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The Buildroot user manual

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Table of Contents

 
I. Getting started

    1. About Buildroot
    2. System requirements

        2.1. Mandatory packages
        2.2. Optional packages

    3. Getting Buildroot
    4. Buildroot quick start
    5. Community resources

II. User guide

    6. Buildroot configuration

        6.1. Cross-compilation toolchain
        6.2. /dev management
        6.3. init system

    7. Configuration of other components
    8. General Buildroot usage

        8.1. make tips
        8.2. Understanding when a full rebuild is necessary
        8.3. Understanding how to rebuild packages
        8.4. Offline builds
        8.5. Building out-of-tree
        8.6. Environment variables
        8.7. Dealing efficiently with filesystem images
        8.8. Graphing the dependencies between packages
        8.9. Graphing the build duration
        8.10. Graphing the filesystem size contribution of packages
        8.11. Top-level parallel build
        8.12. Integration with Eclipse
        8.13. Advanced usage

    9. Project-specific customization

        9.1. Recommended directory structure
        9.2. Keeping customizations outside of Buildroot
        9.3. Storing the Buildroot configuration
        9.4. Storing the configuration of other components
        9.5. Customizing the generated target filesystem
        9.6. Adding custom user accounts
        9.7. Customization after the images have been created
        9.8. Adding project-specific patches
        9.9. Adding project-specific packages
        9.10. Quick guide to storing your project-specific
            customizations

    10. Frequently Asked Questions & Troubleshooting

        10.1. The boot hangs after Starting network…
        10.2. Why is there no compiler on the target?
        10.3. Why are there no development files on the target?
        10.4. Why is there no documentation on the target?
        10.5. Why are some packages not visible in the Buildroot
            config menu?
        10.6. Why not use the target directory as a chroot directory?
        10.7. Why doesn’t Buildroot generate binary packages (.deb,
            .ipkg…)?
        10.8. How to speed-up the build process?

    11. Known issues
    12. Legal notice and licensing

        12.1. Complying with open source licenses
        12.2. Complying with the Buildroot license

    13. Beyond Buildroot

        13.1. Boot the generated images
        13.2. Chroot

III. Developer guide

    14. How Buildroot works
    15. Coding style

        15.1. Config.in file
        15.2. The .mk file
        15.3. The documentation
        15.4. Support scripts

    16. Adding support for a particular board
    17. Adding new packages to Buildroot

        17.1. Package directory
        17.2. Config files
        17.3. The .mk file
        17.4. The .hash file
        17.5. Infrastructure for packages with specific build systems
        17.6. Infrastructure for autotools-based packages
        17.7. Infrastructure for CMake-based packages
        17.8. Infrastructure for Python packages
        17.9. Infrastructure for LuaRocks-based packages
        17.10. Infrastructure for Perl/CPAN packages
        17.11. Infrastructure for virtual packages
        17.12. Infrastructure for packages using kconfig for
            configuration files
        17.13. Infrastructure for rebar-based packages
        17.14. Infrastructure for Waf-based packages
        17.15. Infrastructure for Meson-based packages
        17.16. Integration of Cargo-based packages
        17.17. Infrastructure for Go packages
        17.18. Infrastructure for packages building kernel modules
        17.19. Infrastructure for asciidoc documents
        17.20. Infrastructure specific to the Linux kernel package
        17.21. Hooks available in the various build steps
        17.22. Gettext integration and interaction with packages
        17.23. Tips and tricks
        17.24. Conclusion

    18. Patching a package

        18.1. Providing patches
        18.2. How patches are applied
        18.3. Format and licensing of the package patches
        18.4. Integrating patches found on the Web

    19. Download infrastructure
    20. Debugging Buildroot
    21. Contributing to Buildroot

        21.1. Reproducing, analyzing and fixing bugs
        21.2. Analyzing and fixing autobuild failures
        21.3. Reviewing and testing patches
        21.4. Work on items from the TODO list
        21.5. Submitting patches
        21.6. Reporting issues/bugs or getting help
        21.7. Using the run-tests framework

    22. DEVELOPERS file and get-developers
    23. Release Engineering

        23.1. Releases
        23.2. Development

IV. Appendix

    24. Makedev syntax documentation
    25. Makeusers syntax documentation
    26. Migrating from older Buildroot versions

        26.1. Migrating to 2016.11
        26.2. Migrating to 2017.08

List of Examples

17.1. Config script: divine package
17.2. Config script: imagemagick package:


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Buildroot 2020.02.3 manual generated on 2020-06-03 14:56:43 UTC from
git revision d42f3adaae

The Buildroot manual is written by the Buildroot developers. It is
licensed under the GNU General Public License, version 2. Refer to
the COPYING [http://git.buildroot.org/buildroot/tree/COPYING?id=
d42f3adaae24a6aa3abc2de4f39fa8023f971d31] file in the Buildroot
sources for the full text of this license.

Copyright © 2004-2020 The Buildroot developers

Part I. Getting started

Table of Contents

1. About Buildroot
2. System requirements

    2.1. Mandatory packages
    2.2. Optional packages

3. Getting Buildroot
4. Buildroot quick start
5. Community resources

Chapter 1. About Buildroot

Buildroot is a tool that simplifies and automates the process of
building a complete Linux system for an embedded system, using
cross-compilation.

In order to achieve this, Buildroot is able to generate a
cross-compilation toolchain, a root filesystem, a Linux kernel image
and a bootloader for your target. Buildroot can be used for any
combination of these options, independently (you can for example use
an existing cross-compilation toolchain, and build only your root
filesystem with Buildroot).

Buildroot is useful mainly for people working with embedded systems.
Embedded systems often use processors that are not the regular x86
processors everyone is used to having in his PC. They can be PowerPC
processors, MIPS processors, ARM processors, etc.

Buildroot supports numerous processors and their variants; it also
comes with default configurations for several boards available
off-the-shelf. Besides this, a number of third-party projects are
based on, or develop their BSP ^[1] or SDK ^[2] on top of Buildroot.


---------------------------------------------------------------------

^[1] BSP: Board Support Package

^[2] SDK: Software Development Kit

Chapter 2. System requirements

Buildroot is designed to run on Linux systems.

While Buildroot itself will build most host packages it needs for the
compilation, certain standard Linux utilities are expected to be
already installed on the host system. Below you will find an overview
of the mandatory and optional packages (note that package names may
vary between distributions).

2.1. Mandatory packages

  * Build tools:

      + which
      + sed
      + make (version 3.81 or any later)
      + binutils
      + build-essential (only for Debian based systems)
      + gcc (version 4.8 or any later)
      + g++ (version 4.8 or any later)
      + bash
      + patch
      + gzip
      + bzip2
      + perl (version 5.8.7 or any later)
      + tar
      + cpio
      + unzip
      + rsync
      + file (must be in /usr/bin/file)
      + bc
  * Source fetching tools:

      + wget

2.2. Optional packages

  * Recommended dependencies:

    Some features or utilities in Buildroot, like the legal-info, or
    the graph generation tools, have additional dependencies.
    Although they are not mandatory for a simple build, they are
    still highly recommended:

      + python (version 2.7 or any later)
  * Configuration interface dependencies:

    For these libraries, you need to install both runtime and
    development data, which in many distributions are packaged
    separately. The development packages typically have a -dev or 
    -devel suffix.

      + ncurses5 to use the menuconfig interface
      + qt5 to use the xconfig interface
      + glib2, gtk2 and glade2 to use the gconfig interface
  * Source fetching tools:

    In the official tree, most of the package sources are retrieved
    using wget from ftp, http or https locations. A few packages are
    only available through a version control system. Moreover,
    Buildroot is capable of downloading sources via other tools, like
    rsync or scp (refer to Chapter 19, Download infrastructure for
    more details). If you enable packages using any of these methods,
    you will need to install the corresponding tool on the host
    system:

      + bazaar
      + cvs
      + git
      + mercurial
      + rsync
      + scp
      + subversion
  * Java-related packages, if the Java Classpath needs to be built
    for the target system:

      + The javac compiler
      + The jar tool
  * Documentation generation tools:

      + asciidoc, version 8.6.3 or higher
      + w3m
      + python with the argparse module (automatically present in
        2.7+ and 3.2+)
      + dblatex (required for the pdf manual only)
  * Graph generation tools:

      + graphviz to use graph-depends and <pkg>-graph-depends
      + python-matplotlib to use graph-build

Chapter 3. Getting Buildroot

Buildroot releases are made every 3 months, in February, May, August
and November. Release numbers are in the format YYYY.MM, so for
example 2013.02, 2014.08.

Release tarballs are available at http://buildroot.org/downloads/.

For your convenience, a Vagrantfile [https://www.vagrantup.com/] is
available in support/misc/Vagrantfile in the Buildroot source tree to
quickly set up a virtual machine with the needed dependencies to get
started.

If you want to setup an isolated buildroot environment on Linux or
Mac Os X, paste this line onto your terminal:

curl -O https://buildroot.org/downloads/Vagrantfile; vagrant up

If you are on Windows, paste this into your powershell:

(new-object System.Net.WebClient).DownloadFile(
"https://buildroot.org/downloads/Vagrantfile","Vagrantfile");
vagrant up

If you want to follow development, you can use the daily snapshots or
make a clone of the Git repository. Refer to the Download page [http:
//buildroot.org/download] of the Buildroot website for more details.

Chapter 4. Buildroot quick start

Important: you can and should build everything as a normal user.
There is no need to be root to configure and use Buildroot. By
running all commands as a regular user, you protect your system
against packages behaving badly during compilation and installation.

The first step when using Buildroot is to create a configuration.
Buildroot has a nice configuration tool similar to the one you can
find in the Linux kernel [http://www.kernel.org/] or in BusyBox
[http://www.busybox.net/].

From the buildroot directory, run

 $ make menuconfig

for the original curses-based configurator, or

 $ make nconfig

for the new curses-based configurator, or

 $ make xconfig

for the Qt-based configurator, or

 $ make gconfig

for the GTK-based configurator.

All of these "make" commands will need to build a configuration
utility (including the interface), so you may need to install
"development" packages for relevant libraries used by the
configuration utilities. Refer to Chapter 2, System requirements for
more details, specifically the optional requirements to get the
dependencies of your favorite interface.

For each menu entry in the configuration tool, you can find
associated help that describes the purpose of the entry. Refer to
Chapter 6, Buildroot configuration for details on some specific
configuration aspects.

Once everything is configured, the configuration tool generates a
.config file that contains the entire configuration. This file will
be read by the top-level Makefile.

To start the build process, simply run:

 $ make

By default, Buildroot does not support top-level parallel build, so
running make -jN is not necessary. There is however experimental
support for top-level parallel build, see Section 8.11, “Top-level
parallel build”.

The make command will generally perform the following steps:

  * download source files (as required);
  * configure, build and install the cross-compilation toolchain, or
    simply import an external toolchain;
  * configure, build and install selected target packages;
  * build a kernel image, if selected;
  * build a bootloader image, if selected;
  * create a root filesystem in selected formats.

Buildroot output is stored in a single directory, output/. This
directory contains several subdirectories:

  * images/ where all the images (kernel image, bootloader and root
    filesystem images) are stored. These are the files you need to
    put on your target system.
  * build/ where all the components are built (this includes tools
    needed by Buildroot on the host and packages compiled for the
    target). This directory contains one subdirectory for each of
    these components.
  * host/ contains both the tools built for the host, and the sysroot
    of the target toolchain. The former is an installation of tools
    compiled for the host that are needed for the proper execution of
    Buildroot, including the cross-compilation toolchain. The latter
    is a hierarchy similar to a root filesystem hierarchy. It
    contains the headers and libraries of all user-space packages
    that provide and install libraries used by other packages.
    However, this directory is not intended to be the root filesystem
    for the target: it contains a lot of development files,
    unstripped binaries and libraries that make it far too big for an
    embedded system. These development files are used to compile
    libraries and applications for the target that depend on other
    libraries.
  * staging/ is a symlink to the target toolchain sysroot inside host
    /, which exists for backwards compatibility.
  * target/ which contains almost the complete root filesystem for
    the target: everything needed is present except the device files
    in /dev/ (Buildroot can’t create them because Buildroot doesn’t
    run as root and doesn’t want to run as root). Also, it doesn’t
    have the correct permissions (e.g. setuid for the busybox
    binary). Therefore, this directory should not be used on your
    target. Instead, you should use one of the images built in the
    images/ directory. If you need an extracted image of the root
    filesystem for booting over NFS, then use the tarball image
    generated in images/ and extract it as root. Compared to staging
    /, target/ contains only the files and libraries needed to run
    the selected target applications: the development files (headers,
    etc.) are not present, the binaries are stripped.

These commands, make menuconfig|nconfig|gconfig|xconfig and make, are
the basic ones that allow to easily and quickly generate images
fitting your needs, with all the features and applications you
enabled.

More details about the "make" command usage are given in Section 8.1,
“make tips”.

Chapter 5. Community resources

Like any open source project, Buildroot has different ways to share
information in its community and outside.

Each of those ways may interest you if you are looking for some help,
want to understand Buildroot or contribute to the project.

Mailing List

    Buildroot has a mailing list for discussion and development. It
    is the main method of interaction for Buildroot users and
    developers.

    Only subscribers to the Buildroot mailing list are allowed to
    post to this list. You can subscribe via the mailing list info
    page [http://lists.buildroot.org/mailman/listinfo/buildroot].

    Mails that are sent to the mailing list are also available in the
    mailing list archives [http://lists.buildroot.org/pipermail/
    buildroot] and via Gmane [http://gmane.org], at
    gmane.comp.lib.uclibc.buildroot [http://dir.gmane.org/
    gmane.comp.lib.uclibc.buildroot]. Please search the mailing list
    archives before asking questions, since there is a good chance
    someone else has asked the same question before.

IRC

    The Buildroot IRC channel #buildroot [irc://freenode.net/#
    buildroot] is hosted on Freenode [http://webchat.freenode.net].
    It is a useful place to ask quick questions or discuss on certain
    topics.

    When asking for help on IRC, share relevant logs or pieces of
    code using a code sharing website, such as http://code.bulix.org.

    Note that for certain questions, posting to the mailing list may
    be better as it will reach more people, both developers and
    users.

Bug tracker
    Bugs in Buildroot can be reported via the mailing list or
    alternatively via the Buildroot bugtracker [https://
    bugs.buildroot.org/buglist.cgi?product=buildroot]. Please refer
    to Section 21.6, “Reporting issues/bugs or getting help” before
    creating a bug report.
Wiki
    The Buildroot wiki page [http://elinux.org/Buildroot] is hosted
    on the eLinux [http://elinux.org] wiki. It contains some useful
    links, an overview of past and upcoming events, and a TODO list.
Patchwork

    Patchwork is a web-based patch tracking system designed to
    facilitate the contribution and management of contributions to an
    open-source project. Patches that have been sent to a mailing
    list are 'caught' by the system, and appear on a web page. Any
    comments posted that reference the patch are appended to the
    patch page too. For more information on Patchwork see http://
    jk.ozlabs.org/projects/patchwork/.

    Buildroot’s Patchwork website is mainly for use by Buildroot’s
    maintainer to ensure patches aren’t missed. It is also used by
    Buildroot patch reviewers (see also Section 21.3.1, “Applying
    Patches from Patchwork”). However, since the website exposes
    patches and their corresponding review comments in a clean and
    concise web interface, it can be useful for all Buildroot
    developers.

    The Buildroot patch management interface is available at http://
    patchwork.buildroot.org.

Part II. User guide

Table of Contents

6. Buildroot configuration

    6.1. Cross-compilation toolchain
    6.2. /dev management
    6.3. init system

7. Configuration of other components
8. General Buildroot usage

    8.1. make tips
    8.2. Understanding when a full rebuild is necessary
    8.3. Understanding how to rebuild packages
    8.4. Offline builds
    8.5. Building out-of-tree
    8.6. Environment variables
    8.7. Dealing efficiently with filesystem images
    8.8. Graphing the dependencies between packages
    8.9. Graphing the build duration
    8.10. Graphing the filesystem size contribution of packages
    8.11. Top-level parallel build
    8.12. Integration with Eclipse
    8.13. Advanced usage

9. Project-specific customization

    9.1. Recommended directory structure
    9.2. Keeping customizations outside of Buildroot
    9.3. Storing the Buildroot configuration
    9.4. Storing the configuration of other components
    9.5. Customizing the generated target filesystem
    9.6. Adding custom user accounts
    9.7. Customization after the images have been created
    9.8. Adding project-specific patches
    9.9. Adding project-specific packages
    9.10. Quick guide to storing your project-specific customizations

10. Frequently Asked Questions & Troubleshooting

    10.1. The boot hangs after Starting network…
    10.2. Why is there no compiler on the target?
    10.3. Why are there no development files on the target?
    10.4. Why is there no documentation on the target?
    10.5. Why are some packages not visible in the Buildroot config
        menu?
    10.6. Why not use the target directory as a chroot directory?
    10.7. Why doesn’t Buildroot generate binary packages (.deb,
        .ipkg…)?
    10.8. How to speed-up the build process?

11. Known issues
12. Legal notice and licensing

    12.1. Complying with open source licenses
    12.2. Complying with the Buildroot license

13. Beyond Buildroot

    13.1. Boot the generated images
    13.2. Chroot

Chapter 6. Buildroot configuration

All the configuration options in make *config have a help text
providing details about the option.

The make *config commands also offer a search tool. Read the help
message in the different frontend menus to know how to use it:

  * in menuconfig, the search tool is called by pressing /;
  * in xconfig, the search tool is called by pressing Ctrl + f.

The result of the search shows the help message of the matching
items. In menuconfig, numbers in the left column provide a shortcut
to the corresponding entry. Just type this number to directly jump to
the entry, or to the containing menu in case the entry is not
selectable due to a missing dependency.

Although the menu structure and the help text of the entries should
be sufficiently self-explanatory, a number of topics require
additional explanation that cannot easily be covered in the help text
and are therefore covered in the following sections.

6.1. Cross-compilation toolchain

A compilation toolchain is the set of tools that allows you to
compile code for your system. It consists of a compiler (in our case,
gcc), binary utils like assembler and linker (in our case, binutils)
and a C standard library (for example GNU Libc [http://www.gnu.org/
software/libc/libc.html], uClibc-ng [http://www.uclibc-ng.org/]).

The system installed on your development station certainly already
has a compilation toolchain that you can use to compile an
application that runs on your system. If you’re using a PC, your
compilation toolchain runs on an x86 processor and generates code for
an x86 processor. Under most Linux systems, the compilation toolchain
uses the GNU libc (glibc) as the C standard library. This compilation
toolchain is called the "host compilation toolchain". The machine on
which it is running, and on which you’re working, is called the "host
system" ^[3].

The compilation toolchain is provided by your distribution, and
Buildroot has nothing to do with it (other than using it to build a
cross-compilation toolchain and other tools that are run on the
development host).

As said above, the compilation toolchain that comes with your system
runs on and generates code for the processor in your host system. As
your embedded system has a different processor, you need a
cross-compilation toolchain - a compilation toolchain that runs on
your host system but generates code for your target system (and
target processor). For example, if your host system uses x86 and your
target system uses ARM, the regular compilation toolchain on your
host runs on x86 and generates code for x86, while the
cross-compilation toolchain runs on x86 and generates code for ARM.

Buildroot provides two solutions for the cross-compilation toolchain:

  * The internal toolchain backend, called Buildroot toolchain in the
    configuration interface.
  * The external toolchain backend, called External toolchain in the
    configuration interface.

The choice between these two solutions is done using the Toolchain
Type option in the Toolchain menu. Once one solution has been chosen,
a number of configuration options appear, they are detailed in the
following sections.

6.1.1. Internal toolchain backend

The internal toolchain backend is the backend where Buildroot builds
by itself a cross-compilation toolchain, before building the
userspace applications and libraries for your target embedded system.

This backend supports several C libraries: uClibc-ng [http://
www.uclibc-ng.org], glibc [http://www.gnu.org/software/libc/
libc.html] and musl [http://www.musl-libc.org].

Once you have selected this backend, a number of options appear. The
most important ones allow to:

  * Change the version of the Linux kernel headers used to build the
    toolchain. This item deserves a few explanations. In the process
    of building a cross-compilation toolchain, the C library is being
    built. This library provides the interface between userspace
    applications and the Linux kernel. In order to know how to "talk"
    to the Linux kernel, the C library needs to have access to the 
    Linux kernel headers (i.e. the .h files from the kernel), which
    define the interface between userspace and the kernel (system
    calls, data structures, etc.). Since this interface is backward
    compatible, the version of the Linux kernel headers used to build
    your toolchain do not need to match exactly the version of the
    Linux kernel you intend to run on your embedded system. They only
    need to have a version equal or older to the version of the Linux
    kernel you intend to run. If you use kernel headers that are more
    recent than the Linux kernel you run on your embedded system,
    then the C library might be using interfaces that are not
    provided by your Linux kernel.
  * Change the version of the GCC compiler, binutils and the C
    library.
  * Select a number of toolchain options (uClibc only): whether the
    toolchain should have RPC support (used mainly for NFS),
    wide-char support, locale support (for internationalization), C++
    support or thread support. Depending on which options you choose,
    the number of userspace applications and libraries visible in
    Buildroot menus will change: many applications and libraries
    require certain toolchain options to be enabled. Most packages
    show a comment when a certain toolchain option is required to be
    able to enable those packages. If needed, you can further refine
    the uClibc configuration by running make uclibc-menuconfig. Note
    however that all packages in Buildroot are tested against the
    default uClibc configuration bundled in Buildroot: if you deviate
    from this configuration by removing features from uClibc, some
    packages may no longer build.

It is worth noting that whenever one of those options is modified,
then the entire toolchain and system must be rebuilt. See
Section 8.2, “Understanding when a full rebuild is necessary”.

Advantages of this backend:

  * Well integrated with Buildroot
  * Fast, only builds what’s necessary

Drawbacks of this backend:

  * Rebuilding the toolchain is needed when doing make clean, which
    takes time. If you’re trying to reduce your build time, consider
    using the External toolchain backend.

6.1.2. External toolchain backend

The external toolchain backend allows to use existing pre-built
cross-compilation toolchains. Buildroot knows about a number of
well-known cross-compilation toolchains (from Linaro [http://
www.linaro.org] for ARM, Sourcery CodeBench [http://www.mentor.com/
embedded-software/sourcery-tools/sourcery-codebench/editions/
lite-edition/] for ARM, x86-64, PowerPC, and MIPS, and is capable of
downloading them automatically, or it can be pointed to a custom
toolchain, either available for download or installed locally.

Then, you have three solutions to use an external toolchain:

  * Use a predefined external toolchain profile, and let Buildroot
    download, extract and install the toolchain. Buildroot already
    knows about a few CodeSourcery and Linaro toolchains. Just select
    the toolchain profile in Toolchain from the available ones. This
    is definitely the easiest solution.
  * Use a predefined external toolchain profile, but instead of
    having Buildroot download and extract the toolchain, you can tell
    Buildroot where your toolchain is already installed on your
    system. Just select the toolchain profile in Toolchain through
    the available ones, unselect Download toolchain automatically,
    and fill the Toolchain path text entry with the path to your
    cross-compiling toolchain.
  * Use a completely custom external toolchain. This is particularly
    useful for toolchains generated using crosstool-NG or with
    Buildroot itself. To do this, select the Custom toolchain
    solution in the Toolchain list. You need to fill the Toolchain
    path, Toolchain prefix and External toolchain C library options.
    Then, you have to tell Buildroot what your external toolchain
    supports. If your external toolchain uses the glibc library, you
    only have to tell whether your toolchain supports C++ or not and
    whether it has built-in RPC support. If your external toolchain
    uses the uClibc library, then you have to tell Buildroot if it
    supports RPC, wide-char, locale, program invocation, threads and
    C++. At the beginning of the execution, Buildroot will tell you
    if the selected options do not match the toolchain configuration.

Our external toolchain support has been tested with toolchains from
CodeSourcery and Linaro, toolchains generated by crosstool-NG [http:/
/crosstool-ng.org], and toolchains generated by Buildroot itself. In
general, all toolchains that support the sysroot feature should work.
If not, do not hesitate to contact the developers.

We do not support toolchains or SDK generated by OpenEmbedded or
Yocto, because these toolchains are not pure toolchains (i.e. just
the compiler, binutils, the C and C++ libraries). Instead these
toolchains come with a very large set of pre-compiled libraries and
programs. Therefore, Buildroot cannot import the sysroot of the
toolchain, as it would contain hundreds of megabytes of pre-compiled
libraries that are normally built by Buildroot.

We also do not support using the distribution toolchain (i.e. the gcc
/binutils/C library installed by your distribution) as the toolchain
to build software for the target. This is because your distribution
toolchain is not a "pure" toolchain (i.e. only with the C/C++
library), so we cannot import it properly into the Buildroot build
environment. So even if you are building a system for a x86 or x86_64
target, you have to generate a cross-compilation toolchain with
Buildroot or crosstool-NG.

If you want to generate a custom toolchain for your project, that can
be used as an external toolchain in Buildroot, our recommendation is
to build it either with Buildroot itself (see Section 6.1.3, “Build
an external toolchain with Buildroot”) or with crosstool-NG [http://
crosstool-ng.org].

Advantages of this backend:

  * Allows to use well-known and well-tested cross-compilation
    toolchains.
  * Avoids the build time of the cross-compilation toolchain, which
    is often very significant in the overall build time of an
    embedded Linux system.

Drawbacks of this backend:

  * If your pre-built external toolchain has a bug, may be hard to
    get a fix from the toolchain vendor, unless you build your
    external toolchain by yourself using Buildroot or Crosstool-NG.

6.1.3. Build an external toolchain with Buildroot

The Buildroot internal toolchain option can be used to create an
external toolchain. Here are a series of steps to build an internal
toolchain and package it up for reuse by Buildroot itself (or other
projects).

Create a new Buildroot configuration, with the following details:

  * Select the appropriate Target options for your target CPU
    architecture
  * In the Toolchain menu, keep the default of Buildroot toolchain
    for Toolchain type, and configure your toolchain as desired
  * In the System configuration menu, select None as the Init system
    and none as /bin/sh
  * In the Target packages menu, disable BusyBox
  * In the Filesystem images menu, disable tar the root filesystem

Then, we can trigger the build, and also ask Buildroot to generate a
SDK. This will conveniently generate for us a tarball which contains
our toolchain:

make sdk

This produces the SDK tarball in $(O)/images, with a name similar to
arm-buildroot-linux-uclibcgnueabi_sdk-buildroot.tar.gz. Save this
tarball, as it is now the toolchain that you can re-use as an
external toolchain in other Buildroot projects.

In those other Buildroot projects, in the Toolchain menu:

  * Set Toolchain type to External toolchain
  * Set Toolchain to Custom toolchain
  * Set Toolchain origin to Toolchain to be downloaded and installed
  * Set Toolchain URL to file:///path/to/your/sdk/tarball.tar.gz

6.1.3.1. External toolchain wrapper

When using an external toolchain, Buildroot generates a wrapper
program, that transparently passes the appropriate options (according
to the configuration) to the external toolchain programs. In case you
need to debug this wrapper to check exactly what arguments are
passed, you can set the environment variable BR2_DEBUG_WRAPPER to
either one of:

  * 0, empty or not set: no debug
  * 1: trace all arguments on a single line
  * 2: trace one argument per line

6.2. /dev management

On a Linux system, the /dev directory contains special files, called 
device files, that allow userspace applications to access the
hardware devices managed by the Linux kernel. Without these device
files, your userspace applications would not be able to use the
hardware devices, even if they are properly recognized by the Linux
kernel.

Under System configuration, /dev management, Buildroot offers four
different solutions to handle the /dev directory :

  * The first solution is Static using device table. This is the old
    classical way of handling device files in Linux. With this
    method, the device files are persistently stored in the root
    filesystem (i.e. they persist across reboots), and there is
    nothing that will automatically create and remove those device
    files when hardware devices are added or removed from the system.
    Buildroot therefore creates a standard set of device files using
    a device table, the default one being stored in system/
    device_table_dev.txt in the Buildroot source code. This file is
    processed when Buildroot generates the final root filesystem
    image, and the device files are therefore not visible in the
    output/target directory. The BR2_ROOTFS_STATIC_DEVICE_TABLE
    option allows to change the default device table used by
    Buildroot, or to add an additional device table, so that
    additional device files are created by Buildroot during the
    build. So, if you use this method, and a device file is missing
    in your system, you can for example create a board/<yourcompany>/
    <yourproject>/device_table_dev.txt file that contains the
    description of your additional device files, and then you can set
    BR2_ROOTFS_STATIC_DEVICE_TABLE to system/device_table_dev.txt
    board/<yourcompany>/<yourproject>/device_table_dev.txt. For more
    details about the format of the device table file, see
    Chapter 24, Makedev syntax documentation.
  * The second solution is Dynamic using devtmpfs only. devtmpfs is a
    virtual filesystem inside the Linux kernel that has been
    introduced in kernel 2.6.32 (if you use an older kernel, it is
    not possible to use this option). When mounted in /dev, this
    virtual filesystem will automatically make device files appear
    and disappear as hardware devices are added and removed from the
    system. This filesystem is not persistent across reboots: it is
    filled dynamically by the kernel. Using devtmpfs requires the
    following kernel configuration options to be enabled:
    CONFIG_DEVTMPFS and CONFIG_DEVTMPFS_MOUNT. When Buildroot is in
    charge of building the Linux kernel for your embedded device, it
    makes sure that those two options are enabled. However, if you
    build your Linux kernel outside of Buildroot, then it is your
    responsibility to enable those two options (if you fail to do so,
    your Buildroot system will not boot).
  * The third solution is Dynamic using devtmpfs + mdev. This method
    also relies on the devtmpfs virtual filesystem detailed above (so
    the requirement to have CONFIG_DEVTMPFS and CONFIG_DEVTMPFS_MOUNT
    enabled in the kernel configuration still apply), but adds the
    mdev userspace utility on top of it. mdev is a program part of
    BusyBox that the kernel will call every time a device is added or
    removed. Thanks to the /etc/mdev.conf configuration file, mdev
    can be configured to for example, set specific permissions or
    ownership on a device file, call a script or application whenever
    a device appears or disappear, etc. Basically, it allows 
    userspace to react on device addition and removal events. mdev
    can for example be used to automatically load kernel modules when
    devices appear on the system. mdev is also important if you have
    devices that require a firmware, as it will be responsible for
    pushing the firmware contents to the kernel. mdev is a
    lightweight implementation (with fewer features) of udev. For
    more details about mdev and the syntax of its configuration file,
    see http://git.busybox.net/busybox/tree/docs/mdev.txt.
  * The fourth solution is Dynamic using devtmpfs + eudev. This
    method also relies on the devtmpfs virtual filesystem detailed
    above, but adds the eudev userspace daemon on top of it. eudev is
    a daemon that runs in the background, and gets called by the
    kernel when a device gets added or removed from the system. It is
    a more heavyweight solution than mdev, but provides higher
    flexibility. eudev is a standalone version of udev, the original
    userspace daemon used in most desktop Linux distributions, which
    is now part of Systemd. For more details, see http://
    en.wikipedia.org/wiki/Udev.

The Buildroot developers recommendation is to start with the Dynamic
using devtmpfs only solution, until you have the need for userspace
to be notified when devices are added/removed, or if firmwares are
needed, in which case Dynamic using devtmpfs + mdev is usually a good
solution.

Note that if systemd is chosen as init system, /dev management will
be performed by the udev program provided by systemd.

6.3. init system

The init program is the first userspace program started by the kernel
(it carries the PID number 1), and is responsible for starting the
userspace services and programs (for example: web server, graphical
applications, other network servers, etc.).

Buildroot allows to use three different types of init systems, which
can be chosen from System configuration, Init system:

  * The first solution is BusyBox. Amongst many programs, BusyBox has
    an implementation of a basic init program, which is sufficient
    for most embedded systems. Enabling the BR2_INIT_BUSYBOX will
    ensure BusyBox will build and install its init program. This is
    the default solution in Buildroot. The BusyBox init program will
    read the /etc/inittab file at boot to know what to do. The syntax
    of this file can be found in http://git.busybox.net/busybox/tree/
    examples/inittab (note that BusyBox inittab syntax is special: do
    not use a random inittab documentation from the Internet to learn
    about BusyBox inittab). The default inittab in Buildroot is
    stored in system/skeleton/etc/inittab. Apart from mounting a few
    important filesystems, the main job the default inittab does is
    to start the /etc/init.d/rcS shell script, and start a getty
    program (which provides a login prompt).
  * The second solution is systemV. This solution uses the old
    traditional sysvinit program, packed in Buildroot in package/
    sysvinit. This was the solution used in most desktop Linux
    distributions, until they switched to more recent alternatives
    such as Upstart or Systemd. sysvinit also works with an inittab
    file (which has a slightly different syntax than the one from
    BusyBox). The default inittab installed with this init solution
    is located in package/sysvinit/inittab.
  * The third solution is systemd. systemd is the new generation init
    system for Linux. It does far more than traditional init
    programs: aggressive parallelization capabilities, uses socket
    and D-Bus activation for starting services, offers on-demand
    starting of daemons, keeps track of processes using Linux control
    groups, supports snapshotting and restoring of the system state,
    etc. systemd will be useful on relatively complex embedded
    systems, for example the ones requiring D-Bus and services
    communicating between each other. It is worth noting that systemd
    brings a fairly big number of large dependencies: dbus, udev and
    more. For more details about systemd, see http://
    www.freedesktop.org/wiki/Software/systemd.

The solution recommended by Buildroot developers is to use the 
BusyBox init as it is sufficient for most embedded systems. systemd
can be used for more complex situations.


---------------------------------------------------------------------

^[3] This terminology differs from what is used by GNU configure,
where the host is the machine on which the application will run
(which is usually the same as target)

Chapter 7. Configuration of other components

Before attempting to modify any of the components below, make sure
you have already configured Buildroot itself, and have enabled the
corresponding package.

BusyBox

    If you already have a BusyBox configuration file, you can
    directly specify this file in the Buildroot configuration, using
    BR2_PACKAGE_BUSYBOX_CONFIG. Otherwise, Buildroot will start from
    a default BusyBox configuration file.

    To make subsequent changes to the configuration, use make
    busybox-menuconfig to open the BusyBox configuration editor.

    It is also possible to specify a BusyBox configuration file
    through an environment variable, although this is not
    recommended. Refer to Section 8.6, “Environment variables” for
    more details.

uClibc
    Configuration of uClibc is done in the same way as for BusyBox.
    The configuration variable to specify an existing configuration
    file is BR2_UCLIBC_CONFIG. The command to make subsequent changes
    is make uclibc-menuconfig.
Linux kernel

    If you already have a kernel configuration file, you can directly
    specify this file in the Buildroot configuration, using
    BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG.

    If you do not yet have a kernel configuration file, you can
    either start by specifying a defconfig in the Buildroot
    configuration, using BR2_LINUX_KERNEL_USE_DEFCONFIG, or start by
    creating an empty file and specifying it as custom configuration
    file, using BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG.

    To make subsequent changes to the configuration, use make
    linux-menuconfig to open the Linux configuration editor.

Barebox
    Configuration of Barebox is done in the same way as for the Linux
    kernel. The corresponding configuration variables are
    BR2_TARGET_BAREBOX_USE_CUSTOM_CONFIG and
    BR2_TARGET_BAREBOX_USE_DEFCONFIG. To open the configuration
    editor, use make barebox-menuconfig.
U-Boot
    Configuration of U-Boot (version 2015.04 or newer) is done in the
    same way as for the Linux kernel. The corresponding configuration
    variables are BR2_TARGET_UBOOT_USE_CUSTOM_CONFIG and
    BR2_TARGET_UBOOT_USE_DEFCONFIG. To open the configuration editor,
    use make uboot-menuconfig.

Chapter 8. General Buildroot usage

8.1. make tips

This is a collection of tips that help you make the most of
Buildroot.

Display all commands executed by make: 

 $ make V=1 <target>

Display the list of boards with a defconfig: 

 $ make list-defconfigs

Display all available targets: 

 $ make help

Not all targets are always available, some settings in the .config
file may hide some targets:

  * busybox-menuconfig only works when busybox is enabled;
  * linux-menuconfig and linux-savedefconfig only work when linux is
    enabled;
  * uclibc-menuconfig is only available when the uClibc C library is
    selected in the internal toolchain backend;
  * barebox-menuconfig and barebox-savedefconfig only work when the
    barebox bootloader is enabled.
  * uboot-menuconfig and uboot-savedefconfig only work when the
    U-Boot bootloader is enabled.

Cleaning: Explicit cleaning is required when any of the architecture
or toolchain configuration options are changed.

To delete all build products (including build directories, host,
staging and target trees, the images and the toolchain):

 $ make clean

Generating the manual: The present manual sources are located in the 
docs/manual directory. To generate the manual:

 $ make manual-clean
 $ make manual

The manual outputs will be generated in output/docs/manual.

Notes

  * A few tools are required to build the documentation (see:
    Section 2.2, “Optional packages”).

Resetting Buildroot for a new target: To delete all build products as
well as the configuration:

 $ make distclean

Notes. If ccache is enabled, running make clean or distclean does not
empty the compiler cache used by Buildroot. To delete it, refer to
Section 8.13.3, “Using ccache in Buildroot”.

Dumping the internal make variables: One can dump the variables known
to make, along with their values:

 $ make -s printvars VARS='VARIABLE1 VARIABLE2'
 VARIABLE1=value_of_variable
 VARIABLE2=value_of_variable

It is possible to tweak the output using some variables:

  * VARS will limit the listing to variables which names match the
    specified make-patterns - this must be set else nothing is
    printed
  * QUOTED_VARS, if set to YES, will single-quote the value
  * RAW_VARS, if set to YES, will print the unexpanded value

For example:

 $ make -s printvars VARS=BUSYBOX_%DEPENDENCIES
 BUSYBOX_DEPENDENCIES=skeleton toolchain
 BUSYBOX_FINAL_ALL_DEPENDENCIES=skeleton toolchain
 BUSYBOX_FINAL_DEPENDENCIES=skeleton toolchain
 BUSYBOX_FINAL_PATCH_DEPENDENCIES=
 BUSYBOX_RDEPENDENCIES=ncurses util-linux

 $ make -s printvars VARS=BUSYBOX_%DEPENDENCIES QUOTED_VARS=YES
 BUSYBOX_DEPENDENCIES='skeleton toolchain'
 BUSYBOX_FINAL_ALL_DEPENDENCIES='skeleton toolchain'
 BUSYBOX_FINAL_DEPENDENCIES='skeleton toolchain'
 BUSYBOX_FINAL_PATCH_DEPENDENCIES=''
 BUSYBOX_RDEPENDENCIES='ncurses util-linux'

 $ make -s printvars VARS=BUSYBOX_%DEPENDENCIES RAW_VARS=YES
 BUSYBOX_DEPENDENCIES=skeleton toolchain
 BUSYBOX_FINAL_ALL_DEPENDENCIES=$(sort $(BUSYBOX_FINAL_DEPENDENCIES) $(BUSYBOX_FINAL_PATCH_DEPENDENCIES))
 BUSYBOX_FINAL_DEPENDENCIES=$(sort $(BUSYBOX_DEPENDENCIES))
 BUSYBOX_FINAL_PATCH_DEPENDENCIES=$(sort $(BUSYBOX_PATCH_DEPENDENCIES))
 BUSYBOX_RDEPENDENCIES=ncurses util-linux

The output of quoted variables can be reused in shell scripts, for
example:

 $ eval $(make -s printvars VARS=BUSYBOX_DEPENDENCIES QUOTED_VARS=YES)
 $ echo $BUSYBOX_DEPENDENCIES
 skeleton toolchain

8.2. Understanding when a full rebuild is necessary

Buildroot does not attempt to detect what parts of the system should
be rebuilt when the system configuration is changed through make
menuconfig, make xconfig or one of the other configuration tools. In
some cases, Buildroot should rebuild the entire system, in some
cases, only a specific subset of packages. But detecting this in a
completely reliable manner is very difficult, and therefore the
Buildroot developers have decided to simply not attempt to do this.

Instead, it is the responsibility of the user to know when a full
rebuild is necessary. As a hint, here are a few rules of thumb that
can help you understand how to work with Buildroot:

  * When the target architecture configuration is changed, a complete
    rebuild is needed. Changing the architecture variant, the binary
    format or the floating point strategy for example has an impact
    on the entire system.
  * When the toolchain configuration is changed, a complete rebuild
    generally is needed. Changing the toolchain configuration often
    involves changing the compiler version, the type of C library or
    its configuration, or some other fundamental configuration item,
    and these changes have an impact on the entire system.
  * When an additional package is added to the configuration, a full
    rebuild is not necessarily needed. Buildroot will detect that
    this package has never been built, and will build it. However, if
    this package is a library that can optionally be used by packages
    that have already been built, Buildroot will not automatically
    rebuild those. Either you know which packages should be rebuilt,
    and you can rebuild them manually, or you should do a full
    rebuild. For example, let’s suppose you have built a system with
    the ctorrent package, but without openssl. Your system works, but
    you realize you would like to have SSL support in ctorrent, so
    you enable the openssl package in Buildroot configuration and
    restart the build. Buildroot will detect that openssl should be
    built and will be build it, but it will not detect that ctorrent
    should be rebuilt to benefit from openssl to add OpenSSL support.
    You will either have to do a full rebuild, or rebuild ctorrent
    itself.
  * When a package is removed from the configuration, Buildroot does
    not do anything special. It does not remove the files installed
    by this package from the target root filesystem or from the
    toolchain sysroot. A full rebuild is needed to get rid of this
    package. However, generally you don’t necessarily need this
    package to be removed right now: you can wait for the next lunch
    break to restart the build from scratch.
  * When the sub-options of a package are changed, the package is not
    automatically rebuilt. After making such changes, rebuilding only
    this package is often sufficient, unless enabling the package
    sub-option adds some features to the package that are useful for
    another package which has already been built. Again, Buildroot
    does not track when a package should be rebuilt: once a package
    has been built, it is never rebuilt unless explicitly told to do
    so.
  * When a change to the root filesystem skeleton is made, a full
    rebuild is needed. However, when changes to the root filesystem
    overlay, a post-build script or a post-image script are made,
    there is no need for a full rebuild: a simple make invocation
    will take the changes into account.
  * When a package listed in FOO_DEPENDENCIES is rebuilt or removed,
    the package foo is not automatically rebuilt. For example, if a
    package bar is listed in FOO_DEPENDENCIES with FOO_DEPENDENCIES =
    bar and the configuration of the bar package is changed, the
    configuration change would not result in a rebuild of package foo
    automatically. In this scenario, you may need to either rebuild
    any packages in your build which reference bar in their
    DEPENDENCIES, or perform a full rebuild to ensure any bar
    dependent packages are up to date.

Generally speaking, when you’re facing a build error and you’re
unsure of the potential consequences of the configuration changes
you’ve made, do a full rebuild. If you get the same build error, then
you are sure that the error is not related to partial rebuilds of
packages, and if this error occurs with packages from the official
Buildroot, do not hesitate to report the problem! As your experience
with Buildroot progresses, you will progressively learn when a full
rebuild is really necessary, and you will save more and more time.

For reference, a full rebuild is achieved by running:

$ make clean all

8.3. Understanding how to rebuild packages

One of the most common questions asked by Buildroot users is how to
rebuild a given package or how to remove a package without rebuilding
everything from scratch.

Removing a package is unsupported by Buildroot without rebuilding
from scratch. This is because Buildroot doesn’t keep track of which
package installs what files in the output/staging and output/target
directories, or which package would be compiled differently depending
on the availability of another package.

The easiest way to rebuild a single package from scratch is to remove
its build directory in output/build. Buildroot will then re-extract,
re-configure, re-compile and re-install this package from scratch.
You can ask buildroot to do this with the make <package>-dirclean
command.

On the other hand, if you only want to restart the build process of a
package from its compilation step, you can run make <package>
-rebuild. It will restart the compilation and installation of the
package, but not from scratch: it basically re-executes make and make
install inside the package, so it will only rebuild files that
changed.

If you want to restart the build process of a package from its
configuration step, you can run make <package>-reconfigure. It will
restart the configuration, compilation and installation of the
package.

While <package>-rebuild implies <package>-reinstall and <package>
-reconfigure implies <package>-rebuild, these targets as well as
<package> only act on the said package, and do not trigger
re-creating the root filesystem image. If re-creating the root
filesystem in necessary, one should in addition run make or make all.

Internally, Buildroot creates so-called stamp files to keep track of
which build steps have been completed for each package. They are
stored in the package build directory, output/build/<package>-
<version>/ and are named .stamp_<step-name>. The commands detailed
above simply manipulate these stamp files to force Buildroot to
restart a specific set of steps of a package build process.

Further details about package special make targets are explained in
Section 8.13.5, “Package-specific make targets”.

8.4. Offline builds

If you intend to do an offline build and just want to download all
sources that you previously selected in the configurator (menuconfig,
nconfig, xconfig or gconfig), then issue:

 $ make source

You can now disconnect or copy the content of your dl directory to
the build-host.

8.5. Building out-of-tree

As default, everything built by Buildroot is stored in the directory
output in the Buildroot tree.

Buildroot also supports building out of tree with a syntax similar to
the Linux kernel. To use it, add O=<directory> to the make command
line:

 $ make O=/tmp/build

Or:

 $ cd /tmp/build; make O=$PWD -C path/to/buildroot

All the output files will be located under /tmp/build. If the O path
does not exist, Buildroot will create it.

Note: the O path can be either an absolute or a relative path, but if
it’s passed as a relative path, it is important to note that it is
interpreted relative to the main Buildroot source directory, not the
current working directory.

When using out-of-tree builds, the Buildroot .config and temporary
files are also stored in the output directory. This means that you
can safely run multiple builds in parallel using the same source tree
as long as they use unique output directories.

For ease of use, Buildroot generates a Makefile wrapper in the output
directory - so after the first run, you no longer need to pass O=<…>
and -C <…>, simply run (in the output directory):

 $ make <target>

8.6. Environment variables

Buildroot also honors some environment variables, when they are
passed to make or set in the environment:

  * HOSTCXX, the host C++ compiler to use
  * HOSTCC, the host C compiler to use
  * UCLIBC_CONFIG_FILE=<path/to/.config>, path to the uClibc
    configuration file, used to compile uClibc, if an internal
    toolchain is being built. Note that the uClibc configuration file
    can also be set from the configuration interface, so through the
    Buildroot .config file; this is the recommended way of setting
    it.
  * BUSYBOX_CONFIG_FILE=<path/to/.config>, path to the BusyBox
    configuration file. Note that the BusyBox configuration file can
    also be set from the configuration interface, so through the
    Buildroot .config file; this is the recommended way of setting
    it.
  * BR2_CCACHE_DIR to override the directory where Buildroot stores
    the cached files when using ccache.
  * BR2_DL_DIR to override the directory in which Buildroot stores/
    retrieves downloaded files. Note that the Buildroot download
    directory can also be set from the configuration interface, so
    through the Buildroot .config file. See Section 8.13.4, “Location
    of downloaded packages” for more details on how you can set the
    download directory.
  * BR2_GRAPH_ALT, if set and non-empty, to use an alternate
    color-scheme in build-time graphs
  * BR2_GRAPH_OUT to set the filetype of generated graphs, either pdf
    (the default), or png.
  * BR2_GRAPH_DEPS_OPTS to pass extra options to the dependency
    graph; see Section 8.8, “Graphing the dependencies between
    packages” for the accepted options
  * BR2_GRAPH_DOT_OPTS is passed verbatim as options to the dot
    utility to draw the dependency graph.
  * BR2_GRAPH_SIZE_OPTS to pass extra options to the size graph; see
    Section 8.10, “Graphing the filesystem size contribution of
    packages” for the acepted options

An example that uses config files located in the toplevel directory
and in your $HOME:

 $ make UCLIBC_CONFIG_FILE=uClibc.config BUSYBOX_CONFIG_FILE=$HOME/bb.config

If you want to use a compiler other than the default gcc or g++ for
building helper-binaries on your host, then do

 $ make HOSTCXX=g++-4.3-HEAD HOSTCC=gcc-4.3-HEAD

8.7. Dealing efficiently with filesystem images

Filesystem images can get pretty big, depending on the filesystem you
choose, the number of packages, whether you provisioned free space…
Yet, some locations in the filesystems images may just be empty (e.g.
a long run of zeroes); such a file is called a sparse file.

Most tools can handle sparse files efficiently, and will only store
or write those parts of a sparse file that are not empty.

For example:

  * tar accepts the -S option to tell it to only store non-zero
    blocks of sparse files:

      + tar cf archive.tar -S [files…] will efficiently store sparse
        files in a tarball
      + tar xf archive.tar -S will efficiently store sparse files
        extracted from a tarball
  * cp accepts the --sparse=WHEN option (WHEN is one of auto, never
    or always):

      + cp --sparse=always source.file dest.file will make dest.file
        a sparse file if source.file has long runs of zeroes

Other tools may have similar options. Please consult their respective
man pages.

You can use sparse files if you need to store the filesystem images
(e.g. to transfer from one machine to another), or if you need to
send them (e.g. to the Q&A team).

Note however that flashing a filesystem image to a device while using
the sparse mode of dd may result in a broken filesystem (e.g. the
block bitmap of an ext2 filesystem may be corrupted; or, if you have
sparse files in your filesystem, those parts may not be all-zeroes
when read back). You should only use sparse files when handling files
on the build machine, not when transferring them to an actual device
that will be used on the target.

8.8. Graphing the dependencies between packages

One of Buildroot’s jobs is to know the dependencies between packages,
and make sure they are built in the right order. These dependencies
can sometimes be quite complicated, and for a given system, it is
often not easy to understand why such or such package was brought
into the build by Buildroot.

In order to help understanding the dependencies, and therefore better
understand what is the role of the different components in your
embedded Linux system, Buildroot is capable of generating dependency
graphs.

To generate a dependency graph of the full system you have compiled,
simply run:

make graph-depends

You will find the generated graph in output/graphs/graph-depends.pdf.

If your system is quite large, the dependency graph may be too
complex and difficult to read. It is therefore possible to generate
the dependency graph just for a given package:

make <pkg>-graph-depends

You will find the generated graph in output/graph/<pkg>
-graph-depends.pdf.

Note that the dependency graphs are generated using the dot tool from
the Graphviz project, which you must have installed on your system to
use this feature. In most distributions, it is available as the
graphviz package.

By default, the dependency graphs are generated in the PDF format.
However, by passing the BR2_GRAPH_OUT environment variable, you can
switch to other output formats, such as PNG, PostScript or SVG. All
formats supported by the -T option of the dot tool are supported.

BR2_GRAPH_OUT=svg make graph-depends

The graph-depends behaviour can be controlled by setting options in
the BR2_GRAPH_DEPS_OPTS environment variable. The accepted options
are:

  * --depth N, -d N, to limit the dependency depth to N levels. The
    default, 0, means no limit.
  * --stop-on PKG, -s PKG, to stop the graph on the package PKG. PKG
    can be an actual package name, a glob, the keyword virtual (to
    stop on virtual packages), or the keyword host (to stop on host
    packages). The package is still present on the graph, but its
    dependencies are not.
  * --exclude PKG, -x PKG, like --stop-on, but also omits PKG from
    the graph.
  * --transitive, --no-transitive, to draw (or not) the transitive
    dependencies. The default is to not draw transitive dependencies.
  * --colors R,T,H, the comma-separated list of colors to draw the
    root package (R), the target packages (T) and the host packages
    (H). Defaults to: lightblue,grey,gainsboro

BR2_GRAPH_DEPS_OPTS='-d 3 --no-transitive --colors=red,green,blue' make graph-depends

8.9. Graphing the build duration

When the build of a system takes a long time, it is sometimes useful
to be able to understand which packages are the longest to build, to
see if anything can be done to speed up the build. In order to help
such build time analysis, Buildroot collects the build time of each
step of each package, and allows to generate graphs from this data.

To generate the build time graph after a build, run:

make graph-build

This will generate a set of files in output/graphs :

  * build.hist-build.pdf, a histogram of the build time for each
    package, ordered in the build order.
  * build.hist-duration.pdf, a histogram of the build time for each
    package, ordered by duration (longest first)
  * build.hist-name.pdf, a histogram of the build time for each
    package, order by package name.
  * build.pie-packages.pdf, a pie chart of the build time per package
  * build.pie-steps.pdf, a pie chart of the global time spent in each
    step of the packages build process.

This graph-build target requires the Python Matplotlib and Numpy
libraries to be installed (python-matplotlib and python-numpy on most
distributions), and also the argparse module if you’re using a Python
version older than 2.7 (python-argparse on most distributions).

By default, the output format for the graph is PDF, but a different
format can be selected using the BR2_GRAPH_OUT environment variable.
The only other format supported is PNG:

BR2_GRAPH_OUT=png make graph-build

8.10. Graphing the filesystem size contribution of packages

When your target system grows, it is sometimes useful to understand
how much each Buildroot package is contributing to the overall root
filesystem size. To help with such an analysis, Buildroot collects
data about files installed by each package and using this data,
generates a graph and CSV files detailing the size contribution of
the different packages.

To generate these data after a build, run:

make graph-size

This will generate:

  * output/graphs/graph-size.pdf, a pie chart of the contribution of
    each package to the overall root filesystem size
  * output/graphs/package-size-stats.csv, a CSV file giving the size
    contribution of each package to the overall root filesystem size
  * output/graphs/file-size-stats.csv, a CSV file giving the size
    contribution of each installed file to the package it belongs,
    and to the overall filesystem size.

This graph-size target requires the Python Matplotlib library to be
installed (python-matplotlib on most distributions), and also the
argparse module if you’re using a Python version older than 2.7
(python-argparse on most distributions).

Just like for the duration graph, a BR2_GRAPH_OUT environment
variable is supported to adjust the output file format. See
Section 8.8, “Graphing the dependencies between packages” for details
about this environment variable.

Additionally, one may set the environment variable
BR2_GRAPH_SIZE_OPTS to further control the generated graph. Accepted
options are:

  * --size-limit X, -l X, will group all packages which individual
    contribution is below X percent, to a single entry labelled 
    Others in the graph. By default, X=0.01, which means packages
    each contributing less than 1% are grouped under Others. Accepted
    values are in the range [0.0..1.0].
  * --iec, --binary, --si, --decimal, to use IEC (binary, powers of
    1024) or SI (decimal, powers of 1000; the default) prefixes.
  * --biggest-first, to sort packages in decreasing size order,
    rather than in increasing size order.

Note. The collected filesystem size data is only meaningful after a
complete clean rebuild. Be sure to run make clean all before using
make graph-size.

To compare the root filesystem size of two different Buildroot
compilations, for example after adjusting the configuration or when
switching to another Buildroot release, use the size-stats-compare
script. It takes two file-size-stats.csv files (produced by make
graph-size) as input. Refer to the help text of this script for more
details:

utils/size-stats-compare -h

8.11. Top-level parallel build

Note. This section deals with a very experimental feature, which is
known to break even in some non-unusual situations. Use at your own
risk.

Buildroot has always been capable of using parallel build on a per
package basis: each package is built by Buildroot using make -jN (or
the equivalent invocation for non-make-based build systems). The
level of parallelism is by default number of CPUs + 1, but it can be
adjusted using the BR2_JLEVEL configuration option.

Until 2020.02, Buildroot was however building packages in a serial
fashion: each package was built one after the other, without
parallelization of the build between packages. As of 2020.02,
Buildroot has experimental support for top-level parallel build,
which allows some signicant build time savings by building packages
that have no dependency relationship in parallel. This feature is
however marked as experimental and is known not to work in some
cases.

In order to use top-level parallel build, one must:

 1. Enable the option BR2_PER_PACKAGE_DIRECTORIES in the Buildroot
    configuration
 2. Use make -jN when starting the Buildroot build

Internally, the BR2_PER_PACKAGE_DIRECTORIES will enable a mechanism
called per-package directories, which will have the following
effects:

  * Instead of a global target directory and a global host directory
    common to all packages, per-package target and host directories
    will be used, in $(O)/per-package/<pkg>/target/ and $(O)/
    per-package/<pkg>/host/ respectively. Those folders will be
    populated from the corresponding folders of the package
    dependencies at the beginning of <pkg> build. The compiler and
    all other tools will therefore only be able to see and access
    files installed by dependencies explicitly listed by <pkg>.
  * At the end of the build, the global target and host directories
    will be populated, located in $(O)/target and $(O)/host
    respectively. This means that during the build, those folders
    will be empty and it’s only at the very end of the build that
    they will be populated.

8.12. Integration with Eclipse

While a part of the embedded Linux developers like classical text
editors like Vim or Emacs, and command-line based interfaces, a
number of other embedded Linux developers like richer graphical
interfaces to do their development work. Eclipse being one of the
most popular Integrated Development Environment, Buildroot integrates
with Eclipse in order to ease the development work of Eclipse users.

Our integration with Eclipse simplifies the compilation, remote
execution and remote debugging of applications and libraries that are
built on top of a Buildroot system. It does not integrate the
Buildroot configuration and build processes themselves with Eclipse.
Therefore, the typical usage model of our Eclipse integration would
be:

  * Configure your Buildroot system with make menuconfig, make
    xconfig or any other configuration interface provided with
    Buildroot.
  * Build your Buildroot system by running make.
  * Start Eclipse to develop, execute and debug your own custom
    applications and libraries, that will rely on the libraries built
    and installed by Buildroot.

The Buildroot Eclipse integration installation process and usage is
described in detail at https://github.com/mbats/
eclipse-buildroot-bundle/wiki.

8.13. Advanced usage

8.13.1. Using the generated toolchain outside Buildroot

You may want to compile, for your target, your own programs or other
software that are not packaged in Buildroot. In order to do this you
can use the toolchain that was generated by Buildroot.

The toolchain generated by Buildroot is located by default in output/
host/. The simplest way to use it is to add output/host/bin/ to your
PATH environment variable and then to use ARCH-linux-gcc,
ARCH-linux-objdump, ARCH-linux-ld, etc.

Alternatively, Buildroot can also export the toolchain and the
development files of all selected packages, as an SDK, by running the
command make sdk. This generates a tarball of the content of the host
directory output/host/, named <TARGET-TUPLE>_sdk-buildroot.tar.gz
(which can be overriden by setting the environment variable
BR2_SDK_PREFIX) and located in the output directory output/images/.

This tarball can then be distributed to application developers, when
they want to develop their applications that are not (yet) packaged
as a Buildroot package.

Upon extracting the SDK tarball, the user must run the script
relocate-sdk.sh (located at the top directory of the SDK), to make
sure all paths are updated with the new location.

Alternatively, if you just want to prepare the SDK without generating
the tarball (e.g. because you will just be moving the host directory,
or will be generating the tarball on your own), Buildroot also allows
you to just prepare the SDK with make prepare-sdk without actually
generating a tarball.

8.13.2. Using gdb in Buildroot

Buildroot allows to do cross-debugging, where the debugger runs on
the build machine and communicates with gdbserver on the target to
control the execution of the program.

To achieve this:

  * If you are using an internal toolchain (built by Buildroot), you
    must enable BR2_PACKAGE_HOST_GDB, BR2_PACKAGE_GDB and
    BR2_PACKAGE_GDB_SERVER. This ensures that both the cross gdb and
    gdbserver get built, and that gdbserver gets installed to your
    target.
  * If you are using an external toolchain, you should enable
    BR2_TOOLCHAIN_EXTERNAL_GDB_SERVER_COPY, which will copy the
    gdbserver included with the external toolchain to the target. If
    your external toolchain does not have a cross gdb or gdbserver,
    it is also possible to let Buildroot build them, by enabling the
    same options as for the internal toolchain backend.

Now, to start debugging a program called foo, you should run on the
target:

gdbserver :2345 foo

This will cause gdbserver to listen on TCP port 2345 for a connection
from the cross gdb.

Then, on the host, you should start the cross gdb using the following
command line:

<buildroot>/output/host/bin/<tuple>-gdb -x <buildroot>/output/staging/usr/share/buildroot/gdbinit foo

Of course, foo must be available in the current directory, built with
debugging symbols. Typically you start this command from the
directory where foo is built (and not from output/target/ as the
binaries in that directory are stripped).

The <buildroot>/output/staging/usr/share/buildroot/gdbinit file will
tell the cross gdb where to find the libraries of the target.

Finally, to connect to the target from the cross gdb:

(gdb) target remote <target ip address>:2345

8.13.3. Using ccache in Buildroot

ccache [http://ccache.samba.org] is a compiler cache. It stores the
object files resulting from each compilation process, and is able to
skip future compilation of the same source file (with same compiler
and same arguments) by using the pre-existing object files. When
doing almost identical builds from scratch a number of times, it can
nicely speed up the build process.

ccache support is integrated in Buildroot. You just have to enable
Enable compiler cache in Build options. This will automatically build
ccache and use it for every host and target compilation.

The cache is located in $HOME/.buildroot-ccache. It is stored outside
of Buildroot output directory so that it can be shared by separate
Buildroot builds. If you want to get rid of the cache, simply remove
this directory.

You can get statistics on the cache (its size, number of hits,
misses, etc.) by running make ccache-stats.

The make target ccache-options and the CCACHE_OPTIONS variable
provide more generic access to the ccache. For example

# set cache limit size
make CCACHE_OPTIONS="--max-size=5G" ccache-options

# zero statistics counters
make CCACHE_OPTIONS="--zero-stats" ccache-options

ccache makes a hash of the source files and of the compiler options.
If a compiler option is different, the cached object file will not be
used. Many compiler options, however, contain an absolute path to the
staging directory. Because of this, building in a different output
directory would lead to many cache misses.

To avoid this issue, buildroot has the Use relative paths option
(BR2_CCACHE_USE_BASEDIR). This will rewrite all absolute paths that
point inside the output directory into relative paths. Thus, changing
the output directory no longer leads to cache misses.

A disadvantage of the relative paths is that they also end up to be
relative paths in the object file. Therefore, for example, the
debugger will no longer find the file, unless you cd to the output
directory first.

See the ccache manual’s section on "Compiling in different
directories" [https://ccache.samba.org/manual.html#
_compiling_in_different_directories] for more details about this
rewriting of absolute paths.

8.13.4. Location of downloaded packages

The various tarballs that are downloaded by Buildroot are all stored
in BR2_DL_DIR, which by default is the dl directory. If you want to
keep a complete version of Buildroot which is known to be working
with the associated tarballs, you can make a copy of this directory.
This will allow you to regenerate the toolchain and the target
filesystem with exactly the same versions.

If you maintain several Buildroot trees, it might be better to have a
shared download location. This can be achieved by pointing the
BR2_DL_DIR environment variable to a directory. If this is set, then
the value of BR2_DL_DIR in the Buildroot configuration is overridden.
The following line should be added to <~/.bashrc>.

 export BR2_DL_DIR=<shared download location>

The download location can also be set in the .config file, with the
BR2_DL_DIR option. Unlike most options in the .config file, this
value is overridden by the BR2_DL_DIR environment variable.

8.13.5. Package-specific make targets

Running make <package> builds and installs that particular package
and its dependencies.

For packages relying on the Buildroot infrastructure, there are
numerous special make targets that can be called independently like
this:

make <package>-<target>

The package build targets are (in the order they are executed):

+------------------------------------------------------------+
|command/target |Description                                 |
|---------------+--------------------------------------------|
|    source     |Fetch the source (download the tarball,     |
|               |clone the source repository, etc)           |
|---------------+--------------------------------------------|
|    depends    |Build and install all dependencies required |
|               |to build the package                        |
|---------------+--------------------------------------------|
|    extract    |Put the source in the package build         |
|               |directory (extract the tarball, copy the    |
|               |source, etc)                                |
|---------------+--------------------------------------------|
|     patch     |Apply the patches, if any                   |
|---------------+--------------------------------------------|
|   configure   |Run the configure commands, if any          |
|---------------+--------------------------------------------|
|     build     |Run the compilation commands                |
|---------------+--------------------------------------------|
|install-staging|target package: Run the installation of the |
|               |package in the staging directory, if        |
|               |necessary                                   |
|---------------+--------------------------------------------|
|install-target |target package: Run the installation of the |
|               |package in the target directory, if         |
|               |necessary                                   |
|---------------+--------------------------------------------|
|    install    |target package: Run the 2 previous          |
|               |installation commands                       |
|               |                                            |
|               |host package: Run the installation of the   |
|               |package in the host directory               |
+------------------------------------------------------------+

Additionally, there are some other useful make targets:

+------------------------------------------------------------+
|    command/target     |Description                         |
|-----------------------+------------------------------------|
|     show-depends      |Displays the first-order            |
|                       |dependencies required to build the  |
|                       |package                             |
|-----------------------+------------------------------------|
|show-recursive-depends |Recursively displays the            |
|                       |dependencies required to build the  |
|                       |package                             |
|-----------------------+------------------------------------|
|     show-rdepends     |Displays the first-order reverse    |
|                       |dependencies of the package (i.e    |
|                       |packages that directly depend on it)|
|-----------------------+------------------------------------|
|show-recursive-rdepends|Recursively displays the reverse    |
|                       |dependencies of the package (i.e the|
|                       |packages that depend on it, directly|
|                       |or indirectly)                      |
|-----------------------+------------------------------------|
|     graph-depends     |Generate a dependency graph of the  |
|                       |package, in the context of the      |
|                       |current Buildroot configuration. See|
|                       |this section for more details about |
|                       |dependency graphs.                  |
|-----------------------+------------------------------------|
|    graph-rdepends     |Generate a graph of this package    |
|                       |reverse dependencies (i.e the       |
|                       |packages that depend on it, directly|
|                       |or indirectly)                      |
|-----------------------+------------------------------------|
|       dirclean        |Remove the whole package build      |
|                       |directory                           |
|-----------------------+------------------------------------|
|       reinstall       |Re-run the install commands         |
|-----------------------+------------------------------------|
|        rebuild        |Re-run the compilation commands -   |
|                       |this only makes sense when using the|
|                       |OVERRIDE_SRCDIR feature or when you |
|                       |modified a file directly in the     |
|                       |build directory                     |
|-----------------------+------------------------------------|
|      reconfigure      |Re-run the configure commands, then |
|                       |rebuild - this only makes sense when|
|                       |using the OVERRIDE_SRCDIR feature or|
|                       |when you modified a file directly in|
|                       |the build directory                 |
+------------------------------------------------------------+

8.13.6. Using Buildroot during development

The normal operation of Buildroot is to download a tarball, extract
it, configure, compile and install the software component found
inside this tarball. The source code is extracted in output/build/
<package>-<version>, which is a temporary directory: whenever make
clean is used, this directory is entirely removed, and re-created at
the next make invocation. Even when a Git or Subversion repository is
used as the input for the package source code, Buildroot creates a
tarball out of it, and then behaves as it normally does with
tarballs.

This behavior is well-suited when Buildroot is used mainly as an
integration tool, to build and integrate all the components of an
embedded Linux system. However, if one uses Buildroot during the
development of certain components of the system, this behavior is not
very convenient: one would instead like to make a small change to the
source code of one package, and be able to quickly rebuild the system
with Buildroot.

Making changes directly in output/build/<package>-<version> is not an
appropriate solution, because this directory is removed on make
clean.

Therefore, Buildroot provides a specific mechanism for this use case:
the <pkg>_OVERRIDE_SRCDIR mechanism. Buildroot reads an override
file, which allows the user to tell Buildroot the location of the
source for certain packages.

The default location of the override file is $(CONFIG_DIR)/local.mk,
as defined by the BR2_PACKAGE_OVERRIDE_FILE configuration option. $
(CONFIG_DIR) is the location of the Buildroot .config file, so
local.mk by default lives side-by-side with the .config file, which
means:

  * In the top-level Buildroot source directory for in-tree builds
    (i.e., when O= is not used)
  * In the out-of-tree directory for out-of-tree builds (i.e., when O
    = is used)

If a different location than these defaults is required, it can be
specified through the BR2_PACKAGE_OVERRIDE_FILE configuration option.

In this override file, Buildroot expects to find lines of the form:

<pkg1>_OVERRIDE_SRCDIR = /path/to/pkg1/sources
<pkg2>_OVERRIDE_SRCDIR = /path/to/pkg2/sources

For example:

LINUX_OVERRIDE_SRCDIR = /home/bob/linux/
BUSYBOX_OVERRIDE_SRCDIR = /home/bob/busybox/

When Buildroot finds that for a given package, an <pkg>
_OVERRIDE_SRCDIR has been defined, it will no longer attempt to
download, extract and patch the package. Instead, it will directly
use the source code available in the specified directory and make
clean will not touch this directory. This allows to point Buildroot
to your own directories, that can be managed by Git, Subversion, or
any other version control system. To achieve this, Buildroot will use
rsync to copy the source code of the component from the specified
<pkg>_OVERRIDE_SRCDIR to output/build/<package>-custom/.

This mechanism is best used in conjunction with the make <pkg>
-rebuild and make <pkg>-reconfigure targets. A make <pkg>-rebuild all
sequence will rsync the source code from <pkg>_OVERRIDE_SRCDIR to
output/build/<package>-custom (thanks to rsync, only the modified
files are copied), and restart the build process of just this
package.

In the example of the linux package above, the developer can then
make a source code change in /home/bob/linux and then run:

make linux-rebuild all

and in a matter of seconds gets the updated Linux kernel image in
output/images. Similarly, a change can be made to the BusyBox source
code in /home/bob/busybox, and after:

make busybox-rebuild all

the root filesystem image in output/images contains the updated
BusyBox.

Source trees for big projects often contain hundreds or thousands of
files which are not needed for building, but will slow down the
process of copying the sources with rsync. Optionally, it is possible
define <pkg>_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS to skip syncing certain
files from the source tree. For example, when working on the
webkitgtk package, the following will exclude the tests and in-tree
builds from a local WebKit source tree:

WEBKITGTK_OVERRIDE_SRCDIR = /home/bob/WebKit
WEBKITGTK_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS = \
        --exclude JSTests --exclude ManualTests --exclude PerformanceTests \
        --exclude WebDriverTests --exclude WebKitBuild --exclude WebKitLibraries \
        --exclude WebKit.xcworkspace --exclude Websites --exclude Examples

By default, Buildroot skips syncing of VCS artifacts (e.g., the .git
and .svn directories). Some packages prefer to have these VCS
directories available during build, for example for automatically
determining a precise commit reference for version information. To
undo this built-in filtering at a cost of a slower speed, add these
directories back:

LINUX_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS = --include .git

Chapter 9. Project-specific customization

Typical actions you may need to perform for a given project are:

  * configuring Buildroot (including build options and toolchain,
    bootloader, kernel, package and filesystem image type selection)
  * configuring other components, like the Linux kernel and BusyBox
  * customizing the generated target filesystem

      + adding or overwriting files on the target filesystem (using
        BR2_ROOTFS_OVERLAY)
      + modifying or deleting files on the target filesystem (using
        BR2_ROOTFS_POST_BUILD_SCRIPT)
      + running arbitrary commands prior to generating the filesystem
        image (using BR2_ROOTFS_POST_BUILD_SCRIPT)
      + setting file permissions and ownership (using
        BR2_ROOTFS_DEVICE_TABLE)
      + adding custom devices nodes (using
        BR2_ROOTFS_STATIC_DEVICE_TABLE)
  * adding custom user accounts (using BR2_ROOTFS_USERS_TABLES)
  * running arbitrary commands after generating the filesystem image
    (using BR2_ROOTFS_POST_IMAGE_SCRIPT)
  * adding project-specific patches to some packages (using
    BR2_GLOBAL_PATCH_DIR)
  * adding project-specific packages

An important note regarding such project-specific customizations:
please carefully consider which changes are indeed project-specific
and which changes are also useful to developers outside your project.
The Buildroot community highly recommends and encourages the
upstreaming of improvements, packages and board support to the
official Buildroot project. Of course, it is sometimes not possible
or desirable to upstream because the changes are highly specific or
proprietary.

This chapter describes how to make such project-specific
customizations in Buildroot and how to store them in a way that you
can build the same image in a reproducible way, even after running 
make clean. By following the recommended strategy, you can even use
the same Buildroot tree to build multiple distinct projects!

9.1. Recommended directory structure

When customizing Buildroot for your project, you will be creating one
or more project-specific files that need to be stored somewhere.
While most of these files could be placed in any location as their
path is to be specified in the Buildroot configuration, the Buildroot
developers recommend a specific directory structure which is
described in this section.

Orthogonal to this directory structure, you can choose where you
place this structure itself: either inside the Buildroot tree, or
outside of it using a br2-external tree. Both options are valid, the
choice is up to you.

+-- board/
|   +-- <company>/
|       +-- <boardname>/
|           +-- linux.config
|           +-- busybox.config
|           +-- <other configuration files>
|           +-- post_build.sh
|           +-- post_image.sh
|           +-- rootfs_overlay/
|           |   +-- etc/
|           |   +-- <some file>
|           +-- patches/
|               +-- foo/
|               |   +-- <some patch>
|               +-- libbar/
|                   +-- <some other patches>
|
+-- configs/
|   +-- <boardname>_defconfig
|
+-- package/
|   +-- <company>/
|       +-- Config.in (if not using a br2-external tree)
|       +-- <company>.mk (if not using a br2-external tree)
|       +-- package1/
|       |    +-- Config.in
|       |    +-- package1.mk
|       +-- package2/
|           +-- Config.in
|           +-- package2.mk
|
+-- Config.in (if using a br2-external tree)
+-- external.mk (if using a br2-external tree)
+-- external.desc (if using a br2-external tree)

Details on the files shown above are given further in this chapter.

Note: if you choose to place this structure outside of the Buildroot
tree but in a br2-external tree, the <company> and possibly
<boardname> components may be superfluous and can be left out.

9.1.1. Implementing layered customizations

It is quite common for a user to have several related projects that
partly need the same customizations. Instead of duplicating these
customizations for each project, it is recommended to use a layered
customization approach, as explained in this section.

Almost all of the customization methods available in Buildroot, like
post-build scripts and root filesystem overlays, accept a
space-separated list of items. The specified items are always treated
in order, from left to right. By creating more than one such item,
one for the common customizations and another one for the really
project-specific customizations, you can avoid unnecessary
duplication. Each layer is typically embodied by a separate directory
inside board/<company>/. Depending on your projects, you could even
introduce more than two layers.

An example directory structure for where a user has two customization
layers common and fooboard is:

+-- board/
    +-- <company>/
        +-- common/
        |   +-- post_build.sh
        |   +-- rootfs_overlay/
        |   |   +-- ...
        |   +-- patches/
        |       +-- ...
        |
        +-- fooboard/
            +-- linux.config
            +-- busybox.config
            +-- <other configuration files>
            +-- post_build.sh
            +-- rootfs_overlay/
            |   +-- ...
            +-- patches/
                +-- ...

For example, if the user has the BR2_GLOBAL_PATCH_DIR configuration
option set as:

BR2_GLOBAL_PATCH_DIR="board/<company>/common/patches board/<company>/fooboard/patches"

then first the patches from the common layer would be applied,
followed by the patches from the fooboard layer.

9.2. Keeping customizations outside of Buildroot

As already briefly mentioned in Section 9.1, “Recommended directory
structure”, you can place project-specific customizations in two
locations:

  * directly within the Buildroot tree, typically maintaining them
    using branches in a version control system so that upgrading to a
    newer Buildroot release is easy.
  * outside of the Buildroot tree, using the br2-external mechanism.
    This mechanism allows to keep package recipes, board support and
    configuration files outside of the Buildroot tree, while still
    having them nicely integrated in the build logic. We call this
    location a br2-external tree. This section explains how to use
    the br2-external mechanism and what to provide in a br2-external
    tree.

One can tell Buildroot to use one or more br2-external trees by
setting the BR2_EXTERNAL make variable set to the path(s) of the
br2-external tree(s) to use. It can be passed to any Buildroot make
invocation. It is automatically saved in the hidden .br2-external.mk
file in the output directory. Thanks to this, there is no need to
pass BR2_EXTERNAL at every make invocation. It can however be changed
at any time by passing a new value, and can be removed by passing an
empty value.

Note. The path to a br2-external tree can be either absolute or
relative. If it is passed as a relative path, it is important to note
that it is interpreted relative to the main Buildroot source
directory, not to the Buildroot output directory.

Note: If using an br2-external tree from before Buildroot 2016.11,
you need to convert it before you can use it with Buildroot 2016.11
onward. See Section 26.1, “Migrating to 2016.11” for help on doing
so.

Some examples:

buildroot/ $ make BR2_EXTERNAL=/path/to/foo menuconfig

From now on, definitions from the /path/to/foo br2-external tree will
be used:

buildroot/ $ make
buildroot/ $ make legal-info

We can switch to another br2-external tree at any time:

buildroot/ $ make BR2_EXTERNAL=/where/we/have/bar xconfig

We can also use multiple br2-external trees:

buildroot/ $ make BR2_EXTERNAL=/path/to/foo:/where/we/have/bar menuconfig

Or disable the usage of any br2-external tree:

buildroot/ $ make BR2_EXTERNAL= xconfig

9.2.1. Layout of a br2-external tree

A br2-external tree must contain at least those three files,
described in the following chapters:

  * external.desc
  * external.mk
  * Config.in

Apart from those mandatory files, there may be additional and
optional content that may be present in a br2-external tree, like the
configs/ or provides/ directories. They are described in the
following chapters as well.

A complete example br2-external tree layout is also described later.

9.2.1.1. The external.desc file

That file describes the br2-external tree: the name and description
for that br2-external tree.

The format for this file is line based, with each line starting by a
keyword, followed by a colon and one or more spaces, followed by the
value assigned to that keyword. There are two keywords currently
recognised:

  * name, mandatory, defines the name for that br2-external tree.
    That name must only use ASCII characters in the set [A-Za-z0-9_];
    any other character is forbidden. Buildroot sets the variable
    BR2_EXTERNAL_$(NAME)_PATH to the absolute path of the
    br2-external tree, so that you can use it to refer to your
    br2-external tree. This variable is available both in Kconfig, so
    you can use it to source your Kconfig files (see below) and in
    the Makefile, so that you can use it to include other Makefiles
    (see below) or refer to other files (like data files) from your
    br2-external tree.

    Note: Since it is possible to use multiple br2-external trees at
    once, this name is used by Buildroot to generate variables for
    each of those trees. That name is used to identify your
    br2-external tree, so try to come up with a name that really
    describes your br2-external tree, in order for it to be
    relatively unique, so that it does not clash with another name
    from another br2-external tree, especially if you are planning on
    somehow sharing your br2-external tree with third parties or
    using br2-external trees from third parties.

  * desc, optional, provides a short description for that
    br2-external tree. It shall fit on a single line, is mostly
    free-form (see below), and is used when displaying information
    about a br2-external tree (e.g. above the list of defconfig
    files, or as the prompt in the menuconfig); as such, it should
    relatively brief (40 chars is probably a good upper limit). The
    description is available in the BR2_EXTERNAL_$(NAME)_DESC
    variable.

Examples of names and the corresponding BR2_EXTERNAL_$(NAME)_PATH
variables:

  * FOO → BR2_EXTERNAL_FOO_PATH
  * BAR_42 → BR2_EXTERNAL_BAR_42_PATH

In the following examples, it is assumed the name to be set to
BAR_42.

Note: Both BR2_EXTERNAL_$(NAME)_PATH and BR2_EXTERNAL_$(NAME)_DESC
are available in the Kconfig files and the Makefiles. They are also
exported in the environment so are available in post-build,
post-image and in-fakeroot scripts.

9.2.1.2. The Config.in and external.mk files

Those files (which may each be empty) can be used to define package
recipes (i.e. foo/Config.in and foo/foo.mk like for packages bundled
in Buildroot itself) or other custom configuration options or make
logic.

Buildroot automatically includes the Config.in from each br2-external
tree to make it appear in the top-level configuration menu, and
includes the external.mk from each br2-external tree with the rest of
the makefile logic.

The main usage of this is to store package recipes. The recommended
way to do this is to write a Config.in file that looks like:

source "$BR2_EXTERNAL_BAR_42_PATH/package/package1/Config.in"
source "$BR2_EXTERNAL_BAR_42_PATH/package/package2/Config.in"

Then, have an external.mk file that looks like:

include $(sort $(wildcard $(BR2_EXTERNAL_BAR_42_PATH)/package/*/*.mk))

And then in $(BR2_EXTERNAL_BAR_42_PATH)/package/package1 and $
(BR2_EXTERNAL_BAR_42_PATH)/package/package2 create normal Buildroot
package recipes, as explained in Chapter 17, Adding new packages to
Buildroot. If you prefer, you can also group the packages in
subdirectories called <boardname> and adapt the above paths
accordingly.

You can also define custom configuration options in Config.in and
custom make logic in external.mk.

9.2.1.3. The configs/ directory

One can store Buildroot defconfigs in the configs subdirectory of the
br2-external tree. Buildroot will automatically show them in the
output of make list-defconfigs and allow them to be loaded with the
normal make <name>_defconfig command. They will be visible in the 
make list-defconfigs output, below an External configs label that
contains the name of the br2-external tree they are defined in.

Note: If a defconfig file is present in more than one br2-external
tree, then the one from the last br2-external tree is used. It is
thus possible to override a defconfig bundled in Buildroot or another
br2-external tree.

9.2.1.4. The provides/ directory

For some packages, Buildroot provides a choice between two (or more)
implementations of API-compatible such packages. For example, there
is a choice to choose either libjpeg ot jpeg-turbo, and another
choice between openssl or libressl. Finally, there is a choice to
select one of the known, pre-configured toolchains.

It is possible for a br2-external to extend those choices, by
providing a set of files that define those alternatives:

  * provides/toolchains.in defines the pre-configured toolchains,
    which will then be listed in the toolchain selection;
  * provides/jpeg.in defines the alternative libjpeg implementations;
  * provides/openssl.in defines the alternative openssl
    implementations.

9.2.1.5. Free-form content

One can store all the board-specific configuration files there, such
as the kernel configuration, the root filesystem overlay, or any
other configuration file for which Buildroot allows to set the
location (by using the BR2_EXTERNAL_$(NAME)_PATH variable). For
example, you could set the paths to a global patch directory, to a
rootfs overlay and to the kernel configuration file as follows (e.g.
by running make menuconfig and filling in these options):

BR2_GLOBAL_PATCH_DIR=$(BR2_EXTERNAL_BAR_42_PATH)/patches/
BR2_ROOTFS_OVERLAY=$(BR2_EXTERNAL_BAR_42_PATH)/board/<boardname>/overlay/
BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE=$(BR2_EXTERNAL_BAR_42_PATH)/board/<boardname>/kernel.config

9.2.1.6. Additional Linux kernel extensions

Additional Linux kernel extensions (see Section 17.20.2,
“linux-kernel-extensions”) can be added by storing them in the linux/
directory at the root of a br2-external tree.

9.2.1.7. Example layout

Here is an example layout using all features of br2-external (the
sample content is shown for the file above it, when it is relevant to
explain the br2-external tree; this is all entirely made up just for
the sake of illustration, of course):

/path/to/br2-ext-tree/
  |- external.desc
  |     |name: BAR_42
  |     |desc: Example br2-external tree
  |     `----
  |
  |- Config.in
  |     |source "$BR2_EXTERNAL_BAR_42_PATH/toolchain/toolchain-external-mine/Config.in.options"
  |     |source "$BR2_EXTERNAL_BAR_42_PATH/package/pkg-1/Config.in"
  |     |source "$BR2_EXTERNAL_BAR_42_PATH/package/pkg-2/Config.in"
  |     |source "$BR2_EXTERNAL_BAR_42_PATH/package/my-jpeg/Config.in"
  |     |
  |     |config BAR_42_FLASH_ADDR
  |     |    hex "my-board flash address"
  |     |    default 0x10AD
  |     `----
  |
  |- external.mk
  |     |include $(sort $(wildcard $(BR2_EXTERNAL_BAR_42_PATH)/package/*/*.mk))
  |     |include $(sort $(wildcard $(BR2_EXTERNAL_BAR_42_PATH)/toolchain/*/*.mk))
  |     |
  |     |flash-my-board:
  |     |    $(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/flash-image \
  |     |        --image $(BINARIES_DIR)/image.bin \
  |     |        --address $(BAR_42_FLASH_ADDR)
  |     `----
  |
  |- package/pkg-1/Config.in
  |     |config BR2_PACKAGE_PKG_1
  |     |    bool "pkg-1"
  |     |    help
  |     |      Some help about pkg-1
  |     `----
  |- package/pkg-1/pkg-1.hash
  |- package/pkg-1/pkg-1.mk
  |     |PKG_1_VERSION = 1.2.3
  |     |PKG_1_SITE = /some/where/to/get/pkg-1
  |     |PKG_1_LICENSE = blabla
  |     |
  |     |define PKG_1_INSTALL_INIT_SYSV
  |     |    $(INSTALL) -D -m 0755 $(PKG_1_PKGDIR)/S99my-daemon \
  |     |                          $(TARGET_DIR)/etc/init.d/S99my-daemon
  |     |endef
  |     |
  |     |$(eval $(autotools-package))
  |     `----
  |- package/pkg-1/S99my-daemon
  |
  |- package/pkg-2/Config.in
  |- package/pkg-2/pkg-2.hash
  |- package/pkg-2/pkg-2.mk
  |
  |- provides/jpeg.in
  |     |config BR2_PACKAGE_MY_JPEG
  |     |    bool "my-jpeg"
  |     `----
  |- package/my-jpeg/Config.in
  |     |config BR2_PACKAGE_PROVIDES_JPEG
  |     |    default "my-jpeg" if BR2_PACKAGE_MY_JPEG
  |     `----
  |- package/my-jpeg/my-jpeg.mk
  |     |# This is a normal package .mk file
  |     |MY_JPEG_VERSION = 1.2.3
  |     |MY_JPEG_SITE = https://example.net/some/place
  |     |MY_JPEG_PROVIDES = jpeg
  |     |$(eval $(autotools-package))
  |     `----
  |
  |- provides/toolchains.in
  |     |config BR2_TOOLCHAIN_EXTERNAL_MINE
  |     |    bool "my custom toolchain"
  |     |    depends on BR2_some_arch
  |     |    select BR2_INSTALL_LIBSTDCPP
  |     `----
  |- toolchain/toolchain-external-mine/Config.in.options
  |     |if BR2_TOOLCHAIN_EXTERNAL_MINE
  |     |config BR2_TOOLCHAIN_EXTERNAL_PREFIX
  |     |    default "arch-mine-linux-gnu"
  |     |config BR2_PACKAGE_PROVIDES_TOOLCHAIN_EXTERNAL
  |     |    default "toolchain-external-mine"
  |     |endif
  |     `----
  |- toolchain/toolchain-external-mine/toolchain-external-mine.mk
  |     |TOOLCHAIN_EXTERNAL_MINE_SITE = https://example.net/some/place
  |     |TOOLCHAIN_EXTERNAL_MINE_SOURCE = my-toolchain.tar.gz
  |     |$(eval $(toolchain-external-package))
  |     `----
  |
  |- linux/Config.ext.in
  |     |config BR2_LINUX_KERNEL_EXT_EXAMPLE_DRIVER
  |     |    bool "example-external-driver"
  |     |    help
  |     |      Example external driver
  |     |---
  |- linux/linux-ext-example-driver.mk
  |
  |- configs/my-board_defconfig
  |     |BR2_GLOBAL_PATCH_DIR="$(BR2_EXTERNAL_BAR_42_PATH)/patches/"
  |     |BR2_ROOTFS_OVERLAY="$(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/overlay/"
  |     |BR2_ROOTFS_POST_IMAGE_SCRIPT="$(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/post-image.sh"
  |     |BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="$(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/kernel.config"
  |     `----
  |
  |- patches/linux/0001-some-change.patch
  |- patches/linux/0002-some-other-change.patch
  |- patches/busybox/0001-fix-something.patch
  |
  |- board/my-board/kernel.config
  |- board/my-board/overlay/var/www/index.html
  |- board/my-board/overlay/var/www/my.css
  |- board/my-board/flash-image
  `- board/my-board/post-image.sh
        |#!/bin/sh
        |generate-my-binary-image \
        |    --root ${BINARIES_DIR}/rootfs.tar \
        |    --kernel ${BINARIES_DIR}/zImage \
        |    --dtb ${BINARIES_DIR}/my-board.dtb \
        |    --output ${BINARIES_DIR}/image.bin
        `----

The br2-external tree will then be visible in the menuconfig (with
the layout expanded):

External options  --->
    *** Example br2-external tree (in /path/to/br2-ext-tree/)
    [ ] pkg-1
    [ ] pkg-2
    (0x10AD) my-board flash address

If you are using more than one br2-external tree, it would look like
(with the layout expanded and the second one with name FOO_27 but no
desc: field in external.desc):

External options  --->
    Example br2-external tree  --->
        *** Example br2-external tree (in /path/to/br2-ext-tree)
        [ ] pkg-1
        [ ] pkg-2
        (0x10AD) my-board flash address
    FOO_27  --->
        *** FOO_27 (in /path/to/another-br2-ext)
        [ ] foo
        [ ] bar

Additionally, the jpeg provider will be visible in the jpeg choice:

Target packages  --->
    Libraries  --->
        Graphics  --->
            [*] jpeg support
                jpeg variant ()  --->
                    ( ) jpeg
                    ( ) jpeg-turbo
                        *** jpeg from: Example br2-external tree ***
                    (X) my-jpeg
                        *** jpeg from: FOO_27 ***
                    ( ) another-jpeg

And similarly for the toolchains:

Toolchain  --->
    Toolchain ()  --->
        ( ) Custom toolchain
            *** Toolchains from: Example br2-external tree ***
        (X) my custom toolchain

Note. The toolchain options in toolchain/toolchain-external-mine/
Config.in.options will not appear in the Toolchain menu. They must be
explicitly included from within the br2-external’s top-level
Config.in and will thus appear in the External options menu.

9.3. Storing the Buildroot configuration

The Buildroot configuration can be stored using the command make
savedefconfig.

This strips the Buildroot configuration down by removing
configuration options that are at their default value. The result is
stored in a file called defconfig. If you want to save it in another
place, change the BR2_DEFCONFIG option in the Buildroot configuration
itself, or call make with make savedefconfig BR2_DEFCONFIG=
<path-to-defconfig>.

The recommended place to store this defconfig is configs/<boardname>
_defconfig. If you follow this recommendation, the configuration will
be listed in make help and can be set again by running make
<boardname>_defconfig.

Alternatively, you can copy the file to any other place and rebuild
with make defconfig BR2_DEFCONFIG=<path-to-defconfig-file>.

9.4. Storing the configuration of other components

The configuration files for BusyBox, the Linux kernel, Barebox,
U-Boot and uClibc should be stored as well if changed. For each of
these components, a Buildroot configuration option exists to point to
an input configuration file, e.g.
BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE. To store their configuration,
set these configuration options to a path where you want to save the
configuration files, and then use the helper targets described below
to actually store the configuration.

As explained in Section 9.1, “Recommended directory structure”, the
recommended path to store these configuration files is board/
<company>/<boardname>/foo.config.

Make sure that you create a configuration file before changing the
BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE etc. options. Otherwise,
Buildroot will try to access this config file, which doesn’t exist
yet, and will fail. You can create the configuration file by running
make linux-menuconfig etc.

Buildroot provides a few helper targets to make the saving of
configuration files easier.

  * make linux-update-defconfig saves the linux configuration to the
    path specified by BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE. It
    simplifies the config file by removing default values. However,
    this only works with kernels starting from 2.6.33. For earlier
    kernels, use make linux-update-config.
  * make busybox-update-config saves the busybox configuration to the
    path specified by BR2_PACKAGE_BUSYBOX_CONFIG.
  * make uclibc-update-config saves the uClibc configuration to the
    path specified by BR2_UCLIBC_CONFIG.
  * make barebox-update-defconfig saves the barebox configuration to
    the path specified by BR2_TARGET_BAREBOX_CUSTOM_CONFIG_FILE.
  * make uboot-update-defconfig saves the U-Boot configuration to the
    path specified by BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE.
  * For at91bootstrap3, no helper exists so you have to copy the
    config file manually to
    BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_CONFIG_FILE.

9.5. Customizing the generated target filesystem

Besides changing the configuration through make *config, there are a
few other ways to customize the resulting target filesystem.

The two recommended methods, which can co-exist, are root filesystem
overlay(s) and post build script(s).

Root filesystem overlays (BR2_ROOTFS_OVERLAY)

    A filesystem overlay is a tree of files that is copied directly
    over the target filesystem after it has been built. To enable
    this feature, set config option BR2_ROOTFS_OVERLAY (in the System
    configuration menu) to the root of the overlay. You can even
    specify multiple overlays, space-separated. If you specify a
    relative path, it will be relative to the root of the Buildroot
    tree. Hidden directories of version control systems, like .git,
    .svn, .hg, etc., files called .empty and files ending in ~ are
    excluded from the copy.

    When BR2_ROOTFS_MERGED_USR is enabled, then the overlay must not
    contain the /bin, /lib or /sbin directories, as Buildroot will
    create them as symbolic links to the relevant folders in /usr. In
    such a situation, should the overlay have any programs or
    libraries, they should be placed in /usr/bin, /usr/sbin and /usr/
    lib.

    As shown in Section 9.1, “Recommended directory structure”, the
    recommended path for this overlay is board/<company>/<boardname>/
    rootfs-overlay.

Post-build scripts (BR2_ROOTFS_POST_BUILD_SCRIPT)

    Post-build scripts are shell scripts called after Buildroot
    builds all the selected software, but before the rootfs images
    are assembled. To enable this feature, specify a space-separated
    list of post-build scripts in config option
    BR2_ROOTFS_POST_BUILD_SCRIPT (in the System configuration menu).
    If you specify a relative path, it will be relative to the root
    of the Buildroot tree.

    Using post-build scripts, you can remove or modify any file in
    your target filesystem. You should, however, use this feature
    with care. Whenever you find that a certain package generates
    wrong or unneeded files, you should fix that package rather than
    work around it with some post-build cleanup scripts.

    As shown in Section 9.1, “Recommended directory structure”, the
    recommended path for this script is board/<company>/<boardname>/
    post_build.sh.

    The post-build scripts are run with the main Buildroot tree as
    current working directory. The path to the target filesystem is
    passed as the first argument to each script. If the config option
    BR2_ROOTFS_POST_SCRIPT_ARGS is not empty, these arguments will be
    passed to the script too. All the scripts will be passed the
    exact same set of arguments, it is not possible to pass different
    sets of arguments to each script.

    In addition, you may also use these environment variables:

      + BR2_CONFIG: the path to the Buildroot .config file
      + HOST_DIR, STAGING_DIR, TARGET_DIR: see Section 17.5.2,
        “generic-package reference”
      + BUILD_DIR: the directory where packages are extracted and
        built
      + BINARIES_DIR: the place where all binary files (aka images)
        are stored
      + BASE_DIR: the base output directory

Below three more methods of customizing the target filesystem are
described, but they are not recommended.

Direct modification of the target filesystem

    For temporary modifications, you can modify the target filesystem
    directly and rebuild the image. The target filesystem is
    available under output/target/. After making your changes, run
    make to rebuild the target filesystem image.

    This method allows you to do anything to the target filesystem,
    but if you need to clean your Buildroot tree using make clean,
    these changes will be lost. Such cleaning is necessary in several
    cases, refer to Section 8.2, “Understanding when a full rebuild
    is necessary” for details. This solution is therefore only useful
    for quick tests: changes do not survive the make clean command.
    Once you have validated your changes, you should make sure that
    they will persist after a make clean, using a root filesystem
    overlay or a post-build script.

Custom target skeleton (BR2_ROOTFS_SKELETON_CUSTOM)

    The root filesystem image is created from a target skeleton, on
    top of which all packages install their files. The skeleton is
    copied to the target directory output/target before any package
    is built and installed. The default target skeleton provides the
    standard Unix filesystem layout and some basic init scripts and
    configuration files.

    If the default skeleton (available under system/skeleton) does
    not match your needs, you would typically use a root filesystem
    overlay or post-build script to adapt it. However, if the default
    skeleton is entirely different than what you need, using a custom
    skeleton may be more suitable.

    To enable this feature, enable config option
    BR2_ROOTFS_SKELETON_CUSTOM and set
    BR2_ROOTFS_SKELETON_CUSTOM_PATH to the path of your custom
    skeleton. Both options are available in the System configuration
    menu. If you specify a relative path, it will be relative to the
    root of the Buildroot tree.

    Custom skeletons don’t need to contain the /bin, /lib or /sbin
    directories, since they are created automatically during the
    build. When BR2_ROOTFS_MERGED_USR is enabled, then the custom
    skeleton must not contain the /bin, /lib or /sbin directories, as
    Buildroot will create them as symbolic links to the relevant
    folders in /usr. In such a situation, should the skeleton have
    any programs or libraries, they should be placed in /usr/bin, /
    usr/sbin and /usr/lib.

    This method is not recommended because it duplicates the entire
    skeleton, which prevents taking advantage of the fixes or
    improvements brought to the default skeleton in later Buildroot
    releases.

Post-fakeroot scripts (BR2_ROOTFS_POST_FAKEROOT_SCRIPT)

    When aggregating the final images, some parts of the process
    requires root rights: creating device nodes in /dev, setting
    permissions or ownership to files and directories… To avoid
    requiring actual root rights, Buildroot uses fakeroot to simulate
    root rights. This is not a complete substitute for actually being
    root, but is enough for what Buildroot needs.

    Post-fakeroot scripts are shell scripts that are called at the 
    end of the fakeroot phase, right before the filesystem image
    generator is called. As such, they are called in the fakeroot
    context.

    Post-fakeroot scripts can be useful in case you need to tweak the
    filesystem to do modifications that are usually only available to
    the root user.

    Note: It is recommended to use the existing mechanisms to set
    file permissions or create entries in /dev (see Section 9.5.1,
    “Setting file permissions and ownership and adding custom devices
    nodes”) or to create users (see Section 9.6, “Adding custom user
    accounts”)

    Note: The difference between post-build scripts (above) and
    fakeroot scripts, is that post-build scripts are not called in
    the fakeroot context.

    Note: Using fakeroot is not an absolute substitute for actually
    being root. fakeroot only ever fakes the file access rights and
    types (regular, block-or-char device…) and uid/gid; these are
    emulated in-memory.

9.5.1. Setting file permissions and ownership and adding custom
devices nodes

Sometimes it is needed to set specific permissions or ownership on
files or device nodes. For example, certain files may need to be
owned by root. Since the post-build scripts are not run as root, you
cannot do such changes from there unless you use an explicit fakeroot
from the post-build script.

Instead, Buildroot provides support for so-called permission tables.
To use this feature, set config option BR2_ROOTFS_DEVICE_TABLE to a
space-separated list of permission tables, regular text files
following the makedev syntax.

If you are using a static device table (i.e. not using devtmpfs,
mdev, or (e)udev) then you can add device nodes using the same
syntax, in so-called device tables. To use this feature, set config
option BR2_ROOTFS_STATIC_DEVICE_TABLE to a space-separated list of
device tables.

As shown in Section 9.1, “Recommended directory structure”, the
recommended location for such files is board/<company>/<boardname>/.

It should be noted that if the specific permissions or device nodes
are related to a specific application, you should set variables
FOO_PERMISSIONS and FOO_DEVICES in the package’s .mk file instead
(see Section 17.5.2, “generic-package reference”).

9.6. Adding custom user accounts

Sometimes it is needed to add specific users in the target system. To
cover this requirement, Buildroot provides support for so-called 
users tables. To use this feature, set config option
BR2_ROOTFS_USERS_TABLES to a space-separated list of users tables,
regular text files following the makeusers syntax.

As shown in Section 9.1, “Recommended directory structure”, the
recommended location for such files is board/<company>/<boardname>/.

It should be noted that if the custom users are related to a specific
application, you should set variable FOO_USERS in the package’s .mk
file instead (see Section 17.5.2, “generic-package reference”).

9.7. Customization after the images have been created

While post-build scripts (Section 9.5, “Customizing the generated
target filesystem”) are run before building the filesystem image,
kernel and bootloader, post-image scripts can be used to perform some
specific actions after all images have been created.

Post-image scripts can for example be used to automatically extract
your root filesystem tarball in a location exported by your NFS
server, or to create a special firmware image that bundles your root
filesystem and kernel image, or any other custom action required for
your project.

To enable this feature, specify a space-separated list of post-image
scripts in config option BR2_ROOTFS_POST_IMAGE_SCRIPT (in the System
configuration menu). If you specify a relative path, it will be
relative to the root of the Buildroot tree.

Just like post-build scripts, post-image scripts are run with the
main Buildroot tree as current working directory. The path to the
images output directory is passed as the first argument to each
script. If the config option BR2_ROOTFS_POST_SCRIPT_ARGS is not
empty, these arguments will be passed to the script too. All the
scripts will be passed the exact same set of arguments, it is not
possible to pass different sets of arguments to each script.

Again just like for the post-build scripts, the scripts have access
to the environment variables BR2_CONFIG, HOST_DIR, STAGING_DIR,
TARGET_DIR, BUILD_DIR, BINARIES_DIR and BASE_DIR.

The post-image scripts will be executed as the user that executes
Buildroot, which should normally not be the root user. Therefore, any
action requiring root permissions in one of these scripts will
require special handling (usage of fakeroot or sudo), which is left
to the script developer.

9.8. Adding project-specific patches

It is sometimes useful to apply extra patches to packages - on top of
those provided in Buildroot. This might be used to support custom
features in a project, for example, or when working on a new
architecture.

The BR2_GLOBAL_PATCH_DIR configuration option can be used to specify
a space separated list of one or more directories containing package
patches.

For a specific version <packageversion> of a specific package
<packagename>, patches are applied from BR2_GLOBAL_PATCH_DIR as
follows:

 1. For every directory - <global-patch-dir> - that exists in
    BR2_GLOBAL_PATCH_DIR, a <package-patch-dir> will be determined as
    follows:

      + <global-patch-dir>/<packagename>/<packageversion>/ if the
        directory exists.
      + Otherwise, <global-patch-dir>/<packagename> if the directory
        exists.
 2. Patches will then be applied from a <package-patch-dir> as
    follows:

      + If a series file exists in the package directory, then
        patches are applied according to the series file;
      + Otherwise, patch files matching *.patch are applied in
        alphabetical order. So, to ensure they are applied in the
        right order, it is highly recommended to name the patch files
        like this: <number>-<description>.patch, where <number>
        refers to the apply order.

For information about how patches are applied for a package, see
Section 18.2, “How patches are applied”

The BR2_GLOBAL_PATCH_DIR option is the preferred method for
specifying a custom patch directory for packages. It can be used to
specify a patch directory for any package in buildroot. It should
also be used in place of the custom patch directory options that are
available for packages such as U-Boot and Barebox. By doing this, it
will allow a user to manage their patches from one top-level
directory.

The exception to BR2_GLOBAL_PATCH_DIR being the preferred method for
specifying custom patches is BR2_LINUX_KERNEL_PATCH.
BR2_LINUX_KERNEL_PATCH should be used to specify kernel patches that
are available at a URL. Note: BR2_LINUX_KERNEL_PATCH specifies kernel
patches that are applied after patches available in
BR2_GLOBAL_PATCH_DIR, as it is done from a post-patch hook of the
Linux package.

9.9. Adding project-specific packages

In general, any new package should be added directly in the package
directory and submitted to the Buildroot upstream project. How to add
packages to Buildroot in general is explained in full detail in
Chapter 17, Adding new packages to Buildroot and will not be repeated
here. However, your project may need some proprietary packages that
cannot be upstreamed. This section will explain how you can keep such
project-specific packages in a project-specific directory.

As shown in Section 9.1, “Recommended directory structure”, the
recommended location for project-specific packages is package/
<company>/. If you are using the br2-external tree feature (see
Section 9.2, “Keeping customizations outside of Buildroot”) the
recommended location is to put them in a sub-directory named package/
in your br2-external tree.

However, Buildroot will not be aware of the packages in this
location, unless we perform some additional steps. As explained in
Chapter 17, Adding new packages to Buildroot, a package in Buildroot
basically consists of two files: a .mk file (describing how to build
the package) and a Config.in file (describing the configuration
options for this package).

Buildroot will automatically include the .mk files in first-level
subdirectories of the package directory (using the pattern package/*/
*.mk). If we want Buildroot to include .mk files from deeper
subdirectories (like package/<company>/package1/) then we simply have
to add a .mk file in a first-level subdirectory that includes these
additional .mk files. Therefore, create a file package/<company>/
<company>.mk with following contents (assuming you have only one
extra directory level below package/<company>/):

include $(sort $(wildcard package/<company>/*/*.mk))

For the Config.in files, create a file package/<company>/Config.in
that includes the Config.in files of all your packages. An exhaustive
list has to be provided since wildcards are not supported in the
source command of kconfig. For example:

source "package/<company>/package1/Config.in"
source "package/<company>/package2/Config.in"

Include this new file package/<company>/Config.in from package/
Config.in, preferably in a company-specific menu to make merges with
future Buildroot versions easier.

If using a br2-external tree, refer to Section 9.2, “Keeping
customizations outside of Buildroot” for how to fill in those files.

9.10. Quick guide to storing your project-specific customizations

Earlier in this chapter, the different methods for making
project-specific customizations have been described. This section
will now summarize all this by providing step-by-step instructions to
storing your project-specific customizations. Clearly, the steps that
are not relevant to your project can be skipped.

 1. make menuconfig to configure toolchain, packages and kernel.
 2. make linux-menuconfig to update the kernel config, similar for
    other configuration like busybox, uclibc, …
 3. mkdir -p board/<manufacturer>/<boardname>
 4. Set the following options to board/<manufacturer>/<boardname>/
    <package>.config (as far as they are relevant):

      + BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE
      + BR2_PACKAGE_BUSYBOX_CONFIG
      + BR2_UCLIBC_CONFIG
      + BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_CONFIG_FILE
      + BR2_TARGET_BAREBOX_CUSTOM_CONFIG_FILE
      + BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE
 5. Write the configuration files:

      + make linux-update-defconfig
      + make busybox-update-config
      + make uclibc-update-config
      + cp <output>/build/at91bootstrap3-*/.config board/
        <manufacturer>/<boardname>/at91bootstrap3.config
      + make barebox-update-defconfig
      + make uboot-update-defconfig
 6. Create board/<manufacturer>/<boardname>/rootfs-overlay/ and fill
    it with additional files you need on your rootfs, e.g. board/
    <manufacturer>/<boardname>/rootfs-overlay/etc/inittab. Set
    BR2_ROOTFS_OVERLAY to board/<manufacturer>/<boardname>/
    rootfs-overlay.
 7. Create a post-build script board/<manufacturer>/<boardname>/
    post_build.sh. Set BR2_ROOTFS_POST_BUILD_SCRIPT to board/
    <manufacturer>/<boardname>/post_build.sh
 8. If additional setuid permissions have to be set or device nodes
    have to be created, create board/<manufacturer>/<boardname>/
    device_table.txt and add that path to BR2_ROOTFS_DEVICE_TABLE.
 9. If additional user accounts have to be created, create board/
    <manufacturer>/<boardname>/users_table.txt and add that path to
    BR2_ROOTFS_USERS_TABLES.
10. To add custom patches to certain packages, set
    BR2_GLOBAL_PATCH_DIR to board/<manufacturer>/<boardname>/patches/
    and add your patches for each package in a subdirectory named
    after the package. Each patch should be called <packagename>-
    <num>-<description>.patch.
11. Specifically for the Linux kernel, there also exists the option
    BR2_LINUX_KERNEL_PATCH with as main advantage that it can also
    download patches from a URL. If you do not need this,
    BR2_GLOBAL_PATCH_DIR is preferred. U-Boot, Barebox, at91bootstrap
    and at91bootstrap3 also have separate options, but these do not
    provide any advantage over BR2_GLOBAL_PATCH_DIR and will likely
    be removed in the future.
12. If you need to add project-specific packages, create package/
    <manufacturer>/ and place your packages in that directory. Create
    an overall <manufacturer>.mk file that includes the .mk files of
    all your packages. Create an overall Config.in file that sources
    the Config.in files of all your packages. Include this Config.in
    file from Buildroot’s package/Config.in file.
13. make savedefconfig to save the buildroot configuration.
14. cp defconfig configs/<boardname>_defconfig

Chapter 10. Frequently Asked Questions & Troubleshooting

10.1. The boot hangs after Starting network…

If the boot process seems to hang after the following messages
(messages not necessarily exactly similar, depending on the list of
packages selected):

Freeing init memory: 3972K
Initializing random number generator... done.
Starting network...
Starting dropbear sshd: generating rsa key... generating dsa key... OK

then it means that your system is running, but didn’t start a shell
on the serial console. In order to have the system start a shell on
your serial console, you have to go into the Buildroot configuration,
in System configuration, modify Run a getty (login prompt) after boot
and set the appropriate port and baud rate in the getty options
submenu. This will automatically tune the /etc/inittab file of the
generated system so that a shell starts on the correct serial port.

10.2. Why is there no compiler on the target?

It has been decided that support for the native compiler on the
target would be stopped from the Buildroot-2012.11 release because:

  * this feature was neither maintained nor tested, and often broken;
  * this feature was only available for Buildroot toolchains;
  * Buildroot mostly targets small or very small target hardware with
    limited resource onboard (CPU, ram, mass-storage), for which
    compiling on the target does not make much sense;
  * Buildroot aims at easing the cross-compilation, making native
    compilation on the target unnecessary.

If you need a compiler on your target anyway, then Buildroot is not
suitable for your purpose. In such case, you need a real distribution
and you should opt for something like:

  * openembedded [http://www.openembedded.org]
  * yocto [https://www.yoctoproject.org]
  * emdebian [http://www.emdebian.org]
  * Fedora [https://fedoraproject.org/wiki/Architectures]
  * openSUSE ARM [http://en.opensuse.org/Portal:ARM]
  * Arch Linux ARM [http://archlinuxarm.org]
  * …

10.3. Why are there no development files on the target?

Since there is no compiler available on the target (see Section 10.2,
“Why is there no compiler on the target?”), it does not make sense to
waste space with headers or static libraries.

Therefore, those files are always removed from the target since the
Buildroot-2012.11 release.

10.4. Why is there no documentation on the target?

Because Buildroot mostly targets small or very small target hardware
with limited resource onboard (CPU, ram, mass-storage), it does not
make sense to waste space with the documentation data.

If you need documentation data on your target anyway, then Buildroot
is not suitable for your purpose, and you should look for a real
distribution (see: Section 10.2, “Why is there no compiler on the
target?”).

10.5. Why are some packages not visible in the Buildroot config menu?

If a package exists in the Buildroot tree and does not appear in the
config menu, this most likely means that some of the package’s
dependencies are not met.

To know more about the dependencies of a package, search for the
package symbol in the config menu (see Section 8.1, “make tips”).

Then, you may have to recursively enable several options (which
correspond to the unmet dependencies) to finally be able to select
the package.

If the package is not visible due to some unmet toolchain options,
then you should certainly run a full rebuild (see Section 8.1, “make
tips” for more explanations).

10.6. Why not use the target directory as a chroot directory?

There are plenty of reasons to not use the target directory a chroot
one, among these:

  * file ownerships, modes and permissions are not correctly set in
    the target directory;
  * device nodes are not created in the target directory.

For these reasons, commands run through chroot, using the target
directory as the new root, will most likely fail.

If you want to run the target filesystem inside a chroot, or as an
NFS root, then use the tarball image generated in images/ and extract
it as root.

10.7. Why doesn’t Buildroot generate binary packages (.deb, .ipkg…)?

One feature that is often discussed on the Buildroot list is the
general topic of "package management". To summarize, the idea would
be to add some tracking of which Buildroot package installs what
files, with the goals of:

  * being able to remove files installed by a package when this
    package gets unselected from the menuconfig;
  * being able to generate binary packages (ipk or other format) that
    can be installed on the target without re-generating a new root
    filesystem image.

In general, most people think it is easy to do: just track which
package installed what and remove it when the package is unselected.
However, it is much more complicated than that:

  * It is not only about the target/ directory, but also the sysroot
    in host/<tuple>/sysroot and the host/ directory itself. All files
    installed in those directories by various packages must be
    tracked.
  * When a package is unselected from the configuration, it is not
    sufficient to remove just the files it installed. One must also
    remove all its reverse dependencies (i.e. packages relying on it)
    and rebuild all those packages. For example, package A depends
    optionally on the OpenSSL library. Both are selected, and
    Buildroot is built. Package A is built with crypto support using
    OpenSSL. Later on, OpenSSL gets unselected from the
    configuration, but package A remains (since OpenSSL is an
    optional dependency, this is possible.) If only OpenSSL files are
    removed, then the files installed by package A are broken: they
    use a library that is no longer present on the target. Although
    this is technically doable, it adds a lot of complexity to
    Buildroot, which goes against the simplicity we try to stick to.
  * In addition to the previous problem, there is the case where the
    optional dependency is not even known to Buildroot. For example,
    package A in version 1.0 never used OpenSSL, but in version 2.0
    it automatically uses OpenSSL if available. If the Buildroot .mk
    file hasn’t been updated to take this into account, then package
    A will not be part of the reverse dependencies of OpenSSL and
    will not be removed and rebuilt when OpenSSL is removed. For
    sure, the .mk file of package A should be fixed to mention this
    optional dependency, but in the mean time, you can have
    non-reproducible behaviors.
  * The request is to also allow changes in the menuconfig to be
    applied on the output directory without having to rebuild
    everything from scratch. However, this is very difficult to
    achieve in a reliable way: what happens when the suboptions of a
    package are changed (we would have to detect this, and rebuild
    the package from scratch and potentially all its reverse
    dependencies), what happens if toolchain options are changed,
    etc. At the moment, what Buildroot does is clear and simple so
    its behaviour is very reliable and it is easy to support users.
    If configuration changes done in menuconfig are applied after the
    next make, then it has to work correctly and properly in all
    situations, and not have some bizarre corner cases. The risk is
    to get bug reports like "I have enabled package A, B and C, then
    ran make, then disabled package C and enabled package D and ran
    make, then re-enabled package C and enabled package E and then
    there is a build failure". Or worse "I did some configuration,
    then built, then did some changes, built, some more changes,
    built, some more changes, built, and now it fails, but I don’t
    remember all the changes I did and in which order". This will be
    impossible to support.

For all these reasons, the conclusion is that adding tracking of
installed files to remove them when the package is unselected, or to
generate a repository of binary packages, is something that is very
hard to achieve reliably and will add a lot of complexity.

On this matter, the Buildroot developers make this position
statement:

  * Buildroot strives to make it easy to generate a root filesystem
    (hence the name, by the way.) That is what we want to make
    Buildroot good at: building root filesystems.
  * Buildroot is not meant to be a distribution (or rather, a
    distribution generator.) It is the opinion of most Buildroot
    developers that this is not a goal we should pursue. We believe
    that there are other tools better suited to generate a distro
    than Buildroot is. For example, Open Embedded [http://
    openembedded.org/], or openWRT [https://openwrt.org/], are such
    tools.
  * We prefer to push Buildroot in a direction that makes it easy (or
    even easier) to generate complete root filesystems. This is what
    makes Buildroot stands out in the crowd (among other things, of
    course!)
  * We believe that for most embedded Linux systems, binary packages
    are not necessary, and potentially harmful. When binary packages
    are used, it means that the system can be partially upgraded,
    which creates an enormous number of possible combinations of
    package versions that should be tested before doing the upgrade
    on the embedded device. On the other hand, by doing complete
    system upgrades by upgrading the entire root filesystem image at
    once, the image deployed to the embedded system is guaranteed to
    really be the one that has been tested and validated.

10.8. How to speed-up the build process?

Since Buildroot often involves doing full rebuilds of the entire
system that can be quite long, we provide below a number of tips to
help reduce the build time:

  * Use a pre-built external toolchain instead of the default
    Buildroot internal toolchain. By using a pre-built Linaro
    toolchain (on ARM) or a Sourcery CodeBench toolchain (for ARM,
    x86, x86-64, MIPS, etc.), you will save the build time of the
    toolchain at each complete rebuild, approximately 15 to 20
    minutes. Note that temporarily using an external toolchain does
    not prevent you to switch back to an internal toolchain (that may
    provide a higher level of customization) once the rest of your
    system is working;
  * Use the ccache compiler cache (see: Section 8.13.3, “Using ccache
    in Buildroot”);
  * Learn about rebuilding only the few packages you actually care
    about (see Section 8.3, “Understanding how to rebuild packages”),
    but beware that sometimes full rebuilds are anyway necessary (see
    Section 8.2, “Understanding when a full rebuild is necessary”);
  * Make sure you are not using a virtual machine for the Linux
    system used to run Buildroot. Most of the virtual machine
    technologies are known to cause a significant performance impact
    on I/O, which is really important for building source code;
  * Make sure that you’re using only local files: do not attempt to
    do a build over NFS, which significantly slows down the build.
    Having the Buildroot download folder available locally also helps
    a bit.
  * Buy new hardware. SSDs and lots of RAM are key to speeding up the
    builds.
  * Experiment with top-level parallel build, see Section 8.11,
    “Top-level parallel build”.

Chapter 11. Known issues

  * It is not possible to pass extra linker options via
    BR2_TARGET_LDFLAGS if such options contain a $ sign. For example,
    the following is known to break: BR2_TARGET_LDFLAGS="-Wl,-rpath=
    '$ORIGIN/../lib'"
  * The libffi package is not supported on the SuperH 2 and ARC
    architectures.
  * The prboom package triggers a compiler failure with the SuperH 4
    compiler from Sourcery CodeBench, version 2012.09.

Chapter 12. Legal notice and licensing

12.1. Complying with open source licenses

All of the end products of Buildroot (toolchain, root filesystem,
kernel, bootloaders) contain open source software, released under
various licenses.

Using open source software gives you the freedom to build rich
embedded systems, choosing from a wide range of packages, but also
imposes some obligations that you must know and honour. Some licenses
require you to publish the license text in the documentation of your
product. Others require you to redistribute the source code of the
software to those that receive your product.

The exact requirements of each license are documented in each
package, and it is your responsibility (or that of your legal office)
to comply with those requirements. To make this easier for you,
Buildroot can collect for you some material you will probably need.
To produce this material, after you have configured Buildroot with
make menuconfig, make xconfig or make gconfig, run:

make legal-info

Buildroot will collect legally-relevant material in your output
directory, under the legal-info/ subdirectory. There you will find:

  * A README file, that summarizes the produced material and contains
    warnings about material that Buildroot could not produce.
  * buildroot.config: this is the Buildroot configuration file that
    is usually produced with make menuconfig, and which is necessary
    to reproduce the build.
  * The source code for all packages; this is saved in the sources/
    and host-sources/ subdirectories for target and host packages
    respectively. The source code for packages that set <PKG>
    _REDISTRIBUTE = NO will not be saved. Patches that were applied
    are also saved, along with a file named series that lists the
    patches in the order they were applied. Patches are under the
    same license as the files that they modify. Note: Buildroot
    applies additional patches to Libtool scripts of autotools-based
    packages. These patches can be found under support/libtool in the
    Buildroot source and, due to technical limitations, are not saved
    with the package sources. You may need to collect them manually.
  * A manifest file (one for host and one for target packages)
    listing the configured packages, their version, license and
    related information. Some of this information might not be
    defined in Buildroot; such items are marked as "unknown".
  * The license texts of all packages, in the licenses/ and
    host-licenses/ subdirectories for target and host packages
    respectively. If the license file(s) are not defined in
    Buildroot, the file is not produced and a warning in the README
    indicates this.

Please note that the aim of the legal-info feature of Buildroot is to
produce all the material that is somehow relevant for legal
compliance with the package licenses. Buildroot does not try to
produce the exact material that you must somehow make public.
Certainly, more material is produced than is needed for a strict
legal compliance. For example, it produces the source code for
packages released under BSD-like licenses, that you are not required
to redistribute in source form.

Moreover, due to technical limitations, Buildroot does not produce
some material that you will or may need, such as the toolchain source
code for some of the external toolchains and the Buildroot source
code itself. When you run make legal-info, Buildroot produces
warnings in the README file to inform you of relevant material that
could not be saved.

Finally, keep in mind that the output of make legal-info is based on
declarative statements in each of the packages recipes. The Buildroot
developers try to do their best to keep those declarative statements
as accurate as possible, to the best of their knowledge. However, it
is very well possible that those declarative statements are not all
fully accurate nor exhaustive. You (or your legal department) have to
check the output of make legal-info before using it as your own
compliance delivery. See the NO WARRANTY clauses (clauses 11 and 12)
in the COPYING file at the root of the Buildroot distribution.

12.2. Complying with the Buildroot license

Buildroot itself is an open source software, released under the GNU
General Public License, version 2 [http://www.gnu.org/licenses/
old-licenses/gpl-2.0.html] or (at your option) any later version,
with the exception of the package patches detailed below. However,
being a build system, it is not normally part of the end product: if
you develop the root filesystem, kernel, bootloader or toolchain for
a device, the code of Buildroot is only present on the development
machine, not in the device storage.

Nevertheless, the general view of the Buildroot developers is that
you should release the Buildroot source code along with the source
code of other packages when releasing a product that contains
GPL-licensed software. This is because the GNU GPL [http://
www.gnu.org/licenses/old-licenses/gpl-2.0.html] defines the "complete
source code" for an executable work as "all the source code for all
modules it contains, plus any associated interface definition files,
plus the scripts used to control compilation and installation of the
executable". Buildroot is part of the scripts used to control
compilation and installation of the executable, and as such it is
considered part of the material that must be redistributed.

Keep in mind that this is only the Buildroot developers' opinion, and
you should consult your legal department or lawyer in case of any
doubt.

12.2.1. Patches to packages

Buildroot also bundles patch files, which are applied to the sources
of the various packages. Those patches are not covered by the license
of Buildroot. Instead, they are covered by the license of the
software to which the patches are applied. When said software is
available under multiple licenses, the Buildroot patches are only
provided under the publicly accessible licenses.

See Chapter 18, Patching a package for the technical details.

Chapter 13. Beyond Buildroot

13.1. Boot the generated images

13.1.1. NFS boot

To achieve NFS-boot, enable tar root filesystem in the Filesystem
images menu.

After a complete build, just run the following commands to setup the
NFS-root directory:

sudo tar -xavf /path/to/output_dir/rootfs.tar -C /path/to/nfs_root_dir

Remember to add this path to /etc/exports.

Then, you can execute a NFS-boot from your target.

13.1.2. Live CD

To build a live CD image, enable the iso image option in the 
Filesystem images menu. Note that this option is only available on
the x86 and x86-64 architectures, and if you are building your kernel
with Buildroot.

You can build a live CD image with either IsoLinux, Grub or Grub 2 as
a bootloader, but only Isolinux supports making this image usable
both as a live CD and live USB (through the Build hybrid image
option).

You can test your live CD image using QEMU:

qemu-system-i386 -cdrom output/images/rootfs.iso9660

Or use it as a hard-drive image if it is a hybrid ISO:

qemu-system-i386 -hda output/images/rootfs.iso9660

It can be easily flashed to a USB drive with dd:

dd if=output/images/rootfs.iso9660 of=/dev/sdb

13.2. Chroot

If you want to chroot in a generated image, then there are few thing
you should be aware of:

  * you should setup the new root from the tar root filesystem image;
  * either the selected target architecture is compatible with your
    host machine, or you should use some qemu-* binary and correctly
    set it within the binfmt properties to be able to run the
    binaries built for the target on your host machine;
  * Buildroot does not currently provide host-qemu and binfmt
    correctly built and set for that kind of use.

Part III. Developer guide

Table of Contents

14. How Buildroot works
15. Coding style

    15.1. Config.in file
    15.2. The .mk file
    15.3. The documentation
    15.4. Support scripts

16. Adding support for a particular board
17. Adding new packages to Buildroot

    17.1. Package directory
    17.2. Config files
    17.3. The .mk file
    17.4. The .hash file
    17.5. Infrastructure for packages with specific build systems
    17.6. Infrastructure for autotools-based packages
    17.7. Infrastructure for CMake-based packages
    17.8. Infrastructure for Python packages
    17.9. Infrastructure for LuaRocks-based packages
    17.10. Infrastructure for Perl/CPAN packages
    17.11. Infrastructure for virtual packages
    17.12. Infrastructure for packages using kconfig for
        configuration files
    17.13. Infrastructure for rebar-based packages
    17.14. Infrastructure for Waf-based packages
    17.15. Infrastructure for Meson-based packages
    17.16. Integration of Cargo-based packages
    17.17. Infrastructure for Go packages
    17.18. Infrastructure for packages building kernel modules
    17.19. Infrastructure for asciidoc documents
    17.20. Infrastructure specific to the Linux kernel package
    17.21. Hooks available in the various build steps
    17.22. Gettext integration and interaction with packages
    17.23. Tips and tricks
    17.24. Conclusion

18. Patching a package

    18.1. Providing patches
    18.2. How patches are applied
    18.3. Format and licensing of the package patches
    18.4. Integrating patches found on the Web

19. Download infrastructure
20. Debugging Buildroot
21. Contributing to Buildroot

    21.1. Reproducing, analyzing and fixing bugs
    21.2. Analyzing and fixing autobuild failures
    21.3. Reviewing and testing patches
    21.4. Work on items from the TODO list
    21.5. Submitting patches
    21.6. Reporting issues/bugs or getting help
    21.7. Using the run-tests framework

22. DEVELOPERS file and get-developers
23. Release Engineering

    23.1. Releases
    23.2. Development

Chapter 14. How Buildroot works

As mentioned above, Buildroot is basically a set of Makefiles that
download, configure, and compile software with the correct options.
It also includes patches for various software packages - mainly the
ones involved in the cross-compilation toolchain (gcc, binutils and
uClibc).

There is basically one Makefile per software package, and they are
named with the .mk extension. Makefiles are split into many different
parts.

  * The toolchain/ directory contains the Makefiles and associated
    files for all software related to the cross-compilation
    toolchain: binutils, gcc, gdb, kernel-headers and uClibc.
  * The arch/ directory contains the definitions for all the
    processor architectures that are supported by Buildroot.
  * The package/ directory contains the Makefiles and associated
    files for all user-space tools and libraries that Buildroot can
    compile and add to the target root filesystem. There is one
    sub-directory per package.
  * The linux/ directory contains the Makefiles and associated files
    for the Linux kernel.
  * The boot/ directory contains the Makefiles and associated files
    for the bootloaders supported by Buildroot.
  * The system/ directory contains support for system integration,
    e.g. the target filesystem skeleton and the selection of an init
    system.
  * The fs/ directory contains the Makefiles and associated files for
    software related to the generation of the target root filesystem
    image.

Each directory contains at least 2 files:

  * something.mk is the Makefile that downloads, configures, compiles
    and installs the package something.
  * Config.in is a part of the configuration tool description file.
    It describes the options related to the package.

The main Makefile performs the following steps (once the
configuration is done):

  * Create all the output directories: staging, target, build, etc.
    in the output directory (output/ by default, another value can be
    specified using O=)
  * Generate the toolchain target. When an internal toolchain is
    used, this means generating the cross-compilation toolchain. When
    an external toolchain is used, this means checking the features
    of the external toolchain and importing it into the Buildroot
    environment.
  * Generate all the targets listed in the TARGETS variable. This
    variable is filled by all the individual components' Makefiles.
    Generating these targets will trigger the compilation of the
    userspace packages (libraries, programs), the kernel, the
    bootloader and the generation of the root filesystem images,
    depending on the configuration.

Chapter 15. Coding style

Overall, these coding style rules are here to help you to add new
files in Buildroot or refactor existing ones.

If you slightly modify some existing file, the important thing is to
keep the consistency of the whole file, so you can:

  * either follow the potentially deprecated coding style used in
    this file,
  * or entirely rework it in order to make it comply with these
    rules.

15.1. Config.in file

Config.in files contain entries for almost anything configurable in
Buildroot.

An entry has the following pattern:

config BR2_PACKAGE_LIBFOO
        bool "libfoo"
        depends on BR2_PACKAGE_LIBBAZ
        select BR2_PACKAGE_LIBBAR
        help
          This is a comment that explains what libfoo is. The help text
          should be wrapped.

          http://foosoftware.org/libfoo/

  * The bool, depends on, select and help lines are indented with one
    tab.
  * The help text itself should be indented with one tab and two
    spaces.
  * The help text should be wrapped to fit 72 columns, where tab
    counts for 8, so 62 characters in the text itself.

The Config.in files are the input for the configuration tool used in
Buildroot, which is the regular Kconfig. For further details about
the Kconfig language, refer to http://kernel.org/doc/Documentation/
kbuild/kconfig-language.txt.

15.2. The .mk file

  * Header: The file starts with a header. It contains the module
    name, preferably in lowercase, enclosed between separators made
    of 80 hashes. A blank line is mandatory after the header:

    ################################################################################
    #
    # libfoo
    #
    ################################################################################

  * Assignment: use = preceded and followed by one space:

    LIBFOO_VERSION = 1.0
    LIBFOO_CONF_OPTS += --without-python-support

    Do not align the = signs.

  * Indentation: use tab only:

    define LIBFOO_REMOVE_DOC
            $(RM) -fr $(TARGET_DIR)/usr/share/libfoo/doc \
                    $(TARGET_DIR)/usr/share/man/man3/libfoo*
    endef

    Note that commands inside a define block should always start with
    a tab, so make recognizes them as commands.

  * Optional dependency:

      + Prefer multi-line syntax.

        YES:

        ifeq ($(BR2_PACKAGE_PYTHON),y)
        LIBFOO_CONF_OPTS += --with-python-support
        LIBFOO_DEPENDENCIES += python
        else
        LIBFOO_CONF_OPTS += --without-python-support
        endif

        NO:

        LIBFOO_CONF_OPTS += --with$(if $(BR2_PACKAGE_PYTHON),,out)-python-support
        LIBFOO_DEPENDENCIES += $(if $(BR2_PACKAGE_PYTHON),python,)

      + Keep configure options and dependencies close together.
  * Optional hooks: keep hook definition and assignment together in
    one if block.

    YES:

    ifneq ($(BR2_LIBFOO_INSTALL_DATA),y)
    define LIBFOO_REMOVE_DATA
            $(RM) -fr $(TARGET_DIR)/usr/share/libfoo/data
    endef
    LIBFOO_POST_INSTALL_TARGET_HOOKS += LIBFOO_REMOVE_DATA
    endif

    NO:

    define LIBFOO_REMOVE_DATA
            $(RM) -fr $(TARGET_DIR)/usr/share/libfoo/data
    endef

    ifneq ($(BR2_LIBFOO_INSTALL_DATA),y)
    LIBFOO_POST_INSTALL_TARGET_HOOKS += LIBFOO_REMOVE_DATA
    endif

15.3. The documentation

The documentation uses the asciidoc [http://www.methods.co.nz/
asciidoc/] format.

For further details about the asciidoc syntax, refer to http://
www.methods.co.nz/asciidoc/userguide.html.

15.4. Support scripts

Some scripts in the support/ and utils/ directories are written in
Python and should follow the PEP8 Style Guide for Python Code [https:
//www.python.org/dev/peps/pep-0008/].

Chapter 16. Adding support for a particular board

Buildroot contains basic configurations for several publicly
available hardware boards, so that users of such a board can easily
build a system that is known to work. You are welcome to add support
for other boards to Buildroot too.

To do so, you need to create a normal Buildroot configuration that
builds a basic system for the hardware: toolchain, kernel,
bootloader, filesystem and a simple BusyBox-only userspace. No
specific package should be selected: the configuration should be as
minimal as possible, and should only build a working basic BusyBox
system for the target platform. You can of course use more
complicated configurations for your internal projects, but the
Buildroot project will only integrate basic board configurations.
This is because package selections are highly application-specific.

Once you have a known working configuration, run make savedefconfig.
This will generate a minimal defconfig file at the root of the
Buildroot source tree. Move this file into the configs/ directory,
and rename it <boardname>_defconfig.

It is recommended to use as much as possible upstream versions of the
Linux kernel and bootloaders, and to use as much as possible default
kernel and bootloader configurations. If they are incorrect for your
board, or no default exists, we encourage you to send fixes to the
corresponding upstream projects.

However, in the mean time, you may want to store kernel or bootloader
configuration or patches specific to your target platform. To do so,
create a directory board/<manufacturer> and a subdirectory board/
<manufacturer>/<boardname>. You can then store your patches and
configurations in these directories, and reference them from the main
Buildroot configuration. Refer to Chapter 9, Project-specific
customization for more details.

Chapter 17. Adding new packages to Buildroot

This section covers how new packages (userspace libraries or
applications) can be integrated into Buildroot. It also shows how
existing packages are integrated, which is needed for fixing issues
or tuning their configuration.

When you add a new package, be sure to test it in various conditions
(see Section 17.23.3, “How to test your package”) and also check it
for coding style (see Section 17.23.2, “How to check the coding
style”).

17.1. Package directory

First of all, create a directory under the package directory for your
software, for example libfoo.

Some packages have been grouped by topic in a sub-directory: x11r7,
qt5 and gstreamer. If your package fits in one of these categories,
then create your package directory in these. New subdirectories are
discouraged, however.

17.2. Config files

For the package to be displayed in the configuration tool, you need
to create a Config file in your package directory. There are two
types: Config.in and Config.in.host.

17.2.1. Config.in file

For packages used on the target, create a file named Config.in. This
file will contain the option descriptions related to our libfoo
software that will be used and displayed in the configuration tool.
It should basically contain:

config BR2_PACKAGE_LIBFOO
        bool "libfoo"
        help
          This is a comment that explains what libfoo is. The help text
          should be wrapped.

          http://foosoftware.org/libfoo/

The bool line, help line and other metadata information about the
configuration option must be indented with one tab. The help text
itself should be indented with one tab and two spaces, lines should
be wrapped to fit 72 columns, where tab counts for 8, so 62
characters in the text itself. The help text must mention the
upstream URL of the project after an empty line.

As a convention specific to Buildroot, the ordering of the attributes
is as follows:

 1. The type of option: bool, string… with the prompt
 2. If needed, the default value(s)
 3. Any dependencies on the target in depends on form
 4. Any dependencies on the toolchain in depends on form
 5. Any dependencies on other packages in depends on form
 6. Any dependency of the select form
 7. The help keyword and help text.

You can add other sub-options into a if BR2_PACKAGE_LIBFOO…endif
statement to configure particular things in your software. You can
look at examples in other packages. The syntax of the Config.in file
is the same as the one for the kernel Kconfig file. The documentation
for this syntax is available at http://kernel.org/doc/Documentation/
kbuild/kconfig-language.txt

Finally you have to add your new libfoo/Config.in to package/
Config.in (or in a category subdirectory if you decided to put your
package in one of the existing categories). The files included there
are sorted alphabetically per category and are NOT supposed to
contain anything but the bare name of the package.

source "package/libfoo/Config.in"

17.2.2. Config.in.host file

Some packages also need to be built for the host system. There are
two options here:

  * The host package is only required to satisfy build-time
    dependencies of one or more target packages. In this case, add
    host-foo to the target package’s BAR_DEPENDENCIES variable. No
    Config.in.host file should be created.
  * The host package should be explicitly selectable by the user from
    the configuration menu. In this case, create a Config.in.host
    file for that host package:

    config BR2_PACKAGE_HOST_FOO
            bool "host foo"
            help
              This is a comment that explains what foo for the host is.

              http://foosoftware.org/foo/

    The same coding style and options as for the Config.in file are
    valid.

    Finally you have to add your new libfoo/Config.in.host to package
    /Config.in.host. The files included there are sorted
    alphabetically and are NOT supposed to contain anything but the 
    bare name of the package.

    source "package/foo/Config.in.host"

    The host package will then be available from the Host utilities
    menu.

17.2.3. Choosing depends on or select

The Config.in file of your package must also ensure that dependencies
are enabled. Typically, Buildroot uses the following rules:

  * Use a select type of dependency for dependencies on libraries.
    These dependencies are generally not obvious and it therefore
    make sense to have the kconfig system ensure that the
    dependencies are selected. For example, the libgtk2 package uses
    select BR2_PACKAGE_LIBGLIB2 to make sure this library is also
    enabled. The select keyword expresses the dependency with a
    backward semantic.
  * Use a depends on type of dependency when the user really needs to
    be aware of the dependency. Typically, Buildroot uses this type
    of dependency for dependencies on target architecture, MMU
    support and toolchain options (see Section 17.2.4, “Dependencies
    on target and toolchain options”), or for dependencies on "big"
    things, such as the X.org system. The depends on keyword
    expresses the dependency with a forward semantic.

Note. The current problem with the kconfig language is that these two
dependency semantics are not internally linked. Therefore, it may be
possible to select a package, whom one of its dependencies/
requirement is not met.

An example illustrates both the usage of select and depends on.

config BR2_PACKAGE_RRDTOOL
        bool "rrdtool"
        depends on BR2_USE_WCHAR
        select BR2_PACKAGE_FREETYPE
        select BR2_PACKAGE_LIBART
        select BR2_PACKAGE_LIBPNG
        select BR2_PACKAGE_ZLIB
        help
          RRDtool is the OpenSource industry standard, high performance
          data logging and graphing system for time series data.

          http://oss.oetiker.ch/rrdtool/

comment "rrdtool needs a toolchain w/ wchar"
        depends on !BR2_USE_WCHAR

Note that these two dependency types are only transitive with the
dependencies of the same kind.

This means, in the following example:

config BR2_PACKAGE_A
        bool "Package A"

config BR2_PACKAGE_B
        bool "Package B"
        depends on BR2_PACKAGE_A

config BR2_PACKAGE_C
        bool "Package C"
        depends on BR2_PACKAGE_B

config BR2_PACKAGE_D
        bool "Package D"
        select BR2_PACKAGE_B

config BR2_PACKAGE_E
        bool "Package E"
        select BR2_PACKAGE_D

  * Selecting Package C will be visible if Package B has been
    selected, which in turn is only visible if Package A has been
    selected.
  * Selecting Package E will select Package D, which will select
    Package B, it will not check for the dependencies of Package B,
    so it will not select Package A.
  * Since Package B is selected but Package A is not, this violates
    the dependency of Package B on Package A. Therefore, in such a
    situation, the transitive dependency has to be added explicitly:

config BR2_PACKAGE_D
        bool "Package D"
        select BR2_PACKAGE_B
        depends on BR2_PACKAGE_A

config BR2_PACKAGE_E
        bool "Package E"
        select BR2_PACKAGE_D
        depends on BR2_PACKAGE_A

Overall, for package library dependencies, select should be
preferred.

Note that such dependencies will ensure that the dependency option is
also enabled, but not necessarily built before your package. To do
so, the dependency also needs to be expressed in the .mk file of the
package.

Further formatting details: see the coding style.

17.2.4. Dependencies on target and toolchain options

Many packages depend on certain options of the toolchain: the choice
of C library, C++ support, thread support, RPC support, wchar
support, or dynamic library support. Some packages can only be built
on certain target architectures, or if an MMU is available in the
processor.

These dependencies have to be expressed with the appropriate depends
on statements in the Config.in file. Additionally, for dependencies
on toolchain options, a comment should be displayed when the option
is not enabled, so that the user knows why the package is not
available. Dependencies on target architecture or MMU support should
not be made visible in a comment: since it is unlikely that the user
can freely choose another target, it makes little sense to show these
dependencies explicitly.

The comment should only be visible if the config option itself would
be visible when the toolchain option dependencies are met. This means
that all other dependencies of the package (including dependencies on
target architecture and MMU support) have to be repeated on the
comment definition. To keep it clear, the depends on statement for
these non-toolchain option should be kept separate from the depends
on statement for the toolchain options. If there is a dependency on a
config option in that same file (typically the main package) it is
preferable to have a global if … endif construct rather than
repeating the depends on statement on the comment and other config
options.

The general format of a dependency comment for package foo is:

foo needs a toolchain w/ featA, featB, featC

for example:

mpd needs a toolchain w/ C++, threads, wchar

or

crda needs a toolchain w/ threads

Note that this text is kept brief on purpose, so that it will fit on
a 80-character terminal.

The rest of this section enumerates the different target and
toolchain options, the corresponding config symbols to depend on, and
the text to use in the comment.

  * Target architecture

      + Dependency symbol: BR2_powerpc, BR2_mips, … (see arch/
        Config.in)
      + Comment string: no comment to be added
  * MMU support

      + Dependency symbol: BR2_USE_MMU
      + Comment string: no comment to be added
  * Gcc _sync* built-ins used for atomic operations. They are
    available in variants operating on 1 byte, 2 bytes, 4 bytes and 8
    bytes. Since different architectures support atomic operations on
    different sizes, one dependency symbol is available for each
    size:

      + Dependency symbol: BR2_TOOLCHAIN_HAS_SYNC_1 for 1 byte,
        BR2_TOOLCHAIN_HAS_SYNC_2 for 2 bytes,
        BR2_TOOLCHAIN_HAS_SYNC_4 for 4 bytes,
        BR2_TOOLCHAIN_HAS_SYNC_8 for 8 bytes.
      + Comment string: no comment to be added
  * Gcc _atomic* built-ins used for atomic operations.

      + Dependency symbol: BR2_TOOLCHAIN_HAS_ATOMIC.
      + Comment string: no comment to be added
  * Kernel headers

      + Dependency symbol: BR2_TOOLCHAIN_HEADERS_AT_LEAST_X_Y,
        (replace X_Y with the proper version, see toolchain/
        Config.in)
      + Comment string: headers >= X.Y and/or headers <= X.Y (replace
        X.Y with the proper version)
  * GCC version

      + Dependency symbol: BR2_TOOLCHAIN_GCC_AT_LEAST_X_Y, (replace
        X_Y with the proper version, see toolchain/Config.in)
      + Comment string: gcc >= X.Y and/or gcc <= X.Y (replace X.Y
        with the proper version)
  * Host GCC version

      + Dependency symbol: BR2_HOST_GCC_AT_LEAST_X_Y, (replace X_Y
        with the proper version, see Config.in)
      + Comment string: no comment to be added
      + Note that it is usually not the package itself that has a
        minimum host GCC version, but rather a host-package on which
        it depends.
  * C library

      + Dependency symbol: BR2_TOOLCHAIN_USES_GLIBC,
        BR2_TOOLCHAIN_USES_MUSL, BR2_TOOLCHAIN_USES_UCLIBC
      + Comment string: for the C library, a slightly different
        comment text is used: foo needs a glibc toolchain, or foo
        needs a glibc toolchain w/ C++
  * C++ support

      + Dependency symbol: BR2_INSTALL_LIBSTDCPP
      + Comment string: C++
  * D support

      + Dependency symbol: BR2_TOOLCHAIN_HAS_DLANG
      + Comment string: Dlang
  * Fortran support

      + Dependency symbol: BR2_TOOLCHAIN_HAS_FORTRAN
      + Comment string: fortran
  * thread support

      + Dependency symbol: BR2_TOOLCHAIN_HAS_THREADS
      + Comment string: threads (unless
        BR2_TOOLCHAIN_HAS_THREADS_NPTL is also needed, in which case,
        specifying only NPTL is sufficient)
  * NPTL thread support

      + Dependency symbol: BR2_TOOLCHAIN_HAS_THREADS_NPTL
      + Comment string: NPTL
  * RPC support

      + Dependency symbol: BR2_TOOLCHAIN_HAS_NATIVE_RPC
      + Comment string: RPC
  * wchar support

      + Dependency symbol: BR2_USE_WCHAR
      + Comment string: wchar
  * dynamic library

      + Dependency symbol: !BR2_STATIC_LIBS
      + Comment string: dynamic library

17.2.5. Dependencies on a Linux kernel built by buildroot

Some packages need a Linux kernel to be built by buildroot. These are
typically kernel modules or firmware. A comment should be added in
the Config.in file to express this dependency, similar to
dependencies on toolchain options. The general format is:

foo needs a Linux kernel to be built

If there is a dependency on both toolchain options and the Linux
kernel, use this format:

foo needs a toolchain w/ featA, featB, featC and a Linux kernel to be built

17.2.6. Dependencies on udev /dev management

If a package needs udev /dev management, it should depend on symbol
BR2_PACKAGE_HAS_UDEV, and the following comment should be added:

foo needs udev /dev management

If there is a dependency on both toolchain options and udev /dev
management, use this format:

foo needs udev /dev management and a toolchain w/ featA, featB, featC

17.2.7. Dependencies on features provided by virtual packages

Some features can be provided by more than one package, such as the
openGL libraries.

See Section 17.11, “Infrastructure for virtual packages” for more on
the virtual packages.

17.3. The .mk file

Finally, here’s the hardest part. Create a file named libfoo.mk. It
describes how the package should be downloaded, configured, built,
installed, etc.

Depending on the package type, the .mk file must be written in a
different way, using different infrastructures:

  * Makefiles for generic packages (not using autotools or CMake):
    These are based on an infrastructure similar to the one used for
    autotools-based packages, but require a little more work from the
    developer. They specify what should be done for the
    configuration, compilation and installation of the package. This
    infrastructure must be used for all packages that do not use the
    autotools as their build system. In the future, other specialized
    infrastructures might be written for other build systems. We
    cover them through in a tutorial and a reference.
  * Makefiles for autotools-based software (autoconf, automake,
    etc.): We provide a dedicated infrastructure for such packages,
    since autotools is a very common build system. This
    infrastructure must be used for new packages that rely on the
    autotools as their build system. We cover them through a tutorial
    and reference.
  * Makefiles for cmake-based software: We provide a dedicated
    infrastructure for such packages, as CMake is a more and more
    commonly used build system and has a standardized behaviour. This
    infrastructure must be used for new packages that rely on CMake.
    We cover them through a tutorial and reference.
  * Makefiles for Python modules: We have a dedicated infrastructure
    for Python modules that use either the distutils or the
    setuptools mechanism. We cover them through a tutorial and a
    reference.
  * Makefiles for Lua modules: We have a dedicated infrastructure for
    Lua modules available through the LuaRocks web site. We cover
    them through a tutorial and a reference.

Further formatting details: see the writing rules.

17.4. The .hash file

When possible, you must add a third file, named libfoo.hash, that
contains the hashes of the downloaded files for the libfoo package.
The only reason for not adding a .hash file is when hash checking is
not possible due to how the package is downloaded.

When a package has a version selection choice, then the hash file may
be stored in a subdirectory named after the version, e.g. package/
libfoo/1.2.3/libfoo.hash. This is especially important if the
different versions have different licensing terms, but they are
stored in the same file. Otherwise, the hash file should stay in the
package’s directory.

The hashes stored in that file are used to validate the integrity of
the downloaded files and of the license files.

The format of this file is one line for each file for which to check
the hash, each line with the following three fields separated by two
spaces:

  * the type of hash, one of:

      + md5, sha1, sha224, sha256, sha384, sha512, none
  * the hash of the file:

      + for none, one or more non-space chars, usually just the
        string xxx
      + for md5, 32 hexadecimal characters
      + for sha1, 40 hexadecimal characters
      + for sha224, 56 hexadecimal characters
      + for sha256, 64 hexadecimal characters
      + for sha384, 96 hexadecimal characters
      + for sha512, 128 hexadecimal characters
  * the name of the file:

      + for a source archive: the basename of the file, without any
        directory component,
      + for a license file: the path as it appears in
        FOO_LICENSE_FILES.

Lines starting with a # sign are considered comments, and ignored.
Empty lines are ignored.

There can be more than one hash for a single file, each on its own
line. In this case, all hashes must match.

Note. Ideally, the hashes stored in this file should match the hashes
published by upstream, e.g. on their website, in the e-mail
announcement… If upstream provides more than one type of hash (e.g.
sha1 and sha512), then it is best to add all those hashes in the
.hash file. If upstream does not provide any hash, or only provides
an md5 hash, then compute at least one strong hash yourself
(preferably sha256, but not md5), and mention this in a comment line
above the hashes.

Note. The hashes for license files are used to detect a license
change when a package version is bumped. The hashes are checked
during the make legal-info target run. For a package with multiple
versions (like Qt5), create the hash file in a subdirectory
<packageversion> of that package (see also Section 18.2, “How patches
are applied”).

The none hash type is reserved to those archives downloaded from a
repository, like a git clone, a subversion checkout…

The example below defines a sha1 and a sha256 published by upstream
for the main libfoo-1.2.3.tar.bz2 tarball, an md5 from upstream and a
locally-computed sha256 hashes for a binary blob, a sha256 for a
downloaded patch, and an archive with no hash:

# Hashes from: http://www.foosoftware.org/download/libfoo-1.2.3.tar.bz2.{sha1,sha256}:
sha1  486fb55c3efa71148fe07895fd713ea3a5ae343a  libfoo-1.2.3.tar.bz2
sha256  efc8103cc3bcb06bda6a781532d12701eb081ad83e8f90004b39ab81b65d4369  libfoo-1.2.3.tar.bz2

# md5 from: http://www.foosoftware.org/download/libfoo-1.2.3.tar.bz2.md5, sha256 locally computed:
md5  2d608f3c318c6b7557d551a5a09314f03452f1a1  libfoo-data.bin
sha256  01ba4719c80b6fe911b091a7c05124b64eeece964e09c058ef8f9805daca546b  libfoo-data.bin

# Locally computed:
sha256  ff52101fb90bbfc3fe9475e425688c660f46216d7e751c4bbdb1dc85cdccacb9  libfoo-fix-blabla.patch

# No hash for 1234:
none  xxx  libfoo-1234.tar.gz

# Hash for license files:
sha256  a45a845012742796534f7e91fe623262ccfb99460a2bd04015bd28d66fba95b8  COPYING
sha256  01b1f9f2c8ee648a7a596a1abe8aa4ed7899b1c9e5551bda06da6e422b04aa55  doc/COPYING.LGPL

If the .hash file is present, and it contains one or more hashes for
a downloaded file, the hash(es) computed by Buildroot (after
download) must match the hash(es) stored in the .hash file. If one or
more hashes do not match, Buildroot considers this an error, deletes
the downloaded file, and aborts.

If the .hash file is present, but it does not contain a hash for a
downloaded file, Buildroot considers this an error and aborts.
However, the downloaded file is left in the download directory since
this typically indicates that the .hash file is wrong but the
downloaded file is probably OK.

Hashes are currently checked for files fetched from http/ftp servers,
Git repositories, files copied using scp and local files. Hashes are
not checked for other version control systems (such as Subversion,
CVS, etc.) because Buildroot currently does not generate reproducible
tarballs when source code is fetched from such version control
systems.

Hashes should only be added in .hash files for files that are
guaranteed to be stable. For example, patches auto-generated by
Github are not guaranteed to be stable, and therefore their hashes
can change over time. Such patches should not be downloaded, and
instead be added locally to the package folder.

If the .hash file is missing, then no check is done at all.

17.5. Infrastructure for packages with specific build systems

By packages with specific build systems we mean all the packages
whose build system is not one of the standard ones, such as autotools
or CMake. This typically includes packages whose build system is
based on hand-written Makefiles or shell scripts.

17.5.1. generic-package tutorial

01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_LICENSE = GPL-3.0+
11: LIBFOO_LICENSE_FILES = COPYING
12: LIBFOO_INSTALL_STAGING = YES
13: LIBFOO_CONFIG_SCRIPTS = libfoo-config
14: LIBFOO_DEPENDENCIES = host-libaaa libbbb
15:
16: define LIBFOO_BUILD_CMDS
17:     $(MAKE) $(TARGET_CONFIGURE_OPTS) -C $(@D) all
18: endef
19:
20: define LIBFOO_INSTALL_STAGING_CMDS
21:     $(INSTALL) -D -m 0755 $(@D)/libfoo.a $(STAGING_DIR)/usr/lib/libfoo.a
22:     $(INSTALL) -D -m 0644 $(@D)/foo.h $(STAGING_DIR)/usr/include/foo.h
23:     $(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(STAGING_DIR)/usr/lib
24: endef
25:
26: define LIBFOO_INSTALL_TARGET_CMDS
27:     $(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(TARGET_DIR)/usr/lib
28:     $(INSTALL) -d -m 0755 $(TARGET_DIR)/etc/foo.d
29: endef
30:
31: define LIBFOO_USERS
32:     foo -1 libfoo -1 * - - - LibFoo daemon
33: endef
34:
35: define LIBFOO_DEVICES
36:     /dev/foo  c  666  0  0  42  0  -  -  -
37: endef
38:
39: define LIBFOO_PERMISSIONS
40:     /bin/foo  f  4755  foo  libfoo   -  -  -  -  -
41: endef
42:
43: $(eval $(generic-package))

The Makefile begins on line 7 to 11 with metadata information: the
version of the package (LIBFOO_VERSION), the name of the tarball
containing the package (LIBFOO_SOURCE) (xz-ed tarball recommended)
the Internet location at which the tarball can be downloaded from
(LIBFOO_SITE), the license (LIBFOO_LICENSE) and file with the license
text (LIBFOO_LICENSE_FILES). All variables must start with the same
prefix, LIBFOO_ in this case. This prefix is always the uppercased
version of the package name (see below to understand where the
package name is defined).

On line 12, we specify that this package wants to install something
to the staging space. This is often needed for libraries, since they
must install header files and other development files in the staging
space. This will ensure that the commands listed in the
LIBFOO_INSTALL_STAGING_CMDS variable will be executed.

On line 13, we specify that there is some fixing to be done to some
of the libfoo-config files that were installed during
LIBFOO_INSTALL_STAGING_CMDS phase. These *-config files are
executable shell script files that are located in $(STAGING_DIR)/usr/
bin directory and are executed by other 3rd party packages to find
out the location and the linking flags of this particular package.

The problem is that all these *-config files by default give wrong,
host system linking flags that are unsuitable for cross-compiling.

For example: -I/usr/include instead of -I$(STAGING_DIR)/usr/include
or: -L/usr/lib instead of -L$(STAGING_DIR)/usr/lib

So some sed magic is done to these scripts to make them give correct
flags. The argument to be given to LIBFOO_CONFIG_SCRIPTS is the file
name(s) of the shell script(s) needing fixing. All these names are
relative to $(STAGING_DIR)/usr/bin and if needed multiple names can
be given.

In addition, the scripts listed in LIBFOO_CONFIG_SCRIPTS are removed
from $(TARGET_DIR)/usr/bin, since they are not needed on the target.

Example 17.1. Config script: divine package

Package divine installs shell script $(STAGING_DIR)/usr/bin/
divine-config.

So its fixup would be:

DIVINE_CONFIG_SCRIPTS = divine-config


Example 17.2. Config script: imagemagick package:

Package imagemagick installs the following scripts: $(STAGING_DIR)/
usr/bin/{Magick,Magick++,MagickCore,MagickWand,Wand}-config

So it’s fixup would be:

IMAGEMAGICK_CONFIG_SCRIPTS = \
   Magick-config Magick++-config \
   MagickCore-config MagickWand-config Wand-config


On line 14, we specify the list of dependencies this package relies
on. These dependencies are listed in terms of lower-case package
names, which can be packages for the target (without the host-
prefix) or packages for the host (with the host-) prefix). Buildroot
will ensure that all these packages are built and installed before
the current package starts its configuration.

The rest of the Makefile, lines 16..29, defines what should be done
at the different steps of the package configuration, compilation and
installation. LIBFOO_BUILD_CMDS tells what steps should be performed
to build the package. LIBFOO_INSTALL_STAGING_CMDS tells what steps
should be performed to install the package in the staging space.
LIBFOO_INSTALL_TARGET_CMDS tells what steps should be performed to
install the package in the target space.

All these steps rely on the $(@D) variable, which contains the
directory where the source code of the package has been extracted.

On lines 31..33, we define a user that is used by this package (e.g.
to run a daemon as non-root) (LIBFOO_USERS).

On line 35..37, we define a device-node file used by this package
(LIBFOO_DEVICES).

On line 39..41, we define the permissions to set to specific files
installed by this package (LIBFOO_PERMISSIONS).

Finally, on line 43, we call the generic-package function, which
generates, according to the variables defined previously, all the
Makefile code necessary to make your package working.

17.5.2. generic-package reference

There are two variants of the generic target. The generic-package
macro is used for packages to be cross-compiled for the target. The
host-generic-package macro is used for host packages, natively
compiled for the host. It is possible to call both of them in a
single .mk file: once to create the rules to generate a target
package and once to create the rules to generate a host package:

$(eval $(generic-package))
$(eval $(host-generic-package))

This might be useful if the compilation of the target package
requires some tools to be installed on the host. If the package name
is libfoo, then the name of the package for the target is also
libfoo, while the name of the package for the host is host-libfoo.
These names should be used in the DEPENDENCIES variables of other
packages, if they depend on libfoo or host-libfoo.

The call to the generic-package and/or host-generic-package macro 
must be at the end of the .mk file, after all variable definitions.
The call to host-generic-package must be after the call to
generic-package, if any.

For the target package, the generic-package uses the variables
defined by the .mk file and prefixed by the uppercased package name:
LIBFOO_*. host-generic-package uses the HOST_LIBFOO_* variables. For 
some variables, if the HOST_LIBFOO_ prefixed variable doesn’t exist,
the package infrastructure uses the corresponding variable prefixed
by LIBFOO_. This is done for variables that are likely to have the
same value for both the target and host packages. See below for
details.

The list of variables that can be set in a .mk file to give metadata
information is (assuming the package name is libfoo) :

  * LIBFOO_VERSION, mandatory, must contain the version of the
    package. Note that if HOST_LIBFOO_VERSION doesn’t exist, it is
    assumed to be the same as LIBFOO_VERSION. It can also be a
    revision number or a tag for packages that are fetched directly
    from their version control system. Examples:

      + a version for a release tarball: LIBFOO_VERSION = 0.1.2
      + a sha1 for a git tree: LIBFOO_VERSION =
        cb9d6aa9429e838f0e54faa3d455bcbab5eef057
      + a tag for a git tree LIBFOO_VERSION = v0.1.2

        Note: Using a branch name as FOO_VERSION is not supported,
        because it does not and can not work as people would expect
        it should:

         1. due to local caching, Buildroot will not re-fetch the
            repository, so people who expect to be able to follow the
            remote repository would be quite surprised and
            disappointed;
         2. because two builds can never be perfectly simultaneous,
            and because the remote repository may get new commits on
            the branch anytime, two users, using the same Buildroot
            tree and building the same configuration, may get
            different source, thus rendering the build non
            reproducible, and people would be quite surprised and
            disappointed.
  * LIBFOO_SOURCE may contain the name of the tarball of the package,
    which Buildroot will use to download the tarball from
    LIBFOO_SITE. If HOST_LIBFOO_SOURCE is not specified, it defaults
    to LIBFOO_SOURCE. If none are specified, then the value is
    assumed to be libfoo-$(LIBFOO_VERSION).tar.gz. Example:
    LIBFOO_SOURCE = foobar-$(LIBFOO_VERSION).tar.bz2
  * LIBFOO_PATCH may contain a space-separated list of patch file
    names, that Buildroot will download and apply to the package
    source code. If an entry contains ://, then Buildroot will assume
    it is a full URL and download the patch from this location.
    Otherwise, Buildroot will assume that the patch should be
    downloaded from LIBFOO_SITE. If HOST_LIBFOO_PATCH is not
    specified, it defaults to LIBFOO_PATCH. Note that patches that
    are included in Buildroot itself use a different mechanism: all
    files of the form *.patch present in the package directory inside
    Buildroot will be applied to the package after extraction (see
    patching a package). Finally, patches listed in the LIBFOO_PATCH
    variable are applied before the patches stored in the Buildroot
    package directory.
  * LIBFOO_SITE provides the location of the package, which can be a
    URL or a local filesystem path. HTTP, FTP and SCP are supported
    URL types for retrieving package tarballs. In these cases don’t
    include a trailing slash: it will be added by Buildroot between
    the directory and the filename as appropriate. Git, Subversion,
    Mercurial, and Bazaar are supported URL types for retrieving
    packages directly from source code management systems. There is a
    helper function to make it easier to download source tarballs
    from GitHub (refer to Section 17.23.4, “How to add a package from
    GitHub” for details). A filesystem path may be used to specify
    either a tarball or a directory containing the package source
    code. See LIBFOO_SITE_METHOD below for more details on how
    retrieval works. Note that SCP URLs should be of the form scp://
    [user@]host:filepath, and that filepath is relative to the user’s
    home directory, so you may want to prepend the path with a slash
    for absolute paths: scp://[user@]host:/absolutepath. If
    HOST_LIBFOO_SITE is not specified, it defaults to LIBFOO_SITE.
    Examples: LIBFOO_SITE=http://www.libfoosoftware.org/libfoo
    LIBFOO_SITE=http://svn.xiph.org/trunk/Tremor LIBFOO_SITE=/opt/
    software/libfoo.tar.gz LIBFOO_SITE=$(TOPDIR)/../src/libfoo
  * LIBFOO_DL_OPTS is a space-separated list of additional options to
    pass to the downloader. Useful for retrieving documents with
    server-side checking for user logins and passwords, or to use a
    proxy. All download methods valid for LIBFOO_SITE_METHOD are
    supported; valid options depend on the download method (consult
    the man page for the respective download utilities).
  * LIBFOO_EXTRA_DOWNLOADS is a space-separated list of additional
    files that Buildroot should download. If an entry contains ://
    then Buildroot will assume it is a complete URL and will download
    the file using this URL. Otherwise, Buildroot will assume the
    file to be downloaded is located at LIBFOO_SITE. Buildroot will
    not do anything with those additional files, except download
    them: it will be up to the package recipe to use them from $
    (LIBFOO_DL_DIR).
  * LIBFOO_SITE_METHOD determines the method used to fetch or copy
    the package source code. In many cases, Buildroot guesses the
    method from the contents of LIBFOO_SITE and setting
    LIBFOO_SITE_METHOD is unnecessary. When HOST_LIBFOO_SITE_METHOD
    is not specified, it defaults to the value of LIBFOO_SITE_METHOD.
    The possible values of LIBFOO_SITE_METHOD are:

      + wget for normal FTP/HTTP downloads of tarballs. Used by
        default when LIBFOO_SITE begins with http://, https:// or
        ftp://.
      + scp for downloads of tarballs over SSH with scp. Used by
        default when LIBFOO_SITE begins with scp://.
      + svn for retrieving source code from a Subversion repository.
        Used by default when LIBFOO_SITE begins with svn://. When a
        http:// Subversion repository URL is specified in
        LIBFOO_SITE, one must specify LIBFOO_SITE_METHOD=svn.
        Buildroot performs a checkout which is preserved as a tarball
        in the download cache; subsequent builds use the tarball
        instead of performing another checkout.
      + cvs for retrieving source code from a CVS repository. Used by
        default when LIBFOO_SITE begins with cvs://. The downloaded
        source code is cached as with the svn method. Anonymous
        pserver mode is assumed otherwise explicitly defined on
        LIBFOO_SITE. Both LIBFOO_SITE=cvs://libfoo.net:/cvsroot/
        libfoo and LIBFOO_SITE=cvs://:ext:libfoo.net:/cvsroot/libfoo
        are accepted, on the former anonymous pserver access mode is
        assumed. LIBFOO_SITE must contain the source URL as well as
        the remote repository directory. The module is the package
        name. LIBFOO_VERSION is mandatory and must be a tag, a
        branch, or a date (e.g. "2014-10-20", "2014-10-20 13:45",
        "2014-10-20 13:45+01" see "man cvs" for further details).
      + git for retrieving source code from a Git repository. Used by
        default when LIBFOO_SITE begins with git://. The downloaded
        source code is cached as with the svn method.
      + hg for retrieving source code from a Mercurial repository.
        One must specify LIBFOO_SITE_METHOD=hg when LIBFOO_SITE
        contains a Mercurial repository URL. The downloaded source
        code is cached as with the svn method.
      + bzr for retrieving source code from a Bazaar repository. Used
        by default when LIBFOO_SITE begins with bzr://. The
        downloaded source code is cached as with the svn method.
      + file for a local tarball. One should use this when
        LIBFOO_SITE specifies a package tarball as a local filename.
        Useful for software that isn’t available publicly or in
        version control.
      + local for a local source code directory. One should use this
        when LIBFOO_SITE specifies a local directory path containing
        the package source code. Buildroot copies the contents of the
        source directory into the package’s build directory. Note
        that for local packages, no patches are applied. If you need
        to still patch the source code, use LIBFOO_POST_RSYNC_HOOKS,
        see Section 17.21.1, “Using the POST_RSYNC hook”.
  * LIBFOO_GIT_SUBMODULES can be set to YES to create an archive with
    the git submodules in the repository. This is only available for
    packages downloaded with git (i.e. when LIBFOO_SITE_METHOD=git).
    Note that we try not to use such git submodules when they contain
    bundled libraries, in which case we prefer to use those libraries
    from their own package.
  * LIBFOO_STRIP_COMPONENTS is the number of leading components
    (directories) that tar must strip from file names on extraction.
    The tarball for most packages has one leading component named "
    <pkg-name>-<pkg-version>", thus Buildroot passes
    --strip-components=1 to tar to remove it. For non-standard
    packages that don’t have this component, or that have more than
    one leading component to strip, set this variable with the value
    to be passed to tar. Default: 1.
  * LIBFOO_EXCLUDES is a space-separated list of patterns to exclude
    when extracting the archive. Each item from that list is passed
    as a tar’s --exclude option. By default, empty.
  * LIBFOO_DEPENDENCIES lists the dependencies (in terms of package
    name) that are required for the current target package to
    compile. These dependencies are guaranteed to be compiled and
    installed before the configuration of the current package starts.
    However, modifications to configuration of these dependencies
    will not force a rebuild of the current package. In a similar
    way, HOST_LIBFOO_DEPENDENCIES lists the dependencies for the
    current host package.
  * LIBFOO_EXTRACT_DEPENDENCIES lists the dependencies (in terms of
    package name) that are required for the current target package to
    be extracted. These dependencies are guaranteed to be compiled
    and installed before the extract step of the current package
    starts. This is only used internally by the package
    infrastructure, and should typically not be used directly by
    packages.
  * LIBFOO_PATCH_DEPENDENCIES lists the dependencies (in terms of
    package name) that are required for the current package to be
    patched. These dependencies are guaranteed to be extracted and
    patched (but not necessarily built) before the current package is
    patched. In a similar way, HOST_LIBFOO_PATCH_DEPENDENCIES lists
    the dependencies for the current host package. This is seldom
    used; usually, LIBFOO_DEPENDENCIES is what you really want to
    use.
  * LIBFOO_PROVIDES lists all the virtual packages libfoo is an
    implementation of. See Section 17.11, “Infrastructure for virtual
    packages”.
  * LIBFOO_INSTALL_STAGING can be set to YES or NO (default). If set
    to YES, then the commands in the LIBFOO_INSTALL_STAGING_CMDS
    variables are executed to install the package into the staging
    directory.
  * LIBFOO_INSTALL_TARGET can be set to YES (default) or NO. If set
    to YES, then the commands in the LIBFOO_INSTALL_TARGET_CMDS
    variables are executed to install the package into the target
    directory.
  * LIBFOO_INSTALL_IMAGES can be set to YES or NO (default). If set
    to YES, then the commands in the LIBFOO_INSTALL_IMAGES_CMDS
    variable are executed to install the package into the images
    directory.
  * LIBFOO_CONFIG_SCRIPTS lists the names of the files in $
    (STAGING_DIR)/usr/bin that need some special fixing to make them
    cross-compiling friendly. Multiple file names separated by space
    can be given and all are relative to $(STAGING_DIR)/usr/bin. The
    files listed in LIBFOO_CONFIG_SCRIPTS are also removed from $
    (TARGET_DIR)/usr/bin since they are not needed on the target.
  * LIBFOO_DEVICES lists the device files to be created by Buildroot
    when using the static device table. The syntax to use is the
    makedevs one. You can find some documentation for this syntax in
    the Chapter 24, Makedev syntax documentation. This variable is
    optional.
  * LIBFOO_PERMISSIONS lists the changes of permissions to be done at
    the end of the build process. The syntax is once again the
    makedevs one. You can find some documentation for this syntax in
    the Chapter 24, Makedev syntax documentation. This variable is
    optional.
  * LIBFOO_USERS lists the users to create for this package, if it
    installs a program you want to run as a specific user (e.g. as a
    daemon, or as a cron-job). The syntax is similar in spirit to the
    makedevs one, and is described in the Chapter 25, Makeusers
    syntax documentation. This variable is optional.
  * LIBFOO_LICENSE defines the license (or licenses) under which the
    package is released. This name will appear in the manifest file
    produced by make legal-info. If the license appears in the SPDX
    License List [https://spdx.org/licenses/], use the SPDX short
    identifier to make the manifest file uniform. Otherwise, describe
    the license in a precise and concise way, avoiding ambiguous
    names such as BSD which actually name a family of licenses. This
    variable is optional. If it is not defined, unknown will appear
    in the license field of the manifest file for this package. The
    expected format for this variable must comply with the following
    rules:

      + If different parts of the package are released under
        different licenses, then comma separate licenses (e.g.
        LIBFOO_LICENSE = GPL-2.0+, LGPL-2.1+). If there is clear
        distinction between which component is licensed under what
        license, then annotate the license with that component,
        between parenthesis (e.g. LIBFOO_LICENSE = GPL-2.0+
        (programs), LGPL-2.1+ (libraries)).
      + If some licenses are conditioned on a sub-option being
        enabled, append the conditional licenses with a comma (e.g.:
        FOO_LICENSE += , GPL-2.0+ (programs)); the infrastructure
        will internally remove the space before the comma.
      + If the package is dual licensed, then separate licenses with
        the or keyword (e.g. LIBFOO_LICENSE = AFL-2.1 or GPL-2.0+).
  * LIBFOO_LICENSE_FILES is a space-separated list of files in the
    package tarball that contain the license(s) under which the
    package is released. make legal-info copies all of these files in
    the legal-info directory. See Chapter 12, Legal notice and
    licensing for more information. This variable is optional. If it
    is not defined, a warning will be produced to let you know, and
    not saved will appear in the license files field of the manifest
    file for this package.
  * LIBFOO_ACTUAL_SOURCE_TARBALL only applies to packages whose
    LIBFOO_SITE / LIBTOO_SOURCE pair points to an archive that does
    not actually contain source code, but binary code. This a very
    uncommon case, only known to apply to external toolchains which
    come already compiled, although theoretically it might apply to
    other packages. In such cases a separate tarball is usually
    available with the actual source code. Set
    LIBFOO_ACTUAL_SOURCE_TARBALL to the name of the actual source
    code archive and Buildroot will download it and use it when you
    run make legal-info to collect legally-relevant material. Note
    this file will not be downloaded during regular builds nor by
    make source.
  * LIBFOO_ACTUAL_SOURCE_SITE provides the location of the actual
    source tarball. The default value is LIBFOO_SITE, so you don’t
    need to set this variable if the binary and source archives are
    hosted on the same directory. If LIBFOO_ACTUAL_SOURCE_TARBALL is
    not set, it doesn’t make sense to define
    LIBFOO_ACTUAL_SOURCE_SITE.
  * LIBFOO_REDISTRIBUTE can be set to YES (default) or NO to indicate
    if the package source code is allowed to be redistributed. Set it
    to NO for non-opensource packages: Buildroot will not save the
    source code for this package when collecting the legal-info.
  * LIBFOO_FLAT_STACKSIZE defines the stack size of an application
    built into the FLAT binary format. The application stack size on
    the NOMMU architecture processors can’t be enlarged at run time.
    The default stack size for the FLAT binary format is only 4k
    bytes. If the application consumes more stack, append the
    required number here.
  * LIBFOO_BIN_ARCH_EXCLUDE is a space-separated list of paths
    (relative to the target directory) to ignore when checking that
    the package installs correctly cross-compiled binaries. You
    seldom need to set this variable, unless the package installs
    binary blobs outside the default locations, /lib/firmware, /usr/
    lib/firmware, /lib/modules, /usr/lib/modules, and /usr/share,
    which are automatically excluded.
  * LIBFOO_IGNORE_CVES is a space-separated list of CVEs that tells
    Buildroot CVE tracking tools which CVEs should be ignored for
    this package. This is typically used when the CVE is fixed by a
    patch in the package, or when the CVE for some reason does not
    affect the Buildroot package. A Makefile comment must always
    precede the addition of a CVE to this variable. Example:

# 0001-fix-cve-2020-12345.patch
LIBFOO_IGNORE_CVES += CVE-2020-12345
# only when built with libbaz, which Buildroot doesn't support
LIBFOO_IGNORE_CVES += CVE-2020-54321

The recommended way to define these variables is to use the following
syntax:

LIBFOO_VERSION = 2.32

Now, the variables that define what should be performed at the
different steps of the build process.

  * LIBFOO_EXTRACT_CMDS lists the actions to be performed to extract
    the package. This is generally not needed as tarballs are
    automatically handled by Buildroot. However, if the package uses
    a non-standard archive format, such as a ZIP or RAR file, or has
    a tarball with a non-standard organization, this variable allows
    to override the package infrastructure default behavior.
  * LIBFOO_CONFIGURE_CMDS lists the actions to be performed to
    configure the package before its compilation.
  * LIBFOO_BUILD_CMDS lists the actions to be performed to compile
    the package.
  * HOST_LIBFOO_INSTALL_CMDS lists the actions to be performed to
    install the package, when the package is a host package. The
    package must install its files to the directory given by $
    (HOST_DIR). All files, including development files such as
    headers should be installed, since other packages might be
    compiled on top of this package.
  * LIBFOO_INSTALL_TARGET_CMDS lists the actions to be performed to
    install the package to the target directory, when the package is
    a target package. The package must install its files to the
    directory given by $(TARGET_DIR). Only the files required for 
    execution of the package have to be installed. Header files,
    static libraries and documentation will be removed again when the
    target filesystem is finalized.
  * LIBFOO_INSTALL_STAGING_CMDS lists the actions to be performed to
    install the package to the staging directory, when the package is
    a target package. The package must install its files to the
    directory given by $(STAGING_DIR). All development files should
    be installed, since they might be needed to compile other
    packages.
  * LIBFOO_INSTALL_IMAGES_CMDS lists the actions to be performed to
    install the package to the images directory, when the package is
    a target package. The package must install its files to the
    directory given by $(BINARIES_DIR). Only files that are binary
    images (aka images) that do not belong in the TARGET_DIR but are
    necessary for booting the board should be placed here. For
    example, a package should utilize this step if it has binaries
    which would be similar to the kernel image, bootloader or root
    filesystem images.
  * LIBFOO_INSTALL_INIT_SYSV, LIBFOO_INSTALL_INIT_OPENRC and
    LIBFOO_INSTALL_INIT_SYSTEMD list the actions to install init
    scripts either for the systemV-like init systems (busybox,
    sysvinit, etc.), openrc or for the systemd units. These commands
    will be run only when the relevant init system is installed (i.e.
    if systemd is selected as the init system in the configuration,
    only LIBFOO_INSTALL_INIT_SYSTEMD will be run). The only exception
    is when openrc is chosen as init system and
    LIBFOO_INSTALL_INIT_OPENRC has not been set, in such situation
    LIBFOO_INSTALL_INIT_SYSV will be called, since openrc supports
    sysv init scripts. When systemd is used as the init system,
    buildroot will automatically enable all services using the
    systemctl preset-all command in the final phase of image
    building. You can add preset files to prevent a particular unit
    from being automatically enabled by buildroot.
  * LIBFOO_HELP_CMDS lists the actions to print the package help,
    which is included to the main make help output. These commands
    can print anything in any format. This is seldom used, as
    packages rarely have custom rules. Do not use this variable,
    unless you really know that you need to print help.

The preferred way to define these variables is:

define LIBFOO_CONFIGURE_CMDS
        action 1
        action 2
        action 3
endef

In the action definitions, you can use the following variables:

  * $(LIBFOO_PKGDIR) contains the path to the directory containing
    the libfoo.mk and Config.in files. This variable is useful when
    it is necessary to install a file bundled in Buildroot, like a
    runtime configuration file, a splashscreen image…
  * $(@D), which contains the directory in which the package source
    code has been uncompressed.
  * $(LIBFOO_DL_DIR) contains the path to the directory where all the
    downloads made by Buildroot for libfoo are stored in.
  * $(TARGET_CC), $(TARGET_LD), etc. to get the target
    cross-compilation utilities
  * $(TARGET_CROSS) to get the cross-compilation toolchain prefix
  * Of course the $(HOST_DIR), $(STAGING_DIR) and $(TARGET_DIR)
    variables to install the packages properly. Those variables point
    to the global host, staging and target directories, unless 
    per-package directory support is used, in which case they point
    to the current package host, staging and target directories. In
    both cases, it doesn’t make any difference from the package point
    of view: it should simply use HOST_DIR, STAGING_DIR and
    TARGET_DIR. See Section 8.11, “Top-level parallel build” for more
    details about per-package directory support.

Finally, you can also use hooks. See Section 17.21, “Hooks available
in the various build steps” for more information.

17.6. Infrastructure for autotools-based packages

17.6.1. autotools-package tutorial

First, let’s see how to write a .mk file for an autotools-based
package, with an example :

01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_INSTALL_STAGING = YES
11: LIBFOO_INSTALL_TARGET = NO
12: LIBFOO_CONF_OPTS = --disable-shared
13: LIBFOO_DEPENDENCIES = libglib2 host-pkgconf
14:
15: $(eval $(autotools-package))

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball (xz-ed tarball
recommended) and the location of the tarball on the Web. Buildroot
will automatically download the tarball from this location.

On line 10, we tell Buildroot to install the package to the staging
directory. The staging directory, located in output/staging/ is the
directory where all the packages are installed, including their
development files, etc. By default, packages are not installed to the
staging directory, since usually, only libraries need to be installed
in the staging directory: their development files are needed to
compile other libraries or applications depending on them. Also by
default, when staging installation is enabled, packages are installed
in this location using the make install command.

On line 11, we tell Buildroot to not install the package to the
target directory. This directory contains what will become the root
filesystem running on the target. For purely static libraries, it is
not necessary to install them in the target directory because they
will not be used at runtime. By default, target installation is
enabled; setting this variable to NO is almost never needed. Also by
default, packages are installed in this location using the make
install command.

On line 12, we tell Buildroot to pass a custom configure option, that
will be passed to the ./configure script before configuring and
building the package.

On line 13, we declare our dependencies, so that they are built
before the build process of our package starts.

Finally, on line line 15, we invoke the autotools-package macro that
generates all the Makefile rules that actually allows the package to
be built.

17.6.2. autotools-package reference

The main macro of the autotools package infrastructure is
autotools-package. It is similar to the generic-package macro. The
ability to have target and host packages is also available, with the
host-autotools-package macro.

Just like the generic infrastructure, the autotools infrastructure
works by defining a number of variables before calling the
autotools-package macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the autotools
infrastructure: LIBFOO_VERSION, LIBFOO_SOURCE, LIBFOO_PATCH,
LIBFOO_SITE, LIBFOO_SUBDIR, LIBFOO_DEPENDENCIES,
LIBFOO_INSTALL_STAGING, LIBFOO_INSTALL_TARGET.

A few additional variables, specific to the autotools infrastructure,
can also be defined. Many of them are only useful in very specific
cases, typical packages will therefore only use a few of them.

  * LIBFOO_SUBDIR may contain the name of a subdirectory inside the
    package that contains the configure script. This is useful, if
    for example, the main configure script is not at the root of the
    tree extracted by the tarball. If HOST_LIBFOO_SUBDIR is not
    specified, it defaults to LIBFOO_SUBDIR.
  * LIBFOO_CONF_ENV, to specify additional environment variables to
    pass to the configure script. By default, empty.
  * LIBFOO_CONF_OPTS, to specify additional configure options to pass
    to the configure script. By default, empty.
  * LIBFOO_MAKE, to specify an alternate make command. This is
    typically useful when parallel make is enabled in the
    configuration (using BR2_JLEVEL) but that this feature should be
    disabled for the given package, for one reason or another. By
    default, set to $(MAKE). If parallel building is not supported by
    the package, then it should be set to LIBFOO_MAKE=$(MAKE1).
  * LIBFOO_MAKE_ENV, to specify additional environment variables to
    pass to make in the build step. These are passed before the make
    command. By default, empty.
  * LIBFOO_MAKE_OPTS, to specify additional variables to pass to make
    in the build step. These are passed after the make command. By
    default, empty.
  * LIBFOO_AUTORECONF, tells whether the package should be
    autoreconfigured or not (i.e. if the configure script and
    Makefile.in files should be re-generated by re-running autoconf,
    automake, libtool, etc.). Valid values are YES and NO. By
    default, the value is NO
  * LIBFOO_AUTORECONF_ENV, to specify additional environment
    variables to pass to the autoreconf program if LIBFOO_AUTORECONF=
    YES. These are passed in the environment of the autoreconf
    command. By default, empty.
  * LIBFOO_AUTORECONF_OPTS to specify additional options passed to
    the autoreconf program if LIBFOO_AUTORECONF=YES. By default,
    empty.
  * LIBFOO_GETTEXTIZE, tells whether the package should be
    gettextized or not (i.e. if the package uses a different gettext
    version than Buildroot provides, and it is needed to run 
    gettextize.) Only valid when LIBFOO_AUTORECONF=YES. Valid values
    are YES and NO. The default is NO.
  * LIBFOO_GETTEXTIZE_OPTS, to specify additional options passed to
    the gettextize program, if LIBFOO_GETTEXTIZE=YES. You may use
    that if, for example, the .po files are not located in the
    standard place (i.e. in po/ at the root of the package.) By
    default, -f.
  * LIBFOO_LIBTOOL_PATCH tells whether the Buildroot patch to fix
    libtool cross-compilation issues should be applied or not. Valid
    values are YES and NO. By default, the value is YES
  * LIBFOO_INSTALL_STAGING_OPTS contains the make options used to
    install the package to the staging directory. By default, the
    value is DESTDIR=$(STAGING_DIR) install, which is correct for
    most autotools packages. It is still possible to override it.
  * LIBFOO_INSTALL_TARGET_OPTS contains the make options used to
    install the package to the target directory. By default, the
    value is DESTDIR=$(TARGET_DIR) install. The default value is
    correct for most autotools packages, but it is still possible to
    override it if needed.

With the autotools infrastructure, all the steps required to build
and install the packages are already defined, and they generally work
well for most autotools-based packages. However, when required, it is
still possible to customize what is done in any particular step:

  * By adding a post-operation hook (after extract, patch, configure,
    build or install). See Section 17.21, “Hooks available in the
    various build steps” for details.
  * By overriding one of the steps. For example, even if the
    autotools infrastructure is used, if the package .mk file defines
    its own LIBFOO_CONFIGURE_CMDS variable, it will be used instead
    of the default autotools one. However, using this method should
    be restricted to very specific cases. Do not use it in the
    general case.

17.7. Infrastructure for CMake-based packages

17.7.1. cmake-package tutorial

First, let’s see how to write a .mk file for a CMake-based package,
with an example :

01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_INSTALL_STAGING = YES
11: LIBFOO_INSTALL_TARGET = NO
12: LIBFOO_CONF_OPTS = -DBUILD_DEMOS=ON
13: LIBFOO_DEPENDENCIES = libglib2 host-pkgconf
14:
15: $(eval $(cmake-package))

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball (xz-ed tarball
recommended) and the location of the tarball on the Web. Buildroot
will automatically download the tarball from this location.

On line 10, we tell Buildroot to install the package to the staging
directory. The staging directory, located in output/staging/ is the
directory where all the packages are installed, including their
development files, etc. By default, packages are not installed to the
staging directory, since usually, only libraries need to be installed
in the staging directory: their development files are needed to
compile other libraries or applications depending on them. Also by
default, when staging installation is enabled, packages are installed
in this location using the make install command.

On line 11, we tell Buildroot to not install the package to the
target directory. This directory contains what will become the root
filesystem running on the target. For purely static libraries, it is
not necessary to install them in the target directory because they
will not be used at runtime. By default, target installation is
enabled; setting this variable to NO is almost never needed. Also by
default, packages are installed in this location using the make
install command.

On line 12, we tell Buildroot to pass custom options to CMake when it
is configuring the package.

On line 13, we declare our dependencies, so that they are built
before the build process of our package starts.

Finally, on line line 15, we invoke the cmake-package macro that
generates all the Makefile rules that actually allows the package to
be built.

17.7.2. cmake-package reference

The main macro of the CMake package infrastructure is cmake-package.
It is similar to the generic-package macro. The ability to have
target and host packages is also available, with the
host-cmake-package macro.

Just like the generic infrastructure, the CMake infrastructure works
by defining a number of variables before calling the cmake-package
macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the CMake infrastructure:
LIBFOO_VERSION, LIBFOO_SOURCE, LIBFOO_PATCH, LIBFOO_SITE,
LIBFOO_SUBDIR, LIBFOO_DEPENDENCIES, LIBFOO_INSTALL_STAGING,
LIBFOO_INSTALL_TARGET.

A few additional variables, specific to the CMake infrastructure, can
also be defined. Many of them are only useful in very specific cases,
typical packages will therefore only use a few of them.

  * LIBFOO_SUBDIR may contain the name of a subdirectory inside the
    package that contains the main CMakeLists.txt file. This is
    useful, if for example, the main CMakeLists.txt file is not at
    the root of the tree extracted by the tarball. If
    HOST_LIBFOO_SUBDIR is not specified, it defaults to
    LIBFOO_SUBDIR.
  * LIBFOO_CONF_ENV, to specify additional environment variables to
    pass to CMake. By default, empty.
  * LIBFOO_CONF_OPTS, to specify additional configure options to pass
    to CMake. By default, empty. A number of common CMake options are
    set by the cmake-package infrastructure; so it is normally not
    necessary to set them in the package’s *.mk file unless you want
    to override them:

      + CMAKE_BUILD_TYPE is driven by BR2_ENABLE_DEBUG;
      + CMAKE_INSTALL_PREFIX;
      + BUILD_SHARED_LIBS is driven by BR2_STATIC_LIBS;
      + BUILD_DOC, BUILD_DOCS are disabled;
      + BUILD_EXAMPLE, BUILD_EXAMPLES are disabled;
      + BUILD_TEST, BUILD_TESTS, BUILD_TESTING are disabled.
  * LIBFOO_SUPPORTS_IN_SOURCE_BUILD = NO should be set when the
    package cannot be built inside the source tree but needs a
    separate build directory.
  * LIBFOO_MAKE, to specify an alternate make command. This is
    typically useful when parallel make is enabled in the
    configuration (using BR2_JLEVEL) but that this feature should be
    disabled for the given package, for one reason or another. By
    default, set to $(MAKE). If parallel building is not supported by
    the package, then it should be set to LIBFOO_MAKE=$(MAKE1).
  * LIBFOO_MAKE_ENV, to specify additional environment variables to
    pass to make in the build step. These are passed before the make
    command. By default, empty.
  * LIBFOO_MAKE_OPTS, to specify additional variables to pass to make
    in the build step. These are passed after the make command. By
    default, empty.
  * LIBFOO_INSTALL_STAGING_OPTS contains the make options used to
    install the package to the staging directory. By default, the
    value is DESTDIR=$(STAGING_DIR) install, which is correct for
    most CMake packages. It is still possible to override it.
  * LIBFOO_INSTALL_TARGET_OPTS contains the make options used to
    install the package to the target directory. By default, the
    value is DESTDIR=$(TARGET_DIR) install. The default value is
    correct for most CMake packages, but it is still possible to
    override it if needed.

With the CMake infrastructure, all the steps required to build and
install the packages are already defined, and they generally work
well for most CMake-based packages. However, when required, it is
still possible to customize what is done in any particular step:

  * By adding a post-operation hook (after extract, patch, configure,
    build or install). See Section 17.21, “Hooks available in the
    various build steps” for details.
  * By overriding one of the steps. For example, even if the CMake
    infrastructure is used, if the package .mk file defines its own
    LIBFOO_CONFIGURE_CMDS variable, it will be used instead of the
    default CMake one. However, using this method should be
    restricted to very specific cases. Do not use it in the general
    case.

17.8. Infrastructure for Python packages

This infrastructure applies to Python packages that use the standard
Python setuptools mechanism as their build system, generally
recognizable by the usage of a setup.py script.

17.8.1. python-package tutorial

First, let’s see how to write a .mk file for a Python package, with
an example :

01: ################################################################################
02: #
03: # python-foo
04: #
05: ################################################################################
06:
07: PYTHON_FOO_VERSION = 1.0
08: PYTHON_FOO_SOURCE = python-foo-$(PYTHON_FOO_VERSION).tar.xz
09: PYTHON_FOO_SITE = http://www.foosoftware.org/download
10: PYTHON_FOO_LICENSE = BSD-3-Clause
11: PYTHON_FOO_LICENSE_FILES = LICENSE
12: PYTHON_FOO_ENV = SOME_VAR=1
13: PYTHON_FOO_DEPENDENCIES = libmad
14: PYTHON_FOO_SETUP_TYPE = distutils
15:
16: $(eval $(python-package))

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball (xz-ed tarball
recommended) and the location of the tarball on the Web. Buildroot
will automatically download the tarball from this location.

On line 10 and 11, we give licensing details about the package (its
license on line 10, and the file containing the license text on line
11).

On line 12, we tell Buildroot to pass custom options to the Python
setup.py script when it is configuring the package.

On line 13, we declare our dependencies, so that they are built
before the build process of our package starts.

On line 14, we declare the specific Python build system being used.
In this case the distutils Python build system is used. The two
supported ones are distutils and setuptools.

Finally, on line 16, we invoke the python-package macro that
generates all the Makefile rules that actually allow the package to
be built.

17.8.2. python-package reference

As a policy, packages that merely provide Python modules should all
be named python-<something> in Buildroot. Other packages that use the
Python build system, but are not Python modules, can freely choose
their name (existing examples in Buildroot are scons and supervisor).

Packages that are only compatible with one version of Python (as in:
Python 2 or Python 3) should depend on that version explicitely in
their Config.in file (BR2_PACKAGE_PYTHON for Python 2,
BR2_PACKAGE_PYTHON3 for Python 3). Packages that are compatible with
both versions should not explicitely depend on them in their
Config.in file, since that condition is already expressed for the
whole "External python modules" menu.

The main macro of the Python package infrastructure is
python-package. It is similar to the generic-package macro. It is
also possible to create Python host packages with the
host-python-package macro.

Just like the generic infrastructure, the Python infrastructure works
by defining a number of variables before calling the python-package
or host-python-package macros.

All the package metadata information variables that exist in the
generic package infrastructure also exist in the Python
infrastructure: PYTHON_FOO_VERSION, PYTHON_FOO_SOURCE,
PYTHON_FOO_PATCH, PYTHON_FOO_SITE, PYTHON_FOO_SUBDIR,
PYTHON_FOO_DEPENDENCIES, PYTHON_FOO_LICENSE,
PYTHON_FOO_LICENSE_FILES, PYTHON_FOO_INSTALL_STAGING, etc.

Note that:

  * It is not necessary to add python or host-python in the
    PYTHON_FOO_DEPENDENCIES variable of a package, since these basic
    dependencies are automatically added as needed by the Python
    package infrastructure.
  * Similarly, it is not needed to add host-setuptools to
    PYTHON_FOO_DEPENDENCIES for setuptools-based packages, since it’s
    automatically added by the Python infrastructure as needed.

One variable specific to the Python infrastructure is mandatory:

  * PYTHON_FOO_SETUP_TYPE, to define which Python build system is
    used by the package. The two supported values are distutils and
    setuptools. If you don’t know which one is used in your package,
    look at the setup.py file in your package source code, and see
    whether it imports things from the distutils module or the
    setuptools module.

A few additional variables, specific to the Python infrastructure,
can optionally be defined, depending on the package’s needs. Many of
them are only useful in very specific cases, typical packages will
therefore only use a few of them, or none.

  * PYTHON_FOO_SUBDIR may contain the name of a subdirectory inside
    the package that contains the main setup.py file. This is useful,
    if for example, the main setup.py file is not at the root of the
    tree extracted by the tarball. If HOST_PYTHON_FOO_SUBDIR is not
    specified, it defaults to PYTHON_FOO_SUBDIR.
  * PYTHON_FOO_ENV, to specify additional environment variables to
    pass to the Python setup.py script (for both the build and
    install steps). Note that the infrastructure is automatically
    passing several standard variables, defined in
    PKG_PYTHON_DISTUTILS_ENV (for distutils target packages),
    HOST_PKG_PYTHON_DISTUTILS_ENV (for distutils host packages),
    PKG_PYTHON_SETUPTOOLS_ENV (for setuptools target packages) and
    HOST_PKG_PYTHON_SETUPTOOLS_ENV (for setuptools host packages).
  * PYTHON_FOO_BUILD_OPTS, to specify additional options to pass to
    the Python setup.py script during the build step. For target
    distutils packages, the PKG_PYTHON_DISTUTILS_BUILD_OPTS options
    are already passed automatically by the infrastructure.
  * PYTHON_FOO_INSTALL_TARGET_OPTS, PYTHON_FOO_INSTALL_STAGING_OPTS,
    HOST_PYTHON_FOO_INSTALL_OPTS to specify additional options to
    pass to the Python setup.py script during the target installation
    step, the staging installation step or the host installation,
    respectively. Note that the infrastructure is automatically
    passing some options, defined in
    PKG_PYTHON_DISTUTILS_INSTALL_TARGET_OPTS or
    PKG_PYTHON_DISTUTILS_INSTALL_STAGING_OPTS (for target distutils
    packages), HOST_PKG_PYTHON_DISTUTILS_INSTALL_OPTS (for host
    distutils packages), PKG_PYTHON_SETUPTOOLS_INSTALL_TARGET_OPTS or
    PKG_PYTHON_SETUPTOOLS_INSTALL_STAGING_OPTS (for target setuptools
    packages) and HOST_PKG_PYTHON_SETUPTOOLS_INSTALL_OPTS (for host
    setuptools packages).
  * HOST_PYTHON_FOO_NEEDS_HOST_PYTHON, to define the host python
    interpreter. The usage of this variable is limited to host
    packages. The two supported value are python2 and python3. It
    will ensure the right host python package is available and will
    invoke it for the build. If some build steps are overloaded, the
    right python interpreter must be explicitly called in the
    commands.

With the Python infrastructure, all the steps required to build and
install the packages are already defined, and they generally work
well for most Python-based packages. However, when required, it is
still possible to customize what is done in any particular step:

  * By adding a post-operation hook (after extract, patch, configure,
    build or install). See Section 17.21, “Hooks available in the
    various build steps” for details.
  * By overriding one of the steps. For example, even if the Python
    infrastructure is used, if the package .mk file defines its own
    PYTHON_FOO_BUILD_CMDS variable, it will be used instead of the
    default Python one. However, using this method should be
    restricted to very specific cases. Do not use it in the general
    case.

17.8.3. Generating a python-package from a PyPI repository

If the Python package for which you would like to create a Buildroot
package is available on PyPI, you may want to use the scanpypi tool
located in utils/ to automate the process.

You can find the list of existing PyPI packages here [https://
pypi.python.org].

scanpypi requires Python’s setuptools package to be installed on your
host.

When at the root of your buildroot directory just do :

utils/scanpypi foo bar -o package

This will generate packages python-foo and python-bar in the package
folder if they exist on https://pypi.python.org.

Find the external python modules menu and insert your package inside.
Keep in mind that the items inside a menu should be in alphabetical
order.

Please keep in mind that you’ll most likely have to manually check
the package for any mistakes as there are things that cannot be
guessed by the generator (e.g. dependencies on any of the python core
modules such as BR2_PACKAGE_PYTHON_ZLIB). Also, please take note that
the license and license files are guessed and must be checked. You
also need to manually add the package to the package/Config.in file.

If your Buildroot package is not in the official Buildroot tree but
in a br2-external tree, use the -o flag as follows:

utils/scanpypi foo bar -o other_package_dir

This will generate packages python-foo and python-bar in the
other_package_directory instead of package.

Option -h will list the available options:

utils/scanpypi -h

17.8.4. python-package CFFI backend

C Foreign Function Interface for Python (CFFI) provides a convenient
and reliable way to call compiled C code from Python using interface
declarations written in C. Python packages relying on this backend
can be identified by the appearance of a cffi dependency in the
install_requires field of their setup.py file.

Such a package should:

  * add python-cffi as a runtime dependency in order to install the
    compiled C library wrapper on the target. This is achieved by
    adding select BR2_PACKAGE_PYTHON_CFFI to the package Config.in.

config BR2_PACKAGE_PYTHON_FOO
        bool "python-foo"
        select BR2_PACKAGE_PYTHON_CFFI # runtime

  * add host-python-cffi as a build-time dependency in order to
    cross-compile the C wrapper. This is achieved by adding
    host-python-cffi to the PYTHON_FOO_DEPENDENCIES variable.

################################################################################
#
# python-foo
#
################################################################################

...

PYTHON_FOO_DEPENDENCIES = host-python-cffi

$(eval $(python-package))

17.9. Infrastructure for LuaRocks-based packages

17.9.1. luarocks-package tutorial

First, let’s see how to write a .mk file for a LuaRocks-based
package, with an example :

01: ################################################################################
02: #
03: # lua-foo
04: #
05: ################################################################################
06:
07: LUA_FOO_VERSION = 1.0.2-1
08: LUA_FOO_NAME_UPSTREAM = foo
09: LUA_FOO_DEPENDENCIES = bar
10:
11: LUA_FOO_BUILD_OPTS += BAR_INCDIR=$(STAGING_DIR)/usr/include
12: LUA_FOO_BUILD_OPTS += BAR_LIBDIR=$(STAGING_DIR)/usr/lib
13: LUA_FOO_LICENSE = luaFoo license
14: LUA_FOO_LICENSE_FILES = $(LUA_FOO_SUBDIR)/COPYING
15:
16: $(eval $(luarocks-package))

On line 7, we declare the version of the package (the same as in the
rockspec, which is the concatenation of the upstream version and the
rockspec revision, separated by a hyphen -).

On line 8, we declare that the package is called "foo" on LuaRocks.
In Buildroot, we give Lua-related packages a name that starts with
"lua", so the Buildroot name is different from the upstream name.
LUA_FOO_NAME_UPSTREAM makes the link between the two names.

On line 9, we declare our dependencies against native libraries, so
that they are built before the build process of our package starts.

On lines 11-12, we tell Buildroot to pass custom options to LuaRocks
when it is building the package.

On lines 13-14, we specify the licensing terms for the package.

Finally, on line 16, we invoke the luarocks-package macro that
generates all the Makefile rules that actually allows the package to
be built.

Most of these details can be retrieved from the rock and rockspec.
So, this file and the Config.in file can be generated by running the
command luarocks buildroot foo lua-foo in the Buildroot directory.
This command runs a specific Buildroot addon of luarocks that will
automatically generate a Buildroot package. The result must still be
manually inspected and possibly modified.

  * The package/Config.in file has to be updated manually to include
    the generated Config.in files.

17.9.2. luarocks-package reference

LuaRocks is a deployment and management system for Lua modules, and
supports various build.type: builtin, make and cmake. In the context
of Buildroot, the luarocks-package infrastructure only supports the
builtin mode. LuaRocks packages that use the make or cmake build
mechanisms should instead be packaged using the generic-package and
cmake-package infrastructures in Buildroot, respectively.

The main macro of the LuaRocks package infrastructure is
luarocks-package: like generic-package it works by defining a number
of variables providing metadata information about the package, and
then calling luarocks-package. It is worth mentioning that building
LuaRocks packages for the host is not supported, so the macro
host-luarocks-package is not implemented.

Just like the generic infrastructure, the LuaRocks infrastructure
works by defining a number of variables before calling the
luarocks-package macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the LuaRocks infrastructure:
LUA_FOO_VERSION, LUA_FOO_SOURCE, LUA_FOO_SITE, LUA_FOO_DEPENDENCIES,
LUA_FOO_LICENSE, LUA_FOO_LICENSE_FILES.

Two of them are populated by the LuaRocks infrastructure (for the
download step). If your package is not hosted on the LuaRocks mirror
$(BR2_LUAROCKS_MIRROR), you can override them:

  * LUA_FOO_SITE, which defaults to $(BR2_LUAROCKS_MIRROR)
  * LUA_FOO_SOURCE, which defaults to $(lowercase
    LUA_FOO_NAME_UPSTREAM)-$(LUA_FOO_VERSION).src.rock

A few additional variables, specific to the LuaRocks infrastructure,
are also defined. They can be overridden in specific cases.

  * LUA_FOO_NAME_UPSTREAM, which defaults to lua-foo, i.e. the
    Buildroot package name
  * LUA_FOO_ROCKSPEC, which defaults to $(lowercase
    LUA_FOO_NAME_UPSTREAM)-$(LUA_FOO_VERSION).rockspec
  * LUA_FOO_SUBDIR, which defaults to $(LUA_FOO_NAME_UPSTREAM)-$
    (LUA_FOO_VERSION_WITHOUT_ROCKSPEC_REVISION)
  * LUA_FOO_BUILD_OPTS contains additional build options for the
    luarocks build call.

17.10. Infrastructure for Perl/CPAN packages

17.10.1. perl-package tutorial

First, let’s see how to write a .mk file for a Perl/CPAN package,
with an example :

01: ################################################################################
02: #
03: # perl-foo-bar
04: #
05: ################################################################################
06:
07: PERL_FOO_BAR_VERSION = 0.02
08: PERL_FOO_BAR_SOURCE = Foo-Bar-$(PERL_FOO_BAR_VERSION).tar.gz
09: PERL_FOO_BAR_SITE = $(BR2_CPAN_MIRROR)/authors/id/M/MO/MONGER
10: PERL_FOO_BAR_DEPENDENCIES = perl-strictures
11: PERL_FOO_BAR_LICENSE = Artistic or GPL-1.0+
12: PERL_FOO_BAR_LICENSE_FILES = LICENSE
13: PERL_FOO_BAR_DISTNAME = Foo-Bar
14:
15: $(eval $(perl-package))

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball and the location
of the tarball on a CPAN server. Buildroot will automatically
download the tarball from this location.

On line 10, we declare our dependencies, so that they are built
before the build process of our package starts.

On line 11 and 12, we give licensing details about the package (its
license on line 11, and the file containing the license text on line
12).

On line 13, the name of the distribution as needed by the script
utils/scancpan (in order to regenerate/upgrade these package files).

Finally, on line 15, we invoke the perl-package macro that generates
all the Makefile rules that actually allow the package to be built.

Most of these data can be retrieved from https://metacpan.org/. So,
this file and the Config.in can be generated by running the script
utils/scancpan Foo-Bar in the Buildroot directory (or in a
br2-external tree). This script creates a Config.in file and
foo-bar.mk file for the requested package, and also recursively for
all dependencies specified by CPAN. You should still manually edit
the result. In particular, the following things should be checked.

  * If the perl module links with a shared library that is provided
    by another (non-perl) package, this dependency is not added
    automatically. It has to be added manually to
    PERL_FOO_BAR_DEPENDENCIES.
  * The package/Config.in file has to be updated manually to include
    the generated Config.in files. As a hint, the scancpan script
    prints out the required source "…" statements, sorted
    alphabetically.

17.10.2. perl-package reference

As a policy, packages that provide Perl/CPAN modules should all be
named perl-<something> in Buildroot.

This infrastructure handles various Perl build systems :
ExtUtils-MakeMaker (EUMM), Module-Build (MB) and Module-Build-Tiny.
Build.PL is preferred by default when a package provides a
Makefile.PL and a Build.PL.

The main macro of the Perl/CPAN package infrastructure is
perl-package. It is similar to the generic-package macro. The ability
to have target and host packages is also available, with the
host-perl-package macro.

Just like the generic infrastructure, the Perl/CPAN infrastructure
works by defining a number of variables before calling the
perl-package macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the Perl/CPAN
infrastructure: PERL_FOO_VERSION, PERL_FOO_SOURCE, PERL_FOO_PATCH,
PERL_FOO_SITE, PERL_FOO_SUBDIR, PERL_FOO_DEPENDENCIES,
PERL_FOO_INSTALL_TARGET.

Note that setting PERL_FOO_INSTALL_STAGING to YES has no effect
unless a PERL_FOO_INSTALL_STAGING_CMDS variable is defined. The perl
infrastructure doesn’t define these commands since Perl modules
generally don’t need to be installed to the staging directory.

A few additional variables, specific to the Perl/CPAN infrastructure,
can also be defined. Many of them are only useful in very specific
cases, typical packages will therefore only use a few of them.

  * PERL_FOO_PREFER_INSTALLER/HOST_PERL_FOO_PREFER_INSTALLER,
    specifies the preferred installation method. Possible values are
    EUMM (for Makefile.PL based installation using
    ExtUtils-MakeMaker) and MB (for Build.PL based installation using
    Module-Build). This variable is only used when the package
    provides both installation methods.
  * PERL_FOO_CONF_ENV/HOST_PERL_FOO_CONF_ENV, to specify additional
    environment variables to pass to the perl Makefile.PL or perl
    Build.PL. By default, empty.
  * PERL_FOO_CONF_OPTS/HOST_PERL_FOO_CONF_OPTS, to specify additional
    configure options to pass to the perl Makefile.PL or perl
    Build.PL. By default, empty.
  * PERL_FOO_BUILD_OPTS/HOST_PERL_FOO_BUILD_OPTS, to specify
    additional options to pass to make pure_all or perl Build build
    in the build step. By default, empty.
  * PERL_FOO_INSTALL_TARGET_OPTS, to specify additional options to
    pass to make pure_install or perl Build install in the install
    step. By default, empty.
  * HOST_PERL_FOO_INSTALL_OPTS, to specify additional options to pass
    to make pure_install or perl Build install in the install step.
    By default, empty.

17.11. Infrastructure for virtual packages

In Buildroot, a virtual package is a package whose functionalities
are provided by one or more packages, referred to as providers. The
virtual package management is an extensible mechanism allowing the
user to choose the provider used in the rootfs.

For example, OpenGL ES is an API for 2D and 3D graphics on embedded
systems. The implementation of this API is different for the 
Allwinner Tech Sunxi and the Texas Instruments OMAP35xx platforms. So
libgles will be a virtual package and sunxi-mali and ti-gfx will be
the providers.

17.11.1. virtual-package tutorial

In the following example, we will explain how to add a new virtual
package (something-virtual) and a provider for it (some-provider).

First, let’s create the virtual package.

17.11.2. Virtual package’s Config.in file

The Config.in file of virtual package something-virtual should
contain:

01: config BR2_PACKAGE_HAS_SOMETHING_VIRTUAL
02:     bool
03:
04: config BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL
05:     depends on BR2_PACKAGE_HAS_SOMETHING_VIRTUAL
06:     string

In this file, we declare two options,
BR2_PACKAGE_HAS_SOMETHING_VIRTUAL and
BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL, whose values will be used by
the providers.

17.11.3. Virtual package’s .mk file

The .mk for the virtual package should just evaluate the
virtual-package macro:

01: ################################################################################
02: #
03: # something-virtual
04: #
05: ################################################################################
06:
07: $(eval $(virtual-package))

The ability to have target and host packages is also available, with
the host-virtual-package macro.

17.11.4. Provider’s Config.in file

When adding a package as a provider, only the Config.in file requires
some modifications.

The Config.in file of the package some-provider, which provides the
functionalities of something-virtual, should contain:

01: config BR2_PACKAGE_SOME_PROVIDER
02:     bool "some-provider"
03:     select BR2_PACKAGE_HAS_SOMETHING_VIRTUAL
04:     help
05:       This is a comment that explains what some-provider is.
06:
07:       http://foosoftware.org/some-provider/
08:
09: if BR2_PACKAGE_SOME_PROVIDER
10: config BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL
11:     default "some-provider"
12: endif

On line 3, we select BR2_PACKAGE_HAS_SOMETHING_VIRTUAL, and on line
11, we set the value of BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL to the
name of the provider, but only if it is selected.

17.11.5. Provider’s .mk file

The .mk file should also declare an additional variable
SOME_PROVIDER_PROVIDES to contain the names of all the virtual
packages it is an implementation of:

01: SOME_PROVIDER_PROVIDES = something-virtual

Of course, do not forget to add the proper build and runtime
dependencies for this package!

17.11.6. Notes on depending on a virtual package

When adding a package that requires a certain FEATURE provided by a
virtual package, you have to use depends on BR2_PACKAGE_HAS_FEATURE,
like so:

config BR2_PACKAGE_HAS_FEATURE
    bool

config BR2_PACKAGE_FOO
    bool "foo"
    depends on BR2_PACKAGE_HAS_FEATURE

17.11.7. Notes on depending on a specific provider

If your package really requires a specific provider, then you’ll have
to make your package depends on this provider; you can not select a
provider.

Let’s take an example with two providers for a FEATURE:

config BR2_PACKAGE_HAS_FEATURE
    bool

config BR2_PACKAGE_FOO
    bool "foo"
    select BR2_PACKAGE_HAS_FEATURE

config BR2_PACKAGE_BAR
    bool "bar"
    select BR2_PACKAGE_HAS_FEATURE

And you are adding a package that needs FEATURE as provided by foo,
but not as provided by bar.

If you were to use select BR2_PACKAGE_FOO, then the user would still
be able to select BR2_PACKAGE_BAR in the menuconfig. This would
create a configuration inconsistency, whereby two providers of the
same FEATURE would be enabled at once, one explicitly set by the
user, the other implicitly by your select.

Instead, you have to use depends on BR2_PACKAGE_FOO, which avoids any
implicit configuration inconsistency.

17.12. Infrastructure for packages using kconfig for configuration
files

A popular way for a software package to handle user-specified
configuration is kconfig. Among others, it is used by the Linux
kernel, Busybox, and Buildroot itself. The presence of a .config file
and a menuconfig target are two well-known symptoms of kconfig being
used.

Buildroot features an infrastructure for packages that use kconfig
for their configuration. This infrastructure provides the necessary
logic to expose the package’s menuconfig target as foo-menuconfig in
Buildroot, and to handle the copying back and forth of the
configuration file in a correct way.

The kconfig-package infrastructure is based on the generic-package
infrastructure. All variables supported by generic-package are
available in kconfig-package as well. See Section 17.5.2,
“generic-package reference” for more details.

In order to use the kconfig-package infrastructure for a Buildroot
package, the minimally required lines in the .mk file, in addition to
the variables required by the generic-package infrastructure, are:

FOO_KCONFIG_FILE = reference-to-source-configuration-file

$(eval $(kconfig-package))

This snippet creates the following make targets:

  * foo-menuconfig, which calls the package’s menuconfig target
  * foo-update-config, which copies the configuration back to the
    source configuration file. It is not possible to use this target
    when fragment files are set.
  * foo-update-defconfig, which copies the configuration back to the
    source configuration file. The configuration file will only list
    the options that differ from the default values. It is not
    possible to use this target when fragment files are set.
  * foo-diff-config, which outputs the differences between the
    current configuration and the one defined in the Buildroot
    configuration for this kconfig package. The output is useful to
    identify the configuration changes that may have to be propagated
    to configuration fragments for example.

and ensures that the source configuration file is copied to the build
directory at the right moment.

There are two options to specify a configuration file to use, either
FOO_KCONFIG_FILE (as in the example, above) or FOO_KCONFIG_DEFCONFIG.
It is mandatory to provide either, but not both:

  * FOO_KCONFIG_FILE specifies the path to a defconfig or full-config
    file to be used to configure the package.
  * FOO_KCONFIG_DEFCONFIG specifies the defconfig make rule to call
    to configure the package.

In addition to these minimally required lines, several optional
variables can be set to suit the needs of the package under
consideration:

  * FOO_KCONFIG_EDITORS: a space-separated list of kconfig editors to
    support, for example menuconfig xconfig. By default, menuconfig.
  * FOO_KCONFIG_FRAGMENT_FILES: a space-separated list of
    configuration fragment files that are merged to the main
    configuration file. Fragment files are typically used when there
    is a desire to stay in sync with an upstream (def)config file,
    with some minor modifications.
  * FOO_KCONFIG_OPTS: extra options to pass when calling the kconfig
    editors. This may need to include $(FOO_MAKE_OPTS), for example.
    By default, empty.
  * FOO_KCONFIG_FIXUP_CMDS: a list of shell commands needed to fixup
    the configuration file after copying it or running a kconfig
    editor. Such commands may be needed to ensure a configuration
    consistent with other configuration of Buildroot, for example. By
    default, empty.
  * FOO_KCONFIG_DOTCONFIG: path (with filename) of the .config file,
    relative to the package source tree. The default, .config, should
    be well suited for all packages that use the standard kconfig
    infrastructure as inherited from the Linux kernel; some packages
    use a derivative of kconfig that use a different location.
  * FOO_KCONFIG_DEPENDENCIES: the list of packages (most probably,
    host packages) that need to be built before this package’s
    kconfig is interpreted. Seldom used. By default, empty.

17.13. Infrastructure for rebar-based packages

17.13.1. rebar-package tutorial

First, let’s see how to write a .mk file for a rebar-based package,
with an example :

01: ################################################################################
02: #
03: # erlang-foobar
04: #
05: ################################################################################
06:
07: ERLANG_FOOBAR_VERSION = 1.0
08: ERLANG_FOOBAR_SOURCE = erlang-foobar-$(ERLANG_FOOBAR_VERSION).tar.xz
09: ERLANG_FOOBAR_SITE = http://www.foosoftware.org/download
10: ERLANG_FOOBAR_DEPENDENCIES = host-libaaa libbbb
11:
12: $(eval $(rebar-package))

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball (xz-ed tarball
recommended) and the location of the tarball on the Web. Buildroot
will automatically download the tarball from this location.

On line 10, we declare our dependencies, so that they are built
before the build process of our package starts.

Finally, on line 12, we invoke the rebar-package macro that generates
all the Makefile rules that actually allows the package to be built.

17.13.2. rebar-package reference

The main macro of the rebar package infrastructure is rebar-package.
It is similar to the generic-package macro. The ability to have host
packages is also available, with the host-rebar-package macro.

Just like the generic infrastructure, the rebar infrastructure works
by defining a number of variables before calling the rebar-package
macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the rebar infrastructure:
ERLANG_FOOBAR_VERSION, ERLANG_FOOBAR_SOURCE, ERLANG_FOOBAR_PATCH,
ERLANG_FOOBAR_SITE, ERLANG_FOOBAR_SUBDIR, ERLANG_FOOBAR_DEPENDENCIES,
ERLANG_FOOBAR_INSTALL_STAGING, ERLANG_FOOBAR_INSTALL_TARGET,
ERLANG_FOOBAR_LICENSE and ERLANG_FOOBAR_LICENSE_FILES.

A few additional variables, specific to the rebar infrastructure, can
also be defined. Many of them are only useful in very specific cases,
typical packages will therefore only use a few of them.

  * ERLANG_FOOBAR_USE_AUTOCONF, to specify that the package uses 
    autoconf at the configuration step. When a package sets this
    variable to YES, the autotools infrastructure is used.

    Note. You can also use some of the variables from the autotools
    infrastructure: ERLANG_FOOBAR_CONF_ENV, ERLANG_FOOBAR_CONF_OPTS,
    ERLANG_FOOBAR_AUTORECONF, ERLANG_FOOBAR_AUTORECONF_ENV and
    ERLANG_FOOBAR_AUTORECONF_OPTS.

  * ERLANG_FOOBAR_USE_BUNDLED_REBAR, to specify that the package has
    a bundled version of rebar and that it shall be used. Valid
    values are YES or NO (the default).

    Note. If the package bundles a rebar utility, but can use the
    generic one that Buildroot provides, just say NO (i.e., do not
    specify this variable). Only set if it is mandatory to use the 
    rebar utility bundled in this package.

  * ERLANG_FOOBAR_REBAR_ENV, to specify additional environment
    variables to pass to the rebar utility.
  * ERLANG_FOOBAR_KEEP_DEPENDENCIES, to keep the dependencies
    described in the rebar.config file. Valid values are YES or NO
    (the default). Unless this variable is set to YES, the rebar
    infrastructure removes such dependencies in a post-patch hook to
    ensure rebar does not download nor compile them.

With the rebar infrastructure, all the steps required to build and
install the packages are already defined, and they generally work
well for most rebar-based packages. However, when required, it is
still possible to customize what is done in any particular step:

  * By adding a post-operation hook (after extract, patch, configure,
    build or install). See Section 17.21, “Hooks available in the
    various build steps” for details.
  * By overriding one of the steps. For example, even if the rebar
    infrastructure is used, if the package .mk file defines its own
    ERLANG_FOOBAR_BUILD_CMDS variable, it will be used instead of the
    default rebar one. However, using this method should be
    restricted to very specific cases. Do not use it in the general
    case.

17.14. Infrastructure for Waf-based packages

17.14.1. waf-package tutorial

First, let’s see how to write a .mk file for a Waf-based package,
with an example :

01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_CONF_OPTS = --enable-bar --disable-baz
11: LIBFOO_DEPENDENCIES = bar
12:
13: $(eval $(waf-package))

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball (xz-ed tarball
recommended) and the location of the tarball on the Web. Buildroot
will automatically download the tarball from this location.

On line 10, we tell Buildroot what options to enable for libfoo.

On line 11, we tell Buildroot the depednencies of libfoo.

Finally, on line line 13, we invoke the waf-package macro that
generates all the Makefile rules that actually allows the package to
be built.

17.14.2. waf-package reference

The main macro of the Waf package infrastructure is waf-package. It
is similar to the generic-package macro.

Just like the generic infrastructure, the Waf infrastructure works by
defining a number of variables before calling the waf-package macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the Waf infrastructure:
LIBFOO_VERSION, LIBFOO_SOURCE, LIBFOO_PATCH, LIBFOO_SITE,
LIBFOO_SUBDIR, LIBFOO_DEPENDENCIES, LIBFOO_INSTALL_STAGING,
LIBFOO_INSTALL_TARGET.

An additional variable, specific to the Waf infrastructure, can also
be defined.

  * LIBFOO_SUBDIR may contain the name of a subdirectory inside the
    package that contains the main wscript file. This is useful, if
    for example, the main wscript file is not at the root of the tree
    extracted by the tarball. If HOST_LIBFOO_SUBDIR is not specified,
    it defaults to LIBFOO_SUBDIR.
  * LIBFOO_NEEDS_EXTERNAL_WAF can be set to YES or NO to tell
    Buildroot to use the bundled waf executable. If set to NO, the
    default, then Buildroot will use the waf executable provided in
    the package source tree; if set to YES, then Buildroot will
    download, install waf as a host tool and use it to build the
    package.
  * LIBFOO_WAF_OPTS, to specify additional options to pass to the waf
    script at every step of the package build process: configure,
    build and installation. By default, empty.
  * LIBFOO_CONF_OPTS, to specify additional options to pass to the
    waf script for the configuration step. By default, empty.
  * LIBFOO_BUILD_OPTS, to specify additional options to pass to the
    waf script during the build step. By default, empty.
  * LIBFOO_INSTALL_STAGING_OPTS, to specify additional options to
    pass to the waf script during the staging installation step. By
    default, empty.
  * LIBFOO_INSTALL_TARGET_OPTS, to specify additional options to pass
    to the waf script during the target installation step. By
    default, empty.

17.15. Infrastructure for Meson-based packages

17.15.1. meson-package tutorial

Meson [http://mesonbuild.com] is an open source build system meant to
be both extremely fast, and, even more importantly, as user friendly
as possible. It uses Ninja [https://ninja-build.org] as a companion
tool to perform the actual build operations.

Let’s see how to write a .mk file for a Meson-based package, with an
example:

01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: FOO_VERSION = 1.0
08: FOO_SOURCE = foo-$(FOO_VERSION).tar.gz
09: FOO_SITE = http://www.foosoftware.org/download
10: FOO_LICENSE = GPL-3.0+
11: FOO_LICENSE_FILES = COPYING
12: FOO_INSTALL_STAGING = YES
13:
14: FOO_DEPENDENCIES = host-pkgconf bar
15:
16: ifeq ($(BR2_PACKAGE_BAZ),y)
17: FOO_CONF_OPTS += -Dbaz=true
18: FOO_DEPENDENCIES += baz
19: else
20: FOO_CONF_OPTS += -Dbaz=false
21: endif
22:
23: $(eval $(meson-package))

The Makefile starts with the definition of the standard variables for
package declaration (lines 7 to 11).

On line line 23, we invoke the meson-package macro that generates all
the Makefile rules that actually allows the package to be built.

In the example, host-pkgconf and bar are declared as dependencies in
FOO_DEPENDENCIES at line 14 because the Meson build file of foo uses
pkg-config to determine the compilation flags and libraries of
package bar.

Note that it is not necessary to add host-meson in the
FOO_DEPENDENCIES variable of a package, since this basic dependency
is automatically added as needed by the Meson package infrastructure.

If the "baz" package is selected, then support for the "baz" feature
in "foo" is activated by adding -Dbaz=true to FOO_CONF_OPTS at line
17, as specified in the meson_options.txt file in "foo" source tree.
The "baz" package is also added to FOO_DEPENDENCIES. Note that the
support for baz is explicitly disabled at line 20, if the package is
not selected.

To sum it up, to add a new meson-based package, the Makefile example
can be copied verbatim then edited to replace all occurences of FOO
with the uppercase name of the new package and update the values of
the standard variables.

17.15.2. meson-package reference

The main macro of the Meson package infrastructure is meson-package.
It is similar to the generic-package macro. The ability to have
target and host packages is also available, with the
host-meson-package macro.

Just like the generic infrastructure, the Meson infrastructure works
by defining a number of variables before calling the meson-package
macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the Meson infrastructure:
FOO_VERSION, FOO_SOURCE, FOO_PATCH, FOO_SITE, FOO_SUBDIR,
FOO_DEPENDENCIES, FOO_INSTALL_STAGING, FOO_INSTALL_TARGET.

A few additional variables, specific to the Meson infrastructure, can
also be defined. Many of them are only useful in very specific cases,
typical packages will therefore only use a few of them.

  * FOO_SUBDIR may contain the name of a subdirectory inside the
    package that contains the main meson.build file. This is useful,
    if for example, the main meson.build file is not at the root of
    the tree extracted by the tarball. If HOST_FOO_SUBDIR is not
    specified, it defaults to FOO_SUBDIR.
  * FOO_CONF_ENV, to specify additional environment variables to pass
    to meson for the configuration step. By default, empty.
  * FOO_CONF_OPTS, to specify additional options to pass to meson for
    the configuration step. By default, empty.
  * FOO_CFLAGS, to specify compiler arguments added to the package
    specific cross-compile.conf file c_args property. By default, the
    value of TARGET_CFLAGS.
  * FOO_CXXFLAGS, to specify compiler arguments added to the package
    specific cross-compile.conf file cpp_args property. By default,
    the value of TARGET_CXXFLAGS.
  * FOO_LDFLAGS, to specify compiler arguments added to the package
    specific cross-compile.conf file c_link_args and cpp_link_args
    properties. By default, the value of TARGET_LDFLAGS.
  * FOO_MESON_EXTRA_BINARIES, to specify a space-separated list of
    programs to add to the [binaries] section of the meson
    cross-compilation.conf configuration file. The format is
    program-name='/path/to/program', with no space around the = sign,
    and with the path of the program between single quotes. By
    default, empty. Note that Buildroot already sets the correct
    values for c, cpp, ar, strip, and pkgconfig.
  * FOO_NINJA_ENV, to specify additional environment variables to
    pass to ninja, meson companion tool in charge of the build
    operations. By default, empty.
  * FOO_NINJA_OPTS, to specify a space-separated list of targets to
    build. By default, empty, to build the default target(s).

17.16. Integration of Cargo-based packages

Cargo is the package manager for the Rust programming language. It
allows the user to build programs or libraries written in Rust, but
it also downloads and manages their dependencies, to ensure
repeatable builds. Cargo packages are called "crates".

17.16.1. Cargo-based package’s Config.in file

The Config.in file of Cargo-based package foo should contain:

01: config BR2_PACKAGE_FOO
02:     bool "foo"
03:     depends on BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
04:     select BR2_PACKAGE_HOST_CARGO
05:     help
06:       This is a comment that explains what foo is.
07:
08:       http://foosoftware.org/foo/

17.16.2. Cargo-based package’s .mk file

Buildroot does not (yet) provide a dedicated package infrastructure
for Cargo-based packages. So, we will explain how to write a .mk file
for such a package. Let’s start with an example:

01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: FOO_VERSION = 1.0
08: FOO_SOURCE = foo-$(FOO_VERSION).tar.gz
09: FOO_SITE = http://www.foosoftware.org/download
10: FOO_LICENSE = GPL-3.0+
11: FOO_LICENSE_FILES = COPYING
12:
13: FOO_DEPENDENCIES = host-cargo
14:
15: FOO_CARGO_ENV = CARGO_HOME=$(HOST_DIR)/share/cargo
16: FOO_CARGO_MODE = $(if $(BR2_ENABLE_DEBUG),debug,release)
17:
18: FOO_BIN_DIR = target/$(RUSTC_TARGET_NAME)/$(FOO_CARGO_MODE)
19:
20: FOO_CARGO_OPTS = \
21:   --$(FOO_CARGO_MODE) \
22:     --target=$(RUSTC_TARGET_NAME) \
23:     --manifest-path=$(@D)/Cargo.toml
24:
25: define FOO_BUILD_CMDS
26:     $(TARGET_MAKE_ENV) $(FOO_CARGO_ENV) \
27:             cargo build $(FOO_CARGO_OPTS)
28: endef
29:
30: define FOO_INSTALL_TARGET_CMDS
31:     $(INSTALL) -D -m 0755 $(@D)/$(FOO_BIN_DIR)/foo \
32:             $(TARGET_DIR)/usr/bin/foo
33: endef
34:
35: $(eval $(generic-package))

The Makefile starts with the definition of the standard variables for
package declaration (lines 7 to 11).

As seen in line 35, it is based on the generic-package infrastructure
. So, it defines the variables required by this particular
infrastructure, where Cargo is invoked:

  * FOO_BUILD_CMDS: Cargo is invoked to perform the build. The
    options required to configure the cross-compilation of the
    package are passed via FOO_CONF_OPTS.
  * FOO_INSTALL_TARGET_CMDS: The binary executable generated is
    installed on the target.

In order to have Cargo available for the build, FOO_DEPENDENCIES
needs to contain host-cargo.

To sum it up, to add a new Cargo-based package, the Makefile example
can be copied verbatim then edited to replace all occurences of FOO
with the uppercase name of the new package and update the values of
the standard variables.

17.16.3. About Dependencies Management

A crate can depend on other libraries from crates.io or git
repositories, listed in its Cargo.toml file. Before starting a build,
Cargo usually downloads automatically them. This step can also be
performed independently, via the cargo fetch command.

Cargo maintains a local cache of the registry index and of git
checkouts of the crates, whose location is given by $CARGO_HOME. As
seen in the package Makefile example at line 15, this environment
variable is set to $(HOST_DIR)/share/cargo.

This dependency download mechanism is not convenient when performing
an offline build, as Cargo will fail to fetch the dependencies. In
that case, it is advised to generate a tarball of the dependencies
using the cargo vendor and add it to FOO_EXTRA_DOWNLOADS.

17.17. Infrastructure for Go packages

This infrastructure applies to Go packages that use the standard
build system and use bundled dependencies.

17.17.1. golang-package tutorial

First, let’s see how to write a .mk file for a go package, with an
example :

01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: FOO_VERSION = 1.0
08: FOO_SITE = $(call github,bar,foo,$(FOO_VERSION))
09: FOO_LICENSE = BSD-3-Clause
10: FOO_LICENSE_FILES = LICENSE
11:
12: $(eval $(golang-package))

On line 7, we declare the version of the package.

On line 8, we declare the upstream location of the package, here
fetched from Github, since a large number of Go packages are hosted
on Github.

On line 9 and 10, we give licensing details about the package.

Finally, on line 12, we invoke the golang-package macro that
generates all the Makefile rules that actually allow the package to
be built.

17.17.2. golang-package reference

In their Config.in file, packages using the golang-package
infrastructure should depend on
BR2_PACKAGE_HOST_GO_TARGET_ARCH_SUPPORTS because Buildroot will
automatically add a dependency on host-go to such packages. If you
need CGO support in your package, you must add a dependency on
BR2_PACKAGE_HOST_GO_TARGET_CGO_LINKING_SUPPORTS.

The main macro of the Go package infrastructure is golang-package. It
is similar to the generic-package macro. The ability to build host
packages is also available, with the host-golang-package macro. Host
packages built by host-golang-package macro should depend on
BR2_PACKAGE_HOST_GO_HOST_ARCH_SUPPORTS.

Just like the generic infrastructure, the Go infrastructure works by
defining a number of variables before calling the golang-package.

All the package metadata information variables that exist in the
generic package infrastructure also exist in the Go infrastructure:
FOO_VERSION, FOO_SOURCE, FOO_PATCH, FOO_SITE, FOO_SUBDIR,
FOO_DEPENDENCIES, FOO_LICENSE, FOO_LICENSE_FILES,
FOO_INSTALL_STAGING, etc.

Note that it is not necessary to add host-go in the FOO_DEPENDENCIES
variable of a package, since this basic dependency is automatically
added as needed by the Go package infrastructure.

A few additional variables, specific to the Go infrastructure, can
optionally be defined, depending on the package’s needs. Many of them
are only useful in very specific cases, typical packages will
therefore only use a few of them, or none.

  * If your package need a custom GOPATH to be compiled in, you can
    use the FOO_WORKSPACE variable. The GOPATH being used will be
    <package-srcdir>/<FOO_WORKSPACE>. If FOO_WORKSPACE is not
    specified, it defaults to _gopath.
  * FOO_SRC_SUBDIR is the sub-directory where your source will be
    compiled relatively to the GOPATH. An example value is github.com
    /bar/foo. If FOO_SRC_SUBDIR is not specified, it defaults to a
    value infered from the FOO_SITE variable.
  * FOO_LDFLAGS and FOO_TAGS can be used to pass respectively the
    LDFLAGS or the TAGS to the go build command.
  * FOO_BUILD_TARGETS can be used to pass the list of targets that
    should be built. If FOO_BUILD_TARGETS is not specified, it
    defaults to .. We then have two cases:

      + FOO_BUILD_TARGETS is .. In this case, we assume only one
        binary will be produced, and that by default we name it after
        the package name. If that is not appropriate, the name of the
        produced binary can be overridden using FOO_BIN_NAME.
      + FOO_BUILD_TARGETS is not .. In this case, we iterate over the
        values to build each target, and for each produced a binary
        that is the non-directory component of the target. For
        example if FOO_BUILD_TARGETS = cmd/docker cmd/dockerd the
        binaries produced are docker and dockerd.
  * FOO_INSTALL_BINS can be used to pass the list of binaries that
    should be installed in /usr/bin on the target. If
    FOO_INSTALL_BINS is not specified, it defaults to the lower-case
    name of package.

With the Go infrastructure, all the steps required to build and
install the packages are already defined, and they generally work
well for most Go-based packages. However, when required, it is still
possible to customize what is done in any particular step:

  * By adding a post-operation hook (after extract, patch, configure,
    build or install). See Section 17.21, “Hooks available in the
    various build steps” for details.
  * By overriding one of the steps. For example, even if the Go
    infrastructure is used, if the package .mk file defines its own
    FOO_BUILD_CMDS variable, it will be used instead of the default
    Go one. However, using this method should be restricted to very
    specific cases. Do not use it in the general case.

17.18. Infrastructure for packages building kernel modules

Buildroot offers a helper infrastructure to make it easy to write
packages that build and install Linux kernel modules. Some packages
only contain a kernel module, other packages contain programs and
libraries in addition to kernel modules. Buildroot’s helper
infrastructure supports either case.

17.18.1. kernel-module tutorial

Let’s start with an example on how to prepare a simple package that
only builds a kernel module, and no other component:

01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: FOO_VERSION = 1.2.3
08: FOO_SOURCE = foo-$(FOO_VERSION).tar.xz
09: FOO_SITE = http://www.foosoftware.org/download
10: FOO_LICENSE = GPL-2.0
11: FOO_LICENSE_FILES = COPYING
12:
13: $(eval $(kernel-module))
14: $(eval $(generic-package))

Lines 7-11 define the usual meta-data to specify the version, archive
name, remote URI where to find the package source, licensing
information.

On line 13, we invoke the kernel-module helper infrastructure, that
generates all the appropriate Makefile rules and variables to build
that kernel module.

Finally, on line 14, we invoke the generic-package infrastructure.

The dependency on linux is automatically added, so it is not needed
to specify it in FOO_DEPENDENCIES.

What you may have noticed is that, unlike other package
infrastructures, we explicitly invoke a second infrastructure. This
allows a package to build a kernel module, but also, if needed, use
any one of other package infrastructures to build normal userland
components (libraries, executables…). Using the kernel-module
infrastructure on its own is not sufficient; another package
infrastructure must be used.

Let’s look at a more complex example:

01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: FOO_VERSION = 1.2.3
08: FOO_SOURCE = foo-$(FOO_VERSION).tar.xz
09: FOO_SITE = http://www.foosoftware.org/download
10: FOO_LICENSE = GPL-2.0
11: FOO_LICENSE_FILES = COPYING
12:
13: FOO_MODULE_SUBDIRS = driver/base
14: FOO_MODULE_MAKE_OPTS = KVERSION=$(LINUX_VERSION_PROBED)
15:
16: ifeq ($(BR2_PACKAGE_LIBBAR),y)
17: FOO_DEPENDENCIES = libbar
18: FOO_CONF_OPTS = --enable-bar
19: FOO_MODULE_SUBDIRS += driver/bar
20: else
21: FOO_CONF_OPTS = --disable-bar
22: endif
23:
24: $(eval $(kernel-module))
26: $(eval $(autotools-package))

Here, we see that we have an autotools-based package, that also
builds the kernel module located in sub-directory driver/base and, if
libbar is enabled, the kernel module located in sub-directory driver/
bar, and defines the variable KVERSION to be passed to the Linux
buildsystem when building the module(s).

17.18.2. kernel-module reference

The main macro for the kernel module infrastructure is kernel-module.
Unlike other package infrastructures, it is not stand-alone, and
requires any of the other *-package macros be called after it.

The kernel-module macro defines post-build and post-target-install
hooks to build the kernel modules. If the package’s .mk needs access
to the built kernel modules, it should do so in a post-build hook, 
registered after the call to kernel-module. Similarly, if the
package’s .mk needs access to the kernel module after it has been
installed, it should do so in a post-install hook, registered after
the call to kernel-module. Here’s an example:

$(eval $(kernel-module))

define FOO_DO_STUFF_WITH_KERNEL_MODULE
    # Do something with it...
endef
FOO_POST_BUILD_HOOKS += FOO_DO_STUFF_WITH_KERNEL_MODULE

$(eval $(generic-package))

Finally, unlike the other package infrastructures, there is no
host-kernel-module variant to build a host kernel module.

The following additional variables can optionally be defined to
further configure the build of the kernel module:

  * FOO_MODULE_SUBDIRS may be set to one or more sub-directories
    (relative to the package source top-directory) where the kernel
    module sources are. If empty or not set, the sources for the
    kernel module(s) are considered to be located at the top of the
    package source tree.
  * FOO_MODULE_MAKE_OPTS may be set to contain extra variable
    definitions to pass to the Linux buildsystem.

You may also reference (but you may not set!) those variables:

  * LINUX_DIR contains the path to where the Linux kernel has been
    extracted and built.
  * LINUX_VERSION contains the version string as configured by the
    user.
  * LINUX_VERSION_PROBED contains the real version string of the
    kernel, retrieved with running make -C $(LINUX_DIR) kernelrelease
  * KERNEL_ARCH contains the name of the current architecture, like
    arm, mips…

17.19. Infrastructure for asciidoc documents

The Buildroot manual, which you are currently reading, is entirely
written using the AsciiDoc [http://asciidoc.org/] mark-up syntax. The
manual is then rendered to many formats:

  * html
  * split-html
  * pdf
  * epub
  * text

Although Buildroot only contains one document written in AsciiDoc,
there is, as for packages, an infrastructure for rendering documents
using the AsciiDoc syntax.

Also as for packages, the AsciiDoc infrastructure is available from a
br2-external tree. This allows documentation for a br2-external tree
to match the Buildroot documentation, as it will be rendered to the
same formats and use the same layout and theme.

17.19.1. asciidoc-document tutorial

Whereas package infrastructures are suffixed with -package, the
document infrastructures are suffixed with -document. So, the
AsciiDoc infrastructure is named asciidoc-document.

Here is an example to render a simple AsciiDoc document.

01: ################################################################################
02: #
03: # foo-document
04: #
05: ################################################################################
06:
07: FOO_SOURCES = $(sort $(wildcard $(pkgdir)/*))
08: $(eval $(call asciidoc-document))

On line 7, the Makefile declares what the sources of the document
are. Currently, it is expected that the document’s sources are only
local; Buildroot will not attempt to download anything to render a
document. Thus, you must indicate where the sources are. Usually, the
string above is sufficient for a document with no sub-directory
structure.

On line 8, we call the asciidoc-document function, which generates
all the Makefile code necessary to render the document.

17.19.2. asciidoc-document reference

The list of variables that can be set in a .mk file to give metadata
information is (assuming the document name is foo) :

  * FOO_SOURCES, mandatory, defines the source files for the
    document.
  * FOO_RESOURCES, optional, may contain a space-separated list of
    paths to one or more directories containing so-called resources
    (like CSS or images). By default, empty.
  * FOO_DEPENDENCIES, optional, the list of packages (most probably,
    host-packages) that must be built before building this document.

There are also additional hooks (see Section 17.21, “Hooks available
in the various build steps” for general information on hooks), that a
document may set to define extra actions to be done at various steps:

  * FOO_POST_RSYNC_HOOKS to run additional commands after the sources
    have been copied by Buildroot. This can for example be used to
    generate part of the manual with information extracted from the
    tree. As an example, Buildroot uses this hook to generate the
    tables in the appendices.
  * FOO_CHECK_DEPENDENCIES_HOOKS to run additional tests on required
    components to generate the document. In AsciiDoc, it is possible
    to call filters, that is, programs that will parse an AsciiDoc
    block and render it appropriately (e.g. ditaa [http://
    ditaa.sourceforge.net/] or aafigure [https://pythonhosted.org/
    aafigure/]).
  * FOO_CHECK_DEPENDENCIES_<FMT>_HOOKS, to run additional tests for
    the specified format <FMT> (see the list of rendered formats,
    above).

Here is a complete example that uses all variables and all hooks:

01: ################################################################################
02: #
03: # foo-document
04: #
05: ################################################################################
06:
07: FOO_SOURCES = $(sort $(wildcard $(pkgdir)/*))
08: FOO_RESOURCES = $(sort $(wildcard $(pkgdir)/ressources))
09:
10: define FOO_GEN_EXTRA_DOC
11:     /path/to/generate-script --outdir=$(@D)
12: endef
13: FOO_POST_RSYNC_HOOKS += FOO_GEN_EXTRA_DOC
14:
15: define FOO_CHECK_MY_PROG
16:     if ! which my-prog >/dev/null 2>&1; then \
17:         echo "You need my-prog to generate the foo document"; \
18:         exit 1; \
19:     fi
20: endef
21: FOO_CHECK_DEPENDENCIES_HOOKS += FOO_CHECK_MY_PROG
22:
23: define FOO_CHECK_MY_OTHER_PROG
24:     if ! which my-other-prog >/dev/null 2>&1; then \
25:         echo "You need my-other-prog to generate the foo document as PDF"; \
26:         exit 1; \
27:     fi
28: endef
29: FOO_CHECK_DEPENDENCIES_PDF_HOOKS += FOO_CHECK_MY_OTHER_PROG
30:
31: $(eval $(call asciidoc-document))

17.20. Infrastructure specific to the Linux kernel package

The Linux kernel package can use some specific infrastructures based
on package hooks for building Linux kernel tools or/and building
Linux kernel extensions.

17.20.1. linux-kernel-tools

Buildroot offers a helper infrastructure to build some userspace
tools for the target available within the Linux kernel sources. Since
their source code is part of the kernel source code, a special
package, linux-tools, exists and re-uses the sources of the Linux
kernel that runs on the target.

Let’s look at an example of a Linux tool. For a new Linux tool named
foo, create a new menu entry in the existing package/linux-tools/
Config.in. This file will contain the option descriptions related to
each kernel tool that will be used and displayed in the configuration
tool. It would basically look like:

01: config BR2_PACKAGE_LINUX_TOOLS_FOO
02:     bool "foo"
03:     select BR2_PACKAGE_LINUX_TOOLS
04:     help
05:       This is a comment that explains what foo kernel tool is.
06:
07:       http://foosoftware.org/foo/

The name of the option starts with the prefix
BR2_PACKAGE_LINUX_TOOLS_, followed by the uppercase name of the tool
(like is done for packages).

Note. Unlike other packages, the linux-tools package options appear
in the linux kernel menu, under the Linux Kernel Tools sub-menu, not
under the Target packages main menu.

Then for each linux tool, add a new .mk.in file named package/
linux-tools/linux-tool-foo.mk.in. It would basically look like:

01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: LINUX_TOOLS += foo
08:
09: FOO_DEPENDENCIES = libbbb
10:
11: define FOO_BUILD_CMDS
12:     $(TARGET_MAKE_ENV) $(MAKE) -C $(LINUX_DIR)/tools foo
13: endef
14:
15: define FOO_INSTALL_STAGING_CMDS
16:     $(TARGET_MAKE_ENV) $(MAKE) -C $(LINUX_DIR)/tools \
17:             DESTDIR=$(STAGING_DIR) \
18:             foo_install
19: endef
20:
21: define FOO_INSTALL_TARGET_CMDS
22:     $(TARGET_MAKE_ENV) $(MAKE) -C $(LINUX_DIR)/tools \
23:             DESTDIR=$(TARGET_DIR) \
24:             foo_install
25: endef

On line 7, we register the Linux tool foo to the list of available
Linux tools.

On line 9, we specify the list of dependencies this tool relies on.
These dependencies are added to the Linux package dependencies list
only when the foo tool is selected.

The rest of the Makefile, lines 11-25 defines what should be done at
the different steps of the Linux tool build process like for a
generic package. They will actually be used only when the foo tool is
selected. The only supported commands are _BUILD_CMDS,
_INSTALL_STAGING_CMDS and _INSTALL_TARGET_CMDS.

Note. One must not call $(eval $(generic-package)) or any other
package infrastructure! Linux tools are not packages by themselves,
they are part of the linux-tools package.

17.20.2. linux-kernel-extensions

Some packages provide new features that require the Linux kernel tree
to be modified. This can be in the form of patches to be applied on
the kernel tree, or in the form of new files to be added to the tree.
The Buildroot’s Linux kernel extensions infrastructure provides a
simple solution to automatically do this, just after the kernel
sources are extracted and before the kernel patches are applied.
Examples of extensions packaged using this mechanism are the
real-time extensions Xenomai and RTAI, as well as the set of
out-of-tree LCD screens drivers fbtft.

Let’s look at an example on how to add a new Linux extension foo.

First, create the package foo that provides the extension: this
package is a standard package; see the previous chapters on how to
create such a package. This package is in charge of downloading the
sources archive, checking the hash, defining the licence informations
and building user space tools if any.

Then create the Linux extension proper: create a new menu entry in
the existing linux/Config.ext.in. This file contains the option
descriptions related to each kernel extension that will be used and
displayed in the configuration tool. It would basically look like:

01: config BR2_LINUX_KERNEL_EXT_FOO
02:     bool "foo"
03:     help
04:       This is a comment that explains what foo kernel extension is.
05:
06:       http://foosoftware.org/foo/

Then for each linux extension, add a new .mk file named linux/
linux-ext-foo.mk. It should basically contain:

01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: LINUX_EXTENSIONS += foo
08:
09: define FOO_PREPARE_KERNEL
10:     $(FOO_DIR)/prepare-kernel-tree.sh --linux-dir=$(@D)
11: endef

On line 7, we add the Linux extension foo to the list of available
Linux extensions.

On line 9-11, we define what should be done by the extension to
modify the Linux kernel tree; this is specific to the linux extension
and can use the variables defined by the foo package, like: $
(FOO_DIR) or $(FOO_VERSION)… as well as all the Linux variables,
like: $(LINUX_VERSION) or $(LINUX_VERSION_PROBED), $(KERNEL_ARCH)…
See the definition of those kernel variables.

17.21. Hooks available in the various build steps

The generic infrastructure (and as a result also the derived
autotools and cmake infrastructures) allow packages to specify hooks.
These define further actions to perform after existing steps. Most
hooks aren’t really useful for generic packages, since the .mk file
already has full control over the actions performed in each step of
the package construction.

The following hook points are available:

  * LIBFOO_PRE_DOWNLOAD_HOOKS
  * LIBFOO_POST_DOWNLOAD_HOOKS
  * LIBFOO_PRE_EXTRACT_HOOKS
  * LIBFOO_POST_EXTRACT_HOOKS
  * LIBFOO_PRE_RSYNC_HOOKS
  * LIBFOO_POST_RSYNC_HOOKS
  * LIBFOO_PRE_PATCH_HOOKS
  * LIBFOO_POST_PATCH_HOOKS
  * LIBFOO_PRE_CONFIGURE_HOOKS
  * LIBFOO_POST_CONFIGURE_HOOKS
  * LIBFOO_PRE_BUILD_HOOKS
  * LIBFOO_POST_BUILD_HOOKS
  * LIBFOO_PRE_INSTALL_HOOKS (for host packages only)
  * LIBFOO_POST_INSTALL_HOOKS (for host packages only)
  * LIBFOO_PRE_INSTALL_STAGING_HOOKS (for target packages only)
  * LIBFOO_POST_INSTALL_STAGING_HOOKS (for target packages only)
  * LIBFOO_PRE_INSTALL_TARGET_HOOKS (for target packages only)
  * LIBFOO_POST_INSTALL_TARGET_HOOKS (for target packages only)
  * LIBFOO_PRE_INSTALL_IMAGES_HOOKS
  * LIBFOO_POST_INSTALL_IMAGES_HOOKS
  * LIBFOO_PRE_LEGAL_INFO_HOOKS
  * LIBFOO_POST_LEGAL_INFO_HOOKS

These variables are lists of variable names containing actions to be
performed at this hook point. This allows several hooks to be
registered at a given hook point. Here is an example:

define LIBFOO_POST_PATCH_FIXUP
        action1
        action2
endef

LIBFOO_POST_PATCH_HOOKS += LIBFOO_POST_PATCH_FIXUP

17.21.1. Using the POST_RSYNC hook

The POST_RSYNC hook is run only for packages that use a local source,
either through the local site method or the OVERRIDE_SRCDIR
mechanism. In this case, package sources are copied using rsync from
the local location into the buildroot build directory. The rsync
command does not copy all files from the source directory, though.
Files belonging to a version control system, like the directories
.git, .hg, etc. are not copied. For most packages this is sufficient,
but a given package can perform additional actions using the
POST_RSYNC hook.

In principle, the hook can contain any command you want. One specific
use case, though, is the intentional copying of the version control
directory using rsync. The rsync command you use in the hook can,
among others, use the following variables:

  * $(SRCDIR): the path to the overridden source directory
  * $(@D): the path to the build directory

17.21.2. Target-finalize hook

Packages may also register hooks in LIBFOO_TARGET_FINALIZE_HOOKS.
These hooks are run after all packages are built, but before the
filesystem images are generated. They are seldom used, and your
package probably do not need them.

17.22. Gettext integration and interaction with packages

Many packages that support internationalization use the gettext
library. Dependencies for this library are fairly complicated and
therefore, deserve some explanation.

The glibc C library integrates a full-blown implementation of gettext
, supporting translation. Native Language Support is therefore
built-in in glibc.

On the other hand, the uClibc and musl C libraries only provide a
stub implementation of the gettext functionality, which allows to
compile libraries and programs using gettext functions, but without
providing the translation capabilities of a full-blown gettext
implementation. With such C libraries, if real Native Language
Support is necessary, it can be provided by the libintl library of
the gettext package.

Due to this, and in order to make sure that Native Language Support
is properly handled, packages in Buildroot that can use NLS support
should:

 1. Ensure NLS support is enabled when BR2_SYSTEM_ENABLE_NLS=y. This
    is done automatically for autotools packages and therefore should
    only be done for packages using other package infrastructures.
 2. Add $(TARGET_NLS_DEPENDENCIES) to the package <pkg>_DEPENDENCIES
    variable. This addition should be done unconditionally: the value
    of this variable is automatically adjusted by the core
    infrastructure to contain the relevant list of packages. If NLS
    support is disabled, this variable is empty. If NLS support is
    enabled, this variable contains host-gettext so that tools needed
    to compile translation files are available on the host. In
    addition, if uClibc or musl are used, this variable also contains
    gettext in order to get the full-blown gettext implementation.
 3. If needed, add $(TARGET_NLS_LIBS) to the linker flags, so that
    the package gets linked with libintl. This is generally not
    needed with autotools packages as they usually detect
    automatically that they should link with libintl. However,
    packages using other build systems, or problematic
    autotools-based packages may need this. $(TARGET_NLS_LIBS) should
    be added unconditionally to the linker flags, as the core
    automatically makes it empty or defined to -lintl depending on
    the configuration.

No changes should be made to the Config.in file to support NLS.

Finally, certain packages need some gettext utilities on the target,
such as the gettext program itself, which allows to retrieve
translated strings, from the command line. In such a case, the
package should:

  * use select BR2_PACKAGE_GETTEXT in their Config.in file,
    indicating in a comment above that it’s a runtime dependency
    only.
  * not add any gettext dependency in the DEPENDENCIES variable of
    their .mk file.

17.23. Tips and tricks

17.23.1. Package name, config entry name and makefile variable
relationship

In Buildroot, there is some relationship between:

  * the package name, which is the package directory name (and the
    name of the *.mk file);
  * the config entry name that is declared in the Config.in file;
  * the makefile variable prefix.

It is mandatory to maintain consistency between these elements, using
the following rules:

  * the package directory and the *.mk name are the package name
    itself (e.g.: package/foo-bar_boo/foo-bar_boo.mk);
  * the make target name is the package name itself (e.g.:
    foo-bar_boo);
  * the config entry is the upper case package name with . and -
    characters substituted with _, prefixed with BR2_PACKAGE_ (e.g.:
    BR2_PACKAGE_FOO_BAR_BOO);
  * the *.mk file variable prefix is the upper case package name with
    . and - characters substituted with _ (e.g.:
    FOO_BAR_BOO_VERSION).

17.23.2. How to check the coding style

Buildroot provides a script in utils/check-package that checks new or
changed files for coding style. It is not a complete language
validator, but it catches many common mistakes. It is meant to run in
the actual files you created or modified, before creating the patch
for submission.

This script can be used for packages, filesystem makefiles, Config.in
files, etc. It does not check the files defining the package
infrastructures and some other files containing similar common code.

To use it, run the check-package script, by telling which files you
created or changed:

$ ./utils/check-package package/new-package/*

If you have the utils directory in your path you can also run:

$ cd package/new-package/
$ check-package *

The tool can also be used for packages in a br2-external:

$ check-package -b /path/to/br2-ext-tree/package/my-package/*

17.23.3. How to test your package

Once you have added your new package, it is important that you test
it under various conditions: does it build for all architectures?
Does it build with the different C libraries? Does it need threads,
NPTL? And so on…

Buildroot runs autobuilders [http://autobuild.buildroot.org/] which
continuously test random configurations. However, these only build
the master branch of the git tree, and your new fancy package is not
yet there.

Buildroot provides a script in utils/test-pkg that uses the same base
configurations as used by the autobuilders so you can test your
package in the same conditions.

First, create a config snippet that contains all the necessary
options needed to enable your package, but without any architecture
or toolchain option. For example, let’s create a config snippet that
just enables libcurl, without any TLS backend:

$ cat libcurl.config
BR2_PACKAGE_LIBCURL=y

If your package needs more configuration options, you can add them to
the config snippet. For example, here’s how you would test libcurl
with openssl as a TLS backend and the curl program:

$ cat libcurl.config
BR2_PACKAGE_LIBCURL=y
BR2_PACKAGE_LIBCURL_CURL=y
BR2_PACKAGE_OPENSSL=y

Then run the test-pkg script, by telling it what config snippet to
use and what package to test:

$ ./utils/test-pkg -c libcurl.config -p libcurl

By default, test-pkg will build your package against a subset of the
toolchains used by the autobuilders, which has been selected by the
Buildroot developers as being the most useful and representative
subset. If you want to test all toolchains, pass the -a option. Note
that in any case, internal toolchains are excluded as they take too
long to build.

The output lists all toolchains that are tested and the corresponding
result (excerpt, results are fake):

$ ./utils/test-pkg -c libcurl.config -p libcurl
                armv5-ctng-linux-gnueabi [ 1/11]: OK
              armv7-ctng-linux-gnueabihf [ 2/11]: OK
                        br-aarch64-glibc [ 3/11]: SKIPPED
                           br-arcle-hs38 [ 4/11]: SKIPPED
                            br-arm-basic [ 5/11]: FAILED
                  br-arm-cortex-a9-glibc [ 6/11]: OK
                   br-arm-cortex-a9-musl [ 7/11]: FAILED
                   br-arm-cortex-m4-full [ 8/11]: OK
                             br-arm-full [ 9/11]: OK
                    br-arm-full-nothread [10/11]: FAILED
                      br-arm-full-static [11/11]: OK
11 builds, 2 skipped, 2 build failed, 1 legal-info failed

The results mean:

  * OK: the build was successful.
  * SKIPPED: one or more configuration options listed in the config
    snippet were not present in the final configuration. This is due
    to options having dependencies not satisfied by the toolchain,
    such as for example a package that depends on BR2_USE_MMU with a
    noMMU toolchain. The missing options are reported in
    missing.config in the output build directory (~/br-test-pkg/
    TOOLCHAIN_NAME/ by default).
  * FAILED: the build failed. Inspect the logfile file in the output
    build directory to see what went wrong:

      + the actual build failed,
      + the legal-info failed,
      + one of the preliminary steps (downloading the config file,
        applying the configuration, running dirclean for the package)
        failed.

When there are failures, you can just re-run the script with the same
options (after you fixed your package); the script will attempt to
re-build the package specified with -p for all toolchains, without
the need to re-build all the dependencies of that package.

The test-pkg script accepts a few options, for which you can get some
help by running:

$ ./utils/test-pkg -h

17.23.4. How to add a package from GitHub

Packages on GitHub often don’t have a download area with release
tarballs. However, it is possible to download tarballs directly from
the repository on GitHub. As GitHub is known to have changed download
mechanisms in the past, the github helper function should be used as
shown below.

# Use a tag or a full commit ID
FOO_VERSION = v1.0
FOO_SITE = $(call github,<user>,<package>,$(FOO_VERSION))

Notes

  * The FOO_VERSION can either be a tag or a commit ID.
  * The tarball name generated by github matches the default one from
    Buildroot (e.g.:
    foo-f6fb6654af62045239caed5950bc6c7971965e60.tar.gz), so it is
    not necessary to specify it in the .mk file.
  * When using a commit ID as version, you should use the full 40 hex
    characters.

If the package you wish to add does have a release section on GitHub,
the maintainer may have uploaded a release tarball, or the release
may just point to the automatically generated tarball from the git
tag. If there is a release tarball uploaded by the maintainer, we
prefer to use that since it may be slightly different (e.g. it
contains a configure script so we don’t need to do AUTORECONF).

You can see on the release page if it’s an uploaded tarball or a git
tag:

  * If it looks like the image above then it was uploaded by the
    maintainer and you should use that link (in that example: 
    mongrel2-v1.9.2.tar.bz2) to specify FOO_SITE, and not use the 
    github helper.
  * On the other hand, if there’s is only the "Source code" link,
    then it’s an automatically generated tarball and you should use
    the github helper function.

17.24. Conclusion

As you can see, adding a software package to Buildroot is simply a
matter of writing a Makefile using an existing example and modifying
it according to the compilation process required by the package.

If you package software that might be useful for other people, don’t
forget to send a patch to the Buildroot mailing list (see
Section 21.5, “Submitting patches”)!

Chapter 18. Patching a package

While integrating a new package or updating an existing one, it may
be necessary to patch the source of the software to get it
cross-built within Buildroot.

Buildroot offers an infrastructure to automatically handle this
during the builds. It supports three ways of applying patch sets:
downloaded patches, patches supplied within buildroot and patches
located in a user-defined global patch directory.

18.1. Providing patches

18.1.1. Downloaded

If it is necessary to apply a patch that is available for download,
then add it to the <packagename>_PATCH variable. If an entry contains
://, then Buildroot will assume it is a full URL and download the
patch from this location. Otherwise, Buildroot will assume that the
patch should be downloaded from <packagename>_SITE. It can be a
single patch, or a tarball containing a patch series.

Like for all downloads, a hash should be added to the
<packagename>.hash file.

This method is typically used for packages from Debian.

18.1.2. Within Buildroot

Most patches are provided within Buildroot, in the package directory;
these typically aim to fix cross-compilation, libc support, or other
such issues.

These patch files should be named <number>-<description>.patch.

Notes

  * The patch files coming with Buildroot should not contain any
    package version reference in their filename.
  * The field <number> in the patch file name refers to the apply
    order, and shall start at 1; It is preferred to pad the number
    with zeros up to 4 digits, like git-format-patch does. E.g.:
    0001-foobar-the-buz.patch
  * Previously, it was mandatory for patches to be prefixed with the
    name of the package, like <package>-<number>-<description>.patch,
    but that is no longer the case. Existing packages will be fixed
    as time passes. Do not prefix patches with the package name.
  * Previously, a series file, as used by quilt, could also be added
    in the package directory. In that case, the series file defines
    the patch application order. This is deprecated, and will be
    removed in the future. Do not use a series file.

18.1.3. Global patch directory

The BR2_GLOBAL_PATCH_DIR configuration file option can be used to
specify a space separated list of one or more directories containing
global package patches. See Section 9.8, “Adding project-specific
patches” for details.

18.2. How patches are applied

 1. Run the <packagename>_PRE_PATCH_HOOKS commands if defined;
 2. Cleanup the build directory, removing any existing *.rej files;
 3. If <packagename>_PATCH is defined, then patches from these
    tarballs are applied;
 4. If there are some *.patch files in the package’s Buildroot
    directory or in a package subdirectory named <packageversion>,
    then:

      + If a series file exists in the package directory, then
        patches are applied according to the series file;
      + Otherwise, patch files matching *.patch are applied in
        alphabetical order. So, to ensure they are applied in the
        right order, it is highly recommended to name the patch files
        like this: <number>-<description>.patch, where <number>
        refers to the apply order.
 5. If BR2_GLOBAL_PATCH_DIR is defined, the directories will be
    enumerated in the order they are specified. The patches are
    applied as described in the previous step.
 6. Run the <packagename>_POST_PATCH_HOOKS commands if defined.

If something goes wrong in the steps 3 or 4, then the build fails.

18.3. Format and licensing of the package patches

Patches are released under the same license as the software they
apply to (see Section 12.2, “Complying with the Buildroot license”).

A message explaining what the patch does, and why it is needed,
should be added in the header commentary of the patch.

You should add a Signed-off-by statement in the header of the each
patch to help with keeping track of the changes and to certify that
the patch is released under the same license as the software that is
modified.

If the software is under version control, it is recommended to use
the upstream SCM software to generate the patch set.

Otherwise, concatenate the header with the output of the diff -purN
package-version.orig/ package-version/ command.

If you update an existing patch (e.g. when bumping the package
version), make sure the existing From header and Signed-off-by tags
are not removed, but do update the rest of the patch comment when
appropriate.

At the end, the patch should look like:

configure.ac: add C++ support test

Signed-off-by: John Doe <john.doe@noname.org>

--- configure.ac.orig
+++ configure.ac
@@ -40,2 +40,12 @@

AC_PROG_MAKE_SET
+
+AC_CACHE_CHECK([whether the C++ compiler works],
+               [rw_cv_prog_cxx_works],
+               [AC_LANG_PUSH([C++])
+                AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])],
+                               [rw_cv_prog_cxx_works=yes],
+                               [rw_cv_prog_cxx_works=no])
+                AC_LANG_POP([C++])])
+
+AM_CONDITIONAL([CXX_WORKS], [test "x$rw_cv_prog_cxx_works" = "xyes"])

18.4. Integrating patches found on the Web

When integrating a patch of which you are not the author, you have to
add a few things in the header of the patch itself.

Depending on whether the patch has been obtained from the project
repository itself, or from somewhere on the web, add one of the
following tags:

Backported from: <some commit id>

or

Fetch from: <some url>

It is also sensible to add a few words about any changes to the patch
that may have been necessary.

Chapter 19. Download infrastructure

TODO

Chapter 20. Debugging Buildroot

It is possible to instrument the steps Buildroot does when building
packages. Define the variable BR2_INSTRUMENTATION_SCRIPTS to contain
the path of one or more scripts (or other executables), in a
space-separated list, you want called before and after each step. The
scripts are called in sequence, with three parameters:

  * start or end to denote the start (resp. the end) of a step;
  * the name of the step about to be started, or which just ended;
  * the name of the package.

For example :

make BR2_INSTRUMENTATION_SCRIPTS="/path/to/my/script1 /path/to/my/script2"

The list of steps is:

  * extract
  * patch
  * configure
  * build
  * install-host, when a host-package is installed in $(HOST_DIR)
  * install-target, when a target-package is installed in $
    (TARGET_DIR)
  * install-staging, when a target-package is installed in $
    (STAGING_DIR)
  * install-image, when a target-package installs files in $
    (BINARIES_DIR)

The script has access to the following variables:

  * BR2_CONFIG: the path to the Buildroot .config file
  * HOST_DIR, STAGING_DIR, TARGET_DIR: see Section 17.5.2,
    “generic-package reference”
  * BUILD_DIR: the directory where packages are extracted and built
  * BINARIES_DIR: the place where all binary files (aka images) are
    stored
  * BASE_DIR: the base output directory

Chapter 21. Contributing to Buildroot

There are many ways in which you can contribute to Buildroot:
analyzing and fixing bugs, analyzing and fixing package build
failures detected by the autobuilders, testing and reviewing patches
sent by other developers, working on the items in our TODO list and
sending your own improvements to Buildroot or its manual. The
following sections give a little more detail on each of these items.

If you are interested in contributing to Buildroot, the first thing
you should do is to subscribe to the Buildroot mailing list. This
list is the main way of interacting with other Buildroot developers
and to send contributions to. If you aren’t subscribed yet, then
refer to Chapter 5, Community resources for the subscription link.

If you are going to touch the code, it is highly recommended to use a
git repository of Buildroot, rather than starting from an extracted
source code tarball. Git is the easiest way to develop from and
directly send your patches to the mailing list. Refer to Chapter 3, 
Getting Buildroot for more information on obtaining a Buildroot git
tree.

21.1. Reproducing, analyzing and fixing bugs

A first way of contributing is to have a look at the open bug reports
in the Buildroot bug tracker [https://bugs.buildroot.org/buglist.cgi?
product=buildroot]. As we strive to keep the bug count as small as
possible, all help in reproducing, analyzing and fixing reported bugs
is more than welcome. Don’t hesitate to add a comment to bug reports
reporting your findings, even if you don’t yet see the full picture.

21.2. Analyzing and fixing autobuild failures

The Buildroot autobuilders are a set of build machines that
continuously run Buildroot builds based on random configurations.
This is done for all architectures supported by Buildroot, with
various toolchains, and with a random selection of packages. With the
large commit activity on Buildroot, these autobuilders are a great
help in detecting problems very early after commit.

All build results are available at http://autobuild.buildroot.org,
statistics are at http://autobuild.buildroot.org/stats.php. Every
day, an overview of all failed packages is sent to the mailing list.

Detecting problems is great, but obviously these problems have to be
fixed as well. Your contribution is very welcome here! There are
basically two things that can be done:

  * Analyzing the problems. The daily summary mails do not contain
    details about the actual failures: in order to see what’s going
    on you have to open the build log and check the last output.
    Having someone doing this for all packages in the mail is very
    useful for other developers, as they can make a quick initial
    analysis based on this output alone.
  * Fixing a problem. When fixing autobuild failures, you should
    follow these steps:

     1. Check if you can reproduce the problem by building with the
        same configuration. You can do this manually, or use the
        br-reproduce-build [http://git.buildroot.org/buildroot-test/
        tree/utils/br-reproduce-build] script that will automatically
        clone a Buildroot git repository, checkout the correct
        revision, download and set the right configuration, and start
        the build.
     2. Analyze the problem and create a fix.
     3. Verify that the problem is really fixed by starting from a
        clean Buildroot tree and only applying your fix.
     4. Send the fix to the Buildroot mailing list (see Section 21.5,
        “Submitting patches”). In case you created a patch against
        the package sources, you should also send the patch upstream
        so that the problem will be fixed in a later release, and the
        patch in Buildroot can be removed. In the commit message of a
        patch fixing an autobuild failure, add a reference to the
        build result directory, as follows:

Fixes: http://autobuild.buildroot.org/results/51000a9d4656afe9e0ea6f07b9f8ed374c2e4069

21.3. Reviewing and testing patches

With the amount of patches sent to the mailing list each day, the
maintainer has a very hard job to judge which patches are ready to
apply and which ones aren’t. Contributors can greatly help here by
reviewing and testing these patches.

In the review process, do not hesitate to respond to patch
submissions for remarks, suggestions or anything that will help
everyone to understand the patches and make them better. Please use
internet style replies in plain text emails when responding to patch
submissions.

To indicate approval of a patch, there are three formal tags that
keep track of this approval. To add your tag to a patch, reply to it
with the approval tag below the original author’s Signed-off-by line.
These tags will be picked up automatically by patchwork (see
Section 21.3.1, “Applying Patches from Patchwork”) and will be part
of the commit log when the patch is accepted.

Tested-by
    Indicates that the patch has been tested successfully. You are
    encouraged to specify what kind of testing you performed
    (compile-test on architecture X and Y, runtime test on target A,
    …). This additional information helps other testers and the
    maintainer.
Reviewed-by
    Indicates that you code-reviewed the patch and did your best in
    spotting problems, but you are not sufficiently familiar with the
    area touched to provide an Acked-by tag. This means that there
    may be remaining problems in the patch that would be spotted by
    someone with more experience in that area. Should such problems
    be detected, your Reviewed-by tag remains appropriate and you
    cannot be blamed.
Acked-by
    Indicates that you code-reviewed the patch and you are familiar
    enough with the area touched to feel that the patch can be
    committed as-is (no additional changes required). In case it
    later turns out that something is wrong with the patch, your
    Acked-by could be considered inappropriate. The difference
    between Acked-by and Reviewed-by is thus mainly that you are
    prepared to take the blame on Acked patches, but not on Reviewed
    ones.

If you reviewed a patch and have comments on it, you should simply
reply to the patch stating these comments, without providing a
Reviewed-by or Acked-by tag. These tags should only be provided if
you judge the patch to be good as it is.

It is important to note that neither Reviewed-by nor Acked-by imply
that testing has been performed. To indicate that you both reviewed
and tested the patch, provide two separate tags (Reviewed/Acked-by
and Tested-by).

Note also that any developer can provide Tested/Reviewed/Acked-by
tags, without exception, and we encourage everyone to do this.
Buildroot does not have a defined group of core developers, it just
so happens that some developers are more active than others. The
maintainer will value tags according to the track record of their
submitter. Tags provided by a regular contributor will naturally be
trusted more than tags provided by a newcomer. As you provide tags
more regularly, your trustworthiness (in the eyes of the maintainer)
will go up, but any tag provided is valuable.

Buildroot’s Patchwork website can be used to pull in patches for
testing purposes. Please see Section 21.3.1, “Applying Patches from
Patchwork” for more information on using Buildroot’s Patchwork
website to apply patches.

21.3.1. Applying Patches from Patchwork

The main use of Buildroot’s Patchwork website for a developer is for
pulling in patches into their local git repository for testing
purposes.

When browsing patches in the patchwork management interface, an mbox
link is provided at the top of the page. Copy this link address and
run the following commands:

$ git checkout -b <test-branch-name>
$ wget -O - <mbox-url> | git am

Another option for applying patches is to create a bundle. A bundle
is a set of patches that you can group together using the patchwork
interface. Once the bundle is created and the bundle is made public,
you can copy the mbox link for the bundle and apply the bundle using
the above commands.

21.4. Work on items from the TODO list

If you want to contribute to Buildroot but don’t know where to start,
and you don’t like any of the above topics, you can always work on
items from the Buildroot TODO list [http://elinux.org/Buildroot#
Todo_list]. Don’t hesitate to discuss an item first on the mailing
list or on IRC. Do edit the wiki to indicate when you start working
on an item, so we avoid duplicate efforts.

21.5. Submitting patches

Note

Please, do not attach patches to bugs, send them to the mailing list
instead.

If you made some changes to Buildroot and you would like to
contribute them to the Buildroot project, proceed as follows.

21.5.1. The formatting of a patch

We expect patches to be formatted in a specific way. This is
necessary to make it easy to review patches, to be able to apply them
easily to the git repository, to make it easy to find back in the
history how and why things have changed, and to make it possible to
use git bisect to locate the origin of a problem.

First of all, it is essential that the patch has a good commit
message. The commit message should start with a separate line with a
brief summary of the change, prefixed by the area touched by the
patch. A few examples of good commit titles:

  * package/linuxptp: bump version to 2.0
  * configs/imx23evk: bump Linux version to 4.19
  * package/pkg-generic: postpone evaluation of dependency conditions
  * boot/uboot: needs host-{flex,bison}
  * support/testing: add python-ubjson tests

The description that follows the prefix should start with a lower
case letter (i.e "bump", "needs", "postpone", "add" in the above
examples).

Second, the body of the commit message should describe why this
change is needed, and if necessary also give details about how it was
done. When writing the commit message, think of how the reviewers
will read it, but also think about how you will read it when you look
at this change again a few years down the line.

Third, the patch itself should do only one change, but do it
completely. Two unrelated or weakly related changes should usually be
done in two separate patches. This usually means that a patch affects
only a single package. If several changes are related, it is often
still possible to split them up in small patches and apply them in a
specific order. Small patches make it easier to review, and often
make it easier to understand afterwards why a change was done.
However, each patch must be complete. It is not allowed that the
build is broken when only the first but not the second patch is
applied. This is necessary to be able to use git bisect afterwards.

Of course, while you’re doing your development, you’re probably going
back and forth between packages, and certainly not committing things
immediately in a way that is clean enough for submission. So most
developers rewrite the history of commits to produce a clean set of
commits that is appropriate for submission. To do this, you need to
use interactive rebasing. You can learn about it in the Pro Git book
[https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History].
Sometimes, it is even easier to discard you history with git reset
--soft origin/master and select individual changes with git add -i or
git add -p.

Finally, the patch should be signed off. This is done by adding
Signed-off-by: Your Real Name <> at the end of the commit message.
git commit -s does that for you, if configured properly. The
Signed-off-by tag means that you publish the patch under the
Buildroot license (i.e. GPL-2.0+, except for package patches, which
have the upstream license), and that you are allowed to do so. See
the Developer Certificate of Origin [http://developercertificate.org
/] for details.

When adding new packages, you should submit every package in a
separate patch. This patch should have the update to package/
Config.in, the package Config.in file, the .mk file, the .hash file,
any init script, and all package patches. If the package has many
sub-options, these are sometimes better added as separate follow-up
patches. The summary line should be something like <packagename>: new
package. The body of the commit message can be empty for simple
packages, or it can contain the description of the package (like the
Config.in help text). If anything special has to be done to build the
package, this should also be explained explicitly in the commit
message body.

When you bump a package to a new version, you should also submit a
separate patch for each package. Don’t forget to update the .hash
file, or add it if it doesn’t exist yet. Also don’t forget to check
if the _LICENSE and _LICENSE_FILES are still valid. The summary line
should be something like <packagename>: bump to version <new
version>. If the new version only contains security updates compared
to the existing one, the summary should be <packagename>: security
bump to version <new version> and the commit message body should show
the CVE numbers that are fixed. If some package patches can be
removed in the new version, it should be explained explicitly why
they can be removed, preferably with the upstream commit ID. Also any
other required changes should be explained explicitly, like configure
options that no longer exist or are no longer needed.

If you are interested in getting notified of build failures and of
further changes in the packages you added or modified, please add
yourself to the DEVELOPERS file. This should be done in the same
patch creating or modifying the package. See the DEVELOPERS file for
more information.

Buildroot provides a handy tool to check for common coding style
mistakes on files you created or modified, called check-package (see
Section 17.23.2, “How to check the coding style” for more
information).

21.5.2. Preparing a patch series

Starting from the changes committed in your local git view, rebase
your development branch on top of the upstream tree before generating
a patch set. To do so, run:

$ git fetch --all --tags
$ git rebase origin/master

Now, you are ready to generate then submit your patch set.

To generate it, run:

$ git format-patch -M -n -s -o outgoing origin/master

This will generate patch files in the outgoing subdirectory,
automatically adding the Signed-off-by line.

Once patch files are generated, you can review/edit the commit
message before submitting them, using your favorite text editor.

Buildroot provides a handy tool to know to whom your patches should
be sent, called get-developers (see Chapter 22, DEVELOPERS file and
get-developers for more information). This tool reads your patches
and outputs the appropriate git send-email command to use:

$ ./utils/get-developers outgoing/*

Use the output of get-developers to send your patches:

$ git send-email --to buildroot@buildroot.org --cc bob --cc alice outgoing/*

Alternatively, get-developers -e can be used directly with the
--cc-cmd argument to git send-email to automatically CC the affected
developers:

$ git send-email --to buildroot@buildroot.org \
      --cc-cmd './utils/get-developers -e' origin/master

git can be configured to automatically do this out of the box with:

$ git config sendemail.to buildroot@buildroot.org
$ git config sendemail.ccCmd "$(pwd)/utils/get-developers -e"

And then just do:

$ git send-email origin/master

Note that git should be configured to use your mail account. To
configure git, see man git-send-email or google it.

If you do not use git send-email, make sure posted patches are not
line-wrapped, otherwise they cannot easily be applied. In such a
case, fix your e-mail client, or better yet, learn to use git
send-email.

21.5.3. Cover letter

If you want to present the whole patch set in a separate mail, add
--cover-letter to the git format-patch command (see man
git-format-patch for further information). This will generate a
template for an introduction e-mail to your patch series.

A cover letter may be useful to introduce the changes you propose in
the following cases:

  * large number of commits in the series;
  * deep impact of the changes in the rest of the project;
  * RFC ^[4];
  * whenever you feel it will help presenting your work, your
    choices, the review process, etc.

21.5.4. Patch revision changelog

When improvements are requested, the new revision of each commit
should include a changelog of the modifications between each
submission. Note that when your patch series is introduced by a cover
letter, an overall changelog may be added to the cover letter in
addition to the changelog in the individual commits. The best thing
to rework a patch series is by interactive rebasing: git rebase -i
origin/master. Consult the git manual for more information.

When added to the individual commits, this changelog is added when
editing the commit message. Below the Signed-off-by section, add ---
and your changelog.

Although the changelog will be visible for the reviewers in the mail
thread, as well as in patchwork [http://patchwork.buildroot.org], git
will automatically ignores lines below --- when the patch will be
merged. This is the intended behavior: the changelog is not meant to
be preserved forever in the git history of the project.

Hereafter the recommended layout:

Patch title: short explanation, max 72 chars

A paragraph that explains the problem, and how it manifests itself. If
the problem is complex, it is OK to add more paragraphs. All paragraphs
should be wrapped at 72 characters.

A paragraph that explains the root cause of the problem. Again, more
than one paragraph is OK.

Finally, one or more paragraphs that explain how the problem is solved.
Don't hesitate to explain complex solutions in detail.

Signed-off-by: John DOE <john.doe@example.net>

---
Changes v2 -> v3:
  - foo bar  (suggested by Jane)
  - bar buz

Changes v1 -> v2:
  - alpha bravo  (suggested by John)
  - charly delta

Any patch revision should include the version number. The version
number is simply composed of the letter v followed by an integer
greater or equal to two (i.e. "PATCH v2", "PATCH v3" …).

This can be easily handled with git format-patch by using the option
--subject-prefix:

$ git format-patch --subject-prefix "PATCH v4" \
    -M -s -o outgoing origin/master

Since git version 1.8.1, you can also use -v <n> (where <n> is the
version number):

$ git format-patch -v4 -M -s -o outgoing origin/master

When you provide a new version of a patch, please mark the old one as
superseded in patchwork [http://patchwork.buildroot.org]. You need to
create an account on patchwork [http://patchwork.buildroot.org] to be
able to modify the status of your patches. Note that you can only
change the status of patches you submitted yourself, which means the
email address you register in patchwork [http://
patchwork.buildroot.org] should match the one you use for sending
patches to the mailing list.

You can also add the --in-reply-to <message-id> option when
submitting a patch to the mailing list. The id of the mail to reply
to can be found under the "Message Id" tag on patchwork [http://
patchwork.buildroot.org]. The advantage of in-reply-to is that
patchwork will automatically mark the previous version of the patch
as superseded.

21.6. Reporting issues/bugs or getting help

Before reporting any issue, please check in the mailing list archive
whether someone has already reported and/or fixed a similar problem.

However you choose to report bugs or get help, either by opening a
bug in the bug tracker or by sending a mail to the mailing list,
there are a number of details to provide in order to help people
reproduce and find a solution to the issue.

Try to think as if you were trying to help someone else; in that
case, what would you need?

Here is a short list of details to provide in such case:

  * host machine (OS/release)
  * version of Buildroot
  * target for which the build fails
  * package(s) for which the build fails
  * the command that fails and its output
  * any information you think that may be relevant

Additionally, you should add the .config file (or if you know how, a
defconfig; see Section 9.3, “Storing the Buildroot configuration”).

If some of these details are too large, do not hesitate to use a
pastebin service. Note that not all available pastebin services will
preserve Unix-style line terminators when downloading raw pastes.
Following pastebin services are known to work correctly: - https://
gist.github.com/ - http://code.bulix.org/

21.7. Using the run-tests framework

Buildroot includes a run-time testing framework called run-tests
built upon Python scripting and QEMU runtime execution. There are two
types of test cases within the framework, one for build time tests
and another for run-time tests that have a QEMU dependency. The goals
of the framework are the following:

  * build a well defined configuration
  * optionally, verify some properties of the build output
  * if it is a run-time test:

      + boot it under QEMU
      + run some test condition to verify that a given feature is
        working

The run-tests tool has a series of options documented in the tool’s
help -h description. Some common options include setting the download
folder, the output folder, keeping build output, and for multiple
test cases, you can set the JLEVEL for each.

Here is an example walk through of running a test case.

  * For a first step, let us see what all the test case options are.
    The test cases can be listed by executing support/testing/
    run-tests -l. These tests can all be run individually during test
    development from the console. Both one at a time and selectively
    as a group of a subset of tests.

$ support/testing/run-tests -l
List of tests
test_run (tests.utils.test_check_package.TestCheckPackage)
Test the various ways the script can be called in a simple top to ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainBuildrootMusl) ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainBuildrootuClibc) ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainCCache) ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainCtngMusl) ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainLinaroArm) ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainSourceryArmv4) ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainSourceryArmv5) ... ok
test_run (tests.toolchain.test_external.TestExternalToolchainSourceryArmv7) ... ok
[snip]
test_run (tests.init.test_systemd.TestInitSystemSystemdRoFull) ... ok
test_run (tests.init.test_systemd.TestInitSystemSystemdRoIfupdown) ... ok
test_run (tests.init.test_systemd.TestInitSystemSystemdRoNetworkd) ... ok
test_run (tests.init.test_systemd.TestInitSystemSystemdRwFull) ... ok
test_run (tests.init.test_systemd.TestInitSystemSystemdRwIfupdown) ... ok
test_run (tests.init.test_systemd.TestInitSystemSystemdRwNetworkd) ... ok
test_run (tests.init.test_busybox.TestInitSystemBusyboxRo) ... ok
test_run (tests.init.test_busybox.TestInitSystemBusyboxRoNet) ... ok
test_run (tests.init.test_busybox.TestInitSystemBusyboxRw) ... ok
test_run (tests.init.test_busybox.TestInitSystemBusyboxRwNet) ... ok

Ran 157 tests in 0.021s

OK

Those runtime tests are regularly executed by Buildroot Gitlab CI
infrastructure, see .gitlab.yml and https://gitlab.com/buildroot.org/
buildroot/-/jobs.

21.7.1. Creating a test case

The best way to get familiar with how to create a test case is to
look at a few of the basic file system support/testing/tests/fs/ and
init support/testing/tests/init/ test scripts. Those tests give good
examples of a basic build and build with run type of tests. There are
other more advanced cases that use things like nested br2-external
folders to provide skeletons and additional packages.

The test cases by default use a br-arm-full-* uClibc-ng toolchain and
the prebuild kernel for a armv5/7 cpu. It is recommended to use the
default defconfig test configuration except when Glibc/musl or a
newer kernel are necessary. By using the default it saves build time
and the test would automatically inherit a kernel/std library upgrade
when the default is updated.

The basic test case definition involves

  * Creation of a new test file
  * Defining a unique test class
  * Determining if the default defconfig plus test options can be
    used
  * Implementing a def test_run(self): function to optionally startup
    the emulator and provide test case conditions.

Beyond creating the test script, there are a couple of additional
steps that should be taken once you have your initial test case
script. The first is to add yourself to the DEVELOPERS file to be the
maintainer of that test case. The second is to update the Gitlab CI
yml by executing make .gitlab-ci.yml.

21.7.2. Debugging a test case

Within the Buildroot repository, the testing framework is organized
at the top level in support/testing/ by folders of conf, infra and
tests. All the test cases live under the test folder and are
organized in various folders representing the catagory of test.

Lets walk through an example.

  * Using the Busybox Init system test case with a read/write rootfs
    tests.init.test_busybox.TestInitSystemBusyboxRw
  * A minimal set of command line arguments when debugging a test
    case would include -d which points to your dl folder, -o to an
    output folder, and -k to keep any output on both pass/fail. With
    those options, the test will retain logging and build artifacts
    providing status of the build and execution of the test case.

$ support/testing/run-tests -d dl -o output_folder -k tests.init.test_busybox.TestInitSystemBusyboxRw
15:03:26 TestInitSystemBusyboxRw                  Starting
15:03:28 TestInitSystemBusyboxRw                  Building
15:08:18 TestInitSystemBusyboxRw                  Building done
15:08:27 TestInitSystemBusyboxRw                  Cleaning up
.
Ran 1 test in 301.140s

OK

  * For the case of a successful build, the output_folder would
    contain a <test name> folder with the Buildroot build, build log
    and run-time log. If the build failed, the console output would
    show the stage at which it failed (setup / build / run).
    Depending on the failure stage, the build/run logs and/or
    Buildroot build artifacts can be inspected and instrumented. If
    the QEMU instance needs to be launched for additional testing,
    the first few lines of the run-time log capture it and it would
    allow some incremental testing without re-running support/testing
    /run-tests.
  * You can also make modifications to the current sources inside the
    output_folder (e.g. for debug purposes) and rerun the standard
    Buildroot make targets (in order to regenerate the complete image
    with the new modifications) and then rerun the test. Modifying
    the sources directly can speed up debugging compared to adding
    patch files, wiping the output directoy, and starting the test
    again.

$ ls output_folder/
TestInitSystemBusyboxRw/
TestInitSystemBusyboxRw-build.log
TestInitSystemBusyboxRw-run.log

  * The source file used to implement this example test is found
    under support/testing/tests/init/test_busybox.py. This file
    outlines the minimal defconfig that creates the build, QEMU
    configuration to launch the built images and the test case
    assertions.

To test an existing or new test case within Gitlab CI, there is a
method of invoking a specific test by creating a Buildroot fork in
Gitlab under your account. This can be handy when adding/changing a
run-time test or fixing a bug on a use case tested by a run-time test
case.

In the examples below, the <name> component of the branch name is a
unique string you choose to identify this specific job being created.

  * to trigger all run-test test case jobs:

 $ git push gitlab HEAD:<name>-runtime-tests

  * to trigger one test case job, a specific branch naming string is
    used that includes the full test case name.

 $ git push gitlab HEAD:<name>-<test case name>


---------------------------------------------------------------------

^[4] RFC: (Request for comments) change proposal

Chapter 22. DEVELOPERS file and get-developers

The main Buildroot directory contains a file named DEVELOPERS that
lists the developers involved with various areas of Buildroot. Thanks
to this file, the get-developers tool allows to:

  * Calculate the list of developers to whom patches should be sent,
    by parsing the patches and matching the modified files with the
    relevant developers. See Section 21.5, “Submitting patches” for
    details.
  * Find which developers are taking care of a given architecture or
    package, so that they can be notified when a build failure occurs
    on this architecture or package. This is done in interaction with
    Buildroot’s autobuild infrastructure.

We ask developers adding new packages, new boards, or generally new
functionality in Buildroot, to register themselves in the DEVELOPERS
file. As an example, we expect a developer contributing a new package
to include in his patch the appropriate modification to the
DEVELOPERS file.

The DEVELOPERS file format is documented in detail inside the file
itself.

The get-developers tool, located in utils/ allows to use the
DEVELOPERS file for various tasks:

  * When passing one or several patches as command line argument,
    get-developers will return the appropriate git send-email
    command. If the -e option is passed, only the email addresses are
    printed in a format suitable for git send-email --cc-cmd.
  * When using the -a <arch> command line option, get-developers will
    return the list of developers in charge of the given
    architecture.
  * When using the -p <package> command line option, get-developers
    will return the list of developers in charge of the given
    package.
  * When using the -c command line option, get-developers will look
    at all files under version control in the Buildroot repository,
    and list the ones that are not handled by any developer. The
    purpose of this option is to help completing the DEVELOPERS file.
  * When using without any arguments, it validates the integrity of
    the DEVELOPERS file and will note WARNINGS for items that don’t
    match.

Chapter 23. Release Engineering

23.1. Releases

The Buildroot project makes quarterly releases with monthly bugfix
releases. The first release of each year is a long term support
release, LTS.

  * Quarterly releases: 2020.02, 2020.05, 2020.08, and 2020.11
  * Bugfix releases: 2020.02.1, 2020.02.2, …
  * LTS releases: 2020.02, 2021.02, …

Releases are supported until the first bugfix release of the next
release, e.g., 2020.05.x is EOL when 2020.08.1 is released.

LTS releases are supported until the first bugfix release of the next
LTS, e.g., 2020.02.x is supported until 2021.02.1 is released.

23.2. Development

Each release cycle consist of two months of development on the master
branch and one month stabilization before the release is made. During
this phase no new features are added to master, only bugfixes.

The stabilization phase starts with tagging -rc1, and every week
until the release, another release candidate is tagged.

To handle new features and version bumps during the stabilization
phase, a next branch may be created for these features. Once the
current release has been made, the next branch is merged into master
and the development cycle for the next release continues there.

Part IV. Appendix

Table of Contents

24. Makedev syntax documentation
25. Makeusers syntax documentation
26. Migrating from older Buildroot versions

    26.1. Migrating to 2016.11
    26.2. Migrating to 2017.08

Chapter 24. Makedev syntax documentation

The makedev syntax is used in several places in Buildroot to define
changes to be made for permissions, or which device files to create
and how to create them, in order to avoid calls to mknod.

This syntax is derived from the makedev utility, and more complete
documentation can be found in the package/makedevs/README file.

It takes the form of a space separated list of fields, one file per
line; the fields are:

+--------------------------------------------------+
|name|type|mode|uid|gid|major|minor|start|inc|count|
+--------------------------------------------------+

There are a few non-trivial blocks:

  * name is the path to the file you want to create/modify
  * type is the type of the file, being one of:

      + f: a regular file
      + d: a directory
      + r: a directory recursively
      + c: a character device file
      + b: a block device file
      + p: a named pipe
  * mode are the usual permissions settings (only numerical values
    are allowed)
  * uid and gid are the UID and GID to set on this file; can be
    either numerical values or actual names
  * major and minor are here for device files, set to - for other
    files
  * start, inc and count are for when you want to create a batch of
    files, and can be reduced to a loop, beginning at start,
    incrementing its counter by inc until it reaches count

Let’s say you want to change the permissions of a given file; using
this syntax, you will need to write:

/usr/bin/foo f 755 0 0 - - - - -
/usr/bin/bar f 755 root root - - - - -
/data/buz f 644 buz-user buz-group - - - - -

Alternatively, if you want to change owner/permission of a directory
recursively, you can write (to set UID to foo, GID to bar and access
rights to rwxr-x--- for the directory /usr/share/myapp and all files
and directories below it):

/usr/share/myapp r 750 foo bar - - - - -

On the other hand, if you want to create the device file /dev/hda and
the corresponding 15 files for the partitions, you will need for /dev
/hda:

/dev/hda b 640 root root 3 0 0 0 -

and then for device files corresponding to the partitions of /dev/
hda, /dev/hdaX, X ranging from 1 to 15:

/dev/hda b 640 root root 3 1 1 1 15

Extended attributes are supported if
BR2_ROOTFS_DEVICE_TABLE_SUPPORTS_EXTENDED_ATTRIBUTES is enabled. This
is done by adding a line starting with |xattr after the line
describing the file. Right now, only capability is supported as
extended attribute.

+------------------+
||xattr|capability |
+------------------+

  * |xattr is a "flag" that indicate an extended attribute
  * capability is a capability to add to the previous file

If you want to add the capability cap_sys_admin to the binary foo,
you will write :

/usr/bin/foo f 755 root root - - - - -
|xattr cap_sys_admin+eip

You can add several capabilities to a file by using several |xattr
lines. If you want to add the capability cap_sys_admin and
cap_net_admin to the binary foo, you will write :

/usr/bin/foo f 755 root root - - - - -
|xattr cap_sys_admin+eip
|xattr cap_net_admin+eip

Chapter 25. Makeusers syntax documentation

The syntax to create users is inspired by the makedev syntax, above,
but is specific to Buildroot.

The syntax for adding a user is a space-separated list of fields, one
user per line; the fields are:

+---------------------------------------------------------+
|username|uid|group|gid|password|home|shell|groups|comment|
+---------------------------------------------------------+

Where:

  * username is the desired user name (aka login name) for the user.
    It can not be root, and must be unique. If set to -, then just a
    group will be created.
  * uid is the desired UID for the user. It must be unique, and not
    0. If set to -1, then a unique UID will be computed by Buildroot
    in the range [1000…1999]
  * group is the desired name for the user’s main group. It can not
    be root. If the group does not exist, it will be created.
  * gid is the desired GID for the user’s main group. It must be
    unique, and not 0. If set to -1, and the group does not already
    exist, then a unique GID will be computed by Buildroot in the
    range [1000..1999]
  * password is the crypt(3)-encoded password. If prefixed with !,
    then login is disabled. If prefixed with =, then it is
    interpreted as clear-text, and will be crypt-encoded (using MD5).
    If prefixed with !=, then the password will be crypt-encoded
    (using MD5) and login will be disabled. If set to *, then login
    is not allowed. If set to -, then no password value will be set.
  * home is the desired home directory for the user. If set to -, no
    home directory will be created, and the user’s home will be /.
    Explicitly setting home to / is not allowed.
  * shell is the desired shell for the user. If set to -, then /bin/
    false is set as the user’s shell.
  * groups is the comma-separated list of additional groups the user
    should be part of. If set to -, then the user will be a member of
    no additional group. Missing groups will be created with an
    arbitrary gid.
  * comment (aka GECOS [https://en.wikipedia.org/wiki/Gecos_field]
    field) is an almost-free-form text.

There are a few restrictions on the content of each field:

  * except for comment, all fields are mandatory.
  * except for comment, fields may not contain spaces.
  * no field may contain a colon (:).

If home is not -, then the home directory, and all files below, will
belong to the user and its main group.

Examples:

foo -1 bar -1 !=blabla /home/foo /bin/sh alpha,bravo Foo user

This will create this user:

  * username (aka login name) is: foo
  * uid is computed by Buildroot
  * main group is: bar
  * main group gid is computed by Buildroot
  * clear-text password is: blabla, will be crypt(3)-encoded, and
    login is disabled.
  * home is: /home/foo
  * shell is: /bin/sh
  * foo is also a member of groups: alpha and bravo
  * comment is: Foo user

test 8000 wheel -1 = - /bin/sh - Test user

This will create this user:

  * username (aka login name) is: test
  * uid is : 8000
  * main group is: wheel
  * main group gid is computed by Buildroot, and will use the value
    defined in the rootfs skeleton
  * password is empty (aka no password).
  * home is / but will not belong to test
  * shell is: /bin/sh
  * test is not a member of any additional groups
  * comment is: Test user

Chapter 26. Migrating from older Buildroot versions

Some versions have introduced backward incompatibilities. This
section explains those incompatibilities, and for each explains what
to do to complete the migration.

26.1. Migrating to 2016.11

Before Buildroot 2016.11, it was possible to use only one
br2-external tree at once. With Buildroot 2016.11 came the
possibility to use more than one simultaneously (for details, see
Section 9.2, “Keeping customizations outside of Buildroot”).

This however means that older br2-external trees are not usable
as-is. A minor change has to be made: adding a name to your
br2-external tree.

This can be done very easily in just a few steps:

  * First, create a new file named external.desc, at the root of your
    br2-external tree, with a single line defining the name of your
    br2-external tree:

    $ echo 'name: NAME_OF_YOUR_TREE' >external.desc

    Note. Be careful when choosing a name: It has to be unique and be
    made with only ASCII characters from the set [A-Za-z0-9_].

  * Then, change every occurence of BR2_EXTERNAL in your br2-external
    tree with the new variable:

    $ find . -type f | xargs sed -i 's/BR2_EXTERNAL/BR2_EXTERNAL_NAME_OF_YOUR_TREE_PATH/g'

Now, your br2-external tree can be used with Buildroot 2016.11
onward.

Note: This change makes your br2-external tree incompatible with
Buildroot before 2016.11.

26.2. Migrating to 2017.08

Before Buildroot 2017.08, host packages were installed in $(HOST_DIR)
/usr (with e.g. the autotools' --prefix=$(HOST_DIR)/usr). With
Buildroot 2017.08, they are now installed directly in $(HOST_DIR).

Whenever a package installs an executable that is linked with a
library in $(HOST_DIR)/lib, it must have an RPATH pointing to that
directory.

An RPATH pointing to $(HOST_DIR)/usr/lib is no longer accepted.

