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<chapter id='ref-terms'>
<title>Yocto Project Terms</title>
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:
<emphasis>Append Files:</emphasis>
Files that append build information to a recipe file.
Append files are known as BitBake append files and
<filename>.bbappend</filename> files.
The OpenEmbedded build system expects every append file to have
a corresponding recipe (<filename>.bb</filename>) 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
<filename></filename> and
<para>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
"<ulink url='&YOCTO_DOCS_DEV_URL;#using-bbappend-files'>Using .bbappend Files in Your Layer</ulink>"
section in the Yocto Project Development Tasks Manual.</para>
<para>When you name an append file, you can use the
"<filename>%</filename>" wildcard character to allow for
matching recipe names.
For example, suppose you have an append file named as follows:
<literallayout class='monospaced'>
That append file would match any
version of the recipe.
So, the append file would match any of the following recipe names:
<literallayout class='monospaced'>
The use of the "<filename>%</filename>" character
is limited in that it only works directly in front of the
<filename>.bbappend</filename> portion of the append file's
You cannot use the wildcard character in any other
location of the name.
<listitem><para id='bitbake-term'>
The task executor and scheduler used by the OpenEmbedded build
system to build images.
For more information on BitBake, see the
<ulink url='&YOCTO_DOCS_BB_URL;'>BitBake User Manual</ulink>.
<listitem><para id='board-support-package-bsp-term'>
<emphasis>Board Support Package (BSP):</emphasis>
A group of drivers, definitions, and other components that
provide support for a specific hardware configuration.
For more information on BSPs, see the
<ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>.
<para id='build-directory'>
<emphasis>Build Directory:</emphasis>
This term refers to the area used by the OpenEmbedded build
system for builds.
The area is created when you <filename>source</filename> the
setup environment script that is found in the Source Directory
(i.e. <link linkend='structure-core-script'><filename>&OE_INIT_FILE;</filename></link>).
<link linkend='var-TOPDIR'><filename>TOPDIR</filename></link>
variable points to the Build Directory.</para>
<para>You have a lot of flexibility when creating the Build
Following are some examples that show how to create the
The examples assume your
<link linkend='source-directory'>Source Directory</link> is
named <filename>poky</filename>:
<listitem><para>Create the Build Directory inside your
Source Directory and let the name of the Build
Directory default to <filename>build</filename>:
<literallayout class='monospaced'>
$ cd $HOME/poky
$ source &OE_INIT_FILE;
<listitem><para>Create the Build Directory inside your
home directory and specifically name it
<literallayout class='monospaced'>
$ 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
in your home directory within the existing
directory <filename>mybuilds</filename>:
<literallayout class='monospaced'>
$ cd $HOME
$ source $HOME/poky/&OE_INIT_FILE; $HOME/mybuilds/YP-&POKYVERSION;
By default, the Build Directory contains
<link linkend='var-TMPDIR'><filename>TMPDIR</filename></link>,
which is a temporary directory the build system uses for
its work.
<filename>TMPDIR</filename> 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 <filename>TMPDIR</filename>
in your <filename>local.conf</filename> file
to use a local drive.
Doing so effectively separates <filename>TMPDIR</filename>
from <filename>TOPDIR</filename>, which is the Build
<listitem><para id='hardware-build-system-term'>
<emphasis>Build Host:</emphasis>
The system used to build images in a Yocto Project
Development environment.
The build system is sometimes referred to as the
<firstterm>development host</firstterm>.
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
"<link linkend='ref-classes'>Classes</link>" chapter.
Class files end with the <filename>.bbclass</filename>
filename extension.
<emphasis>Configuration File:</emphasis>
Files that hold global definitions of variables,
user-defined variables, and hardware configuration
These files tell the OpenEmbedded build system what to
build and what to put into the image to support a
particular platform.</para>
<para>Configuration files end with a <filename>.conf</filename>
filename extension.
The <filename>conf/local.conf</filename> configuration file in
<link linkend='build-directory'>Build Directory</link>
contains user-defined variables that affect every build.
The <filename>meta-poky/conf/distro/poky.conf</filename>
configuration file defines Yocto "distro" configuration
variables used only when building with this policy.
Machine configuration files, which
are located throughout the
<link linkend='source-directory'>Source Directory</link>, define
variables for specific hardware and are only used when building
for that target (e.g. the
<filename>machine/beaglebone.conf</filename> configuration
file defines variables for the Texas Instruments ARM Cortex-A8
development board).
<listitem><para id='term-container-layer'>
<emphasis>Container Layer:</emphasis>
Layers that hold other layers.
An example of a container layer is OpenEmbedded's
<ulink url=''><filename>meta-openembedded</filename></ulink>
The <filename>meta-openembedded</filename> layer contains
many <filename>meta-*</filename> layers.
<listitem><para id='cross-development-toolchain'>
<emphasis>Cross-Development Toolchain:</emphasis>
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.</para>
<para>The Yocto Project supports two different cross-development
A toolchain only used by and within
BitBake when building an image for a target
<listitem><para>A relocatable toolchain used outside of
BitBake by developers when developing applications
that will run on a targeted device.
<para>Creation of these toolchains is simple and automated.
For information on toolchain concepts as they apply to the
Yocto Project, see the
"<ulink url='&YOCTO_DOCS_OM_URL;#cross-development-toolchain-generation'>Cross-Development Toolchain Generation</ulink>"
section in the Yocto Project Overview and Concepts Manual.
You can also find more information on using the
relocatable toolchain in the
<ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Application Development and the Extensible Software Development Kit (eSDK)</ulink>
<emphasis>Extensible Software Development Kit (eSDK):</emphasis>
A custom SDK for application developers.
This eSDK allows developers to incorporate their library
and programming changes back into the image to make
their code available to other application developers.</para>
<para>For information on the eSDK, see the
<ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Application Development and the Extensible Software Development Kit (eSDK)</ulink>
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
"<link linkend='ref-images'>Images</link>"
A collection of related recipes.
Layers allow you to consolidate related metadata to
customize your build.
Layers also isolate information used when building
for multiple architectures.
Layers are hierarchical in their ability to override
previous specifications.
You can include any number of available layers from the
Yocto Project and customize the build by adding your
layers after them.
You can search the Layer Index for layers used within
Yocto Project.</para>
<para>For introductory information on layers, see the
"<ulink url='&YOCTO_DOCS_OM_URL;#the-yocto-project-layer-model'>The Yocto Project Layer Model</ulink>"
section in the Yocto Project Overview and Concepts Manual.
For more detailed information on layers, see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>"
section in the Yocto Project Development Tasks Manual.
For a discussion specifically on BSP Layers, see the
"<ulink url='&YOCTO_DOCS_BSP_URL;#bsp-layers'>BSP Layers</ulink>"
section in the Yocto Project Board Support Packages (BSP)
Developer's Guide.
<listitem><para id='metadata'>
A key element of the Yocto Project is the Metadata that
is used to construct a Linux distribution and is contained
in the files that the
<link linkend='build-system-term'>OpenEmbedded build system</link>
parses when building an image.
In general, Metadata includes recipes, configuration
files, and other information that refers to the build
instructions themselves, as well as the data used to
control what things get built and the effects of the
Metadata also includes commands and data used to
indicate what versions of software are used, from
where they are obtained, and changes or additions to the
software itself (patches or auxiliary files) that
are used to fix bugs or customize the software for use
in a particular situation.
OpenEmbedded-Core is an important set of validated
<para>In the context of the kernel ("kernel Metadata"), the
term refers to the kernel config fragments and features
contained in the
<ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi/yocto-kernel-cache'><filename>yocto-kernel-cache</filename></ulink>
Git repository.
<listitem><para id='oe-core'>
<emphasis>OpenEmbedded-Core (OE-Core):</emphasis>
OE-Core is metadata comprised of foundational recipes,
classes, and associated files that are meant to be
common among many different OpenEmbedded-derived systems,
including the Yocto Project.
OE-Core is a curated subset of an original repository
developed by the OpenEmbedded community that has been
pared down into a smaller, core set of continuously
validated recipes.
The result is a tightly controlled and an quality-assured
core set of recipes.</para>
<para>You can see the Metadata in the
<filename>meta</filename> directory of the Yocto Project
<ulink url=''>Source Repositories</ulink>.
<listitem><para id='build-system-term'>
<emphasis>OpenEmbedded Build System:</emphasis>
The build system specific to the Yocto Project.
The OpenEmbedded build system is based on another project known
as "Poky", which uses
<link linkend='bitbake-term'>BitBake</link> as the task
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
For some historical information about Poky, see the
<link linkend='poky'>Poky</link> term.
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.</para>
<para>It is worth noting that the term "package" can,
in general, have subtle meanings.
For example, the packages referred to in the
"<link linkend='required-packages-for-the-build-host'>Required Packages for the Build Host</link>"
section are compiled binaries that, when installed, add
functionality to your Linux distribution.</para>
<para>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
(e.g. <link linkend='var-PR'><filename>PR</filename></link>,
<link linkend='var-PV'><filename>PV</filename></link>, and
<link linkend='var-PE'><filename>PE</filename></link>).
<emphasis>Package Groups:</emphasis>
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
A package group is really just another recipe.
Because package group files are recipes, they end with the
<filename>.bb</filename> filename extension.
<listitem><para id='poky'>
Poky, which is pronounced <emphasis>Pock</emphasis>-ee,
is a reference embedded distribution and a reference
test configuration.
Poky provides the following:
A base-level functional distro used to illustrate
how to customize a distribution.
A means by which to test the Yocto Project
components (i.e. Poky is used to validate
the Yocto Project).
A vehicle through which you can download
the Yocto Project.
Poky is not a product level distro.
Rather, it is a good starting point for customization.
Poky began as an open-source
project initially developed by OpenedHand.
OpenedHand developed Poky from 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.
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 represent the logical unit of execution, the software
to build, the images to build, and use the
<filename>.bb</filename> file extension.
<listitem><para id='reference-kit-term'>
<emphasis>Reference Kit:</emphasis>
A working example of a system, which includes a
<link linkend='board-support-package-bsp-term'>BSP</link>
as well as a
<link linkend='hardware-build-system-term'>build host</link>
and other components, that can work on specific hardware.
<para id='source-directory'>
<emphasis>Source Directory:</emphasis>
This term refers to the directory structure created as a result
of creating a local copy of the <filename>poky</filename> Git
repository <filename>git://</filename>
or expanding a released <filename>poky</filename> tarball.
Creating a local copy of the <filename>poky</filename>
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.
<para>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.</para>
<para>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 <filename>poky</filename> Git
repository results in a local Git repository whose top-level
folder is also named "poky".</para>
<para>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
<filename>&YOCTO_POKY_TARBALL;</filename> results in a
Source Directory whose root folder is named
<para>It is important to understand the differences between the
Source Directory created by unpacking a released tarball as
compared to cloning
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 <filename>poky</filename>
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 <filename>poky</filename> Git
<para>For more information on concepts related to Git
repositories, branches, and tags, see the
"<ulink url='&YOCTO_DOCS_OM_URL;#repositories-tags-and-branches'>Repositories, Tags, and Branches</ulink>"
section in the Yocto Project Overview and Concepts Manual.
A unit of execution for BitBake (e.g.
<link linkend='ref-tasks-compile'><filename>do_compile</filename></link>,
<link linkend='ref-tasks-fetch'><filename>do_fetch</filename></link>,
<link linkend='ref-tasks-patch'><filename>do_patch</filename></link>,
and so forth).
<listitem><para id='toaster-term'><emphasis>Toaster:</emphasis>
A web interface to the Yocto Project's
<link linkend='build-system-term'>OpenEmbedded Build System</link>.
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
<ulink url='&YOCTO_DOCS_TOAST_URL;'>Toaster User Manual</ulink>.
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
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.
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