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" section.
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.
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-build-env
or
oe-init-build-env-memres
).
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-build-env
Create the Build Directory inside your
home directory and specifically name it
test-builds
:
$ cd $HOME $ source poky/oe-init-build-env 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-14.0.2
in your home directory within the existing
directory mybuilds
:
$cd $HOME $ source $HOME/poky/oe-init-build-env $HOME/mybuilds/YP-14.0.2
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.
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 of the
Yocto Project Reference Manual.
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-yocto/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. Sometimes this relocatable cross-development toolchain is referred to as the meta-toolchain.
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 in the Yocto Project Reference Manual. You can also find more information on using the relocatable toolchain in the Yocto Project Application Developer's Guide.
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 in the Yocto Project Reference Manual.
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.
Meta-Toolchain: A term sometimes used for Cross-Development Toolchain.
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"),
it refers to Metadata in the meta
branches of the kernel source Git repositories.
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.
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 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" can mean several things. In its most general sense, it is an open-source project that was initially developed by OpenedHand. With OpenedHand, poky was developed off of the existing OpenEmbedded build system becoming a commercially supportable build system for embedded Linux. After Intel Corporation acquired OpenedHand, the project poky became the basis for the Yocto Project's build system.
Within the Yocto Project source repositories,
poky
exists as a separate Git
repository you can clone to yield a local copy on your
host system.
Thus, "poky" can refer to the local copy of the Source
Directory 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.
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.
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 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
poky-jethro-14.0.2.tar.bz2
results in a
Source Directory whose root folder is named
poky-jethro-14.0.2
.
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).
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.