel Embedded Media and Graphics Driver (Intel EMGD) that overrides Mesa DRI. Due to lack of EGL support, Weston 1.0.3 will not run directly on the emulated QEMU hardware. However, this version of Weston will run under X emulation without issues.

To enable Wayland, you need to enable it to be built and enable it to be included in the image.

To cause Mesa to build the wayland-egl platform and Weston to build Wayland with Kernel Mode Setting (KMS) support, include the "wayland" flag in the DISTRO_FEATURES statement in your local.conf file: DISTRO_FEATURES_append = " wayland" If X11 has been enabled elsewhere, Weston will build Wayland with X11 support

To install the Wayland feature into an image, you must include the following CORE_IMAGE_EXTRA_INSTALL statement in your local.conf file: CORE_IMAGE_EXTRA_INSTALL += "wayland weston"

To run Weston inside X11, enabling it as described earlier and building a Sato image is sufficient. If you are running your image under Sato, a Weston Launcher appears in the "Utility" category. Alternatively, you can run Weston through the command-line interpretor (CLI), which is better suited for development work. To run Weston under the CLI you need to do the following after your image is built: Run these commands to export XDG_RUNTIME_DIR: mkdir -p /tmp/$USER-weston chmod 0700 /tmp/$USER-weston export XDG_RUNTIME_DIR=/tmp/$USER=weston Launch Weston in the shell: weston

This section describes the mechanism by which the OpenEmbedded build system tracks changes to licensing text. The section also describes how to enable commercially licensed recipes, which by default are disabled. For information that can help you maintain compliance with various open source licensing during the lifecycle of the product, see the "Maintaining Open Source License Compliance During Your Project's Lifecycle" section in the Yocto Project Development Manual.

The license of an upstream project might change in the future. In order to prevent these changes going unnoticed, the LIC_FILES_CHKSUM variable tracks changes to the license text. The checksums are validated at the end of the configure step, and if the checksums do not match, the build will fail.

The LIC_FILES_CHKSUM variable contains checksums of the license text in the source code for the recipe. Following is an example of how to specify LIC_FILES_CHKSUM: LIC_FILES_CHKSUM = "file://COPYING;md5=xxxx \ file://licfile1.txt;beginline=5;endline=29;md5=yyyy \ file://licfile2.txt;endline=50;md5=zzzz \ ..." The build system uses the S variable as the default directory used when searching files listed in LIC_FILES_CHKSUM. The previous example employs the default directory. You can also use relative paths as shown in the following example: LIC_FILES_CHKSUM = "file://src/ls.c;beginline=5;endline=16;\ md5=bb14ed3c4cda583abc85401304b5cd4e" LIC_FILES_CHKSUM = "file://../license.html;md5=5c94767cedb5d6987c902ac850ded2c6" In this example, the first line locates a file in ${S}/src/ls.c. The second line refers to a file in WORKDIR, which is the parent of S. Note that LIC_FILES_CHKSUM variable is mandatory for all recipes, unless the LICENSE variable is set to "CLOSED".

As mentioned in the previous section, the LIC_FILES_CHKSUM variable lists all the important files that contain the license text for the source code. It is possible to specify a checksum for an entire file, or a specific section of a file (specified by beginning and ending line numbers with the "beginline" and "endline" parameters, respectively). The latter is useful for source files with a license notice header, README documents, and so forth. If you do not use the "beginline" parameter, then it is assumed that the text begins on the first line of the file. Similarly, if you do not use the "endline" parameter, it is assumed that the license text ends with the last line of the file. The "md5" parameter stores the md5 checksum of the license text. If the license text changes in any way as compared to this parameter then a mismatch occurs. This mismatch triggers a build failure and notifies the developer. Notification allows the developer to review and address the license text changes. Also note that if a mismatch occurs during the build, the correct md5 checksum is placed in the build log and can be easily copied to the recipe. There is no limit to how many files you can specify using the LIC_FILES_CHKSUM variable. Generally, however, every project requires a few specifications for license tracking. Many projects have a "COPYING" file that stores the license information for all the source code files. This practice allows you to just track the "COPYING" file as long as it is kept up to date. If you specify an empty or invalid "md5" parameter, BitBake returns an md5 mis-match error and displays the correct "md5" parameter value during the build. The correct parameter is also captured in the build log. If the whole file contains only license text, you do not need to use the "beginline" and "endline" parameters.

By default, the OpenEmbedded build system disables components that have commercial or other special licensing requirements. Such requirements are defined on a recipe-by-recipe basis through the LICENSE_FLAGS variable definition in the affected recipe. For instance, the $HOME/poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly recipe contains the following statement: LICENSE_FLAGS = "commercial" Here is a slightly more complicated example that contains both an explicit recipe name and version (after variable expansion): LICENSE_FLAGS = "license_${PN}_${PV}" In order for a component restricted by a LICENSE_FLAGS definition to be enabled and included in an image, it needs to have a matching entry in the global LICENSE_FLAGS_WHITELIST variable, which is a variable typically defined in your local.conf file. For example, to enable the $HOME/poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly package, you could add either the string "commercial_gst-plugins-ugly" or the more general string "commercial" to LICENSE_FLAGS_WHITELIST. See the "License Flag Matching" section for a full explanation of how LICENSE_FLAGS matching works. Here is the example: LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly" Likewise, to additionally enable the package built from the recipe containing LICENSE_FLAGS = "license_${PN}_${PV}", and assuming that the actual recipe name was emgd_1.10.bb, the following string would enable that package as well as the original gst-plugins-ugly package: LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly license_emgd_1.10" As a convenience, you do not need to specify the complete license string in the whitelist for every package. you can use an abbreviated form, which consists of just the first portion or portions of the license string before the initial underscore character or characters. A partial string will match any license that contains the given string as the first portion of its license. For example, the following whitelist string will also match both of the packages previously mentioned as well as any other packages that have licenses starting with "commercial" or "license". LICENSE_FLAGS_WHITELIST = "commercial license"

License flag matching allows you to control what recipes the OpenEmbedded build system includes in the build. Fundamentally, the build system attempts to match LICENSE_FLAG strings found in recipes against LICENSE_FLAGS_WHITELIST strings found in the whitelist. A match, causes the build system to include a recipe in the build, while failure to find a match causes the build system to exclude a recipe. In general, license flag matching is simple. However, understanding some concepts will help you correctly and effectively use matching. Before a flag defined by a particular recipe is tested against the contents of the whitelist, the expanded string _${PN} is appended to the flag. This expansion makes each LICENSE_FLAGS value recipe-specific. After expansion, the string is then matched against the whitelist. Thus, specifying LICENSE_FLAGS = "commercial" in recipe "foo", for example, results in the string "commercial_foo". And, to create a match, that string must appear in the whitelist. Judicious use of the LICENSE_FLAGS strings and the contents of the LICENSE_FLAGS_WHITELIST variable allows you a lot of flexibility for including or excluding recipes based on licensing. For example, you can broaden the matching capabilities by using license flags string subsets in the whitelist. When using a string subset, be sure to use the part of the expanded string that precedes the appended underscore character (e.g. usethispart_1.3, usethispart_1.4, and so forth). For example, simply specifying the string "commercial" in the whitelist matches any expanded LICENSE_FLAGS definition that starts with the string "commercial" such as "commercial_foo" and "commercial_bar", which are the strings the build system automatically generates for hypothetical recipes named "foo" and "bar" assuming those recipes simply specify the following: LICENSE_FLAGS = "commercial" Thus, you can choose to exhaustively enumerate each license flag in the whitelist and allow only specific recipes into the image, or you can use a string subset that causes a broader range of matches to allow a range of recipes into the image. This scheme works even if the LICENSE_FLAG string already has _${PN} appended. For example, the build system turns the license flag "commercial_1.2_foo" into "commercial_1.2_foo_foo" and would match both the general "commercial" and the specific "commercial_1.2_foo" strings found in the whitelist, as expected. Here are some other scenarios: You can specify a versioned string in the recipe such as "commercial_foo_1.2" in a "foo" recipe. The build system expands this string to "commercial_foo_1.2_foo". Combine this license flag with a whitelist that has the string "commercial" and you match the flag along with any other flag that starts with the string "commercial". Under the same circumstances, you can use "commercial_foo" in the whitelist and the build system not only matches "commercial_foo_1.2" but also matches any license flag with the string "commercial_foo", regardless of the version. You can be very specific and use both the package and version parts in the whitelist (e.g. "commercial_foo_1.2") to specifically match a versioned recipe.

Other helpful variables related to commercial license handling exist and are defined in the $HOME/poky/meta/conf/distro/include/default-distrovars.inc file: COMMERCIAL_AUDIO_PLUGINS ?= "" COMMERCIAL_VIDEO_PLUGINS ?= "" COMMERCIAL_QT = "" If you want to enable these components, you can do so by making sure you have statements similar to the following in your local.conf configuration file: COMMERCIAL_AUDIO_PLUGINS = "gst-plugins-ugly-mad \ gst-plugins-ugly-mpegaudioparse" COMMERCIAL_VIDEO_PLUGINS = "gst-plugins-ugly-mpeg2dec \ gst-plugins-ugly-mpegstream gst-plugins-bad-mpegvideoparse" COMMERCIAL_QT ?= "qmmp" LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly commercial_gst-plugins-bad commercial_qmmp" Of course, you could also create a matching whitelist for those components using the more general "commercial" in the whitelist, but that would also enable all the other packages with LICENSE_FLAGS containing "commercial", which you may or may not want: LICENSE_FLAGS_WHITELIST = "commercial" Specifying audio and video plug-ins as part of the COMMERCIAL_AUDIO_PLUGINS and COMMERCIAL_VIDEO_PLUGINS statements or commercial Qt components as part of the COMMERCIAL_QT statement (along with the enabling LICENSE_FLAGS_WHITELIST) includes the plug-ins or components into built images, thus adding support for media formats or components.