.. SPDX-License-Identifier: CC-BY-SA-2.0-UK

Writing a New Recipe
********************

Recipes (``.bb`` files) are fundamental components in the Yocto Project
environment. Each software component built by the OpenEmbedded build
system requires a recipe to define the component. This section describes
how to create, write, and test a new recipe.

.. note::

   For information on variables that are useful for recipes and for
   information about recipe naming issues, see the
   ":ref:`ref-manual/varlocality:recipes`" section of the Yocto Project
   Reference Manual.

Overview
========

The following figure shows the basic process for creating a new recipe.
The remainder of the section provides details for the steps.

.. image:: figures/recipe-workflow.png
   :align: center
   :width: 50%

Locate or Automatically Create a Base Recipe
============================================

You can always write a recipe from scratch. However, there are three choices
that can help you quickly get started with a new recipe:

-  ``devtool add``: A command that assists in creating a recipe and an
   environment conducive to development.

-  ``recipetool create``: A command provided by the Yocto Project that
   automates creation of a base recipe based on the source files.

-  *Existing Recipes:* Location and modification of an existing recipe
   that is similar in function to the recipe you need.

.. note::

   For information on recipe syntax, see the
   ":ref:`dev-manual/new-recipe:recipe syntax`" section.

Creating the Base Recipe Using ``devtool add``
----------------------------------------------

The ``devtool add`` command uses the same logic for auto-creating the
recipe as ``recipetool create``, which is listed below. Additionally,
however, ``devtool add`` sets up an environment that makes it easy for
you to patch the source and to make changes to the recipe as is often
necessary when adding a recipe to build a new piece of software to be
included in a build.

You can find a complete description of the ``devtool add`` command in
the ":ref:`sdk-manual/extensible:a closer look at \`\`devtool add\`\``" section
in the Yocto Project Application Development and the Extensible Software
Development Kit (eSDK) manual.

Creating the Base Recipe Using ``recipetool create``
----------------------------------------------------

``recipetool create`` automates creation of a base recipe given a set of
source code files. As long as you can extract or point to the source
files, the tool will construct a recipe and automatically configure all
pre-build information into the recipe. For example, suppose you have an
application that builds using Autotools. Creating the base recipe using
``recipetool`` results in a recipe that has the pre-build dependencies,
license requirements, and checksums configured.

To run the tool, you just need to be in your :term:`Build Directory` and
have sourced the build environment setup script (i.e.
:ref:`structure-core-script`). To get help on the tool, use the following
command::

   $ recipetool -h
   NOTE: Starting bitbake server...
   usage: recipetool [-d] [-q] [--color COLOR] [-h] <subcommand> ...

   OpenEmbedded recipe tool

   options:
     -d, --debug     Enable debug output
     -q, --quiet     Print only errors
     --color COLOR   Colorize output (where COLOR is auto, always, never)
     -h, --help      show this help message and exit

   subcommands:
     create          Create a new recipe
     newappend       Create a bbappend for the specified target in the specified
                       layer
     setvar          Set a variable within a recipe
     appendfile      Create/update a bbappend to replace a target file
     appendsrcfiles  Create/update a bbappend to add or replace source files
     appendsrcfile   Create/update a bbappend to add or replace a source file
   Use recipetool <subcommand> --help to get help on a specific command

Running ``recipetool create -o OUTFILE`` creates the base recipe and
locates it properly in the layer that contains your source files.
Following are some syntax examples:

 - Use this syntax to generate a recipe based on source. Once generated,
   the recipe resides in the existing source code layer::

      recipetool create -o OUTFILE source

 - Use this syntax to generate a recipe using code that
   you extract from source. The extracted code is placed in its own layer
   defined by :term:`EXTERNALSRC`::

      recipetool create -o OUTFILE -x EXTERNALSRC source

 - Use this syntax to generate a recipe based on source. The options
   direct ``recipetool`` to generate debugging information. Once generated,
   the recipe resides in the existing source code layer::

      recipetool create -d -o OUTFILE source

Locating and Using a Similar Recipe
-----------------------------------

Before writing a recipe from scratch, it is often useful to discover
whether someone else has already written one that meets (or comes close
to meeting) your needs. The Yocto Project and OpenEmbedded communities
maintain many recipes that might be candidates for what you are doing.
You can find a good central index of these recipes in the
:oe_layerindex:`OpenEmbedded Layer Index <>`.

Working from an existing recipe or a skeleton recipe is the best way to
get started. Here are some points on both methods:

-  *Locate and modify a recipe that is close to what you want to do:*
   This method works when you are familiar with the current recipe
   space. The method does not work so well for those new to the Yocto
   Project or writing recipes.

   Some risks associated with this method are using a recipe that has
   areas totally unrelated to what you are trying to accomplish with
   your recipe, not recognizing areas of the recipe that you might have
   to add from scratch, and so forth. All these risks stem from
   unfamiliarity with the existing recipe space.

-  *Use and modify the following skeleton recipe:* If for some reason
   you do not want to use ``recipetool`` and you cannot find an existing
   recipe that is close to meeting your needs, you can use the following
   structure to provide the fundamental areas of a new recipe::

      DESCRIPTION = ""
      HOMEPAGE = ""
      LICENSE = ""
      SECTION = ""
      DEPENDS = ""
      LIC_FILES_CHKSUM = ""

      SRC_URI = ""

Storing and Naming the Recipe
=============================

Once you have your base recipe, you should put it in your own layer and
name it appropriately. Locating it correctly ensures that the
OpenEmbedded build system can find it when you use BitBake to process
the recipe.

-  *Storing Your Recipe:* The OpenEmbedded build system locates your
   recipe through the layer's ``conf/layer.conf`` file and the
   :term:`BBFILES` variable. This
   variable sets up a path from which the build system can locate
   recipes. Here is the typical use::

      BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
                  ${LAYERDIR}/recipes-*/*/*.bbappend"

   Consequently, you need to be sure you locate your new recipe inside
   your layer such that it can be found.

   You can find more information on how layers are structured in the
   ":ref:`dev-manual/layers:understanding and creating layers`" section.

-  *Naming Your Recipe:* When you name your recipe, you need to follow
   this naming convention::

      basename_version.bb

   Use lower-cased characters and do not include the reserved suffixes
   ``-native``, ``-cross``, ``-initial``, or ``-dev`` casually (i.e. do not use
   them as part of your recipe name unless the string applies). Here are some
   examples:

   .. code-block:: none

      cups_1.7.0.bb
      gawk_4.0.2.bb
      irssi_0.8.16-rc1.bb

Running a Build on the Recipe
=============================

Creating a new recipe is usually an iterative process that requires
using BitBake to process the recipe multiple times in order to
progressively discover and add information to the recipe file.

Assuming you have sourced the build environment setup script (i.e.
:ref:`structure-core-script`) and you are in the :term:`Build Directory`, use
BitBake to process your recipe. All you need to provide is the
``basename`` of the recipe as described in the previous section::

   $ bitbake basename

During the build, the OpenEmbedded build system creates a temporary work
directory for each recipe
(``${``\ :term:`WORKDIR`\ ``}``)
where it keeps extracted source files, log files, intermediate
compilation and packaging files, and so forth.

The path to the per-recipe temporary work directory depends on the
context in which it is being built. The quickest way to find this path
is to have BitBake return it by running the following::

   $ bitbake -e basename | grep ^WORKDIR=

As an example, assume a Source Directory
top-level folder named ``poky``, a default :term:`Build Directory` at
``poky/build``, and a ``qemux86-poky-linux`` machine target system.
Furthermore, suppose your recipe is named ``foo_1.3.0.bb``. In this
case, the work directory the build system uses to build the package
would be as follows::

   poky/build/tmp/work/qemux86-poky-linux/foo/1.3.0-r0

Inside this directory you can find sub-directories such as ``image``,
``packages-split``, and ``temp``. After the build, you can examine these
to determine how well the build went.

.. note::

   You can find log files for each task in the recipe's ``temp``
   directory (e.g. ``poky/build/tmp/work/qemux86-poky-linux/foo/1.3.0-r0/temp``).
   Log files are named ``log.taskname`` (e.g. ``log.do_configure``,
   ``log.do_fetch``, and ``log.do_compile``).

You can find more information about the build process in
":doc:`/overview-manual/development-environment`"
chapter of the Yocto Project Overview and Concepts Manual.

Fetching Code
=============

The first thing your recipe must do is specify how to fetch the source
files. Fetching is controlled mainly through the
:term:`SRC_URI` variable. Your recipe
must have a :term:`SRC_URI` variable that points to where the source is
located. For a graphical representation of source locations, see the
":ref:`overview-manual/concepts:sources`" section in
the Yocto Project Overview and Concepts Manual.

The :ref:`ref-tasks-fetch` task uses the prefix of each entry in the
:term:`SRC_URI` variable value to determine which
:ref:`fetcher <bitbake-user-manual/bitbake-user-manual-fetching:fetchers>`
to use to get your source files. It is the :term:`SRC_URI` variable that triggers
the fetcher. The :ref:`ref-tasks-patch` task uses the variable after source is
fetched to apply patches. The OpenEmbedded build system uses
:term:`FILESOVERRIDES` for scanning directory locations for local files in
:term:`SRC_URI`.

The :term:`SRC_URI` variable in your recipe must define each unique location
for your source files. It is good practice to not hard-code version
numbers in a URL used in :term:`SRC_URI`. Rather than hard-code these
values, use ``${``\ :term:`PV`\ ``}``,
which causes the fetch process to use the version specified in the
recipe filename. Specifying the version in this manner means that
upgrading the recipe to a future version is as simple as renaming the
recipe to match the new version.

Here is a simple example from the
``meta/recipes-devtools/strace/strace_5.5.bb`` recipe where the source
comes from a single tarball. Notice the use of the
:term:`PV` variable::

   SRC_URI = "https://strace.io/files/${PV}/strace-${PV}.tar.xz \

Files mentioned in :term:`SRC_URI` whose names end in a typical archive
extension (e.g. ``.tar``, ``.tar.gz``, ``.tar.bz2``, ``.zip``, and so
forth), are automatically extracted during the
:ref:`ref-tasks-unpack` task. For
another example that specifies these types of files, see the
":ref:`dev-manual/new-recipe:building an autotooled package`" section.

Another way of specifying source is from an SCM. For Git repositories,
you must specify :term:`SRCREV` and you should specify :term:`PV` to include
the revision with :term:`SRCPV`. Here is an example from the recipe
``meta/recipes-core/musl/gcompat_git.bb``::

   SRC_URI = "git://git.adelielinux.org/adelie/gcompat.git;protocol=https;branch=current"

   PV = "1.0.0+1.1+git${SRCPV}"
   SRCREV = "af5a49e489fdc04b9cf02547650d7aeaccd43793"

If your :term:`SRC_URI` statement includes URLs pointing to individual files
fetched from a remote server other than a version control system,
BitBake attempts to verify the files against checksums defined in your
recipe to ensure they have not been tampered with or otherwise modified
since the recipe was written. Two checksums are used:
``SRC_URI[md5sum]`` and ``SRC_URI[sha256sum]``.

If your :term:`SRC_URI` variable points to more than a single URL (excluding
SCM URLs), you need to provide the ``md5`` and ``sha256`` checksums for
each URL. For these cases, you provide a name for each URL as part of
the :term:`SRC_URI` and then reference that name in the subsequent checksum
statements. Here is an example combining lines from the files
``git.inc`` and ``git_2.24.1.bb``::

   SRC_URI = "${KERNELORG_MIRROR}/software/scm/git/git-${PV}.tar.gz;name=tarball \
              ${KERNELORG_MIRROR}/software/scm/git/git-manpages-${PV}.tar.gz;name=manpages"

   SRC_URI[tarball.md5sum] = "166bde96adbbc11c8843d4f8f4f9811b"
   SRC_URI[tarball.sha256sum] = "ad5334956301c86841eb1e5b1bb20884a6bad89a10a6762c958220c7cf64da02"
   SRC_URI[manpages.md5sum] = "31c2272a8979022497ba3d4202df145d"
   SRC_URI[manpages.sha256sum] = "9a7ae3a093bea39770eb96ca3e5b40bff7af0b9f6123f089d7821d0e5b8e1230"

Proper values for ``md5`` and ``sha256`` checksums might be available
with other signatures on the download page for the upstream source (e.g.
``md5``, ``sha1``, ``sha256``, ``GPG``, and so forth). Because the
OpenEmbedded build system only deals with ``sha256sum`` and ``md5sum``,
you should verify all the signatures you find by hand.

If no :term:`SRC_URI` checksums are specified when you attempt to build the
recipe, or you provide an incorrect checksum, the build will produce an
error for each missing or incorrect checksum. As part of the error
message, the build system provides the checksum string corresponding to
the fetched file. Once you have the correct checksums, you can copy and
paste them into your recipe and then run the build again to continue.

.. note::

   As mentioned, if the upstream source provides signatures for
   verifying the downloaded source code, you should verify those
   manually before setting the checksum values in the recipe and
   continuing with the build.

This final example is a bit more complicated and is from the
``meta/recipes-sato/rxvt-unicode/rxvt-unicode_9.20.bb`` recipe. The
example's :term:`SRC_URI` statement identifies multiple files as the source
files for the recipe: a tarball, a patch file, a desktop file, and an icon::

   SRC_URI = "http://dist.schmorp.de/rxvt-unicode/Attic/rxvt-unicode-${PV}.tar.bz2 \
              file://xwc.patch \
              file://rxvt.desktop \
              file://rxvt.png"

When you specify local files using the ``file://`` URI protocol, the
build system fetches files from the local machine. The path is relative
to the :term:`FILESPATH` variable
and searches specific directories in a certain order:
``${``\ :term:`BP`\ ``}``,
``${``\ :term:`BPN`\ ``}``, and
``files``. The directories are assumed to be subdirectories of the
directory in which the recipe or append file resides. For another
example that specifies these types of files, see the
"`building a single .c file package`_" section.

The previous example also specifies a patch file. Patch files are files
whose names usually end in ``.patch`` or ``.diff`` but can end with
compressed suffixes such as ``diff.gz`` and ``patch.bz2``, for example.
The build system automatically applies patches as described in the
":ref:`dev-manual/new-recipe:patching code`" section.

Fetching Code Through Firewalls
-------------------------------

Some users are behind firewalls and need to fetch code through a proxy.
See the ":doc:`/ref-manual/faq`" chapter for advice.

Limiting the Number of Parallel Connections
-------------------------------------------

Some users are behind firewalls or use servers where the number of parallel
connections is limited. In such cases, you can limit the number of fetch
tasks being run in parallel by adding the following to your ``local.conf``
file::

   do_fetch[number_threads] = "4"

Unpacking Code
==============

During the build, the
:ref:`ref-tasks-unpack` task unpacks
the source with ``${``\ :term:`S`\ ``}``
pointing to where it is unpacked.

If you are fetching your source files from an upstream source archived
tarball and the tarball's internal structure matches the common
convention of a top-level subdirectory named
``${``\ :term:`BPN`\ ``}-${``\ :term:`PV`\ ``}``,
then you do not need to set :term:`S`. However, if :term:`SRC_URI` specifies to
fetch source from an archive that does not use this convention, or from
an SCM like Git or Subversion, your recipe needs to define :term:`S`.

If processing your recipe using BitBake successfully unpacks the source
files, you need to be sure that the directory pointed to by ``${S}``
matches the structure of the source.

Patching Code
=============

Sometimes it is necessary to patch code after it has been fetched. Any
files mentioned in :term:`SRC_URI` whose names end in ``.patch`` or
``.diff`` or compressed versions of these suffixes (e.g. ``diff.gz`` are
treated as patches. The
:ref:`ref-tasks-patch` task
automatically applies these patches.

The build system should be able to apply patches with the "-p1" option
(i.e. one directory level in the path will be stripped off). If your
patch needs to have more directory levels stripped off, specify the
number of levels using the "striplevel" option in the :term:`SRC_URI` entry
for the patch. Alternatively, if your patch needs to be applied in a
specific subdirectory that is not specified in the patch file, use the
"patchdir" option in the entry.

As with all local files referenced in
:term:`SRC_URI` using ``file://``,
you should place patch files in a directory next to the recipe either
named the same as the base name of the recipe
(:term:`BP` and
:term:`BPN`) or "files".

Licensing
=========

Your recipe needs to have both the
:term:`LICENSE` and
:term:`LIC_FILES_CHKSUM`
variables:

-  :term:`LICENSE`: This variable specifies the license for the software.
   If you do not know the license under which the software you are
   building is distributed, you should go to the source code and look
   for that information. Typical files containing this information
   include ``COPYING``, :term:`LICENSE`, and ``README`` files. You could
   also find the information near the top of a source file. For example,
   given a piece of software licensed under the GNU General Public
   License version 2, you would set :term:`LICENSE` as follows::

      LICENSE = "GPL-2.0-only"

   The licenses you specify within :term:`LICENSE` can have any name as long
   as you do not use spaces, since spaces are used as separators between
   license names. For standard licenses, use the names of the files in
   ``meta/files/common-licenses/`` or the :term:`SPDXLICENSEMAP` flag names
   defined in ``meta/conf/licenses.conf``.

-  :term:`LIC_FILES_CHKSUM`: The OpenEmbedded build system uses this
   variable to make sure the license text has not changed. If it has,
   the build produces an error and it affords you the chance to figure
   it out and correct the problem.

   You need to specify all applicable licensing files for the software.
   At the end of the configuration step, the build process will compare
   the checksums of the files to be sure the text has not changed. Any
   differences result in an error with the message containing the
   current checksum. For more explanation and examples of how to set the
   :term:`LIC_FILES_CHKSUM` variable, see the
   ":ref:`dev-manual/licenses:tracking license changes`" section.

   To determine the correct checksum string, you can list the
   appropriate files in the :term:`LIC_FILES_CHKSUM` variable with incorrect
   md5 strings, attempt to build the software, and then note the
   resulting error messages that will report the correct md5 strings.
   See the ":ref:`dev-manual/new-recipe:fetching code`" section for
   additional information.

   Here is an example that assumes the software has a ``COPYING`` file::

      LIC_FILES_CHKSUM = "file://COPYING;md5=xxx"

   When you try to build the
   software, the build system will produce an error and give you the
   correct string that you can substitute into the recipe file for a
   subsequent build.

Dependencies
============

Most software packages have a short list of other packages that they
require, which are called dependencies. These dependencies fall into two
main categories: build-time dependencies, which are required when the
software is built; and runtime dependencies, which are required to be
installed on the target in order for the software to run.

Within a recipe, you specify build-time dependencies using the
:term:`DEPENDS` variable. Although there are nuances,
items specified in :term:`DEPENDS` should be names of other
recipes. It is important that you specify all build-time dependencies
explicitly.

Another consideration is that configure scripts might automatically
check for optional dependencies and enable corresponding functionality
if those dependencies are found. If you wish to make a recipe that is
more generally useful (e.g. publish the recipe in a layer for others to
use), instead of hard-disabling the functionality, you can use the
:term:`PACKAGECONFIG` variable to allow functionality and the
corresponding dependencies to be enabled and disabled easily by other
users of the recipe.

Similar to build-time dependencies, you specify runtime dependencies
through a variable -
:term:`RDEPENDS`, which is
package-specific. All variables that are package-specific need to have
the name of the package added to the end as an override. Since the main
package for a recipe has the same name as the recipe, and the recipe's
name can be found through the
``${``\ :term:`PN`\ ``}`` variable, then
you specify the dependencies for the main package by setting
``RDEPENDS:${PN}``. If the package were named ``${PN}-tools``, then you
would set ``RDEPENDS:${PN}-tools``, and so forth.

Some runtime dependencies will be set automatically at packaging time.
These dependencies include any shared library dependencies (i.e. if a
package "example" contains "libexample" and another package "mypackage"
contains a binary that links to "libexample" then the OpenEmbedded build
system will automatically add a runtime dependency to "mypackage" on
"example"). See the
":ref:`overview-manual/concepts:automatically added runtime dependencies`"
section in the Yocto Project Overview and Concepts Manual for further
details.

Configuring the Recipe
======================

Most software provides some means of setting build-time configuration
options before compilation. Typically, setting these options is
accomplished by running a configure script with options, or by modifying
a build configuration file.

.. note::

   As of Yocto Project Release 1.7, some of the core recipes that
   package binary configuration scripts now disable the scripts due to
   the scripts previously requiring error-prone path substitution. The
   OpenEmbedded build system uses ``pkg-config`` now, which is much more
   robust. You can find a list of the ``*-config`` scripts that are disabled
   in the ":ref:`migration-1.7-binary-configuration-scripts-disabled`" section
   in the Yocto Project Reference Manual.

A major part of build-time configuration is about checking for
build-time dependencies and possibly enabling optional functionality as
a result. You need to specify any build-time dependencies for the
software you are building in your recipe's
:term:`DEPENDS` value, in terms of
other recipes that satisfy those dependencies. You can often find
build-time or runtime dependencies described in the software's
documentation.

The following list provides configuration items of note based on how
your software is built:

-  *Autotools:* If your source files have a ``configure.ac`` file, then
   your software is built using Autotools. If this is the case, you just
   need to modify the configuration.

   When using Autotools, your recipe needs to inherit the
   :ref:`ref-classes-autotools` class and it does not have to
   contain a :ref:`ref-tasks-configure` task. However, you might still want to
   make some adjustments. For example, you can set :term:`EXTRA_OECONF` or
   :term:`PACKAGECONFIG_CONFARGS` to pass any needed configure options that
   are specific to the recipe.

-  *CMake:* If your source files have a ``CMakeLists.txt`` file, then
   your software is built using CMake. If this is the case, you just
   need to modify the configuration.

   When you use CMake, your recipe needs to inherit the
   :ref:`ref-classes-cmake` class and it does not have to contain a
   :ref:`ref-tasks-configure` task. You can make some adjustments by setting
   :term:`EXTRA_OECMAKE` to pass any needed configure options that are
   specific to the recipe.

   .. note::

      If you need to install one or more custom CMake toolchain files
      that are supplied by the application you are building, install the
      files to ``${D}${datadir}/cmake/Modules`` during :ref:`ref-tasks-install`.

-  *Other:* If your source files do not have a ``configure.ac`` or
   ``CMakeLists.txt`` file, then your software is built using some
   method other than Autotools or CMake. If this is the case, you
   normally need to provide a
   :ref:`ref-tasks-configure` task
   in your recipe unless, of course, there is nothing to configure.

   Even if your software is not being built by Autotools or CMake, you
   still might not need to deal with any configuration issues. You need
   to determine if configuration is even a required step. You might need
   to modify a Makefile or some configuration file used for the build to
   specify necessary build options. Or, perhaps you might need to run a
   provided, custom configure script with the appropriate options.

   For the case involving a custom configure script, you would run
   ``./configure --help`` and look for the options you need to set.

Once configuration succeeds, it is always good practice to look at the
``log.do_configure`` file to ensure that the appropriate options have
been enabled and no additional build-time dependencies need to be added
to :term:`DEPENDS`. For example, if the configure script reports that it
found something not mentioned in :term:`DEPENDS`, or that it did not find
something that it needed for some desired optional functionality, then
you would need to add those to :term:`DEPENDS`. Looking at the log might
also reveal items being checked for, enabled, or both that you do not
want, or items not being found that are in :term:`DEPENDS`, in which case
you would need to look at passing extra options to the configure script
as needed. For reference information on configure options specific to
the software you are building, you can consult the output of the
``./configure --help`` command within ``${S}`` or consult the software's
upstream documentation.

Using Headers to Interface with Devices
=======================================

If your recipe builds an application that needs to communicate with some
device or needs an API into a custom kernel, you will need to provide
appropriate header files. Under no circumstances should you ever modify
the existing
``meta/recipes-kernel/linux-libc-headers/linux-libc-headers.inc`` file.
These headers are used to build ``libc`` and must not be compromised
with custom or machine-specific header information. If you customize
``libc`` through modified headers all other applications that use
``libc`` thus become affected.

.. note::

   Never copy and customize the ``libc`` header file (i.e.
   ``meta/recipes-kernel/linux-libc-headers/linux-libc-headers.inc``).

The correct way to interface to a device or custom kernel is to use a
separate package that provides the additional headers for the driver or
other unique interfaces. When doing so, your application also becomes
responsible for creating a dependency on that specific provider.

Consider the following:

-  Never modify ``linux-libc-headers.inc``. Consider that file to be
   part of the ``libc`` system, and not something you use to access the
   kernel directly. You should access ``libc`` through specific ``libc``
   calls.

-  Applications that must talk directly to devices should either provide
   necessary headers themselves, or establish a dependency on a special
   headers package that is specific to that driver.

For example, suppose you want to modify an existing header that adds I/O
control or network support. If the modifications are used by a small
number programs, providing a unique version of a header is easy and has
little impact. When doing so, bear in mind the guidelines in the
previous list.

.. note::

   If for some reason your changes need to modify the behavior of the ``libc``,
   and subsequently all other applications on the system, use a ``.bbappend``
   to modify the ``linux-kernel-headers.inc`` file. However, take care to not
   make the changes machine specific.

Consider a case where your kernel is older and you need an older
``libc`` ABI. The headers installed by your recipe should still be a
standard mainline kernel, not your own custom one.

When you use custom kernel headers you need to get them from
:term:`STAGING_KERNEL_DIR`,
which is the directory with kernel headers that are required to build
out-of-tree modules. Your recipe will also need the following::

   do_configure[depends] += "virtual/kernel:do_shared_workdir"

Compilation
===========

During a build, the :ref:`ref-tasks-compile` task happens after source is fetched,
unpacked, and configured. If the recipe passes through :ref:`ref-tasks-compile`
successfully, nothing needs to be done.

However, if the compile step fails, you need to diagnose the failure.
Here are some common issues that cause failures.

.. note::

   For cases where improper paths are detected for configuration files
   or for when libraries/headers cannot be found, be sure you are using
   the more robust ``pkg-config``. See the note in section
   ":ref:`dev-manual/new-recipe:Configuring the Recipe`" for additional information.

-  *Parallel build failures:* These failures manifest themselves as
   intermittent errors, or errors reporting that a file or directory
   that should be created by some other part of the build process could
   not be found. This type of failure can occur even if, upon
   inspection, the file or directory does exist after the build has
   failed, because that part of the build process happened in the wrong
   order.

   To fix the problem, you need to either satisfy the missing dependency
   in the Makefile or whatever script produced the Makefile, or (as a
   workaround) set :term:`PARALLEL_MAKE` to an empty string::

      PARALLEL_MAKE = ""

   For information on parallel Makefile issues, see the
   ":ref:`dev-manual/debugging:debugging parallel make races`" section.

-  *Improper host path usage:* This failure applies to recipes building
   for the target or ":ref:`ref-classes-nativesdk`" only. The
   failure occurs when the compilation process uses improper headers,
   libraries, or other files from the host system when cross-compiling for
   the target.

   To fix the problem, examine the ``log.do_compile`` file to identify
   the host paths being used (e.g. ``/usr/include``, ``/usr/lib``, and
   so forth) and then either add configure options, apply a patch, or do
   both.

-  *Failure to find required libraries/headers:* If a build-time
   dependency is missing because it has not been declared in
   :term:`DEPENDS`, or because the
   dependency exists but the path used by the build process to find the
   file is incorrect and the configure step did not detect it, the
   compilation process could fail. For either of these failures, the
   compilation process notes that files could not be found. In these
   cases, you need to go back and add additional options to the
   configure script as well as possibly add additional build-time
   dependencies to :term:`DEPENDS`.

   Occasionally, it is necessary to apply a patch to the source to
   ensure the correct paths are used. If you need to specify paths to
   find files staged into the sysroot from other recipes, use the
   variables that the OpenEmbedded build system provides (e.g.
   :term:`STAGING_BINDIR`, :term:`STAGING_INCDIR`, :term:`STAGING_DATADIR`, and so
   forth).

Installing
==========

During :ref:`ref-tasks-install`, the task copies the built files along with their
hierarchy to locations that would mirror their locations on the target
device. The installation process copies files from the
``${``\ :term:`S`\ ``}``,
``${``\ :term:`B`\ ``}``, and
``${``\ :term:`WORKDIR`\ ``}``
directories to the ``${``\ :term:`D`\ ``}``
directory to create the structure as it should appear on the target
system.

How your software is built affects what you must do to be sure your
software is installed correctly. The following list describes what you
must do for installation depending on the type of build system used by
the software being built:

-  *Autotools and CMake:* If the software your recipe is building uses
   Autotools or CMake, the OpenEmbedded build system understands how to
   install the software. Consequently, you do not have to have a
   :ref:`ref-tasks-install` task as part of your recipe. You just need to make
   sure the install portion of the build completes with no issues.
   However, if you wish to install additional files not already being
   installed by ``make install``, you should do this using a
   ``do_install:append`` function using the install command as described
   in the "Manual" bulleted item later in this list.

-  *Other (using* ``make install``\ *)*: You need to define a :ref:`ref-tasks-install`
   function in your recipe. The function should call
   ``oe_runmake install`` and will likely need to pass in the
   destination directory as well. How you pass that path is dependent on
   how the ``Makefile`` being run is written (e.g. ``DESTDIR=${D}``,
   ``PREFIX=${D}``, ``INSTALLROOT=${D}``, and so forth).

   For an example recipe using ``make install``, see the
   ":ref:`dev-manual/new-recipe:building a makefile-based package`" section.

-  *Manual:* You need to define a :ref:`ref-tasks-install` function in your
   recipe. The function must first use ``install -d`` to create the
   directories under
   ``${``\ :term:`D`\ ``}``. Once the
   directories exist, your function can use ``install`` to manually
   install the built software into the directories.

   You can find more information on ``install`` at
   https://www.gnu.org/software/coreutils/manual/html_node/install-invocation.html.

For the scenarios that do not use Autotools or CMake, you need to track
the installation and diagnose and fix any issues until everything
installs correctly. You need to look in the default location of
``${D}``, which is ``${WORKDIR}/image``, to be sure your files have been
installed correctly.

.. note::

   -  During the installation process, you might need to modify some of
      the installed files to suit the target layout. For example, you
      might need to replace hard-coded paths in an initscript with
      values of variables provided by the build system, such as
      replacing ``/usr/bin/`` with ``${bindir}``. If you do perform such
      modifications during :ref:`ref-tasks-install`, be sure to modify the
      destination file after copying rather than before copying.
      Modifying after copying ensures that the build system can
      re-execute :ref:`ref-tasks-install` if needed.

   -  ``oe_runmake install``, which can be run directly or can be run
      indirectly by the :ref:`ref-classes-autotools` and
      :ref:`ref-classes-cmake` classes, runs ``make install`` in parallel.
      Sometimes, a Makefile can have missing dependencies between targets that
      can result in race conditions. If you experience intermittent failures
      during :ref:`ref-tasks-install`, you might be able to work around them by
      disabling parallel Makefile installs by adding the following to the
      recipe::

         PARALLEL_MAKEINST = ""

      See :term:`PARALLEL_MAKEINST` for additional information.

   -  If you need to install one or more custom CMake toolchain files
      that are supplied by the application you are building, install the
      files to ``${D}${datadir}/cmake/Modules`` during
      :ref:`ref-tasks-install`.

Enabling System Services
========================

If you want to install a service, which is a process that usually starts
on boot and runs in the background, then you must include some
additional definitions in your recipe.

If you are adding services and the service initialization script or the
service file itself is not installed, you must provide for that
installation in your recipe using a ``do_install:append`` function. If
your recipe already has a :ref:`ref-tasks-install` function, update the function
near its end rather than adding an additional ``do_install:append``
function.

When you create the installation for your services, you need to
accomplish what is normally done by ``make install``. In other words,
make sure your installation arranges the output similar to how it is
arranged on the target system.

The OpenEmbedded build system provides support for starting services two
different ways:

-  *SysVinit:* SysVinit is a system and service manager that manages the
   init system used to control the very basic functions of your system.
   The init program is the first program started by the Linux kernel
   when the system boots. Init then controls the startup, running and
   shutdown of all other programs.

   To enable a service using SysVinit, your recipe needs to inherit the
   :ref:`ref-classes-update-rc.d` class. The class helps
   facilitate safely installing the package on the target.

   You will need to set the
   :term:`INITSCRIPT_PACKAGES`,
   :term:`INITSCRIPT_NAME`,
   and
   :term:`INITSCRIPT_PARAMS`
   variables within your recipe.

-  *systemd:* System Management Daemon (systemd) was designed to replace
   SysVinit and to provide enhanced management of services. For more
   information on systemd, see the systemd homepage at
   https://freedesktop.org/wiki/Software/systemd/.

   To enable a service using systemd, your recipe needs to inherit the
   :ref:`ref-classes-systemd` class. See the ``systemd.bbclass`` file
   located in your :term:`Source Directory` section for more information.

Packaging
=========

Successful packaging is a combination of automated processes performed
by the OpenEmbedded build system and some specific steps you need to
take. The following list describes the process:

-  *Splitting Files*: The :ref:`ref-tasks-package` task splits the files produced
   by the recipe into logical components. Even software that produces a
   single binary might still have debug symbols, documentation, and
   other logical components that should be split out. The :ref:`ref-tasks-package`
   task ensures that files are split up and packaged correctly.

-  *Running QA Checks*: The :ref:`ref-classes-insane` class adds a
   step to the package generation process so that output quality
   assurance checks are generated by the OpenEmbedded build system. This
   step performs a range of checks to be sure the build's output is free
   of common problems that show up during runtime. For information on
   these checks, see the :ref:`ref-classes-insane` class and
   the ":ref:`ref-manual/qa-checks:qa error and warning messages`"
   chapter in the Yocto Project Reference Manual.

-  *Hand-Checking Your Packages*: After you build your software, you
   need to be sure your packages are correct. Examine the
   ``${``\ :term:`WORKDIR`\ ``}/packages-split``
   directory and make sure files are where you expect them to be. If you
   discover problems, you can set
   :term:`PACKAGES`,
   :term:`FILES`,
   ``do_install(:append)``, and so forth as needed.

-  *Splitting an Application into Multiple Packages*: If you need to
   split an application into several packages, see the
   ":ref:`dev-manual/new-recipe:splitting an application into multiple packages`"
   section for an example.

-  *Installing a Post-Installation Script*: For an example showing how
   to install a post-installation script, see the
   ":ref:`dev-manual/new-recipe:post-installation scripts`" section.

-  *Marking Package Architecture*: Depending on what your recipe is
   building and how it is configured, it might be important to mark the
   packages produced as being specific to a particular machine, or to
   mark them as not being specific to a particular machine or
   architecture at all.

   By default, packages apply to any machine with the same architecture
   as the target machine. When a recipe produces packages that are
   machine-specific (e.g. the
   :term:`MACHINE` value is passed
   into the configure script or a patch is applied only for a particular
   machine), you should mark them as such by adding the following to the
   recipe::

      PACKAGE_ARCH = "${MACHINE_ARCH}"

   On the other hand, if the recipe produces packages that do not
   contain anything specific to the target machine or architecture at
   all (e.g. recipes that simply package script files or configuration
   files), you should use the :ref:`ref-classes-allarch` class to
   do this for you by adding this to your recipe::

      inherit allarch

   Ensuring that the package architecture is correct is not critical
   while you are doing the first few builds of your recipe. However, it
   is important in order to ensure that your recipe rebuilds (or does
   not rebuild) appropriately in response to changes in configuration,
   and to ensure that you get the appropriate packages installed on the
   target machine, particularly if you run separate builds for more than
   one target machine.

Sharing Files Between Recipes
=============================

Recipes often need to use files provided by other recipes on the build
host. For example, an application linking to a common library needs
access to the library itself and its associated headers. The way this
access is accomplished is by populating a sysroot with files. Each
recipe has two sysroots in its work directory, one for target files
(``recipe-sysroot``) and one for files that are native to the build host
(``recipe-sysroot-native``).

.. note::

   You could find the term "staging" used within the Yocto project
   regarding files populating sysroots (e.g. the :term:`STAGING_DIR`
   variable).

Recipes should never populate the sysroot directly (i.e. write files
into sysroot). Instead, files should be installed into standard
locations during the
:ref:`ref-tasks-install` task within
the ``${``\ :term:`D`\ ``}`` directory. The
reason for this limitation is that almost all files that populate the
sysroot are cataloged in manifests in order to ensure the files can be
removed later when a recipe is either modified or removed. Thus, the
sysroot is able to remain free from stale files.

A subset of the files installed by the :ref:`ref-tasks-install` task are
used by the :ref:`ref-tasks-populate_sysroot` task as defined by the
:term:`SYSROOT_DIRS` variable to automatically populate the sysroot. It
is possible to modify the list of directories that populate the sysroot.
The following example shows how you could add the ``/opt`` directory to
the list of directories within a recipe::

   SYSROOT_DIRS += "/opt"

.. note::

   The `/sysroot-only` is to be used by recipes that generate artifacts
   that are not included in the target filesystem, allowing them to share
   these artifacts without needing to use the :term:`DEPLOY_DIR`.

For a more complete description of the :ref:`ref-tasks-populate_sysroot`
task and its associated functions, see the
:ref:`staging <ref-classes-staging>` class.

Using Virtual Providers
=======================

Prior to a build, if you know that several different recipes provide the
same functionality, you can use a virtual provider (i.e. ``virtual/*``)
as a placeholder for the actual provider. The actual provider is
determined at build-time.

A common scenario where a virtual provider is used would be for the kernel
recipe. Suppose you have three kernel recipes whose :term:`PN` values map to
``kernel-big``, ``kernel-mid``, and ``kernel-small``. Furthermore, each of
these recipes in some way uses a :term:`PROVIDES` statement that essentially
identifies itself as being able to provide ``virtual/kernel``. Here is one way
through the :ref:`ref-classes-kernel` class::

   PROVIDES += "virtual/kernel"

Any recipe that inherits the :ref:`ref-classes-kernel` class is
going to utilize a :term:`PROVIDES` statement that identifies that recipe as
being able to provide the ``virtual/kernel`` item.

Now comes the time to actually build an image and you need a kernel
recipe, but which one? You can configure your build to call out the
kernel recipe you want by using the :term:`PREFERRED_PROVIDER` variable. As
an example, consider the :yocto_git:`x86-base.inc
</poky/tree/meta/conf/machine/include/x86/x86-base.inc>` include file, which is a
machine (i.e. :term:`MACHINE`) configuration file. This include file is the
reason all x86-based machines use the ``linux-yocto`` kernel. Here are the
relevant lines from the include file::

   PREFERRED_PROVIDER_virtual/kernel ??= "linux-yocto"
   PREFERRED_VERSION_linux-yocto ??= "4.15%"

When you use a virtual provider, you do not have to "hard code" a recipe
name as a build dependency. You can use the
:term:`DEPENDS` variable to state the
build is dependent on ``virtual/kernel`` for example::

   DEPENDS = "virtual/kernel"

During the build, the OpenEmbedded build system picks
the correct recipe needed for the ``virtual/kernel`` dependency based on
the :term:`PREFERRED_PROVIDER` variable. If you want to use the small kernel
mentioned at the beginning of this section, configure your build as
follows::

   PREFERRED_PROVIDER_virtual/kernel ??= "kernel-small"

.. note::

   Any recipe that :term:`PROVIDES` a ``virtual/*`` item that is ultimately not
   selected through :term:`PREFERRED_PROVIDER` does not get built. Preventing these
   recipes from building is usually the desired behavior since this mechanism's
   purpose is to select between mutually exclusive alternative providers.

The following lists specific examples of virtual providers:

-  ``virtual/kernel``: Provides the name of the kernel recipe to use
   when building a kernel image.

-  ``virtual/bootloader``: Provides the name of the bootloader to use
   when building an image.

-  ``virtual/libgbm``: Provides ``gbm.pc``.

-  ``virtual/egl``: Provides ``egl.pc`` and possibly ``wayland-egl.pc``.

-  ``virtual/libgl``: Provides ``gl.pc`` (i.e. libGL).

-  ``virtual/libgles1``: Provides ``glesv1_cm.pc`` (i.e. libGLESv1_CM).

-  ``virtual/libgles2``: Provides ``glesv2.pc`` (i.e. libGLESv2).

.. note::

   Virtual providers only apply to build time dependencies specified with
   :term:`PROVIDES` and :term:`DEPENDS`. They do not apply to runtime
   dependencies specified with :term:`RPROVIDES` and :term:`RDEPENDS`.

Properly Versioning Pre-Release Recipes
=======================================

Sometimes the name of a recipe can lead to versioning problems when the
recipe is upgraded to a final release. For example, consider the
``irssi_0.8.16-rc1.bb`` recipe file in the list of example recipes in
the ":ref:`dev-manual/new-recipe:storing and naming the recipe`" section.
This recipe is at a release candidate stage (i.e. "rc1"). When the recipe is
released, the recipe filename becomes ``irssi_0.8.16.bb``. The version
change from ``0.8.16-rc1`` to ``0.8.16`` is seen as a decrease by the
build system and package managers, so the resulting packages will not
correctly trigger an upgrade.

In order to ensure the versions compare properly, the recommended
convention is to set :term:`PV` within the
recipe to "previous_version+current_version". You can use an additional
variable so that you can use the current version elsewhere. Here is an
example::

   REALPV = "0.8.16-rc1"
   PV = "0.8.15+${REALPV}"

Post-Installation Scripts
=========================

Post-installation scripts run immediately after installing a package on
the target or during image creation when a package is included in an
image. To add a post-installation script to a package, add a
``pkg_postinst:``\ `PACKAGENAME`\ ``()`` function to the recipe file
(``.bb``) and replace `PACKAGENAME` with the name of the package you want
to attach to the ``postinst`` script. To apply the post-installation
script to the main package for the recipe, which is usually what is
required, specify
``${``\ :term:`PN`\ ``}`` in place of
PACKAGENAME.

A post-installation function has the following structure::

   pkg_postinst:PACKAGENAME() {
       # Commands to carry out
   }

The script defined in the post-installation function is called when the
root filesystem is created. If the script succeeds, the package is
marked as installed.

.. note::

   Any RPM post-installation script that runs on the target should
   return a 0 exit code. RPM does not allow non-zero exit codes for
   these scripts, and the RPM package manager will cause the package to
   fail installation on the target.

Sometimes it is necessary for the execution of a post-installation
script to be delayed until the first boot. For example, the script might
need to be executed on the device itself. To delay script execution
until boot time, you must explicitly mark post installs to defer to the
target. You can use ``pkg_postinst_ontarget()`` or call
``postinst_intercept delay_to_first_boot`` from ``pkg_postinst()``. Any
failure of a ``pkg_postinst()`` script (including exit 1) triggers an
error during the
:ref:`ref-tasks-rootfs` task.

If you have recipes that use ``pkg_postinst`` function and they require
the use of non-standard native tools that have dependencies during
root filesystem construction, you need to use the
:term:`PACKAGE_WRITE_DEPS`
variable in your recipe to list these tools. If you do not use this
variable, the tools might be missing and execution of the
post-installation script is deferred until first boot. Deferring the
script to the first boot is undesirable and impossible for read-only
root filesystems.

.. note::

   There is equivalent support for pre-install, pre-uninstall, and post-uninstall
   scripts by way of ``pkg_preinst``, ``pkg_prerm``, and ``pkg_postrm``,
   respectively. These scrips work in exactly the same way as does
   ``pkg_postinst`` with the exception that they run at different times. Also,
   because of when they run, they are not applicable to being run at image
   creation time like ``pkg_postinst``.

Testing
=======

The final step for completing your recipe is to be sure that the
software you built runs correctly. To accomplish runtime testing, add
the build's output packages to your image and test them on the target.

For information on how to customize your image by adding specific
packages, see ":ref:`dev-manual/customizing-images:customizing images`" section.

Examples
========

To help summarize how to write a recipe, this section provides some
recipe examples given various scenarios:

-  `Building a single .c file package`_

-  `Building a Makefile-based package`_

-  `Building an Autotooled package`_

-  `Building a Meson package`_

-  `Splitting an application into multiple packages`_

-  `Packaging externally produced binaries`_

Building a Single .c File Package
---------------------------------

Building an application from a single file that is stored locally (e.g. under
``files``) requires a recipe that has the file listed in the :term:`SRC_URI`
variable. Additionally, you need to manually write the :ref:`ref-tasks-compile`
and :ref:`ref-tasks-install` tasks. The :term:`S` variable defines the
directory containing the source code, which is set to :term:`WORKDIR` in this
case --- the directory BitBake uses for the build::

   SUMMARY = "Simple helloworld application"
   SECTION = "examples"
   LICENSE = "MIT"
   LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"

   SRC_URI = "file://helloworld.c"

   S = "${WORKDIR}"

   do_compile() {
       ${CC} ${LDFLAGS} helloworld.c -o helloworld
   }

   do_install() {
       install -d ${D}${bindir}
       install -m 0755 helloworld ${D}${bindir}
   }

By default, the ``helloworld``, ``helloworld-dbg``, and ``helloworld-dev`` packages
are built. For information on how to customize the packaging process, see the
":ref:`dev-manual/new-recipe:splitting an application into multiple packages`"
section.

Building a Makefile-Based Package
---------------------------------

Applications built with GNU ``make`` require a recipe that has the source archive
listed in :term:`SRC_URI`. You do not need to add a :ref:`ref-tasks-compile`
step since by default BitBake starts the ``make`` command to compile the
application. If you need additional ``make`` options, you should store them in
the :term:`EXTRA_OEMAKE` or :term:`PACKAGECONFIG_CONFARGS` variables. BitBake
passes these options into the GNU ``make`` invocation. Note that a
:ref:`ref-tasks-install` task is still required. Otherwise, BitBake runs an
empty :ref:`ref-tasks-install` task by default.

Some applications might require extra parameters to be passed to the
compiler. For example, the application might need an additional header
path. You can accomplish this by adding to the :term:`CFLAGS` variable. The
following example shows this::

   CFLAGS:prepend = "-I ${S}/include "

In the following example, ``lz4`` is a makefile-based package::

   SUMMARY = "Extremely Fast Compression algorithm"
   DESCRIPTION = "LZ4 is a very fast lossless compression algorithm, providing compression speed at 400 MB/s per core, scalable with multi-cores CPU. It also features an extremely fast decoder, with speed in multiple GB/s per core, typically reaching RAM speed limits on multi-core systems."
   HOMEPAGE = "https://github.com/lz4/lz4"

   LICENSE = "BSD-2-Clause | GPL-2.0-only"
   LIC_FILES_CHKSUM = "file://lib/LICENSE;md5=ebc2ea4814a64de7708f1571904b32cc \
                       file://programs/COPYING;md5=b234ee4d69f5fce4486a80fdaf4a4263 \
                       file://LICENSE;md5=d57c0d21cb917fb4e0af2454aa48b956 \
                       "

   PE = "1"

   SRCREV = "d44371841a2f1728a3f36839fd4b7e872d0927d3"

   SRC_URI = "git://github.com/lz4/lz4.git;branch=release;protocol=https \
              file://CVE-2021-3520.patch \
              "
   UPSTREAM_CHECK_GITTAGREGEX = "v(?P<pver>.*)"

   S = "${WORKDIR}/git"

   # Fixed in r118, which is larger than the current version.
   CVE_CHECK_IGNORE += "CVE-2014-4715"

   EXTRA_OEMAKE = "PREFIX=${prefix} CC='${CC}' CFLAGS='${CFLAGS}' DESTDIR=${D} LIBDIR=${libdir} INCLUDEDIR=${includedir} BUILD_STATIC=no"

   do_install() {
           oe_runmake install
   }

   BBCLASSEXTEND = "native nativesdk"

Building an Autotooled Package
------------------------------

Applications built with the Autotools such as ``autoconf`` and ``automake``
require a recipe that has a source archive listed in :term:`SRC_URI` and also
inherit the :ref:`ref-classes-autotools` class, which contains the definitions
of all the steps needed to build an Autotool-based application. The result of
the build is automatically packaged. And, if the application uses NLS for
localization, packages with local information are generated (one package per
language). Following is one example: (``hello_2.3.bb``)::

   SUMMARY = "GNU Helloworld application"
   SECTION = "examples"
   LICENSE = "GPL-2.0-or-later"
   LIC_FILES_CHKSUM = "file://COPYING;md5=751419260aa954499f7abaabaa882bbe"

   SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.gz"

   inherit autotools gettext

The variable :term:`LIC_FILES_CHKSUM` is used to track source license changes
as described in the ":ref:`dev-manual/licenses:tracking license changes`"
section in the Yocto Project Overview and Concepts Manual. You can quickly
create Autotool-based recipes in a manner similar to the previous example.

Building a Meson Package
------------------------

Applications built with the `Meson build system <https://mesonbuild.com/>`__
just need a recipe that has sources described in :term:`SRC_URI` and inherits
the :ref:`ref-classes-meson` class.

The :oe_git:`ipcalc recipe </meta-openembedded/tree/meta-networking/recipes-support/ipcalc>`
is a simple example of an application without dependencies::

   SUMMARY = "Tool to assist in network address calculations for IPv4 and IPv6."
   HOMEPAGE = "https://gitlab.com/ipcalc/ipcalc"

   SECTION = "net"

   LICENSE = "GPL-2.0-only"
   LIC_FILES_CHKSUM = "file://COPYING;md5=b234ee4d69f5fce4486a80fdaf4a4263"

   SRC_URI = "git://gitlab.com/ipcalc/ipcalc.git;protocol=https;branch=master"
   SRCREV = "4c4261a47f355946ee74013d4f5d0494487cc2d6"

   S = "${WORKDIR}/git"

   inherit meson

Applications with dependencies are likely to inherit the
:ref:`ref-classes-pkgconfig` class, as ``pkg-config`` is the default method
used by Meson to find dependencies and compile applications against them.

Splitting an Application into Multiple Packages
-----------------------------------------------

You can use the variables :term:`PACKAGES` and :term:`FILES` to split an
application into multiple packages.

Following is an example that uses the ``libxpm`` recipe. By default,
this recipe generates a single package that contains the library along
with a few binaries. You can modify the recipe to split the binaries
into separate packages::

   require xorg-lib-common.inc

   SUMMARY = "Xpm: X Pixmap extension library"
   LICENSE = "MIT"
   LIC_FILES_CHKSUM = "file://COPYING;md5=51f4270b012ecd4ab1a164f5f4ed6cf7"
   DEPENDS += "libxext libsm libxt"
   PE = "1"

   XORG_PN = "libXpm"

   PACKAGES =+ "sxpm cxpm"
   FILES:cxpm = "${bindir}/cxpm"
   FILES:sxpm = "${bindir}/sxpm"

In the previous example, we want to ship the ``sxpm`` and ``cxpm``
binaries in separate packages. Since ``bindir`` would be packaged into
the main :term:`PN` package by default, we prepend the :term:`PACKAGES` variable
so additional package names are added to the start of list. This results
in the extra ``FILES:*`` variables then containing information that
define which files and directories go into which packages. Files
included by earlier packages are skipped by latter packages. Thus, the
main :term:`PN` package does not include the above listed files.

Packaging Externally Produced Binaries
--------------------------------------

Sometimes, you need to add pre-compiled binaries to an image. For
example, suppose that there are binaries for proprietary code,
created by a particular division of a company. Your part of the company
needs to use those binaries as part of an image that you are building
using the OpenEmbedded build system. Since you only have the binaries
and not the source code, you cannot use a typical recipe that expects to
fetch the source specified in
:term:`SRC_URI` and then compile it.

One method is to package the binaries and then install them as part of
the image. Generally, it is not a good idea to package binaries since,
among other things, it can hinder the ability to reproduce builds and
could lead to compatibility problems with ABI in the future. However,
sometimes you have no choice.

The easiest solution is to create a recipe that uses the
:ref:`ref-classes-bin-package` class and to be sure that you are using default
locations for build artifacts.  In most cases, the
:ref:`ref-classes-bin-package` class handles "skipping" the configure and
compile steps as well as sets things up to grab packages from the appropriate
area. In particular, this class sets ``noexec`` on both the
:ref:`ref-tasks-configure` and :ref:`ref-tasks-compile` tasks, sets
``FILES:${PN}`` to "/" so that it picks up all files, and sets up a
:ref:`ref-tasks-install` task, which effectively copies all files from ``${S}``
to ``${D}``. The :ref:`ref-classes-bin-package` class works well when the files
extracted into ``${S}`` are already laid out in the way they should be laid out
on the target. For more information on these variables, see the :term:`FILES`,
:term:`PN`, :term:`S`, and :term:`D` variables in the Yocto Project Reference
Manual's variable glossary.

.. note::

   -  Using :term:`DEPENDS` is a good
      idea even for components distributed in binary form, and is often
      necessary for shared libraries. For a shared library, listing the
      library dependencies in :term:`DEPENDS` makes sure that the libraries
      are available in the staging sysroot when other recipes link
      against the library, which might be necessary for successful
      linking.

   -  Using :term:`DEPENDS` also allows runtime dependencies between
      packages to be added automatically. See the
      ":ref:`overview-manual/concepts:automatically added runtime dependencies`"
      section in the Yocto Project Overview and Concepts Manual for more
      information.

If you cannot use the :ref:`ref-classes-bin-package` class, you need to be sure you are
doing the following:

-  Create a recipe where the
   :ref:`ref-tasks-configure` and
   :ref:`ref-tasks-compile` tasks do
   nothing: It is usually sufficient to just not define these tasks in
   the recipe, because the default implementations do nothing unless a
   Makefile is found in
   ``${``\ :term:`S`\ ``}``.

   If ``${S}`` might contain a Makefile, or if you inherit some class
   that replaces :ref:`ref-tasks-configure` and :ref:`ref-tasks-compile` with custom
   versions, then you can use the
   ``[``\ :ref:`noexec <bitbake-user-manual/bitbake-user-manual-metadata:variable flags>`\ ``]``
   flag to turn the tasks into no-ops, as follows::

      do_configure[noexec] = "1"
      do_compile[noexec] = "1"

   Unlike :ref:`bitbake-user-manual/bitbake-user-manual-metadata:deleting a task`,
   using the flag preserves the dependency chain from the :ref:`ref-tasks-fetch`,
   :ref:`ref-tasks-unpack`, and :ref:`ref-tasks-patch` tasks to the
   :ref:`ref-tasks-install` task.

-  Make sure your :ref:`ref-tasks-install` task installs the binaries
   appropriately.

-  Ensure that you set up :term:`FILES`
   (usually
   ``FILES:${``\ :term:`PN`\ ``}``) to
   point to the files you have installed, which of course depends on
   where you have installed them and whether those files are in
   different locations than the defaults.

Following Recipe Style Guidelines
=================================

When writing recipes, it is good to conform to existing style
guidelines. The :oe_wiki:`OpenEmbedded Styleguide </Styleguide>` wiki page
provides rough guidelines for preferred recipe style.

It is common for existing recipes to deviate a bit from this style.
However, aiming for at least a consistent style is a good idea. Some
practices, such as omitting spaces around ``=`` operators in assignments
or ordering recipe components in an erratic way, are widely seen as poor
style.

Recipe Syntax
=============

Understanding recipe file syntax is important for writing recipes. The
following list overviews the basic items that make up a BitBake recipe
file. For more complete BitBake syntax descriptions, see the
":doc:`bitbake:bitbake-user-manual/bitbake-user-manual-metadata`"
chapter of the BitBake User Manual.

-  *Variable Assignments and Manipulations:* Variable assignments allow
   a value to be assigned to a variable. The assignment can be static
   text or might include the contents of other variables. In addition to
   the assignment, appending and prepending operations are also
   supported.

   The following example shows some of the ways you can use variables in
   recipes::

      S = "${WORKDIR}/postfix-${PV}"
      CFLAGS += "-DNO_ASM"
      CFLAGS:append = " --enable-important-feature"

-  *Functions:* Functions provide a series of actions to be performed.
   You usually use functions to override the default implementation of a
   task function or to complement a default function (i.e. append or
   prepend to an existing function). Standard functions use ``sh`` shell
   syntax, although access to OpenEmbedded variables and internal
   methods are also available.

   Here is an example function from the ``sed`` recipe::

      do_install () {
          autotools_do_install
          install -d ${D}${base_bindir}
          mv ${D}${bindir}/sed ${D}${base_bindir}/sed
          rmdir ${D}${bindir}/
      }

   It is
   also possible to implement new functions that are called between
   existing tasks as long as the new functions are not replacing or
   complementing the default functions. You can implement functions in
   Python instead of shell. Both of these options are not seen in the
   majority of recipes.

-  *Keywords:* BitBake recipes use only a few keywords. You use keywords
   to include common functions (``inherit``), load parts of a recipe
   from other files (``include`` and ``require``) and export variables
   to the environment (``export``).

   The following example shows the use of some of these keywords::

      export POSTCONF = "${STAGING_BINDIR}/postconf"
      inherit autoconf
      require otherfile.inc

-  *Comments (#):* Any lines that begin with the hash character (``#``)
   are treated as comment lines and are ignored::

      # This is a comment

This next list summarizes the most important and most commonly used
parts of the recipe syntax. For more information on these parts of the
syntax, you can reference the
":doc:`bitbake:bitbake-user-manual/bitbake-user-manual-metadata`" chapter
in the BitBake User Manual.

-  *Line Continuation (\\):* Use the backward slash (``\``) character to
   split a statement over multiple lines. Place the slash character at
   the end of the line that is to be continued on the next line::

       VAR = "A really long \
              line"

   .. note::

      You cannot have any characters including spaces or tabs after the
      slash character.

-  *Using Variables (${VARNAME}):* Use the ``${VARNAME}`` syntax to
   access the contents of a variable::

      SRC_URI = "${SOURCEFORGE_MIRROR}/libpng/zlib-${PV}.tar.gz"

   .. note::

      It is important to understand that the value of a variable
      expressed in this form does not get substituted automatically. The
      expansion of these expressions happens on-demand later (e.g.
      usually when a function that makes reference to the variable
      executes). This behavior ensures that the values are most
      appropriate for the context in which they are finally used. On the
      rare occasion that you do need the variable expression to be
      expanded immediately, you can use the
      :=
      operator instead of
      =
      when you make the assignment, but this is not generally needed.

-  *Quote All Assignments ("value"):* Use double quotes around values in
   all variable assignments (e.g. ``"value"``). Following is an example::

      VAR1 = "${OTHERVAR}"
      VAR2 = "The version is ${PV}"

-  *Conditional Assignment (?=):* Conditional assignment is used to
   assign a value to a variable, but only when the variable is currently
   unset. Use the question mark followed by the equal sign (``?=``) to
   make a "soft" assignment used for conditional assignment. Typically,
   "soft" assignments are used in the ``local.conf`` file for variables
   that are allowed to come through from the external environment.

   Here is an example where ``VAR1`` is set to "New value" if it is
   currently empty. However, if ``VAR1`` has already been set, it
   remains unchanged::

      VAR1 ?= "New value"

   In this next example, ``VAR1`` is left with the value "Original value"::

      VAR1 = "Original value"
      VAR1 ?= "New value"

-  *Appending (+=):* Use the plus character followed by the equals sign
   (``+=``) to append values to existing variables.

   .. note::

      This operator adds a space between the existing content of the
      variable and the new content.

   Here is an example::

      SRC_URI += "file://fix-makefile.patch"

-  *Prepending (=+):* Use the equals sign followed by the plus character
   (``=+``) to prepend values to existing variables.

   .. note::

      This operator adds a space between the new content and the
      existing content of the variable.

   Here is an example::

      VAR =+ "Starts"

-  *Appending (:append):* Use the ``:append`` operator to append values
   to existing variables. This operator does not add any additional
   space. Also, the operator is applied after all the ``+=``, and ``=+``
   operators have been applied and after all ``=`` assignments have
   occurred. This means that if ``:append`` is used in a recipe, it can
   only be overridden by another layer using the  special ``:remove``
   operator, which in turn will prevent further layers from adding it back.

   The following example shows the space being explicitly added to the
   start to ensure the appended value is not merged with the existing
   value::

      CFLAGS:append = " --enable-important-feature"

   You can also use
   the ``:append`` operator with overrides, which results in the actions
   only being performed for the specified target or machine::

      CFLAGS:append:sh4 = " --enable-important-sh4-specific-feature"

-  *Prepending (:prepend):* Use the ``:prepend`` operator to prepend
   values to existing variables. This operator does not add any
   additional space. Also, the operator is applied after all the ``+=``,
   and ``=+`` operators have been applied and after all ``=``
   assignments have occurred.

   The following example shows the space being explicitly added to the
   end to ensure the prepended value is not merged with the existing
   value::

      CFLAGS:prepend = "-I${S}/myincludes "

   You can also use the
   ``:prepend`` operator with overrides, which results in the actions
   only being performed for the specified target or machine::

      CFLAGS:prepend:sh4 = "-I${S}/myincludes "

-  *Overrides:* You can use overrides to set a value conditionally,
   typically based on how the recipe is being built. For example, to set
   the :term:`KBRANCH` variable's
   value to "standard/base" for any target
   :term:`MACHINE`, except for
   qemuarm where it should be set to "standard/arm-versatile-926ejs",
   you would do the following::

      KBRANCH = "standard/base"
      KBRANCH:qemuarm = "standard/arm-versatile-926ejs"

   Overrides are also used to separate
   alternate values of a variable in other situations. For example, when
   setting variables such as
   :term:`FILES` and
   :term:`RDEPENDS` that are
   specific to individual packages produced by a recipe, you should
   always use an override that specifies the name of the package.

-  *Indentation:* Use spaces for indentation rather than tabs. For
   shell functions, both currently work. However, it is a policy
   decision of the Yocto Project to use tabs in shell functions. Realize
   that some layers have a policy to use spaces for all indentation.

-  *Using Python for Complex Operations:* For more advanced processing,
   it is possible to use Python code during variable assignments (e.g.
   search and replacement on a variable).

   You indicate Python code using the ``${@python_code}`` syntax for the
   variable assignment::

      SRC_URI = "ftp://ftp.info-zip.org/pub/infozip/src/zip${@d.getVar('PV',1).replace('.', '')}.tgz

-  *Shell Function Syntax:* Write shell functions as if you were writing
   a shell script when you describe a list of actions to take. You
   should ensure that your script works with a generic ``sh`` and that
   it does not require any ``bash`` or other shell-specific
   functionality. The same considerations apply to various system
   utilities (e.g. ``sed``, ``grep``, ``awk``, and so forth) that you
   might wish to use. If in doubt, you should check with multiple
   implementations --- including those from BusyBox.