Yocto Project Mega-Manual

The following list describes features and advantages of the Yocto Project:

The following list presents challenges you might encounter when developing using the Yocto Project:

The following list consists of tools that help you develop images and applications using the Yocto Project:

The following list consists of tools that help production related activities using the Yocto Project:

The following list consists of components associated with the OpenEmbedded build system:

Poky is the Yocto Project reference distribution. It contains the Open-Embedded build system (BitBake and OE-Core) as well as a set of metadata to get you started building your own distribution. See the figure in "What is the Yocto Project?" section for an illustration that shows Poky and its relationship with other parts of the Yocto Project.

To use the Yocto Project tools and components, you can download (clone) Poky and use it to bootstrap your own distribution.

You can read more about Poky in the "Reference Embedded Distribution (Poky)" section.

The following lists components associated with packages for finished targets:

The Build Appliance is a virtual machine image that enables you to build and boot a custom embedded Linux image with the Yocto Project using a non-Linux development system.

Historically, the Build Appliance was the second of three methods by which you could use the Yocto Project on a system that was not native to Linux.

As mentioned briefly in the previous section and also in the "Git Workflows and the Yocto Project" section, the Yocto Project maintains source repositories at http://git.yoctoproject.org. If you look at this web-interface of the repositories, each item is a separate Git repository.

Git repositories use branching techniques that track content change (not files) within a project (e.g. a new feature or updated documentation). Creating a tree-like structure based on project divergence allows for excellent historical information over the life of a project. This methodology also allows for an environment from which you can do lots of local experimentation on projects as you develop changes or new features.

A Git repository represents all development efforts for a given project. For example, the Git repository poky contains all changes and developments for that repository over the course of its entire life. That means that all changes that make up all releases are captured. The repository maintains a complete history of changes.

You can create a local copy of any repository by "cloning" it with the git clone command. When you clone a Git repository, you end up with an identical copy of the repository on your development system. Once you have a local copy of a repository, you can take steps to develop locally. For examples on how to clone Git repositories, see the "Locating Yocto Project Source Files" section in the Yocto Project Development Tasks Manual.

It is important to understand that Git tracks content change and not files. Git uses "branches" to organize different development efforts. For example, the poky repository has several branches that include the current "sumo" branch, the "master" branch, and many branches for past Yocto Project releases. You can see all the branches by going to http://git.yoctoproject.org/cgit.cgi/poky/ and clicking on the [...] link beneath the "Branch" heading.

Each of these branches represents a specific area of development. The "master" branch represents the current or most recent development. All other branches represent offshoots of the "master" branch.

When you create a local copy of a Git repository, the copy has the same set of branches as the original. This means you can use Git to create a local working area (also called a branch) that tracks a specific development branch from the upstream source Git repository. in other words, you can define your local Git environment to work on any development branch in the repository. To help illustrate, consider the following example Git commands:

     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git checkout -b sumo origin/sumo
            

In the previous example after moving to the home directory, the git clone command creates a local copy of the upstream poky Git repository. By default, Git checks out the "master" branch for your work. After changing the working directory to the new local repository (i.e. poky), the git checkout command creates and checks out a local branch named "sumo", which tracks the upstream "origin/sumo" branch. Changes you make while in this branch would ultimately affect the upstream "sumo" branch of the poky repository.

It is important to understand that when you create and checkout a local working branch based on a branch name, your local environment matches the "tip" of that particular development branch at the time you created your local branch, which could be different from the files in the "master" branch of the upstream repository. In other words, creating and checking out a local branch based on the "sumo" branch name is not the same as checking out the "master" branch in the repository. Keep reading to see how you create a local snapshot of a Yocto Project Release.

Git uses "tags" to mark specific changes in a repository branch structure. Typically, a tag is used to mark a special point such as the final change (or commit) before a project is released. You can see the tags used with the poky Git repository by going to http://git.yoctoproject.org/cgit.cgi/poky/ and clicking on the [...] link beneath the "Tag" heading.

Some key tags for the poky repository are jethro-14.0.3, morty-16.0.1, pyro-17.0.0, and sumo-20.0.3. These tags represent Yocto Project releases.

When you create a local copy of the Git repository, you also have access to all the tags in the upstream repository. Similar to branches, you can create and checkout a local working Git branch based on a tag name. When you do this, you get a snapshot of the Git repository that reflects the state of the files when the change was made associated with that tag. The most common use is to checkout a working branch that matches a specific Yocto Project release. Here is an example:

     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git fetch --tags
     $ git checkout tags/rocko-18.0.0 -b my_rocko-18.0.0
            

In this example, the name of the top-level directory of your local Yocto Project repository is poky. After moving to the poky directory, the git fetch command makes all the upstream tags available locally in your repository. Finally, the git checkout command creates and checks out a branch named "my-rocko-18.0.0" that is based on the upstream branch whose "HEAD" matches the commit in the repository associated with the "rocko-18.0.0" tag. The files in your repository now exactly match that particular Yocto Project release as it is tagged in the upstream Git repository. It is important to understand that when you create and checkout a local working branch based on a tag, your environment matches a specific point in time and not the entire development branch (i.e. from the "tip" of the branch backwards).

Git has an extensive set of commands that lets you manage changes and perform collaboration over the life of a project. Conveniently though, you can manage with a small set of basic operations and workflows once you understand the basic philosophy behind Git. You do not have to be an expert in Git to be functional. A good place to look for instruction on a minimal set of Git commands is here.

The following list of Git commands briefly describes some basic Git operations as a way to get started. As with any set of commands, this list (in most cases) simply shows the base command and omits the many arguments it supports. See the Git documentation for complete descriptions and strategies on how to use these commands:

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.

This section briefly introduces BitBake. If you want more information on BitBake, see the BitBake User Manual.

To see a list of the options BitBake supports, use either of the following commands:

     $ bitbake -h
     $ 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"). The target often equates to the first part of a recipe's filename (e.g. "foo" for a recipe named foo_1.3.0-r0.bb). So, to process the matchbox-desktop_1.2.3.bb recipe 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" section of the BitBake User Manual.

BitBake also tries to execute any dependent tasks first. So for example, before building matchbox-desktop, BitBake would build a cross compiler and glibc if they had not already been built.

A useful BitBake option to consider is the -k or --continue option. This option instructs BitBake to try and continue processing the job as long 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.

Files that have the .bb suffix are "recipes" files. In general, a recipe contains information about a single piece of software. This information includes the location from which to download the unaltered source, any source patches to be applied to that source (if needed), which special configuration options to apply, how to compile the source files, and how to package the compiled output.

The term "package" is sometimes used to refer to recipes. However, since the word "package" 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 recipes files. An example is the autotools class, which contains common settings for any application that Autotools uses. The "Classes" chapter in the Yocto Project Reference Manual provides details about 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 conf/local.conf, which is found in the Build Directory.

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 general workflow figure:

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 build/conf directory of 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.

The meta-poky 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 oe-init-build-env, which is the build environment script.

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 build environment setup 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 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-poky layer:

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 Tasks Manual.

The files site.conf and auto.conf are not created by the environment initialization script. If you want the site.conf file, you need to create that yourself. The auto.conf file is typically created by an autobuilder:

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. It is important to understand that the OpenEmbedded build system reads the configuration files in a specific order: site.conf, auto.conf, and local.conf. And, the build system applies the normal assignment statement rules as described in the "Syntax and Operators" chapter of the BitBake User Manual. Because the files are parsed in a specific order, variable assignments for the same variable could be affected. For example, if the auto.conf file and the local.conf set variable1 to different values, because the build system parses local.conf after auto.conf, variable1 is assigned the value from the local.conf file.

The previous section described the user configurations that define 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 policies.

In general, three types of layer input exists. You can see them below the "User Configuration" box in the general workflow figure:

The following figure shows an expanded representation of these three layers from the general workflow figure:

In general, all layers have a similar structure. They all contain a licensing file (e.g. COPYING.MIT) 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. You can learn about the general structure for layers used with the Yocto Project in the "Creating Your Own Layer" section in the Yocto Project Development Tasks Manual. For a general discussion on layers and the many layers from which you can draw, see the "Layers" and "The Yocto Project Layer Model" sections both earlier in this manual.

If you explored the previous links, you discovered some areas where many layers that work with the Yocto Project exist. The Source Repositories also shows layers categorized under "Yocto Metadata Layers."

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.

In order for the OpenEmbedded build system to create an image or any target, it must be able to access source files. The general workflow 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 Materials" 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 come from is pointed to by the DL_DIR variable. This area is a cache that can hold previously downloaded source. You can also instruct the OpenEmbedded build system to create tarballs from Git repositories, which is not the default behavior, and store them in the DL_DIR by using the BB_GENERATE_MIRROR_TARBALLS variable.

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 general workflow figure:

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 general workflow 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. The OpenEmbedded build system provides classes to generate different package types, and you specify which classes to enable through 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 class.

The package feed area resides in the Build Directory. The directory the build system uses to temporarily store packages is determined by a combination of variables and the particular package manager in use. See the "Package Feeds" box in the illustration and note the information to the right of that area. In particular, the following defines where package files are kept:

BitBake uses the do_package_write_* tasks to generate packages and place them into the package holding area (e.g. do_package_write_ipk for IPK packages). See the "do_package_write_deb", "do_package_write_ipk", "do_package_write_rpm", and "do_package_write_tar" sections in the Yocto Project Reference Manual for additional information. As an example, consider a scenario where an IPK packaging manager is being used and package architecture support for both i586 and qemux86 exist. Packages for the i586 architecture are placed in build/tmp/deploy/ipk/i586, while packages for the qemux86 architecture are placed in build/tmp/deploy/ipk/qemux86.

The OpenEmbedded build system uses BitBake to produce images and Software Development Kits (SDKs). You can see from the general workflow figure, the BitBake area consists of several functional areas. This section takes a closer look at each of those areas.

4.3.5.1. Source Fetching¶

The first stages of building a recipe are to fetch and unpack the source code:

The do_fetch and do_unpack tasks fetch the source files and unpack them into the Build Directory.

Note

For every local file (e.g. file://) that is part of a recipe's SRC_URI statement, the OpenEmbedded build system takes a checksum of the file for the recipe and inserts the checksum into the signature for the do_fetch task. If any local file has been modified, the do_fetch task and all tasks that depend on it are re-executed.

By default, everything is accomplished in the Build Directory, which has a defined structure. For additional general information on the Build Directory, see the "build/" section in the Yocto Project Reference Manual.

Each recipe has an area in the Build Directory where the unpacked source code resides. The S variable points to this area for a recipe's unpacked source code. The name of that directory for any given recipe is defined from several different variables. The preceding figure and the following list describe the Build Directory's hierarchy:

• TMPDIR: The base directory where the OpenEmbedded build system performs all its work during the build. The default base directory is the tmp directory.

• PACKAGE_ARCH: The architecture of the built package or packages. Depending on the eventual destination of the package or packages (i.e. machine architecture, build host, SDK, or specific machine), PACKAGE_ARCH varies. See the variable's description for details.

• TARGET_OS: The operating system of the target device. A typical value would be "linux" (e.g. "qemux86-poky-linux").

• PN: The name of the recipe used to build the package. This variable can have multiple meanings. However, when used in the context of input files, PN represents the the name of the recipe.

• WORKDIR: The location where the OpenEmbedded build system builds a recipe (i.e. does the work to create the package).

  • PV: The version of the recipe used to build the package.

  • PR: The revision of the recipe used to build the package.

• S: Contains the unpacked source files for a given recipe.

  • BPN: The name of the recipe used to build the package. The BPN variable is a version of the PN variable but with common prefixes and suffixes removed.

  • PV: The version of the recipe used to build the package.

Note

In the previous figure, notice that two sample hierarchies exist: one based on package architecture (i.e. PACKAGE_ARCH) and one based on a machine (i.e. MACHINE). The underlying structures are identical. The differentiator being what the OpenEmbedded build system is using as a build target (e.g. general architecture, a build host, an SDK, or a specific machine).

4.3.5.2. Patching¶

Once source code is fetched and unpacked, BitBake locates patch files and applies them to the source files:

The do_patch task uses a recipe's SRC_URI statements and the FILESPATH variable to locate applicable patch files.

Default processing for patch files assumes the files have either *.patch or *.diff file types. You can use SRC_URI parameters to change the way the build system recognizes patch files. See the do_patch task for more information.

BitBake finds and applies multiple patches for a single recipe in the order in which it locates the patches. The FILESPATH variable defines the default set of directories that the build system uses to search for patch files. Once found, patches are applied to the recipe's source files, which are located in the S directory.

For more information on how the source directories are created, see the "Source Fetching" section. For more information on how to create patches and how the build system processes patches, see the "Patching Code" section in the Yocto Project Development Tasks Manual. You can also see the "Use devtool modify to Modify the Source of an Existing Component" section in the Yocto Project Application Development and the Extensible Software Development Kit (SDK) manual and the "Using Traditional Kernel Development to Patch the Kernel" section in the Yocto Project Linux Kernel Development Manual.

4.3.5.3. Configuration, Compilation, and Staging¶

After source code is patched, BitBake executes tasks that configure and compile the source code. Once compilation occurs, the files are copied to a holding area (staged) in preparation for packaging:

This step in the build process consists of the following tasks:

• do_prepare_recipe_sysroot: This task sets up the two sysroots in ${WORKDIR} (i.e. recipe-sysroot and recipe-sysroot-native) so that during the packaging phase the sysroots can contain the contents of the do_populate_sysroot tasks of the recipes on which the recipe containing the tasks depends. A sysroot exists for both the target and for the native binaries, which run on the host system.

• do_configure: This task configures the source by enabling and disabling any build-time and configuration options for the software being built. Configurations can come from the recipe itself as well as from an inherited class. Additionally, the software itself might configure itself depending on the target for which it is being built.

  The configurations handled by the do_configure task are specific to configurations for the source code being built by the recipe.

  If you are using the autotools class, you can add additional configuration options by using the EXTRA_OECONF or PACKAGECONFIG_CONFARGS variables. For information on how this variable works within that class, see the autotools class here.

• do_compile: Once a configuration task has been satisfied, BitBake compiles the source using the do_compile task. Compilation occurs in the directory pointed to by the B variable. Realize that the B directory is, by default, the same as the S directory.

• do_install: After compilation completes, BitBake executes the do_install task. This task copies files from the B directory and places them in a holding area pointed to by the D variable. Packaging occurs later using files from this holding directory.

4.3.5.4. Package Splitting¶

After source code is configured, compiled, and staged, the build system analyzes the results and splits the output into packages:

The do_package and do_packagedata tasks combine to analyze the files found in the D directory and split them into subsets based on available packages and files. Analysis involves the following as well as other items: splitting out debugging symbols, looking at shared library dependencies between packages, and looking at package relationships.

The do_packagedata task creates package metadata based on the analysis such that the build system can generate the final packages. The do_populate_sysroot task stages (copies) a subset of the files installed by the do_install task into the appropriate sysroot. Working, staged, and intermediate results of the analysis and package splitting process use several areas:

• PKGD: The destination directory (i.e. package) for packages before they are split into individual packages.

• PKGDESTWORK: A temporary work area (i.e. pkgdata) used by the do_package task to save package metadata.

• PKGDEST: The parent directory (i.e. packages-split) for packages after they have been split.

• PKGDATA_DIR: A shared, global-state directory that holds packaging metadata generated during the packaging process. The packaging process copies metadata from PKGDESTWORK to the PKGDATA_DIR area where it becomes globally available.

• STAGING_DIR_HOST: The path for the sysroot for the system on which a component is built to run (i.e. recipe-sysroot).

• STAGING_DIR_NATIVE: The path for the sysroot used when building components for the build host (i.e. recipe-sysroot-native).

• STAGING_DIR_TARGET: The path for the sysroot used when a component that is built to execute on a system and it generates code for yet another machine (e.g. cross-canadian recipes).

The FILES variable defines the files that go into each package in PACKAGES. If you want details on how this is accomplished, you can look at package.bbclass.

Depending on the type of packages being created (RPM, DEB, or IPK), the do_package_write_* task creates the actual packages and places them in the Package Feed area, which is ${TMPDIR}/deploy. You can see the "Package Feeds" section for more detail on that part of the build process.

Note

Support for creating feeds directly from the deploy/* directories does not exist. Creating such feeds usually requires some kind of feed maintenance mechanism that would upload the new packages into an official package feed (e.g. the Ångström distribution). This functionality is highly distribution-specific and thus is not provided out of the box.

4.3.5.5. Image Generation¶

Once packages are split and stored in the Package Feeds area, the build system uses BitBake to generate the root filesystem image:

The image generation process consists of several stages and depends on several tasks and variables. The do_rootfs task creates the root filesystem (file and directory structure) for an image. This task uses several key variables to help create the list of packages to actually install:

• IMAGE_INSTALL: Lists out the base set of packages from which to install from the Package Feeds area.

• PACKAGE_EXCLUDE: Specifies packages that should not be installed into the image.

• IMAGE_FEATURES: Specifies features to include in the image. Most of these features map to additional packages for installation.

• PACKAGE_CLASSES: Specifies the package backend (e.g. RPM, DEB, or IPK) to use and consequently helps determine where to locate packages within the Package Feeds area.

• IMAGE_LINGUAS: Determines the language(s) for which additional language support packages are installed.

• PACKAGE_INSTALL: The final list of packages passed to the package manager for installation into the image.

With IMAGE_ROOTFS pointing to the location of the filesystem under construction and the PACKAGE_INSTALL variable providing the final list of packages to install, the root file system is created.

Package installation is under control of the package manager (e.g. dnf/rpm, opkg, or apt/dpkg) regardless of whether or not package management is enabled for the target. At the end of the process, if package management is not enabled for the target, the package manager's data files are deleted from the root filesystem. As part of the final stage of package installation, post installation scripts that are part of the packages are run. Any scripts that fail to run on the build host are run on the target when the target system is first booted. If you are using a read-only root filesystem, all the post installation scripts must succeed on the build host during the package installation phase since the root filesystem on the target is read-only.

The final stages of the do_rootfs task handle post processing. Post processing includes creation of a manifest file and optimizations.

The manifest file (.manifest) resides in the same directory as the root filesystem image. This file lists out, line-by-line, the installed packages. The manifest file is useful for the testimage class, for example, to determine whether or not to run specific tests. See the IMAGE_MANIFEST variable for additional information.

Optimizing processes that are run across the image include mklibs, prelink, and any other post-processing commands as defined by the ROOTFS_POSTPROCESS_COMMAND variable. The mklibs process optimizes the size of the libraries, while the prelink process optimizes the dynamic linking of shared libraries to reduce start up time of executables.

After the root filesystem is built, processing begins on the image through the do_image task. The build system runs any pre-processing commands as defined by the IMAGE_PREPROCESS_COMMAND variable. This variable specifies a list of functions to call before the build system creates the final image output files.

The build system dynamically creates do_image_* tasks as needed, based on the image types specified in the IMAGE_FSTYPES variable. The process turns everything into an image file or a set of image files and can compress the root filesystem image to reduce the overall size of the image. The formats used for the root filesystem depend on the IMAGE_FSTYPES variable. Compression depends on whether the formats support compression.

As an example, a dynamically created task when creating a particular image type would take the following form:

     do_image_type
                    

So, if the type as specified by the IMAGE_FSTYPES were ext4, the dynamically generated task would be as follows:

     do_image_ext4
                    

The final task involved in image creation is the do_image_complete task. This task completes the image by applying any image post processing as defined through the IMAGE_POSTPROCESS_COMMAND variable. The variable specifies a list of functions to call once the build system has created the final image output files.

Note

The entire image generation process is run under Pseudo. Running under Pseudo ensures that the files in the root filesystem have correct ownership.

4.3.5.6. SDK Generation¶

The OpenEmbedded build system uses BitBake to generate the Software Development Kit (SDK) installer scripts for both the standard SDK and the extensible SDK (eSDK):

Note

For more information on the cross-development toolchain generation, see the "Cross-Development Toolchain Generation" section. For information on advantages gained when building a cross-development toolchain using the do_populate_sdk task, see the "Building an SDK Installer" section in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual.

Like image generation, the SDK script process consists of several stages and depends on many variables. The do_populate_sdk and do_populate_sdk_ext tasks use these key variables to help create the list of packages to actually install. For information on the variables listed in the figure, see the "Application Development SDK" section.

The do_populate_sdk task helps create the standard SDK and handles two parts: a target part and a host part. The target part is the part built for the target hardware and includes libraries and headers. The host part is the part of the SDK that runs on the SDKMACHINE.

The do_populate_sdk_ext task helps create the extensible SDK and handles host and target parts differently than its counter part does for the standard SDK. For the extensible SDK, the task encapsulates the build system, which includes everything needed (host and target) for the SDK.

Regardless of the type of SDK being constructed, the tasks perform some cleanup after which a cross-development environment setup script and any needed configuration files are created. The final output is the Cross-development toolchain installation script (.sh file), which includes the environment setup script.

The images produced by the build system are compressed forms of the root filesystem and are ready to boot on a target device. You can see from the general workflow figure that BitBake output, in part, consists of images. This section takes a closer look at this output:

The build process writes images out to the Build Directory inside the tmp/deploy/images/machine/ 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, while the DEPLOY_DIR_IMAGE variable points to the appropriate directory containing images for the current configuration.

In the general workflow figure, the output labeled "Application Development SDK" represents an SDK. The SDK generation process differs depending on whether you build an extensible SDK (e.g. bitbake -c populate_sdk_ext imagename) or a standard SDK (e.g. bitbake -c populate_sdk imagename). This section takes a closer look at this output:

The specific form of this output is a set of files that includes a self-extracting SDK installer (*.sh), host and target manifest files, and files used for SDK testing. When the SDK installer file is run, it installs the SDK. The SDK 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-toolchain as the "host" part because it runs 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 environment setup script is added so that you can initialize the environment before using the tools.

All the output files for an SDK are written to the deploy/sdk folder inside the Build Directory as shown in the previous figure. Depending on the type of SDK, several variables exist that help configure these files. The following list shows the variables associated with an extensible SDK:

This next list, shows the variables associated with a standard SDK:

When determining what parts of the system need to be built, BitBake works on a per-task basis rather than 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, the do_install and do_package task outputs 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 solution. 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 work 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 work 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, solutions for shell scripts exist. 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 situations, 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 you 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.

As an example, consider a case with in-line Python 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, the question of a task's indirect inputs still exits - items 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, a variety of ways exist by which both the basehash and the dependent task hashes can be influenced. Within the BitBake configuration file, you 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 (i.e. variables never included in any checksum):

     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 WORKDIR since that variable 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, a dummy "noop" signature handler is 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:

Checksums and dependencies, as discussed in the previous section, solve half the problem of supporting a shared state. The other half of the problem is being able to use checksum information during the build and being able to reuse or rebuild specific components.

The sstate class 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. In other words, the build process does not need to worry about its origin.

Two types of output exist. One type 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 class. From a user's perspective, adding shared state wrapping to a task is as simple as this do_deploy example taken from the deploy class:

     DEPLOYDIR = "${WORKDIR}/deploy-${PN}"
     SSTATETASKS += "do_deploy"
     do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"
     do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"

     python do_deploy_setscene () {
         sstate_setscene(d)
     }
     addtask do_deploy_setscene
     do_deploy[dirs] = "${DEPLOYDIR} ${B}"
     do_deploy[stamp-extra-info] = "${MACHINE_ARCH}"
                

The following list explains the previous example:

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;downloadfilename=PATH \n \
     file://.* file:///some/local/dir/sstate/PATH"
                

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.

Follow these steps to prepare a native Linux machine as your Yocto Project development host:

Once you have completed the previous steps, you are ready to continue using a given development path on your native Linux machine. If you are going to use BitBake, see the "Cloning the poky Repository" section. If you are going to use the Extensible SDK, see the "Using the Extensible SDK" Chapter in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual. If you want to work on the kernel, see the Yocto Project Linux Kernel Development Manual. If you are going to use Toaster, see the "Setting Up and Using Toaster" section in the Toaster User Manual.

With CROPS, which leverages Docker Containers, you can create a Yocto Project development environment that is operating system agnostic. You can set up a container in which you can develop using the Yocto Project on a Windows, Mac, or Linux machine.

Follow these general steps to prepare a Windows, Mac, or Linux machine as your Yocto Project development host:

Once you have a container set up, everything is in place to develop just as if you were running on a native Linux machine. If you are going to use the Poky container, see the "Cloning the poky Repository" section. If you are going to use the Extensible SDK container, see the "Using the Extensible SDK" Chapter in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual. If you are going to use the Toaster container, see the "Setting Up and Using Toaster" section in the Toaster User Manual.

Working from a copy of the upstream Yocto Project Source Repositories is the preferred method for obtaining and using a Yocto Project release. You can view the Yocto Project Source Repositories at http://git.yoctoproject.org. In particular, you can find the poky repository at http://git.yoctoproject.org/cgit/cgit.cgi/poky/.

Use the following procedure to locate the latest upstream copy of the poky Git repository:

Yocto Project maintains an Index of Releases area that contains related files that contribute to the Yocto Project. Rather than Git repositories, these files are tarballs that represent snapshots in time of a given component.

The Yocto Project Website uses a "DOWNLOADS" page from which you can locate and download tarballs of any Yocto Project release. Rather than Git repositories, these files represent snapshot tarballs.

Yocto Project maintains an area for nightly builds that contains tarball releases at https://autobuilder.yocto.io//pub/nightly/. These builds include Yocto Project releases, SDK installation scripts, and experimental builds.

Should you ever want to access a nightly build of a particular Yocto Project component, use the following procedure:

Follow these steps to create a local version of the upstream poky Git repository.

When you clone the upstream poky repository, you have access to all its development branches. Each development branch in a repository is unique as it forks off the "master" branch. To see and use the files of a particular development branch locally, you need to know the branch name and then specifically check out that development branch.

Similar to branches, the upstream repository uses tags to mark specific commits associated with significant points in a development branch (i.e. a release point or stage of a release). You might want to set up a local branch based on one of those points in the repository. The process is similar to checking out by branch name except you use tag names.

It is very easy to create your own layers to use with the OpenEmbedded build system. The Yocto Project ships with tools that speed up creating layers. This section describes the steps you perform by hand to create layers so that you can better understand them. For information about the layer-creation tools, see the "Creating a New BSP Layer Using the bitbake-layers Script" section in the Yocto Project Board Support Package (BSP) Developer's Guide and the "Creating a General Layer Using the bitbake-layers Script" section further down in this manual.

Follow these general steps to create your layer without using tools:

To create layers that are easier to maintain and that will not impact builds for other machines, you should consider the information in the following list:

When you create a layer used with the Yocto Project, it is advantageous to make sure that the layer interacts well with existing Yocto Project layers (i.e. the layer is compatible with the Yocto Project). Ensuring compatibility makes the layer easy to be consumed by others in the Yocto Project community and could allow you permission to use the Yocto Project Compatible Logo.

The Yocto Project Compatibility Program consists of a layer application process that requests permission to use the Yocto Project Compatibility Logo for your layer and application. The process consists of two parts:

To be granted permission to use the logo, you need to satisfy the following:

The remainder of this section presents information on the registration form and on the yocto-check-layer script.

     $ source oe-init-build-env
     $ yocto-check-layer your_layer_directory
                    

Before the OpenEmbedded build system can use your new layer, you need to enable it. To enable your layer, simply add your layer's path to the BBLAYERS variable in your conf/bblayers.conf file, which is found in the Build Directory. The following example shows how to enable a layer named meta-mylayer:

     # POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
     # changes incompatibly
     POKY_BBLAYERS_CONF_VERSION = "2"

     BBPATH = "${TOPDIR}"
     BBFILES ?= ""

     BBLAYERS ?= " \
       /home/user/poky/meta \
       /home/user/poky/meta-poky \
       /home/user/poky/meta-yocto-bsp \
       /home/user/poky/meta-mylayer \
       "
                

BitBake parses each conf/layer.conf file from the top down as specified in the BBLAYERS variable within the conf/bblayers.conf file. During the processing of each conf/layer.conf file, BitBake adds the recipes, classes and configurations contained within the particular layer to the source directory.

A recipe that appends Metadata to another recipe is called a BitBake append file. A BitBake append file uses the .bbappend file type suffix, while the corresponding recipe to which Metadata is being appended uses the .bb file type suffix.

You can use a .bbappend file in your layer to make additions or changes to the content of another layer's recipe without having to copy the other layer's recipe into your layer. Your .bbappend file resides in your layer, while the main .bb recipe file to which you are appending Metadata resides in a different layer.

Being able to append information to an existing recipe not only avoids duplication, but also automatically applies recipe changes from a different layer into your layer. If you were copying recipes, you would have to manually merge changes as they occur.

When you create an append file, you must use the same root name as the corresponding recipe file. For example, the append file someapp_2.5.3.bbappend must apply to someapp_2.5.3.bb. This means the original recipe and append file names are version number-specific. If the corresponding recipe is renamed to update to a newer version, you must also rename and possibly update the corresponding .bbappend as well. During the build process, BitBake displays an error on starting if it detects a .bbappend file that does not have a corresponding recipe with a matching name. See the BB_DANGLINGAPPENDS_WARNONLY variable for information on how to handle this error.

As an example, consider the main formfactor recipe and a corresponding formfactor append file both from the Source Directory. Here is the main formfactor recipe, which is named formfactor_0.0.bb and located in the "meta" layer at meta/recipes-bsp/formfactor:

     SUMMARY = "Device formfactor information"
     SECTION = "base"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
     PR = "r45"

     SRC_URI = "file://config file://machconfig"
     S = "${WORKDIR}"

     PACKAGE_ARCH = "${MACHINE_ARCH}"
     INHIBIT_DEFAULT_DEPS = "1"

     do_install() {
	     # Install file only if it has contents
             install -d ${D}${sysconfdir}/formfactor/
             install -m 0644 ${S}/config ${D}${sysconfdir}/formfactor/
	     if [ -s "${S}/machconfig" ]; then
	             install -m 0644 ${S}/machconfig ${D}${sysconfdir}/formfactor/
	     fi
     }                

In the main recipe, note the SRC_URI variable, which tells the OpenEmbedded build system where to find files during the build.

Following is the append file, which is named formfactor_0.0.bbappend and is from the Raspberry Pi BSP Layer named meta-raspberrypi. The file is in the layer at recipes-bsp/formfactor:

     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
                

By default, the build system uses the FILESPATH variable to locate files. This append file extends the locations by setting the FILESEXTRAPATHS variable. Setting this variable in the .bbappend file is the most reliable and recommended method for adding directories to the search path used by the build system to find files.

The statement in this example extends the directories to include ${THISDIR}/${PN}, which resolves to a directory named formfactor in the same directory in which the append file resides (i.e. meta-raspberrypi/recipes-bsp/formfactor. This implies that you must have the supporting directory structure set up that will contain any files or patches you will be including from the layer.

Using the immediate expansion assignment operator := is important because of the reference to THISDIR. The trailing colon character is important as it ensures that items in the list remain colon-separated.

Each layer is assigned a priority value. Priority values control which layer takes precedence if there are recipe files with the same name in multiple layers. For these cases, the recipe file from the layer with a higher priority number takes precedence. Priority values also affect the order in which multiple .bbappend files for the same recipe are applied. You can either specify the priority manually, or allow the build system to calculate it based on the layer's dependencies.

To specify the layer's priority manually, use the BBFILE_PRIORITY variable and append the layer's root name:

     BBFILE_PRIORITY_mylayer = "1"
                

You can use the BitBake layer management tool bitbake-layers to provide a view into the structure of recipes across a multi-layer project. Being able to generate output that reports on configured layers with their paths and priorities and on .bbappend files and their applicable recipes can help to reveal potential problems.

For help on the BitBake layer management tool, use the following command:

     $ bitbake-layers --help
     NOTE: Starting bitbake server...
     usage: bitbake-layers [-d] [-q] [-F] [--color COLOR] [-h] <subcommand> ...

     BitBake layers utility

     optional arguments:
       -d, --debug           Enable debug output
       -q, --quiet           Print only errors
       -F, --force           Force add without recipe parse verification
       --color COLOR         Colorize output (where COLOR is auto, always, never)
       -h, --help            show this help message and exit

     subcommands:
       <subcommand>
         show-layers         show current configured layers.
         show-overlayed      list overlayed recipes (where the same recipe exists
                             in another layer)
         show-recipes        list available recipes, showing the layer they are
                             provided by
         show-appends        list bbappend files and recipe files they apply to
         show-cross-depends  Show dependencies between recipes that cross layer
                             boundaries.
         add-layer           Add one or more layers to bblayers.conf.
         remove-layer        Remove one or more layers from bblayers.conf.
         flatten             flatten layer configuration into a separate output
                             directory.
         layerindex-fetch    Fetches a layer from a layer index along with its
                             dependent layers, and adds them to conf/bblayers.conf.
         layerindex-show-depends
                             Find layer dependencies from layer index.
         create-layer        Create a basic layer

     Use bitbake-layers <subcommand> --help to get help on a specific command
                

The following list describes the available commands:

The bitbake-layers script with the create-layer subcommand simplifies creating a new general layer.

The default mode of the script's operation with this subcommand is to create a layer with the following:

In its simplest form, you can use the following command form to create a layer. The command creates a layer whose name corresponds to your_layer_name in the current directory:

     $ bitbake-layers create-layer your_layer_name
                

As an example, the following command creates a layer named meta-scottrif in your home directory:

     $ cd /usr/home
     $ bitbake-layers create-layer meta-scottrif
     NOTE: Starting bitbake server...
     Add your new layer with 'bitbake-layers add-layer meta-scottrif'
                

If you want to set the priority of the layer to other than the default value of "6", you can either use the ‐‐priority option or you can edit the BBFILE_PRIORITY value in the conf/layer.conf after the script creates it. Furthermore, if you want to give the example recipe file some name other than the default, you can use the ‐‐example-recipe-name option.

The easiest way to see how the bitbake-layers create-layer command works is to experiment with the script. You can also read the usage information by entering the following:

     $ bitbake-layers create-layer --help
     NOTE: Starting bitbake server...
     usage: bitbake-layers create-layer [-h] [--priority PRIORITY]
                                        [--example-recipe-name EXAMPLERECIPE]
                                        layerdir

     Create a basic layer

     positional arguments:
       layerdir              Layer directory to create

     optional arguments:
       -h, --help            show this help message and exit
       --priority PRIORITY, -p PRIORITY
                             Layer directory to create
       --example-recipe-name EXAMPLERECIPE, -e EXAMPLERECIPE
                             Filename of the example recipe
                

Once you create your general layer, you must add it to your bblayers.conf file. Adding the layer to this configuration file makes the OpenEmbedded build system aware of your layer so that it can search it for metadata.

Add your layer by using the bitbake-layers add-layer command:

     $ bitbake-layers add-layer your_layer_name
                

Here is an example that adds a layer named meta-scottrif to the configuration file. Following the command that adds the layer is another bitbake-layers command that shows the layers that are in your bblayers.conf file:

     $ bitbake-layers add-layer meta-scottrif
     NOTE: Starting bitbake server...
     Parsing recipes: 100% |##########################################################| Time: 0:00:49
     Parsing of 1441 .bb files complete (0 cached, 1441 parsed). 2055 targets, 56 skipped, 0 masked, 0 errors.
     $ bitbake-layers show-layers
     NOTE: Starting bitbake server...
     layer                 path                                      priority
     ==========================================================================
     meta                  /home/scottrif/poky/meta                  5
     meta-poky             /home/scottrif/poky/meta-poky             5
     meta-yocto-bsp        /home/scottrif/poky/meta-yocto-bsp        5
     workspace             /home/scottrif/poky/build/workspace       99
     meta-scottrif         /home/scottrif/poky/build/meta-scottrif   6
                

Adding the layer to this file enables the build system to locate the layer during the build.

Probably the easiest way to customize an image is to add a package by way of the local.conf configuration file. Because it is limited to local use, this method generally only allows you to add packages and is not as flexible as creating your own customized image. When you add packages using local variables this way, you need to realize that these variable changes are in effect for every build and consequently affect all images, which might not be what you require.

To add a package to your image using the local configuration file, use the IMAGE_INSTALL variable with the _append operator:

     IMAGE_INSTALL_append = " strace"
                

Use of the syntax is important - specifically, the space between the quote and the package name, which is strace in this example. This space is required since the _append operator does not add the space.

Furthermore, you must use _append instead of the += operator if you want to avoid ordering issues. The reason for this is because doing so unconditionally appends to the variable and avoids ordering problems due to the variable being set in image recipes and .bbclass files with operators like ?=. Using _append ensures the operation takes affect.

As shown in its simplest use, IMAGE_INSTALL_append affects all images. It is possible to extend the syntax so that the variable applies to a specific image only. Here is an example:

     IMAGE_INSTALL_append_pn-core-image-minimal = " strace"
                

This example adds strace to the core-image-minimal image only.

You can add packages using a similar approach through the CORE_IMAGE_EXTRA_INSTALL variable. If you use this variable, only core-image-* images are affected.

Another method for customizing your image is to enable or disable high-level image features by using the IMAGE_FEATURES and EXTRA_IMAGE_FEATURES variables. Although the functions for both variables are nearly equivalent, best practices dictate using IMAGE_FEATURES from within a recipe and using EXTRA_IMAGE_FEATURES from within your local.conf file, which is found in the Build Directory.

To understand how these features work, the best reference is meta/classes/core-image.bbclass. This class lists out the available IMAGE_FEATURES of which most map to package groups while some, such as debug-tweaks and read-only-rootfs, resolve as general configuration settings.

In summary, the file looks at the contents of the IMAGE_FEATURES variable and then maps or configures the feature accordingly. Based on this information, the build system automatically adds the appropriate packages or configurations to the IMAGE_INSTALL variable. Effectively, you are enabling extra features by extending the class or creating a custom class for use with specialized image .bb files.

Use the EXTRA_IMAGE_FEATURES variable from within your local configuration file. Using a separate area from which to enable features with this variable helps you avoid overwriting the features in the image recipe that are enabled with IMAGE_FEATURES. The value of EXTRA_IMAGE_FEATURES is added to IMAGE_FEATURES within meta/conf/bitbake.conf.

To illustrate how you can use these variables to modify your image, consider an example that selects the SSH server. The Yocto Project ships with two SSH servers you can use with your images: Dropbear and OpenSSH. Dropbear is a minimal SSH server appropriate for resource-constrained environments, while OpenSSH is a well-known standard SSH server implementation. By default, the core-image-sato image is configured to use Dropbear. The core-image-full-cmdline and core-image-lsb images both include OpenSSH. The core-image-minimal image does not contain an SSH server.

You can customize your image and change these defaults. Edit the IMAGE_FEATURES variable in your recipe or use the EXTRA_IMAGE_FEATURES in your local.conf file so that it configures the image you are working with to include ssh-server-dropbear or ssh-server-openssh.

You can also customize an image by creating a custom recipe that defines additional software as part of the image. The following example shows the form for the two lines you need:

     IMAGE_INSTALL = "packagegroup-core-x11-base package1 package2"

     inherit core-image
                

Defining the software using a custom recipe gives you total control over the contents of the image. It is important to use the correct names of packages in the IMAGE_INSTALL variable. You must use the OpenEmbedded notation and not the Debian notation for the names (e.g. glibc-dev instead of libc6-dev).

The other method for creating a custom image is to base it on an existing image. For example, if you want to create an image based on core-image-sato but add the additional package strace to the image, copy the meta/recipes-sato/images/core-image-sato.bb to a new .bb and add the following line to the end of the copy:

     IMAGE_INSTALL += "strace"
                

For complex custom images, the best approach for customizing an image is to create a custom package group recipe that is used to build the image or images. A good example of a package group recipe is meta/recipes-core/packagegroups/packagegroup-base.bb.

If you examine that recipe, you see that the PACKAGES variable lists the package group packages to produce. The inherit packagegroup statement sets appropriate default values and automatically adds -dev, -dbg, and -ptest complementary packages for each package specified in the PACKAGES statement.

For each package you specify in PACKAGES, you can use RDEPENDS and RRECOMMENDS entries to provide a list of packages the parent task package should contain. You can see examples of these further down in the packagegroup-base.bb recipe.

Here is a short, fabricated example showing the same basic pieces:

     DESCRIPTION = "My Custom Package Groups"

     inherit packagegroup

     PACKAGES = "\
         packagegroup-custom-apps \
         packagegroup-custom-tools \
         "

     RDEPENDS_packagegroup-custom-apps = "\
         dropbear \
         portmap \
         psplash"

     RDEPENDS_packagegroup-custom-tools = "\
         oprofile \
         oprofileui-server \
         lttng-tools"

     RRECOMMENDS_packagegroup-custom-tools = "\
         kernel-module-oprofile"
                

In the previous example, two package group packages are created with their dependencies and their recommended package dependencies listed: packagegroup-custom-apps, and packagegroup-custom-tools. To build an image using these package group packages, you need to add packagegroup-custom-apps and/or packagegroup-custom-tools to IMAGE_INSTALL. For other forms of image dependencies see the other areas of this section.

By default, the configured hostname (i.e. /etc/hostname) in an image is the same as the machine name. For example, if MACHINE equals "qemux86", the configured hostname written to /etc/hostname is "qemux86".

You can customize this name by altering the value of the "hostname" variable in the base-files recipe using either an append file or a configuration file. Use the following in an append file:

     hostname="myhostname"
                

Use the following in a configuration file:

     hostname_pn-base-files = "myhostname"
                

Changing the default value of the variable "hostname" can be useful in certain situations. For example, suppose you need to do extensive testing on an image and you would like to easily identify the image under test from existing images with typical default hostnames. In this situation, you could change the default hostname to "testme", which results in all the images using the name "testme". Once testing is complete and you do not need to rebuild the image for test any longer, you can easily reset the default hostname.

Another point of interest is that if you unset the variable, the image will have no default hostname in the filesystem. Here is an example that unsets the variable in a configuration file:

     hostname_pn-base-files = ""
                

Having no default hostname in the filesystem is suitable for environments that use dynamic hostnames such as virtual machines.

The following figure shows the basic process for creating a new recipe. The remainder of the section provides details for the steps.

You can always write a recipe from scratch. However, three choices exist that can help you quickly get a start on a new recipe:

     $ recipetool -h
     NOTE: Starting bitbake server...
     usage: recipetool [-d] [-q] [--color COLOR] [-h] <subcommand> ...

     OpenEmbedded recipe tool

     options:
       -d, --debug     Enable debug output
       -q, --quiet     Print only errors
       --color COLOR   Colorize output (where COLOR is auto, always, never)
       -h, --help      show this help message and exit

     subcommands:
       create          Create a new recipe
       newappend       Create a bbappend for the specified target in the specified
                       layer
       setvar          Set a variable within a recipe
       appendfile      Create/update a bbappend to replace a target file
       appendsrcfiles  Create/update a bbappend to add or replace source files
       appendsrcfile   Create/update a bbappend to add or replace a source file
     Use recipetool <subcommand> --help to get help on a specific command
                    

     recipetool create -o OUTFILE source
                    

     recipetool create -o OUTFILE -x EXTERNALSRC source
                    

     recipetool create -d -o OUTFILE source
                    

Once you have your base recipe, you should put it in your own layer and name it appropriately. Locating it correctly ensures that the OpenEmbedded build system can find it when you use BitBake to process the recipe.

Creating a new recipe is usually an iterative process that requires using BitBake to process the recipe multiple times in order to progressively discover and add information to the recipe file.

Assuming you have sourced the build environment setup script (i.e. oe-init-build-env) and you are in the Build Directory, use BitBake to process your recipe. All you need to provide is the basename of the recipe as described in the previous section:

     $ bitbake basename
                

During the build, the OpenEmbedded build system creates a temporary work directory for each recipe (${WORKDIR}) where it keeps extracted source files, log files, intermediate compilation and packaging files, and so forth.

The path to the per-recipe temporary work directory depends on the context in which it is being built. The quickest way to find this path is to have BitBake return it by running the following:

     $ bitbake -e basename | grep ^WORKDIR=
                

As an example, assume a Source Directory top-level folder named poky, a default Build Directory at poky/build, and a qemux86-poky-linux machine target system. Furthermore, suppose your recipe is named foo_1.3.0.bb. In this case, the work directory the build system uses to build the package would be as follows:

     poky/build/tmp/work/qemux86-poky-linux/foo/1.3.0-r0
                

Inside this directory you can find sub-directories such as image, packages-split, and temp. After the build, you can examine these to determine how well the build went.

You can find more information about the build process in "The Yocto Project Development Environment" chapter of the Yocto Project Overview and Concepts Manual.

The first thing your recipe must do is specify how to fetch the source files. Fetching is controlled mainly through the SRC_URI variable. Your recipe must have a SRC_URI variable that points to where the source is located. For a graphical representation of source locations, see the "Sources" section in the Yocto Project Overview and Concepts Manual.

The do_fetch task uses the prefix of each entry in the SRC_URI variable value to determine which fetcher to use to get your source files. It is the SRC_URI variable that triggers the fetcher. The do_patch task uses the variable after source is fetched to apply patches. The OpenEmbedded build system uses FILESOVERRIDES for scanning directory locations for local files in SRC_URI.

The SRC_URI variable in your recipe must define each unique location for your source files. It is good practice to not hard-code pathnames in an URL used in SRC_URI. Rather than hard-code these paths, use ${PV}, which causes the fetch process to use the version specified in the recipe filename. Specifying the version in this manner means that upgrading the recipe to a future version is as simple as renaming the recipe to match the new version.

Here is a simple example from the meta/recipes-devtools/cdrtools/cdrtools-native_3.01a20.bb recipe where the source comes from a single tarball. Notice the use of the PV variable:

     SRC_URI = "ftp://ftp.berlios.de/pub/cdrecord/alpha/cdrtools-${PV}.tar.bz2"
                

Files mentioned in SRC_URI whose names end in a typical archive extension (e.g. .tar, .tar.gz, .tar.bz2, .zip, and so forth), are automatically extracted during the do_unpack task. For another example that specifies these types of files, see the "Autotooled Package" section.

Another way of specifying source is from an SCM. For Git repositories, you must specify SRCREV and you should specify PV to include the revision with SRCPV. Here is an example from the recipe meta/recipes-kernel/blktrace/blktrace_git.bb:

     SRCREV = "d6918c8832793b4205ed3bfede78c2f915c23385"

     PR = "r6"
     PV = "1.0.5+git${SRCPV}"

     SRC_URI = "git://git.kernel.dk/blktrace.git \
                file://ldflags.patch"
                

If your SRC_URI statement includes URLs pointing to individual files fetched from a remote server other than a version control system, BitBake attempts to verify the files against checksums defined in your recipe to ensure they have not been tampered with or otherwise modified since the recipe was written. Two checksums are used: SRC_URI[md5sum] and SRC_URI[sha256sum].

If your SRC_URI variable points to more than a single URL (excluding SCM URLs), you need to provide the md5 and sha256 checksums for each URL. For these cases, you provide a name for each URL as part of the SRC_URI and then reference that name in the subsequent checksum statements. Here is an example:

     SRC_URI = "${DEBIAN_MIRROR}/main/a/apmd/apmd_3.2.2.orig.tar.gz;name=tarball \
                ${DEBIAN_MIRROR}/main/a/apmd/apmd_${PV}.diff.gz;name=patch"

     SRC_URI[tarball.md5sum] = "b1e6309e8331e0f4e6efd311c2d97fa8"
     SRC_URI[tarball.sha256sum] = "7f7d9f60b7766b852881d40b8ff91d8e39fccb0d1d913102a5c75a2dbb52332d"

     SRC_URI[patch.md5sum] = "57e1b689264ea80f78353519eece0c92"
     SRC_URI[patch.sha256sum] = "7905ff96be93d725544d0040e425c42f9c05580db3c272f11cff75b9aa89d430"
                

Proper values for md5 and sha256 checksums might be available with other signatures on the download page for the upstream source (e.g. md5, sha1, sha256, GPG, and so forth). Because the OpenEmbedded build system only deals with sha256sum and md5sum, you should verify all the signatures you find by hand.

If no SRC_URI checksums are specified when you attempt to build the recipe, or you provide an incorrect checksum, the build will produce an error for each missing or incorrect checksum. As part of the error message, the build system provides the checksum string corresponding to the fetched file. Once you have the correct checksums, you can copy and paste them into your recipe and then run the build again to continue.

This final example is a bit more complicated and is from the meta/recipes-sato/rxvt-unicode/rxvt-unicode_9.20.bb recipe. The example's SRC_URI statement identifies multiple files as the source files for the recipe: a tarball, a patch file, a desktop file, and an icon.

     SRC_URI = "http://dist.schmorp.de/rxvt-unicode/Attic/rxvt-unicode-${PV}.tar.bz2 \
                file://xwc.patch \
                file://rxvt.desktop \
                file://rxvt.png"
                

When you specify local files using the file:// URI protocol, the build system fetches files from the local machine. The path is relative to the FILESPATH variable and searches specific directories in a certain order: ${BP}, ${BPN}, and files. The directories are assumed to be subdirectories of the directory in which the recipe or append file resides. For another example that specifies these types of files, see the "Single .c File Package (Hello World!)" section.

The previous example also specifies a patch file. Patch files are files whose names usually end in .patch or .diff but can end with compressed suffixes such as diff.gz and patch.bz2, for example. The build system automatically applies patches as described in the "Patching Code" section.

During the build, the do_unpack task unpacks the source with ${S} pointing to where it is unpacked.

If you are fetching your source files from an upstream source archived tarball and the tarball's internal structure matches the common convention of a top-level subdirectory named ${BPN}-${PV}, then you do not need to set S. However, if SRC_URI specifies to fetch source from an archive that does not use this convention, or from an SCM like Git or Subversion, your recipe needs to define S.

If processing your recipe using BitBake successfully unpacks the source files, you need to be sure that the directory pointed to by ${S} matches the structure of the source.

Sometimes it is necessary to patch code after it has been fetched. Any files mentioned in SRC_URI whose names end in .patch or .diff or compressed versions of these suffixes (e.g. diff.gz are treated as patches. The do_patch task automatically applies these patches.

The build system should be able to apply patches with the "-p1" option (i.e. one directory level in the path will be stripped off). If your patch needs to have more directory levels stripped off, specify the number of levels using the "striplevel" option in the SRC_URI entry for the patch. Alternatively, if your patch needs to be applied in a specific subdirectory that is not specified in the patch file, use the "patchdir" option in the entry.

As with all local files referenced in SRC_URI using file://, you should place patch files in a directory next to the recipe either named the same as the base name of the recipe (BP and BPN) or "files".

Your recipe needs to have both the LICENSE and LIC_FILES_CHKSUM variables:

Most software packages have a short list of other packages that they require, which are called dependencies. These dependencies fall into two main categories: build-time dependencies, which are required when the software is built; and runtime dependencies, which are required to be installed on the target in order for the software to run.

Within a recipe, you specify build-time dependencies using the DEPENDS variable. Although nuances exist, items specified in DEPENDS should be names of other recipes. It is important that you specify all build-time dependencies explicitly. If you do not, due to the parallel nature of BitBake's execution, you can end up with a race condition where the dependency is present for one task of a recipe (e.g. do_configure) and then gone when the next task runs (e.g. do_compile).

Another consideration is that configure scripts might automatically check for optional dependencies and enable corresponding functionality if those dependencies are found. This behavior means that to ensure deterministic results and thus avoid more race conditions, you need to either explicitly specify these dependencies as well, or tell the configure script explicitly to disable the functionality. If you wish to make a recipe that is more generally useful (e.g. publish the recipe in a layer for others to use), instead of hard-disabling the functionality, you can use the PACKAGECONFIG variable to allow functionality and the corresponding dependencies to be enabled and disabled easily by other users of the recipe.

Similar to build-time dependencies, you specify runtime dependencies through a variable - RDEPENDS, which is package-specific. All variables that are package-specific need to have the name of the package added to the end as an override. Since the main package for a recipe has the same name as the recipe, and the recipe's name can be found through the ${PN} variable, then you specify the dependencies for the main package by setting RDEPENDS_${PN}. If the package were named ${PN}-tools, then you would set RDEPENDS_${PN}-tools, and so forth.

Some runtime dependencies will be set automatically at packaging time. These dependencies include any shared library dependencies (i.e. if a package "example" contains "libexample" and another package "mypackage" contains a binary that links to "libexample" then the OpenEmbedded build system will automatically add a runtime dependency to "mypackage" on "example"). See the "Automatically Added Runtime Dependencies" section in the Yocto Project Overview and Concepts Manual for further details.

Most software provides some means of setting build-time configuration options before compilation. Typically, setting these options is accomplished by running a configure script with some options, or by modifying a build configuration file.

A major part of build-time configuration is about checking for build-time dependencies and possibly enabling optional functionality as a result. You need to specify any build-time dependencies for the software you are building in your recipe's DEPENDS value, in terms of other recipes that satisfy those dependencies. You can often find build-time or runtime dependencies described in the software's documentation.

The following list provides configuration items of note based on how your software is built:

Once configuration succeeds, it is always good practice to look at the log.do_configure file to ensure that the appropriate options have been enabled and no additional build-time dependencies need to be added to DEPENDS. For example, if the configure script reports that it found something not mentioned in DEPENDS, or that it did not find something that it needed for some desired optional functionality, then you would need to add those to DEPENDS. Looking at the log might also reveal items being checked for, enabled, or both that you do not want, or items not being found that are in DEPENDS, in which case you would need to look at passing extra options to the configure script as needed. For reference information on configure options specific to the software you are building, you can consult the output of the ./configure --help command within ${S} or consult the software's upstream documentation.

If your recipe builds an application that needs to communicate with some device or needs an API into a custom kernel, you will need to provide appropriate header files. Under no circumstances should you ever modify the existing meta/recipes-kernel/linux-libc-headers/linux-libc-headers.inc file. These headers are used to build libc and must not be compromised with custom or machine-specific header information. If you customize libc through modified headers all other applications that use libc thus become affected.

The correct way to interface to a device or custom kernel is to use a separate package that provides the additional headers for the driver or other unique interfaces. When doing so, your application also becomes responsible for creating a dependency on that specific provider.

Consider the following:

For example, suppose you want to modify an existing header that adds I/O control or network support. If the modifications are used by a small number programs, providing a unique version of a header is easy and has little impact. When doing so, bear in mind the guidelines in the previous list.

Consider a case where your kernel is older and you need an older libc ABI. The headers installed by your recipe should still be a standard mainline kernel, not your own custom one.

When you use custom kernel headers you need to get them from STAGING_KERNEL_DIR, which is the directory with kernel headers that are required to build out-of-tree modules. Your recipe will also need the following:

     do_configure[depends] += "virtual/kernel:do_shared_workdir"
                

During a build, the do_compile task happens after source is fetched, unpacked, and configured. If the recipe passes through do_compile successfully, nothing needs to be done.

However, if the compile step fails, you need to diagnose the failure. Here are some common issues that cause failures.

During do_install, the task copies the built files along with their hierarchy to locations that would mirror their locations on the target device. The installation process copies files from the ${S}, ${B}, and ${WORKDIR} directories to the ${D} directory to create the structure as it should appear on the target system.

How your software is built affects what you must do to be sure your software is installed correctly. The following list describes what you must do for installation depending on the type of build system used by the software being built:

For the scenarios that do not use Autotools or CMake, you need to track the installation and diagnose and fix any issues until everything installs correctly. You need to look in the default location of ${D}, which is ${WORKDIR}/image, to be sure your files have been installed correctly.

If you want to install a service, which is a process that usually starts on boot and runs in the background, then you must include some additional definitions in your recipe.

If you are adding services and the service initialization script or the service file itself is not installed, you must provide for that installation in your recipe using a do_install_append function. If your recipe already has a do_install function, update the function near its end rather than adding an additional do_install_append function.

When you create the installation for your services, you need to accomplish what is normally done by make install. In other words, make sure your installation arranges the output similar to how it is arranged on the target system.

The OpenEmbedded build system provides support for starting services two different ways:

Successful packaging is a combination of automated processes performed by the OpenEmbedded build system and some specific steps you need to take. The following list describes the process:

Recipes often need to use files provided by other recipes on the build host. For example, an application linking to a common library needs access to the library itself and its associated headers. The way this access is accomplished is by populating a sysroot with files. Each recipe has two sysroots in its work directory, one for target files (recipe-sysroot) and one for files that are native to the build host (recipe-sysroot-native).

Recipes should never populate the sysroot directly (i.e. write files into sysroot). Instead, files should be installed into standard locations during the do_install task within the ${D} directory. The reason for this limitation is that almost all files that populate the sysroot are cataloged in manifests in order to ensure the files can be removed later when a recipe is either modified or removed. Thus, the sysroot is able to remain free from stale files.

A subset of the files installed by the do_install task are used by the do_populate_sysroot task as defined by the the SYSROOT_DIRS variable to automatically populate the sysroot. It is possible to modify the list of directories that populate the sysroot. The following example shows how you could add the /opt directory to the list of directories within a recipe:

     SYSROOT_DIRS += "/opt"
                

For a more complete description of the do_populate_sysroot task and its associated functions, see the staging class.

Prior to a build, if you know that several different recipes provide the same functionality, you can use a virtual provider (i.e. virtual/*) as a placeholder for the actual provider. The actual provider is determined at build-time.

A common scenario where a virtual provider is used would be for the kernel recipe. Suppose you have three kernel recipes whose PN values map to kernel-big, kernel-mid, and kernel-small. Furthermore, each of these recipes in some way uses a PROVIDES statement that essentially identifies itself as being able to provide virtual/kernel. Here is one way through the kernel class:

     PROVIDES += "${@ "virtual/kernel" if (d.getVar("KERNEL_PACKAGE_NAME") == "kernel") else "" }"
                

Any recipe that inherits the kernel class is going to utilize a PROVIDES statement that identifies that recipe as being able to provide the virtual/kernel item.

Now comes the time to actually build an image and you need a kernel recipe, but which one? You can configure your build to call out the kernel recipe you want by using the PREFERRED_PROVIDER variable. As an example, consider the x86-base.inc include file, which is a machine (i.e. MACHINE) configuration file. This include file is the reason all x86-based machines use the linux-yocto kernel. Here are the relevant lines from the include file:

     PREFERRED_PROVIDER_virtual/kernel ??= "linux-yocto"
     PREFERRED_VERSION_linux-yocto ??= "4.15%"
                

When you use a virtual provider, you do not have to "hard code" a recipe name as a build dependency. You can use the DEPENDS variable to state the build is dependent on virtual/kernel for example:

     DEPENDS = "virtual/kernel"
                

During the build, the OpenEmbedded build system picks the correct recipe needed for the virtual/kernel dependency based on the PREFERRED_PROVIDER variable. If you want to use the small kernel mentioned at the beginning of this section, configure your build as follows:

     PREFERRED_PROVIDER_virtual/kernel ??= "kernel-small"
                

The following lists specific examples of virtual providers:

Sometimes the name of a recipe can lead to versioning problems when the recipe is upgraded to a final release. For example, consider the irssi_0.8.16-rc1.bb recipe file in the list of example recipes in the "Storing and Naming the Recipe" section. This recipe is at a release candidate stage (i.e. "rc1"). When the recipe is released, the recipe filename becomes irssi_0.8.16.bb. The version change from 0.8.16-rc1 to 0.8.16 is seen as a decrease by the build system and package managers, so the resulting packages will not correctly trigger an upgrade.

In order to ensure the versions compare properly, the recommended convention is to set PV within the recipe to "previous_version+current_version". You can use an additional variable so that you can use the current version elsewhere. Here is an example:

     REALPV = "0.8.16-rc1"
     PV = "0.8.15+${REALPV}"
                

Post-installation scripts run immediately after installing a package on the target or during image creation when a package is included in an image. To add a post-installation script to a package, add a pkg_postinst_PACKAGENAME() function to the recipe file (.bb) and replace PACKAGENAME with the name of the package you want to attach to the postinst script. To apply the post-installation script to the main package for the recipe, which is usually what is required, specify ${PN} in place of PACKAGENAME.

A post-installation function has the following structure:

     pkg_postinst_PACKAGENAME() {
     # Commands to carry out
     }
                

The script defined in the post-installation function is called when the root filesystem is created. If the script succeeds, the package is marked as installed. If the script fails, the package is marked as unpacked and the script is executed when the image boots again.

Sometimes it is necessary for the execution of a post-installation script to be delayed until the first boot. For example, the script might need to be executed on the device itself. To delay script execution until boot time, use the following structure in the post-installation script:

     pkg_postinst_PACKAGENAME() {
     if [ x"$D" = "x" ]; then
          # Actions to carry out on the device go here
     else
          exit 1
     fi
     }
                

The previous example delays execution until the image boots again because the environment variable D points to the directory containing the image when the root filesystem is created at build time but is unset when executed on the first boot.

If you have recipes that use pkg_postinst scripts and they require the use of non-standard native tools that have dependencies during rootfs construction, you need to use the PACKAGE_WRITE_DEPS variable in your recipe to list these tools. If you do not use this variable, the tools might be missing and execution of the post-installation script is deferred until first boot. Deferring the script to first boot is undesirable and for read-only rootfs impossible.

The final step for completing your recipe is to be sure that the software you built runs correctly. To accomplish runtime testing, add the build's output packages to your image and test them on the target.

For information on how to customize your image by adding specific packages, see the "Customizing Images" section.

To help summarize how to write a recipe, this section provides some examples given various scenarios:

     SUMMARY = "Simple helloworld application"
     SECTION = "examples"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"

     SRC_URI = "file://helloworld.c"

     S = "${WORKDIR}"

     do_compile() {
     	${CC} helloworld.c -o helloworld
     }

     do_install() {
     	install -d ${D}${bindir}
     	install -m 0755 helloworld ${D}${bindir}
     }
                    

     SUMMARY = "GNU Helloworld application"
     SECTION = "examples"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=751419260aa954499f7abaabaa882bbe"

     SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.gz"

     inherit autotools gettext
                     

     CFLAGS_prepend = "-I ${S}/include "
                    

     SUMMARY = "Tools for managing memory technology devices"
     SECTION = "base"
     DEPENDS = "zlib lzo e2fsprogs util-linux"
     HOMEPAGE = "http://www.linux-mtd.infradead.org/"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=0636e73ff0215e8d672dc4c32c317bb3 \
                         file://include/common.h;beginline=1;endline=17;md5=ba05b07912a44ea2bf81ce409380049c"

     # Use the latest version at 26 Oct, 2013
     SRCREV = "9f107132a6a073cce37434ca9cda6917dd8d866b"
     SRC_URI = "git://git.infradead.org/mtd-utils.git \
                     file://add-exclusion-to-mkfs-jffs2-git-2.patch \
     "

     PV = "1.5.1+git${SRCPV}"

     S = "${WORKDIR}/git"

     EXTRA_OEMAKE = "'CC=${CC}' 'RANLIB=${RANLIB}' 'AR=${AR}' 'CFLAGS=${CFLAGS} -I${S}/include -DWITHOUT_XATTR' 'BUILDDIR=${S}'"

     do_install () {
             oe_runmake install DESTDIR=${D} SBINDIR=${sbindir} MANDIR=${mandir} INCLUDEDIR=${includedir}
     }

     PACKAGES =+ "mtd-utils-jffs2 mtd-utils-ubifs mtd-utils-misc"

     FILES_mtd-utils-jffs2 = "${sbindir}/mkfs.jffs2 ${sbindir}/jffs2dump ${sbindir}/jffs2reader ${sbindir}/sumtool"
     FILES_mtd-utils-ubifs = "${sbindir}/mkfs.ubifs ${sbindir}/ubi*"
     FILES_mtd-utils-misc = "${sbindir}/nftl* ${sbindir}/ftl* ${sbindir}/rfd* ${sbindir}/doc* ${sbindir}/serve_image ${sbindir}/recv_image"

     PARALLEL_MAKE = ""

     BBCLASSEXTEND = "native"
                    

     require xorg-lib-common.inc

     SUMMARY = "Xpm: X Pixmap extension library"
     LICENSE = "BSD"
     LIC_FILES_CHKSUM = "file://COPYING;md5=51f4270b012ecd4ab1a164f5f4ed6cf7"
     DEPENDS += "libxext libsm libxt"
     PE = "1"

     XORG_PN = "libXpm"

     PACKAGES =+ "sxpm cxpm"
     FILES_cxpm = "${bindir}/cxpm"
     FILES_sxpm = "${bindir}/sxpm"
                    

When writing recipes, it is good to conform to existing style guidelines. The OpenEmbedded Styleguide wiki page provides rough guidelines for preferred recipe style.

It is common for existing recipes to deviate a bit from this style. However, aiming for at least a consistent style is a good idea. Some practices, such as omitting spaces around = operators in assignments or ordering recipe components in an erratic way, are widely seen as poor style.

Understanding recipe file syntax is important for writing recipes. The following list overviews the basic items that make up a BitBake recipe file. For more complete BitBake syntax descriptions, see the "Syntax and Operators" chapter of the BitBake User Manual.

This next list summarizes the most important and most commonly used parts of the recipe syntax. For more information on these parts of the syntax, you can reference the Syntax and Operators chapter in the BitBake User Manual.

To add a new machine, you need to add a new machine configuration file to the layer's conf/machine directory. This configuration file provides details about the device you are adding.

The OpenEmbedded build system uses the root name of the machine configuration file to reference the new machine. For example, given a machine configuration file named crownbay.conf, the build system recognizes the machine as "crownbay".

The most important variables you must set in your machine configuration file or include from a lower-level configuration file are as follows:

You might also need these variables:

You can find full details on these variables in the reference section. You can leverage existing machine .conf files from meta-yocto-bsp/conf/machine/.

The OpenEmbedded build system needs to be able to build a kernel for the machine. You need to either create a new kernel recipe for this machine, or extend an existing kernel recipe. You can find several kernel recipe examples in the Source Directory at meta/recipes-kernel/linux that you can use as references.

If you are creating a new kernel recipe, normal recipe-writing rules apply for setting up a SRC_URI. Thus, you need to specify any necessary patches and set S to point at the source code. You need to create a do_configure task that configures the unpacked kernel with a defconfig file. You can do this by using a make defconfig command or, more commonly, by copying in a suitable defconfig file and then running make oldconfig. By making use of inherit kernel and potentially some of the linux-*.inc files, most other functionality is centralized and the defaults of the class normally work well.

If you are extending an existing kernel recipe, it is usually a matter of adding a suitable defconfig file. The file needs to be added into a location similar to defconfig files used for other machines in a given kernel recipe. A possible way to do this is by listing the file in the SRC_URI and adding the machine to the expression in COMPATIBLE_MACHINE:

     COMPATIBLE_MACHINE = '(qemux86|qemumips)'
                

For more information on defconfig files, see the "Changing the Configuration" section in the Yocto Project Linux Kernel Development Manual.

A formfactor configuration file provides information about the target hardware for which the image is being built and information that the build system cannot obtain from other sources such as the kernel. Some examples of information contained in a formfactor configuration file include framebuffer orientation, whether or not the system has a keyboard, the positioning of the keyboard in relation to the screen, and the screen resolution.

The build system uses reasonable defaults in most cases. However, if customization is necessary, you need to create a machconfig file in the meta/recipes-bsp/formfactor/files directory. This directory contains directories for specific machines such as qemuarm and qemux86. For information about the settings available and the defaults, see the meta/recipes-bsp/formfactor/files/config file found in the same area.

Following is an example for "qemuarm" machine:

     HAVE_TOUCHSCREEN=1
     HAVE_KEYBOARD=1

     DISPLAY_CAN_ROTATE=0
     DISPLAY_ORIENTATION=0
     #DISPLAY_WIDTH_PIXELS=640
     #DISPLAY_HEIGHT_PIXELS=480
     #DISPLAY_BPP=16
     DISPLAY_DPI=150
     DISPLAY_SUBPIXEL_ORDER=vrgb
                

The AUH utility works in conjunction with the OpenEmbedded build system in order to automatically generate upgrades for recipes based on new versions being published upstream. Use AUH when you want to create a service that performs the upgrades automatically and optionally sends you an email with the results.

AUH allows you to update several recipes with a single use. You can also optionally perform build and integration tests using images with the results saved to your hard drive and emails of results optionally sent to recipe maintainers. Finally, AUH creates Git commits with appropriate commit messages in the layer's tree for the changes made to recipes.

The following steps describe how to set up the AUH utility:

This next set of examples describes how to use the AUH:

Once you have run the AUH utility, you can find the results in the AUH build directory:

     ${BUILDDIR}/upgrade-helper/timestamp
                

The AUH utility also creates recipe update commits from successful upgrade attempts in the layer tree.

You can easily set up to run the AUH utility on a regular basis by using a cron job. See the weeklyjob.sh file distributed with the utility for an example.

As mentioned earlier, an alternative method for upgrading recipes to newer versions is to use devtool upgrade. You can read about devtool upgrade in general in the "Use devtool upgrade to Create a Version of the Recipe that Supports a Newer Version of the Software" section in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) Manual.

To see all the command-line options available with devtool upgrade, use the following help command:

     $ devtool upgrade -h
                

If you want to find out what version a recipe is currently at upstream without any attempt to upgrade your local version of the recipe, you can use the following command:

     $ devtool latest-version recipe_name
                

As mentioned in the previous section describing AUH, devtool upgrade works in a less-automated manner than AUH. Specifically, devtool upgrade only works on a single recipe that you name on the command line, cannot perform build and integration testing using images, and does not automatically generate commits for changes in the source tree. Despite all these "limitations", devtool upgrade updates the recipe file to the new upstream version and attempts to rebase custom patches contained by the recipe as needed.

A typical scenario involves having used Git to clone an upstream repository that you use during build operations. Because you are (or have) built the recipe in the past, the layer is likely added to your configuration already. If for some reason, the layer is not added, you could add it easily using the bitbake-layers script. For example, suppose you use the nano.bb recipe from the meta-oe layer in the meta-openembedded repository. For this example, assume that the layer has been cloned into following area:

     /home/scottrif/meta-openembedded
                

The following command from your Build Directory adds the layer to your build configuration (i.e. ${BUILDDIR}/conf/bblayers.conf):

     $ bitbake-layers add-layer /home/scottrif/meta-openembedded/meta-oe
     NOTE: Starting bitbake server...
     Parsing recipes: 100% |##########################################| Time: 0:00:55
     Parsing of 1431 .bb files complete (0 cached, 1431 parsed). 2040 targets, 56 skipped, 0 masked, 0 errors.
     Removing 12 recipes from the x86_64 sysroot: 100% |##############| Time: 0:00:00
     Removing 1 recipes from the x86_64_i586 sysroot: 100% |##########| Time: 0:00:00
     Removing 5 recipes from the i586 sysroot: 100% |#################| Time: 0:00:00
     Removing 5 recipes from the qemux86 sysroot: 100% |##############| Time: 0:00:00
                

For this example, assume that the nano.bb recipe that is upstream has a 2.9.3 version number. However, the version in the local repository is 2.7.4. The following command from your build directory automatically upgrades the recipe for you:

     $ devtool upgrade nano -V 2.9.3
     NOTE: Starting bitbake server...
     NOTE: Creating workspace layer in /home/scottrif/poky/build/workspace
     Parsing recipes: 100% |##########################################| Time: 0:00:46
     Parsing of 1431 .bb files complete (0 cached, 1431 parsed). 2040 targets, 56 skipped, 0 masked, 0 errors.
     NOTE: Extracting current version source...
     NOTE: Resolving any missing task queue dependencies
            .
            .
            .
     NOTE: Executing SetScene Tasks
     NOTE: Executing RunQueue Tasks
     NOTE: Tasks Summary: Attempted 74 tasks of which 72 didn't need to be rerun and all succeeded.
     Adding changed files: 100% |#####################################| Time: 0:00:00
     NOTE: Upgraded source extracted to /home/scottrif/poky/build/workspace/sources/nano
     NOTE: New recipe is /home/scottrif/poky/build/workspace/recipes/nano/nano_2.9.3.bb
                

Continuing with this example, you can use devtool build to build the newly upgraded recipe:

     $ devtool build nano
     NOTE: Starting bitbake server...
     Loading cache: 100% |################################################################################################| Time: 0:00:01
     Loaded 2040 entries from dependency cache.
     Parsing recipes: 100% |##############################################################################################| Time: 0:00:00
     Parsing of 1432 .bb files complete (1431 cached, 1 parsed). 2041 targets, 56 skipped, 0 masked, 0 errors.
     NOTE: Resolving any missing task queue dependencies
            .
            .
            .
     NOTE: Executing SetScene Tasks
     NOTE: Executing RunQueue Tasks
     NOTE: nano: compiling from external source tree /home/scottrif/poky/build/workspace/sources/nano
     NOTE: Tasks Summary: Attempted 520 tasks of which 304 didn't need to be rerun and all succeeded.
                

Within the devtool upgrade workflow, opportunity exists to deploy and test your rebuilt software. For this example, however, running devtool finish cleans up the workspace once the source in your workspace is clean. This usually means using Git to stage and submit commits for the changes generated by the upgrade process.

Once the tree is clean, you can clean things up in this example with the following command from the ${BUILDDIR}/workspace/sources/nano directory:

     $ devtool finish nano meta-oe
     NOTE: Starting bitbake server...
     Loading cache: 100% |################################################################################################| Time: 0:00:00
     Loaded 2040 entries from dependency cache.
     Parsing recipes: 100% |##############################################################################################| Time: 0:00:01
     Parsing of 1432 .bb files complete (1431 cached, 1 parsed). 2041 targets, 56 skipped, 0 masked, 0 errors.
     NOTE: Adding new patch 0001-nano.bb-Stuff-I-changed-when-upgrading-nano.bb.patch
     NOTE: Updating recipe nano_2.9.3.bb
     NOTE: Removing file /home/scottrif/meta-openembedded/meta-oe/recipes-support/nano/nano_2.7.4.bb
     NOTE: Moving recipe file to /home/scottrif/meta-openembedded/meta-oe/recipes-support/nano
     NOTE: Leaving source tree /home/scottrif/poky/build/workspace/sources/nano as-is; if you no longer need it then please delete it manually
                

Using the devtool finish command cleans up the workspace and creates a patch file based on your commits. The tool puts all patch files back into the source directory in a sub-directory named nano in this case.

If for some reason you choose not to upgrade recipes using the Auto Upgrade Helper (AUH) or by using devtool upgrade, you can manually edit the recipe files to upgrade the versions.

To manually upgrade recipe versions, follow these general steps:

In the development environment, you need to build an image whenever you change hardware support, add or change system libraries, or add or change services that have dependencies. Several methods exist that allow you to build an image within the Yocto Project. This section presents the basic steps you need to build a simple image using BitBake from a build host running Linux.

The build process creates an entire Linux distribution from source and places it in your Build Directory under tmp/deploy/images. For detailed information on the build process using BitBake, see the "Images" section in the Yocto Project Overview and Concepts Manual.

The following figure and list overviews the build process:

Bitbake also has functionality that allows you to build multiple targets at the same time, where each target uses a different configuration.

In order to accomplish this, you setup each of the configurations you need to use in parallel by placing the configuration files in your current build directory alongside the usual local.conf file.

Follow these guidelines to create an environment that supports multiple configurations:

Support for multiple configurations in this current release of the Yocto Project (Sumo 2.5.3) has some known issues:

An initial RAM filesystem (initramfs) image provides a temporary root filesystem used for early system initialization (e.g. loading of modules needed to locate and mount the "real" root filesystem).

Follow these steps to create an initramfs image:

Very small distributions have some significant advantages such as requiring less on-die or in-package memory (cheaper), better performance through efficient cache usage, lower power requirements due to less memory, faster boot times, and reduced development overhead. Some real-world examples where a very small distribution gives you distinct advantages are digital cameras, medical devices, and small headless systems.

This section presents information that shows you how you can trim your distribution to even smaller sizes than the poky-tiny distribution, which is around 5 Mbytes, that can be built out-of-the-box using the Yocto Project.