# Introduction

## Welcome

Welcome to the Yocto Project Reference Manual. This manual provides reference information for the current release of the Yocto Project. This manual is best used after you have an understanding of the basics of the Yocto Project. The manual is neither meant to be read as a starting point to the Yocto Project nor read from start to finish. Use this manual to find concepts, variable definitions, class descriptions, and so forth as needed during the course of using the Yocto Project.

For introductory information on the Yocto Project, see the Yocto Project Website and the "Introducing the Yocto Project Development Environment" section.

If you want to use the Yocto Project to test run building an image without having to understand concepts, work through the Yocto Project Quick Start. You can find "how-to" information in the Yocto Project Development Tasks Manual.

### Tip

For more information about the Yocto Project Documentation set, see the "Links and Related Documentation" section.

## Introducing the Yocto Project Development Environment

The Yocto Project is an open-source collaboration project whose focus is for developers of embedded Linux systems. Among other things, the Yocto Project uses an OpenEmbedded build system. The build system, which is based on the OpenEmbedded (OE) project and uses the BitBake tool, constructs complete Linux images for architectures based on ARM, MIPS, PowerPC, x86 and x86-64.

Historically, the OpenEmbedded build system, which is the combination of BitBake and OE components, formed a reference build host that was known as "Poky" (Pah-kee). The term "Poky", as used throughout the Yocto Project Documentation set, can have different meanings.

The Yocto Project provides various ancillary tools for the embedded developer and also features the Sato reference User Interface, which is optimized for stylus-driven, low-resolution screens.

Here are some highlights for the Yocto Project:

- Provides a recent Linux kernel along with a set of system commands and libraries suitable for the embedded environment.

- Makes available system components such as X11, GTK+, Qt, Clutter, and SDL (among others) so you can create a rich user experience on devices that have display hardware. For devices that do not have a display or where you wish to use alternative UI frameworks, these components need not be installed.

- Creates a focused and stable core compatible with the OpenEmbedded project with which you can easily and reliably build and develop.

- Fully supports a wide range of hardware and device emulation through the Quick EMUlator (QEMU).

- Provides a layer mechanism that allows you to easily extend the system, make customizations, and keep them organized.

You can use the Yocto Project to generate images for many kinds of devices. As mentioned earlier, the Yocto Project supports creation of reference images that you can boot within and emulate using QEMU. The standard example machines target QEMU full-system emulation for 32-bit and 64-bit variants of x86, ARM, MIPS, and PowerPC architectures. Beyond emulation, you can use the layer mechanism to extend support to just about any platform that Linux can run on and that a toolchain can target.

Another Yocto Project feature is the Sato reference User Interface. This optional UI that is based on GTK+ is intended for devices with restricted screen sizes and is included as part of the OpenEmbedded Core layer so that developers can test parts of the software stack.

While the Yocto Project does not provide a strict testing framework, it does provide or generate for you artifacts that let you perform target-level and emulated testing and debugging. Additionally, if you are an Eclipse IDE user, you can install an Eclipse Yocto Plug-in to allow you to develop within that familiar environment.

By default, using the Yocto Project to build an image creates a Poky distribution. However, you can create your own distribution by providing key Metadata. A good example is Angstrom, which has had a distribution based on the Yocto Project since its inception. Other examples include commercial distributions like Wind River Linux, Mentor Embedded Linux, ENEA Linux and others. See the "Creating Your Own Distribution" section in the Yocto Project Development Tasks Manual for more information.

## System Requirements

For general Yocto Project system requirements, see the "Setting Up to Use the Yocto Project" section in the Yocto Project Quick Start. The remainder of this section provides details on system requirements not covered in the Yocto Project Quick Start.

### Supported Linux Distributions

Currently, the Yocto Project is supported on the following distributions:

Yocto Project releases are tested against the stable Linux distributions in the following list. The Yocto Project should work on other distributions but validation is not performed against them.

In particular, the Yocto Project does not support and currently has no plans to support rolling-releases or development distributions due to their constantly changing nature. We welcome patches and bug reports, but keep in mind that our priority is on the supported platforms listed below.

If you encounter problems, please go to Yocto Project Bugzilla and submit a bug. We are interested in hearing about your experience.

- Ubuntu 14.04 (LTS)

- Ubuntu 14.10

- Ubuntu 15.04

- Ubuntu 15.10

- Ubuntu 16.04

- Fedora release 22

- Fedora release 23

- CentOS release 7.x

- Debian GNU/Linux 8.x (Jessie)

- Debian GNU/Linux 9.x (Stretch)

- openSUSE 13.2

- openSUSE 42.1

While the Yocto Project Team attempts to ensure all Yocto Project releases are one hundred percent compatible with each officially supported Linux distribution, instances might exist where you encounter a problem while using the Yocto Project on a specific distribution.

### Required Packages for the Host Development System

The list of packages you need on the host development system can be large when covering all build scenarios using the Yocto Project. This section provides required packages according to Linux distribution and function.

#### Ubuntu and Debian

The following list shows the required packages by function given a supported Ubuntu or Debian Linux distribution:

If your build system has the `oss4-dev` package installed, you might experience QEMU build failures due to the package installing its own custom `/usr/include/linux/soundcard.h` on the Debian system. If you run into this situation, either of the following solutions exist:

         $ sudo apt-get build-dep qemu
         $ sudo apt-get remove oss4-dev

- Essentials: Packages needed to build an image on a headless system:

           $ sudo apt-get install &UBUNTU_HOST_PACKAGES_ESSENTIAL;

- Graphical and Eclipse Plug-In Extras: Packages recommended if the host system has graphics support or if you are going to use the Eclipse IDE:

           $ sudo apt-get install libsdl1.2-dev xterm

- Documentation: Packages needed if you are going to build out the Yocto Project documentation manuals:

           $ sudo apt-get install make xsltproc docbook-utils fop dblatex xmlto

- OpenEmbedded Self-Test (`oe-selftest`): Packages needed if you are going to run `oe-selftest`:

           $ sudo apt-get install python-git

#### Fedora Packages

The following list shows the required packages by function given a supported Fedora Linux distribution:

- Essentials: Packages needed to build an image for a headless system:

           $ sudo dnf install &FEDORA_HOST_PACKAGES_ESSENTIAL;

- Graphical and Eclipse Plug-In Extras: Packages recommended if the host system has graphics support or if you are going to use the Eclipse IDE:

           $ sudo dnf install SDL-devel xterm

- Documentation: Packages needed if you are going to build out the Yocto Project documentation manuals:

           $ sudo dnf install make docbook-style-dsssl docbook-style-xsl \
           docbook-dtds docbook-utils fop libxslt dblatex xmlto

- OpenEmbedded Self-Test (`oe-selftest`): Packages needed if you are going to run `oe-selftest`:

           $ sudo dnf install python3-GitPython

#### openSUSE Packages

The following list shows the required packages by function given a supported openSUSE Linux distribution:

- Essentials: Packages needed to build an image for a headless system:

           $ sudo zypper install &OPENSUSE_HOST_PACKAGES_ESSENTIAL;

- Graphical and Eclipse Plug-In Extras: Packages recommended if the host system has graphics support or if you are going to use the Eclipse IDE:

           $ sudo zypper install libSDL-devel xterm

- Documentation: Packages needed if you are going to build out the Yocto Project documentation manuals:

           $ sudo zypper install make dblatex xmlto

- OpenEmbedded Self-Test (`oe-selftest`): Packages needed if you are going to run `oe-selftest`:

           $ sudo zypper install python-GitPython

Sanity testing, through the testimage classes, does not work on systems using the Wicked network manager.

#### CentOS Packages

The following list shows the required packages by function given a supported CentOS Linux distribution:

For CentOS 6.x, some of the versions of the components provided by the distribution are too old (e.g. Git, Python, and tar). It is recommended that you install the buildtools in order to provide versions that will work with the OpenEmbedded build system. For information on how to install the buildtools tarball, see the "Required Git, Tar, and Python Versions" section.

- Essentials: Packages needed to build an image for a headless system:

           $ sudo yum install &CENTOS_HOST_PACKAGES_ESSENTIAL; SDL-devel xterm

  ##### Notes

  - Extra Packages for Enterprise Linux (i.e. `epel-release`) is a collection of packages from Fedora built on RHEL/CentOS for easy installation of packages not included in enterprise Linux by default. You need to install these packages separately.

  - The `makecache` command consumes additional Metadata from `epel-release`.

- Graphical and Eclipse Plug-In Extras: Packages recommended if the host system has graphics support or if you are going to use the Eclipse IDE:

           $ sudo yum install SDL-devel xterm

- Documentation: Packages needed if you are going to build out the Yocto Project documentation manuals:

           $ sudo yum install make docbook-style-dsssl docbook-style-xsl \
           docbook-dtds docbook-utils fop libxslt dblatex xmlto

- OpenEmbedded Self-Test (`oe-selftest`): Packages needed if you are going to run `oe-selftest`:

           $ sudo yum install GitPython

### Required Git, tar, and Python Versions

In order to use the build system, your host development system must meet the following version requirements for Git, tar, and Python:

- Git 1.8.3.1 or greater

- tar 1.27 or greater

- Python 3.4.0 or greater

If your host development system does not meet all these requirements, you can resolve this by installing a `buildtools` tarball that contains these tools. You can get the tarball one of two ways: download a pre-built tarball or use BitBake to build the tarball.

#### Downloading a Pre-Built `buildtools` Tarball

Downloading and running a pre-built buildtools installer is the easiest of the two methods by which you can get these tools:

1. Locate and download the `*.sh` at .

2. Execute the installation script. Here is an example:

            $ sh poky-glibc-x86_64-buildtools-tarball-x86_64-buildtools-nativesdk-standalone-&DISTRO;.sh

   During execution, a prompt appears that allows you to choose the installation directory. For example, you could choose the following:

            /home/your-username/buildtools

3. Source the tools environment setup script by using a command like the following:

            $ source /home/your_username/buildtools/environment-setup-i586-poky-linux

   Of course, you need to supply your installation directory and be sure to use the right file (i.e. i585 or x86-64).

   After you have sourced the setup script, the tools are added to `PATH` and any other environment variables required to run the tools are initialized. The results are working versions versions of Git, tar, Python and `chrpath`.

#### Building Your Own `buildtools` Tarball

Building and running your own buildtools installer applies only when you have a build host that can already run BitBake. In this case, you use that machine to build the `.sh` file and then take steps to transfer and run it on a machine that does not meet the minimal Git, tar, and Python requirements.

Here are the steps to take to build and run your own buildtools installer:

1. On the machine that is able to run BitBake, be sure you have set up your build environment with the setup script (`&OE_INIT_FILE;`).

2. Run the BitBake command to build the tarball:

            $ bitbake buildtools-tarball

   The `SDKMACHINE` variable in your `local.conf` file determines whether you build tools for a 32-bit or 64-bit system.

   Once the build completes, you can find the `.sh` file that installs the tools in the `tmp/deploy/sdk` subdirectory of the Build Directory. The installer file has the string "buildtools" in the name.

3. Transfer the `.sh` file from the build host to the machine that does not meet the Git, tar, or Python requirements.

4. On the machine that does not meet the requirements, run the `.sh` file to install the tools. Here is an example:

            $ sh poky-glibc-x86_64-buildtools-tarball-x86_64-buildtools-nativesdk-standalone-&DISTRO;.sh

   During execution, a prompt appears that allows you to choose the installation directory. For example, you could choose the following:

            /home/your_username/buildtools

5. Source the tools environment setup script by using a command like the following:

            $ source /home/your_username/buildtools/environment-setup-i586-poky-linux

   Of course, you need to supply your installation directory and be sure to use the right file (i.e. i585 or x86-64).

   After you have sourced the setup script, the tools are added to `PATH` and any other environment variables required to run the tools are initialized. The results are working versions versions of Git, tar, Python and `chrpath`.

## Obtaining the Yocto Project

The Yocto Project development team makes the Yocto Project available through a number of methods:

- Source Repositories: Working from a copy of the upstream `poky` repository is the preferred method for obtaining and using a Yocto Project release. You can view the Yocto Project Source Repositories at . In particular, you can find the `poky` repository at .

- Releases: Stable, tested releases are available as tarballs through .

- Nightly Builds: These tarball releases are available at . These builds include Yocto Project releases, SDK installation scripts, and experimental builds.

- Yocto Project Website: You can find tarball releases of the Yocto Project and supported BSPs at the Yocto Project website. Along with these downloads, you can find lots of other information at this site.

## Development Checkouts

Development using the Yocto Project requires a local Source Directory. You can set up the Source Directory by cloning a copy of the upstream poky Git repository. For information on how to do this, see the "Working With Yocto Project Source Files" section in the Yocto Project Development Tasks Manual.

## Yocto Project Terms

Following is a list of terms and definitions users new to the Yocto Project development environment might find helpful. While some of these terms are universal, the list includes them just in case:

- Append Files: Files that append build information to a recipe file. Append files are known as BitBake append files and `.bbappend` files. The OpenEmbedded build system expects every append file to have a corresponding recipe (`.bb`) file. Furthermore, the append file and corresponding recipe file must use the same root filename. The filenames can differ only in the file type suffix used (e.g. `formfactor_0.0.bb` and `formfactor_0.0.bbappend`).

  Information in append files extends or overrides the information in the similarly-named recipe file. For an example of an append file in use, see the "Using .bbappend Files in Your Layer" section in the Yocto Project Development Tasks Manual.

  Append files can also use wildcard patterns in their version numbers so they can be applied to more than one version of the underlying recipe file.

- BitBake: The task executor and scheduler used by the OpenEmbedded build system to build images. For more information on BitBake, see the BitBake User Manual.

- Board Support Package (BSP): A group of drivers, definitions, and other components that provide support for a specific hardware configuration. For more information on BSPs, see the Yocto Project Board Support Package (BSP) Developer's Guide.

- Build Directory: This term refers to the area used by the OpenEmbedded build system for builds. The area is created when you `source` the setup environment script that is found in the Source Directory (i.e. `&OE_INIT_FILE;`). The `TOPDIR` variable points to the Build Directory.

  You have a lot of flexibility when creating the Build Directory. Following are some examples that show how to create the directory. The examples assume your Source Directory is named `poky`:

  - Create the Build Directory inside your Source Directory and let the name of the Build Directory default to `build`:

             $ cd $HOME/poky
             $ source &OE_INIT_FILE;

  - Create the Build Directory inside your home directory and specifically name it `test-builds`:

             $ cd $HOME
             $ source poky/&OE_INIT_FILE; test-builds

  - Provide a directory path and specifically name the Build Directory. Any intermediate folders in the pathname must exist. This next example creates a Build Directory named `YP-&POKYVERSION;` in your home directory within the existing directory `mybuilds`:

             $cd $HOME
             $ source $HOME/poky/&OE_INIT_FILE; $HOME/mybuilds/YP-&POKYVERSION;

  By default, the Build Directory contains `TMPDIR`, which is a temporary directory the build system uses for its work. `TMPDIR` cannot be under NFS. Thus, by default, the Build Directory cannot be under NFS. However, if you need the Build Directory to be under NFS, you can set this up by setting `TMPDIR` in your `local.conf` file to use a local drive. Doing so effectively separates `TMPDIR` from `TOPDIR`, which is the Build Directory.

- Build System: The system used to build images in a Yocto Project Development environment. The build system is sometimes referred to as the development host.

- Classes: Files that provide for logic encapsulation and inheritance so that commonly used patterns can be defined once and then easily used in multiple recipes. For reference information on the Yocto Project classes, see the "Classes" chapter. Class files end with the `.bbclass` filename extension.

- Configuration File: Configuration information in various `.conf` files provides global definitions of variables. The `conf/local.conf` configuration file in the Build Directory contains user-defined variables that affect every build. The `meta-poky/conf/distro/poky.conf` configuration file defines Yocto "distro" configuration variables used only when building with this policy. Machine configuration files, which are located throughout the Source Directory, define variables for specific hardware and are only used when building for that target (e.g. the `machine/beaglebone.conf` configuration file defines variables for the Texas Instruments ARM Cortex-A8 development board). Configuration files end with a `.conf` filename extension.

- Cross-Development Toolchain: In general, a cross-development toolchain is a collection of software development tools and utilities that run on one architecture and allow you to develop software for a different, or targeted, architecture. These toolchains contain cross-compilers, linkers, and debuggers that are specific to the target architecture.

  The Yocto Project supports two different cross-development toolchains:

  - A toolchain only used by and within BitBake when building an image for a target architecture.

  - A relocatable toolchain used outside of BitBake by developers when developing applications that will run on a targeted device.

  Creation of these toolchains is simple and automated. For information on toolchain concepts as they apply to the Yocto Project, see the "Cross-Development Toolchain Generation" section. You can also find more information on using the relocatable toolchain in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual.

- Image: An image is an artifact of the BitBake build process given a collection of recipes and related Metadata. Images are the binary output that run on specific hardware or QEMU and are used for specific use-cases. For a list of the supported image types that the Yocto Project provides, see the "Images" chapter.

- Layer: A collection of recipes representing the core, a BSP, or an application stack. For a discussion specifically on BSP Layers, see the "BSP Layers" section in the Yocto Project Board Support Packages (BSP) Developer's Guide.

- Metadata: The files that BitBake parses when building an image. In general, Metadata includes recipes, classes, and configuration files. In the context of the kernel ("kernel Metadata"), the term refers to the kernel config fragments and features contained in the `yocto-kernel-cache` Git repository.

- OE-Core: A core set of Metadata originating with OpenEmbedded (OE) that is shared between OE and the Yocto Project. This Metadata is found in the `meta` directory of the Source Directory.

- OpenEmbedded Build System: The build system specific to the Yocto Project. The OpenEmbedded build system is based on another project known as "Poky", which uses BitBake as the task executor. Throughout the Yocto Project documentation set, the OpenEmbedded build system is sometimes referred to simply as "the build system". If other build systems, such as a host or target build system are referenced, the documentation clearly states the difference.

  For some historical information about Poky, see the Poky term.

- Package: In the context of the Yocto Project, this term refers to a recipe's packaged output produced by BitBake (i.e. a "baked recipe"). A package is generally the compiled binaries produced from the recipe's sources. You "bake" something by running it through BitBake.

  It is worth noting that the term "package" can, in general, have subtle meanings. For example, the packages referred to in the "The Build Host Packages" section in the Yocto Project Quick Start are compiled binaries that, when installed, add functionality to your Linux distribution.

  Another point worth noting is that historically within the Yocto Project, recipes were referred to as packages - thus, the existence of several BitBake variables that are seemingly mis-named, (e.g. `PR`, `PV`, and `PE`).

- Package Groups: Arbitrary groups of software Recipes. You use package groups to hold recipes that, when built, usually accomplish a single task. For example, a package group could contain the recipes for a company’s proprietary or value-add software. Or, the package group could contain the recipes that enable graphics. A package group is really just another recipe. Because package group files are recipes, they end with the `.bb` filename extension.

- Poky: The term "poky", which is pronounced Pah-kee, can mean several things:

  - In its most general sense, poky is an open-source project that was initially developed by OpenedHand. OpenedHand developed poky off of the existing OpenEmbedded build system to create a commercially supportable build system for embedded Linux. After Intel Corporation acquired OpenedHand, the poky project became the basis for the Yocto Project's build system.

  - Within the Yocto Project Source Repositories, "poky" exists as a separate Git repository from which you can clone to yield a local Git repository that is a copy on your host system. Thus, "poky" can refer to the upstream or local copy of the files used for development within the Yocto Project.

  - Finally, "poky" can refer to the default `DISTRO` (i.e. distribution) created when you use the Yocto Project in conjunction with the `poky` repository to build an image.

- Recipe: A set of instructions for building packages. A recipe describes where you get source code, which patches to apply, how to configure the source, how to compile it and so on. Recipes also describe dependencies for libraries or for other recipes. Recipes represent the logical unit of execution, the software to build, the images to build, and use the `.bb` file extension.

- Reference Kit: A working example of a system, which includes a BSP as well as a build system and other components, that can work on specific hardware.

- Source Directory: This term refers to the directory structure created as a result of creating a local copy of the `poky` Git repository `git://git.yoctoproject.org/poky` or expanding a released `poky` tarball.

  Creating a local copy of the `poky` Git repository is the recommended method for setting up your Source Directory.

  Sometimes you might hear the term "poky directory" used to refer to this directory structure.

  The OpenEmbedded build system does not support file or directory names that contain spaces. Be sure that the Source Directory you use does not contain these types of names.

  The Source Directory contains BitBake, Documentation, Metadata and other files that all support the Yocto Project. Consequently, you must have the Source Directory in place on your development system in order to do any development using the Yocto Project.

  When you create a local copy of the Git repository, you can name the repository anything you like. Throughout much of the documentation, "poky" is used as the name of the top-level folder of the local copy of the poky Git repository. So, for example, cloning the `poky` Git repository results in a local Git repository whose top-level folder is also named "poky".

  While it is not recommended that you use tarball expansion to set up the Source Directory, if you do, the top-level directory name of the Source Directory is derived from the Yocto Project release tarball. For example, downloading and unpacking `&YOCTO_POKY_TARBALL;` results in a Source Directory whose root folder is named `&YOCTO_POKY;`.

  It is important to understand the differences between the Source Directory created by unpacking a released tarball as compared to cloning `git://git.yoctoproject.org/poky`. When you unpack a tarball, you have an exact copy of the files based on the time of release - a fixed release point. Any changes you make to your local files in the Source Directory are on top of the release and will remain local only. On the other hand, when you clone the `poky` Git repository, you have an active development repository with access to the upstream repository's branches and tags. In this case, any local changes you make to the local Source Directory can be later applied to active development branches of the upstream `poky` Git repository.

  For more information on concepts related to Git repositories, branches, and tags, see the "Repositories, Tags, and Branches" section.

- Task: A unit of execution for BitBake (e.g. `do_compile`, `do_fetch`, `do_patch`, and so forth).

- Toaster: A web interface to the Yocto Project's OpenEmbedded Build System. The interface enables you to configure and run your builds. Information about builds is collected and stored in a database. For information on Toaster, see the Yocto Project Toaster Manual.

- Upstream: A reference to source code or repositories that are not local to the development system but located in a master area that is controlled by the maintainer of the source code. For example, in order for a developer to work on a particular piece of code, they need to first get a copy of it from an "upstream" source.