# A Closer Look at the Yocto Project Development Environment

This chapter takes a more detailed look at the Yocto Project development environment. The following diagram represents the development environment at a high level. The remainder of this chapter expands on the fundamental input, output, process, and Metadata) blocks in the Yocto Project development environment.

The generalized Yocto Project Development Environment consists of several functional areas:

- User Configuration: Metadata you can use to control the build process.

- Metadata Layers: Various layers that provide software, machine, and distro Metadata.

- Source Files: Upstream releases, local projects, and SCMs.

- Build System: Processes under the control of BitBake. This block expands on how BitBake fetches source, applies patches, completes compilation, analyzes output for package generation, creates and tests packages, generates images, and generates cross-development tools.

- Package Feeds: Directories containing output packages (RPM, DEB or IPK), which are subsequently used in the construction of an image or SDK, produced by the build system. These feeds can also be copied and shared using a web server or other means to facilitate extending or updating existing images on devices at runtime if runtime package management is enabled.

- Images: Images produced by the development process.

- Application Development SDK: Cross-development tools that are produced along with an image or separately with BitBake.

## User Configuration

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 Yocto Project Development Environment 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 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 Poky repository is primarily an aggregation of existing repositories. It is not a canonical upstream source.

The `meta-yocto` layer inside Poky contains a `conf` directory that has example configuration files. These example files are used as a basis for creating actual configuration files when you source the build environment script (i.e. `&OE_INIT_FILE;` or `oe-init-build-env-memres`).

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_FILE;` or `oe-init-build-env-memres` script in the context of separate OpenEmbedded-Core and BitBake repositories rather than a single Poky repository. This discussion assumes the script is executed from within a cloned or unpacked version of Poky.

Depending on where the script is sourced, different sub-scripts are called to set up the Build Directory (Yocto or OpenEmbedded). Specifically, the script `scripts/oe-setup-builddir` inside the poky directory sets up the Build Directory and seeds the directory (if necessary) with configuration files appropriate for the Yocto Project development environment.

The `scripts/oe-setup-builddir` script uses the `$TEMPLATECONF` variable to determine which sample configuration files to locate.

The `local.conf` file provides many basic variables that define a build environment. Here is a list of a few. To see the default configurations in a `local.conf` file created by the build environment script, see the `local.conf.sample` in the `meta-yocto` layer:

- Parallelism Options: Controlled by the `BB_NUMBER_THREADS` and `PARALLEL_MAKE` variables.

- Target Machine Selection: Controlled by the `MACHINE` variable.

- Download Directory: Controlled by the `DL_DIR` variable.

- Shared State Directory: Controlled by the `SSTATE_DIR` variable.

- Build Output: Controlled by the `TMPDIR` variable.

Configurations set in the `conf/local.conf` file can also be set in the `conf/site.conf` and `conf/auto.conf` configuration files.

The `bblayers.conf` file tells BitBake what layers you want considered during the build. By default, the layers listed in this file include layers minimally needed by the build system. However, you must manually add any custom layers you have created. You can find more information on working with the `bblayers.conf` file in the "Enabling Your Layer" section in the Yocto Project Development Manual.

The files `site.conf` and `auto.conf` are not created by the environment initialization script. If you want these configuration files, you must create them yourself:

- `site.conf`: You can use the `conf/site.conf` configuration file to configure multiple build directories. For example, suppose you had several build environments and they shared some common features. You can set these default build properties here. A good example is perhaps the level of parallelism you want to use through the `BB_NUMBER_THREADS` and `PARALLEL_MAKE` variables.

  One useful scenario for using the `conf/site.conf` file is to extend your `BBPATH` variable to include the path to a `conf/site.conf`. Then, when BitBake looks for Metadata using `BBPATH`, it finds the `conf/site.conf` file and applies your common configurations found in the file. To override configurations in a particular build directory, alter the similar configurations within that build directory's `conf/local.conf` file.

- `auto.conf`: This file is not hand-created. Rather, the file is usually created and written to by an autobuilder. The settings put into the file are typically the same as you would find in the `conf/local.conf` or the `conf/site.conf` files.

You can edit all configuration files to further define any particular build environment. This process is represented by the "User Configuration Edits" box in the figure.

When you launch your build with the `bitbake <target>` command, BitBake sorts out the configurations to ultimately define your build environment.

## Metadata, Machine Configuration, and Policy Configuration

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 policy.

In general, three types of layer input exist:

- Policy Configuration: Distribution Layers provide top-level or general policies for the image or SDK being built. For example, this layer would dictate whether BitBake produces RPM or IPK packages.

- Machine Configuration: Board Support Package (BSP) layers provide machine configurations. This type of information is specific to a particular target architecture.

- Metadata: Software layers contain user-supplied recipe files, patches, and append files.

The following figure shows an expanded representation of the Metadata, Machine Configuration, and Policy Configuration input (layers) boxes of the general Yocto Project Development Environment figure:

In general, all layers have a similar structure. They all contain a licensing file (e.g. `COPYING`) if the layer is to be distributed, a `README` file as good practice and especially if the layer is to be distributed, a configuration directory, and recipe directories.

The Yocto Project has many layers that can be used. You can see a web-interface listing of them on the Source Repositories page. The layers are shown at the bottom categorized under "Yocto Metadata Layers." These layers are fundamentally a subset of the OpenEmbedded Metadata Index, which lists all layers provided by the OpenEmbedded community.

Layers exist in the Yocto Project Source Repositories that cannot be found in the OpenEmbedded Metadata Index. These layers are either deprecated or experimental in nature.

BitBake uses the `conf/bblayers.conf` file, which is part of the user configuration, to find what layers it should be using as part of the build.

For more information on layers, see the "Understanding and Creating Layers" section in the Yocto Project Development Manual.

### Distro Layer

The distribution layer provides policy configurations for your distribution. Best practices dictate that you isolate these types of configurations into their own layer. Settings you provide in `conf/distro/<distro>.conf` override similar settings that BitBake finds in your `conf/local.conf` file in the Build Directory.

The following list provides some explanation and references for what you typically find in the distribution layer:

- classes: Class files (`.bbclass`) hold common functionality that can be shared among recipes in the distribution. When your recipes inherit a class, they take on the settings and functions for that class. You can read more about class files in the "Classes" section.

- conf: This area holds configuration files for the layer (`conf/layer.conf`), the distribution (`conf/distro/<distro>.conf`), and any distribution-wide include files.

- recipes-*: Recipes and append files that affect common functionality across the distribution. This area could include recipes and append files to add distribution-specific configuration, initialization scripts, custom image recipes, and so forth.

### BSP Layer

The BSP Layer provides machine configurations. Everything in this layer is specific to the machine for which you are building the image or the SDK. A common structure or form is defined for BSP layers. You can learn more about this structure in the Yocto Project Board Support Package (BSP) Developer's Guide.

In order for a BSP layer to be considered compliant with the Yocto Project, it must meet some structural requirements.

The BSP Layer's configuration directory contains configuration files for the machine (`conf/machine/<machine>.conf`) and, of course, the layer (`conf/layer.conf`).

The remainder of the layer is dedicated to specific recipes by function: `recipes-bsp`, `recipes-core`, `recipes-graphics`, and `recipes-kernel`. Metadata can exist for multiple formfactors, graphics support systems, and so forth.

While the figure shows several `recipes-*` directories, not all these directories appear in all BSP layers.

### Software Layer

The software layer provides the Metadata for additional software packages used during the build. This layer does not include Metadata that is specific to the distribution or the machine, which are found in their respective layers.

This layer contains any new recipes that your project needs in the form of recipe files.

## Sources

In order for the OpenEmbedded build system to create an image or any target, it must be able to access source files. The general Yocto Project Development Environment figure represents source files using the "Upstream Project Releases", "Local Projects", and "SCMs (optional)" boxes. The figure represents mirrors, which also play a role in locating source files, with the "Source Mirror(s)" box.

The method by which source files are ultimately organized is a function of the project. For example, for released software, projects tend to use tarballs or other archived files that can capture the state of a release guaranteeing that it is statically represented. On the other hand, for a project that is more dynamic or experimental in nature, a project might keep source files in a repository controlled by a Source Control Manager (SCM) such as Git. Pulling source from a repository allows you to control the point in the repository (the revision) from which you want to build software. Finally, a combination of the two might exist, which would give the consumer a choice when deciding where to get source files.

BitBake uses the `SRC_URI` variable to point to source files regardless of their location. Each recipe must have a `SRC_URI` variable that points to the source.

Another area that plays a significant role in where source files 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 base figure:

### Upstream Project Releases

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

### Local Projects

Local projects are custom bits of software the user provides. These bits reside somewhere local to a project - perhaps a directory into which the user checks in items (e.g. a local directory containing a development source tree used by the group).

The canonical method through which to include a local project is to use the `externalsrc` class to include that local project. You use either the `local.conf` or a recipe's append file to override or set the recipe to point to the local directory on your disk to pull in the whole source tree.

For information on how to use the `externalsrc` class, see the "`externalsrc.bbclass`" section.

### Source Control Managers (Optional)

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

For information on how to have the OpenEmbedded build system generate tarballs for Git repositories and place them in the `DL_DIR` directory, see the `BB_GENERATE_MIRROR_TARBALLS` variable.

When fetching a repository, BitBake uses the `SRCREV` variable to determine the specific revision from which to build.

### Source Mirror(s)

Two kinds of mirrors exist: pre-mirrors and regular mirrors. The `PREMIRRORS` and `MIRRORS` variables point to these, respectively. BitBake checks pre-mirrors before looking upstream for any source files. Pre-mirrors are appropriate when you have a shared directory that is not a directory defined by the `DL_DIR` variable. A Pre-mirror typically points to a shared directory that is local to your organization.

Regular mirrors can be any site across the Internet that is used as an alternative location for source code should the primary site not be functioning for some reason or another.

## Package Feeds

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 Yocto Project Development Environment figure shows this package feeds area in the upper-right corner.

This section looks a little closer into the package feeds area used by the build system. Here is a more detailed look at the area:

Package feeds are an intermediary step in the build process. BitBake generates packages whose types are defined by the `PACKAGE_CLASSES` variable. Before placing the packages into package feeds, the build process validates them with generated output quality assurance checks through the `insane` class.

The package feed area resides in `tmp/deploy` of the Build Directory. Folders are created that correspond to the package type (IPK, DEB, or RPM) created. Further organization is derived through the value of the `PACKAGE_ARCH` variable for each package. For example, packages can exist for the i586 or qemux86 architectures. The package files themselves reside within the appropriate architecture folder.

BitBake uses the `do_package_write_*` tasks to place generated packages 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 for additional information.

## BitBake

The OpenEmbedded build system uses BitBake to produce images. You can see from the general Yocto Project Development Environment figure, the BitBake area consists of several functional areas. This section takes a closer look at each of those areas.

Separate documentation exists for the BitBake tool. See the BitBake User Manual for reference material on BitBake.

### 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 work directory.

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`. 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.

Unpacked source files are pointed to by the `S` variable. Each recipe has an area in the Build Directory where the unpacked source code resides. The name of that directory for any given recipe is defined from several different variables. You can see the variables that define these directories by looking at the figure:

- `TMPDIR` - The base directory where the OpenEmbedded build system performs all its work during the build.

- `PACKAGE_ARCH` - The architecture of the built package or packages.

- `TARGET_OS` - The operating system of the target device.

- `PN` - The name of the built package.

- `PV` - The version of the recipe used to build the package.

- `PR` - The revision of the recipe used to build the package.

- `WORKDIR` - The location within `TMPDIR` where a specific package is built.

- `S` - Contains the unpacked source files for a given recipe.

### Patching

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

The `do_patch` task processes recipes by using the `SRC_URI` variable to locate applicable patch files, which by default are `*.patch` or `*.diff` files, or any file if "apply=yes" is specified for the file in `SRC_URI`.

BitBake finds and applies multiple patches for a single recipe in the order in which it finds the patches. Patches are applied to the recipe's source files located in the `S` directory.

For more information on how the source directories are created, see the "Source Fetching" section.

### Configuration and Compilation

After source code is patched, BitBake executes tasks that configure and compile the source code:

This step in the build process consists of three tasks:

- `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 source code configuration 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` variable. For information on how this variable works within that class, see the `meta/classes/autotools.bbclass` file.

- `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`: Once compilation is done, 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.

### Package Splitting

After source code is configured and compiled, the OpenEmbedded 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. The analyzing process 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 OpenEmbedded build system can generate the final packages. Working, staged, and intermediate results of the analysis and package splitting process use these areas:

- `PKGD` - The destination directory for packages before they are split.

- `PKGDATA_DIR` - A shared, global-state directory that holds data generated during the packaging process.

- `PKGDESTWORK` - A temporary work area used by the `do_package` task.

- `PKGDEST` - The parent directory for packages after they have been split.

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 the `package` class.

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.

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.

### Image Generation

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

The image generation process consists of several stages and depends on many variables. The `do_rootfs` task uses these key variables to help create the list of packages to actually install:

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

- `PACKAGE_EXCLUDE`: Specifies packages that should not be installed.

- `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 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 installation is under control of the package manager (e.g. smart/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.

During image generation, the build system attempts to run all post-installation scripts. Any 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 during the package installation phase since the root filesystem is read-only.

During Optimization, optimizing processes are run across the image. These processes include `mklibs` and `prelink`. The `mklibs` process optimizes the size of the libraries. A `prelink` process optimizes the dynamic linking of shared libraries to reduce start up time of executables.

Along with writing out the root filesystem image, the `do_rootfs` task creates a manifest file (`.manifest`) in the same directory as the root filesystem image that lists out, line-by-line, the installed packages. This 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.

Part of the image generation process includes compressing the root filesystem image. Compression is accomplished through several optimization routines designed to reduce the overall size of the image.

After the root filesystem has been constructed, the image generation process turns everything into an image file or a set of image files. The formats used for the root filesystem depend on the `IMAGE_FSTYPES` variable.

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

### SDK Generation

The OpenEmbedded build system uses BitBake to generate the Software Development Kit (SDK) installer script:

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 "Optionally Building a Toolchain Installer" section in the Yocto Project Application Developer's Guide.

Like image generation, the SDK script process consists of several stages and depends on many variables. The `do_populate_sdk` task uses 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 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`.

Once both parts are constructed, the `do_populate_sdk` task performs some cleanup on both parts. After the cleanup, the task creates a cross-development environment setup script and any configuration files that might be needed.

The final output of the task is the Cross-development toolchain installation script (`.sh` file), which includes the environment setup script.

## Images

The images produced by the OpenEmbedded build system are compressed forms of the root filesystem that are ready to boot on a target device. You can see from the general Yocto Project Development Environment figure that BitBake output, in part, consists of images. This section is going to look more closely at this output:

For a list of example images that the Yocto Project provides, see the "Images" chapter.

Images are written 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.

- `<kernel-image>`: A kernel binary file. The `KERNEL_IMAGETYPE` variable setting determines the naming scheme for the kernel image file. Depending on that variable, the file could begin with a variety of naming strings. The `deploy/images/<machine>` directory can contain multiple image files for the machine.

- `<root-filesystem-image>`: Root filesystems for the target device (e.g. `*.ext3` or `*.bz2` files). The `IMAGE_FSTYPES` variable setting determines the root filesystem image type. The `deploy/images/<machine>` directory can contain multiple root filesystems for the machine.

- `<kernel-modules>`: Tarballs that contain all the modules built for the kernel. Kernel module tarballs exist for legacy purposes and can be suppressed by setting the `MODULE_TARBALL_DEPLOY` variable to "0". The `deploy/images/<machine>` directory can contain multiple kernel module tarballs for the machine.

- `<bootloaders>`: Bootloaders supporting the image, if applicable to the target machine. The `deploy/images/<machine>` directory can contain multiple bootloaders for the machine.

- `<symlinks>`: The `deploy/images/<machine>` folder contains a symbolic link that points to the most recently built file for each machine. These links might be useful for external scripts that need to obtain the latest version of each file.

## Application Development SDK

In the general Yocto Project Development Environment figure, the output labeled "Application Development SDK" represents an SDK. This section is going to take a closer look at this output:

The specific form of this output is a self-extracting SDK installer (`*.sh`) that, when run, installs the SDK, which consists of a cross-development toolchain, a set of libraries and headers, and an SDK environment setup script. Running this installer essentially sets up your cross-development environment. You can think of the cross-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 setup script is added so that you can initialize the environment before using the tools.

The Yocto Project supports several methods by which you can set up this cross-development environment. These methods include downloading pre-built SDK installers, building and installing your own SDK installer, or running an Application Development Toolkit (ADT) installer to install not just cross-development toolchains but also additional tools to help in this type of development.

For background information on cross-development toolchains in the Yocto Project development environment, see the "Cross-Development Toolchain Generation" section. For information on setting up a cross-development environment, see the "Installing the ADT and Toolchains" section in the Yocto Project Application Developer's Guide.

Once built, the SDK installers are written out to the `deploy/sdk` folder inside the Build Directory as shown in the figure at the beginning of this section. Several variables exist that help configure these files:

- `DEPLOY_DIR`: Points to the `deploy` directory.

- `SDKMACHINE`: Specifies the architecture of the machine on which the cross-development tools are run to create packages for the target hardware.

- `SDKIMAGE_FEATURES`: Lists the features to include in the "target" part of the SDK.

- `TOOLCHAIN_HOST_TASK`: Lists packages that make up the host part of the SDK (i.e. the part that runs on the `SDKMACHINE`). When you use `bitbake -c populate_sdk <imagename>` to create the SDK, a set of default packages apply. This variable allows you to add more packages.

- `TOOLCHAIN_TARGET_TASK`: Lists packages that make up the target part of the SDK (i.e. the part built for the target hardware).

- `SDKPATH`: Defines the default SDK installation path offered by the installation script.