Yocto Project Introduction

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This is an introduction about the Yocto Project, written from the point of view of someone somewhat familiar with embedded Linux development and embedded Linux distribution maintenance. In my own investigation of the Yocto Project, I found that there were a few key concepts that I didn't find discussed or presented anywhere. I'd like to fix that with this page. The purpose of this page is to give a broad overview of how the project technically works, so that the learning curve for people approaching the Yocto Project for the first time is lessened.

[As of November 2012, this document is still under construction...]

The big picture

The Yocto Project is a collection of tools and meta-data (defined in a bit) that allows a developer to build their own custom distribution of Linux for their embedded platform. This could be a developer at a semi-conductor company, who wishes to develop board support for one of their hardware platforms, or it could be an independent developer writing a complete software stack for a product they are making. It could also be a group of engineers developing a distribution for use in multiple devices or products -- such as an embedded Linux distribution company, or the "systems" team at a company that produces multiple embedded Linux products.

The main parts

The main parts of the Yocto Project are the build system, the package meta-data, and the developer tools. The build system uses a tool called "bitbake" to process the meta-data and produce a complete Linux distribution. By design, the build system produces not just the software that will run on the target, but also the development tools used to build that software. It basically starts completely from scratch, building all the tools needed to construct the software, and then using those to build the kernel, libraries, and programs that comprise a Linux distribution. Finally, it prepares the resulting software by placing it into appropriate bundles (including packages, images, or both) for deployment to the target device and in preparation for application development and debugging. The Yocto Project also includes various additional tools used to develop embedded Linux or applications on top of it. This includes things such as emulators, IDEs and host/target (cross) agents and debug tools.

Let's start by describing some of the concepts of the build system...

Build system

The primary tool used in the build system is called 'bitbake'. Bitbake has a user-manual at http://docs.openembedded.org/bitbake/html/ . (As of August, 2012, this document appears to be a bit dated, and missing a few items of importance.) Basically, bitbake can be thought of as "make" on steroids. It basically performs the same type of functionality as make - which is: determining the actions to perform based on 1) what the user requests at the command line, 2) the project data and 3) the existing build state, and then performing those actions.

Bitbake uses files with it's own new syntax for expressing:

  • the tasks to perform
  • the relationships (dependencies) between those tasks
  • the variables that control how the tasks are performed
  • the actual build instructions (e.g. compiler commands, linker commands, packaging commands, etc.)

Bitbake differs from 'make' in several key ways. The first is that it has a global view of the task list for a distribution. That is, it reads the entire set of files related to the distribution, and determines the global task list for a particular high-level build operation. This is considerably different from 'make', which processes just a single Makefile at a time. (Admittedly, 'make' can be made to work with extremely large projects, using complex include schemes and nested invocations. However, even these types of systems are rarely used for something as complex as a complete Linux distribution build.)

Another apparent difference is that the syntax of the files that bitbake processes allows for a very high degree of flexibility in defining the tasks that should be performed and the variables that control the build process and output.

[note to consider: maybe too much flexibility?]

A variety of mechanisms (described in the bitbake manual) are used to control what operations are performed and what variables are used to control them. bitbake supports inheritance mechanisms, to allow for a class-like definition of common operations. These common operations can be inherited and customized for specific situations. Bitbake also supports conditional definition of new tasks, and has the ability to customize or eliminate (mask) tasks based on variables computed during the build.

Bitbake is written in Python, and some aspects of the build system can be written using short Python snippets as well. Many aspects of the system are written in snippets of shell code as well.

A cheat-sheet for bitbake is at: Bitbake Cheat Sheet

The meta-data

A key element of the Yocto Project is the meta-data which is used to construct a Linux distribution.

Meta-data refers to the build instructions themselves, as well as the data used to control what things get built and to affect how they are built. The meta-data also includes commands and data used to indicate what versions of software are used, and where they are obtained from. The meta-data also consist of changes or additions to the software itself (patches or auxiliary files) which are used to fix bugs or customize the software for use in a particular situation.

The build instructions consists of commands to execute the compiler, linker, archiver, packaging tools and other programs.

The Yocto Project provides the build tool (bitbake) and meta-data itself, for a few distributions. Much of this meta-data can be re-used when someone is building their own distribution. The Yocto Project is related to the OpenEmbedded project, where the bitbake tool, much of the meta-data, and many of the meta-data concepts originated.

Each software component on the system (such as an individual program) has associated with it one or more files to express it's meta-data (dependencies, patches, build instructions). A top-level (usually single) file that defines the 'tasks' for the software is provided in a ".bb" (bitbake) file. This is referred to as that component's "recipe" file. This file may be terse, as the system allows for inheritance and inclusion. Class files (.bbclass) are used to express the meta-data for commonly-used types of build and packaging operations. These files are 'inherit'ed into a recipe. Include files (.inc) are sometimes used to provide a common set of definitions, which can be customized for a particular version of the software. These files are included into a recipe using a 'require' command. Finally, patches and auxiliary files may be associated with recipe. These files can be referenced by the build instructions (eg. applied to the software after fetching it and before compiling it in the case of patches) of the recipe.

A set of bitbake files that are related to a particular feature area can be organized into a "layer". This represents a collection of software or build tasks that can included into an overall distribution.

Finally, a user selects the individual bitbake files to include, or sets of files from different layers, by referencing them in their local configuration (conf/bblayers.conf) and defining build control variables - also in their local configuration (conf/local.conf).

Build stages

The recipes for a distribution define a number of discreet tasks that are performed to accomplish the build. The tasks have names such as 'fetch', 'configure', 'compile'. Internally, these tasks are prefixed with 'do_' (eg. do_configure)

There can be a large number of tasks associated with a software component. By default, bitbake performs all tasks associated with building a software component and preparing it for deployment. However, bitbake can be used to perform just a single task relative to a component, using the '-c' command line option.

For example, to just perform the 'configure' task for the busybox software, you can do the following:

bitbake busybox -c configure

Note that if busybox has already been built, this command might not do anything (because bitbake avoids re-running tasks that are not needed, like 'make'). To force it, you could use the '-f' option:

bitbake busybox -c configure -f

Not every software component needs every build stage, so some might not be defined for a component. To see the list of tasks that a recipe defines for a component, execute the 'listtasks' task for that component:

bitbake busybox -c listtasks

This is a list of the tasks associated with busybox (as of Poky version 7.0)


In general, a build proceed through the stages in order: fetch, configure, compile, install, and package the software.

Build work areas

The area where each software component is built is dependent on the recipe name, the distro name, build tool and the target architecture for the component. During the build, some items are built for the host machine and some are built for the target machine. In non-embedded Linux, the machine you build and debug on and the machine you compile for (run the software on) are very often the same. However, in embedded Linux these are often different machines, and often they even have different CPU architectures.

The directory used for an individual component is something like:


For example, in a build of the busybox code for the ARM qemu emulator platform with yocto version 7.0 was located in: <build_top_dir>/tmp/work/armv5te-poky-linux-gnueabi/busybox-1.19.4-r2

The build directory for the sqlite database tool, for use on my Ubuntu 12.04 (x86_64) host, was located at: <build_top_dir>/tmp/work/x86_64-linux/sqlite3-native-3.7.10-r2

The work directory for a software component contains not only the source code for the item, but additional data related to building it, staging it for installation into a package or image, and a 'temp' directory which has logs of the various build tasks. It is useful to know the work directory if you want to customize the software (for example if you need to make a bugfix or apply some external patch), or determine why some build operation (such as fetching, compiling, packaging, etc.) is failing.

In the 'temp' directory under the work directory are both 'run' files and log files. The 'run' files are actually shell scripts or python scripts that are executed during the different build stages. The log files have the output from the commands that were used to build the software.

Other directories under the work directory are used for packaging the software's output files, or staging them for inclusion in a system image, or for staging other information about the package, such as it's software license.

Build output

The software for an embedded device can be packaged multiple different ways in preparation for deployment to the actual device. For some systems, the built software is packaged into a kernel and a filesystem image, which can then be directly written to flash or storage media on the device. For other systems, the software may be bundled as packages (similar to the way desktop distributions of Linux are delivered). These might require developer or even end-user installation on the target device. The Yocto Project build system is capable of producing images and packages in several formats, that are selectable by the developer in the local configuration for the distribution. For example, three of the different package formats that can be built by yocto are: debian packages (.deb), redhat package manager files (.rpm), and itsy packages (.ipk).

The directory where the software resides after a typical build is: <build_top_dir>/tmp/deploy and <build_top_dir>/tmp/sysroots. The 'sysroots' directory contains the directory structure and contents for the root filesystem for a target build. (Except it seems to be missing /dev, and it doesn't have root ownership - it probably needs pseudo to see the full contents). The 'deploy' directory has the images or packages that are ready to be installed on the device.

Development tools

[need to write about building and SDK, and the IDE here]

Now for some details

[it would be good to fill out the following sections]

How recipes and tasks are found, parsed, and processed

Bitbake reads the entire set of recipe files that are specified by the local configuration, and parses them all into a global task namespace. This includes all the class files and include files as well. These are specified by the BBFILES and BBLAYERS variables in the conf/bblayers.conf file. Within the layers directories, the <layer-dir>/conf/layer.conf file is used to indicate the set of .bb files to parse for that layer.

Note that this global parse of the entire set of recipe files is quite different from 'make', which usually operated on a single Makefile. (This is also why bitbake startup is a little slow).

Information in the meta-data (the DEPENDS and PROVIDES lines) determine package ordering. Where packages are independent of each other, the build order is dependent (I believe) on file parse order. In this case, processing is not required to be in any particular order (and, in fact, can be parallelized). You can have bitbake produce the dependency graph of the packages for your build by using the -g option. This produces output in 'dot' syntax (suitable for processing using some graphviz visualizer).

The list of tasks to perform within a package appears to come from the common class meta-data (see meta/classes/utility-tasks.bbclass, for example) and the meta-data for the individual package These are added by the 'addtask' keyword. You can see a list of tasks for an individual package with: bitbake <pkg> -c listtasks

fetching tips

[info about fetch mechanisms, proxies, mirrors, etc.]

recipe features that control the build

[document the 'important' variables that show up everywhere in the .bb files (eg PN)] [how does append work, how does masking work, how do constraints work]]

extracting stuff

[where to pull stuff out - maybe covered already]

changing stuff

[how to adjust your recipe list, how to adjust the configurations of the packages, how to adjust the included files]

adding stuff

[how to add a 'hello world' package, your own kernel, your own library]

More neat stuff

[mention HOB, the build appliance, build servers, shared state]

Notes that need fleshing out... (this area a work-in-progress)

list recipes

How to see the recipes that are visible for a configuration:

  • bitbake-layers show-recipes

list available images

How to see the available images in the layers that are in a repository: ls meta*/recipes*/images/*.bb

Apparently, a distribution is defined with an "image" recipe.

The image recipe includes, among other things, a definition for IMAGE_FEATURES, which lists some items that will be included.

The items listed in the IMAGE_FEATURES and IMAGE_FEATURE_EXTRAS are ??? (things that are provided by other recipes??)

list available distros

The main item in a distro is the definition of DISTRO_FEATURES, which specifies the things to include in that distribution. These items correspond to ??? (things that are provided by other recipes???) To see list of existing distro, find inside meta-yocto/conf/distro folder

the flow of control

local.conf specifies:

  • MACHINE - this indicates the target platform (board or emulator) for the build
  • DISTRO - this indicates the base distribution for the build
  • IMAGE_EXTRA_FEATURES - adds any extra items to any images that are built

bblayers.conf specifies:

  • BBLAYERS - the list of layer directories where classes and recipes are found
  • How to know what machines are available?
    • 'ls meta*/conf/machine/*.conf'

The meta-ti layer adds the following machines:

  • How to know what layers are available?
    • ls -d1 meta*
  • How to know what distros are available?
    • ls meta*/conf/distro/*.conf
  • How to know what images are available?
    • ls meta*/recipe*/images/*.bb
  • How to know what recipes are available (overall)?
    • ls meta*/recipe*/*/*.bb
  • How to know what recipes are visible to your build (based on your configuration settings)?
    • bitbake-layers show-recipes
  • How to list the tasks are visible to your build based on your configuration settings?
    • bitbake -c listtasks <image-or-recipe-name>

build variables

  • bitbake -e <recipe> | grep "^S=" - print the source directory for a recipe
  • bitbake -e <recipe> | grep "^WORKDIR=" - print the working directory for a recipe

Comparison with other systems

It would be nice to compare and contrast Yocto Project Poky with other build systems, specifically: