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<h1>Buildroot</h1>
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<p>Usage and documentation by Thomas Petazzoni. Contributions from
Karsten Kruse, Ned Ludd, Martin Herren.</p>
<p><small>Last modification : $Date: 2004/12/27 19:01:52 $</small></p>
<ul>
<li><a href="#about">About Buildroot</a></li>
<li><a href="#download">Obtaining Buildroot</a></li>
<li><a href="#using">Using Buildroot</a></li>
<li><a href="#custom_targetfs">Customizing the target filesystem</a></li>
<li><a href="#custom_busybox">Customizing the Busybox
configuration</a></li>
<li><a href="#custom_uclibc">Customizing the uClibc
configuration</a></li>
<li><a href="#buildroot_innards">How Buildroot works</a></li>
<li><a href="#toolchain_standalone">Using the uClibc toolchain without
Buildroot</a></li>
<li><a href="#downloaded_packages">Location of downloaded packages</a></li>
<li><a href="#add_software">Extending Buildroot with more
Software</a></li>
<li><a href="#links">Ressources</a></li>
</ul>
<h2><a name="about" id="about"></a>About Buildroot</h2>
<p>Buildroot is a set of Makefiles and patches that allows to easily
generate both a cross-compilation toolchain and a root filesystem for your
target. The cross-compilation toolchain uses uClibc (<a href=
"http://www.uclibc.org/">http://www.uclibc.org/</a>), a tiny C standard
library.</p>
<p>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 have on his PC. It can be PowerPC
processors, MIPS processors, ARM processors, etc.</p>
<p>A compilation toolchain is the set of tools that allows to
compile code for your system. It consists of a compiler (in our
case, <code>gcc</code>), binary utils like assembler and linker
(in our case, <code>binutils</code>) and a C standard library (for
example <a href="http://www.gnu.org/software/libc/libc.html">GNU
Libc</a>, <a href="http://www.uclibc.org">uClibc</a> or <a
href="http://www.fefe.de/dietlibc/">dietlibc</a>). The system
installed on your development station certainly already has a
compilation toolchain that you can use to compile application that
runs on your system. If you're using a PC, your compilation
toolchain runs on an x86 processor and generates code for a x86
processor. Under most Linux systems, the compilation toolchain
uses the GNU libc as C standard library. This compilation
toolchain is called the "host compilation toolchain", and more
generally, the machine on which it is running, and on which you're
working is called the "host system". The compilation toolchain is
provided by your distribution, and Buildroot has nothing to do
with it.</p>
<p>As said above, the compilation toolchain that comes with your system
runs and generates code for the processor of your host system. As your
embedded system has a different processor, you need a cross-compilation
toolchain: it's a compilation toolchain that runs on your host system but
that 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 of your host runs on x86 and generates code
for x86, while the cross-compilation toolchain runs on x86 and generates
code for ARM.</p>
<p>Even if your embedded system uses a x86 processor, you might interested
in Buildroot, for two reasons:</p>
<ul>
<li>The compilation toolchain of your host certainly uses the GNU Libc
which is a complete but huge C standard library. Instead of using GNU
Libc on your target system, you can use uClibc which is a tiny C standard
library. If you want to use this C library, then you need a compilation
toolchain to generate binaries linked with it. Buildroot can do it for
you.</li>
<li>Buildroot automates the building of a root filesystem with all needed
tools like busybox. It makes it much easier than doing it by hand.</li>
</ul>
<p>You might wonder why such a tool is needed when you can compile
<code>gcc</code>, <code>binutils</code>, uClibc and all the tools by hand.
Of course, doing so is possible. But dealing with all configure options,
with all problems of every <code>gcc</code> or <code>binutils</code>
version it very time-consuming and uninteresting. Buildroot automates this
process through the use of Makefiles, and has a collection of patches for
each <code>gcc</code> and <code>binutils</code> version to make them work
on most architectures.</p>
<h2><a name="download" id="download"></a>Obtaining Buildroot</h2>
<p>Buildroot is available as daily CVS snapshots or directly using
CVS.</p>
<p>The latest snapshot is always available at <a
href="http://uclibc.org/downloads/snapshots/buildroot-snapshot.tar.bz2">http://uclibc.org/downloads/snapshots/buildroot-snapshot.tar.bz2</a>,
and previous snapshots are also available at <a
href="http://uclibc.org/downloads/snapshots/">http://uclibc.org/downloads/snapshots/</a>.</p>
<p>To download Buildroot using CVS, you can simply follow
the rules described on the "Accessing CVS"-page (<a href=
"http://www.uclibc.org/cvs_anon.html">http://www.uclibc.org/cvs_anon.html</a>)
of the uClibc website (<a href=
"http://www.uclibc.org">http://www.uclibc.org</a>), and download the
<code>buildroot</code> CVS module. For the impatient, here's a quick
recipe:</p>
<pre>
$ cvs -d:pserver:anonymous@uclibc.org:/var/cvs login
$ cvs -z3 -d:pserver:anonymous@uclibc.org:/var/cvs co buildroot
</pre>
<h2><a name="using" id="using"></a>Using Buildroot</h2>
<p>Buildroot has a nice configuration tool similar to the one you can find
in the Linux Kernel (<a href=
"http://www.kernel.org/">http://www.kernel.org/</a>) or in Busybox
(<a href="http://www.busybox.org/">http://www.busybox.org/</a>). Note that
you can run everything as a normal user. There is no need to be root to
configure and use Buildroot. The first step is to run the configuration
assistant:</p>
<pre>
$ make menuconfig
</pre>
<p>For each entry of the configuration tool, you can find associated help
that describes the purpose of the entry.</p>
<p>Once everything is configured, the configuration tool has generated a
<code>.config</code> file that contains the description of your
configuration. It will be used by the Makefiles to do what's needed.</p>
<p>Let's go:</p>
<pre>
$ make
</pre>
<p>This command will download, configure and compile all the selected
tools, and finally generate a target filesystem. The target filesystem will
be named <code>root_fs_ARCH.EXT</code> where <code>ARCH</code> is your
architecture and <code>EXT</code> depends on the type of target filesystem
selected in the <code>Target options</code> section of the configuration
tool.</p>
<h2><a name="custom_targetfs" id="custom_targetfs"></a>Customizing the
target filesystem</h2>
<p>There are two ways to customize the resulting target filesystem:</p>
<ul>
<li>Customize the target filesystem directly, and rebuild the image. The
target filesystem is available under <code>build_ARCH/root/</code> where
<code>ARCH</code> is the chosen target architecture. You can simply make
your changes here, and run make afterwards, which will rebuild the target
filesystem image. This method allows to do everything on the target
filesystem, but if you decide to completely rebuild your toolchain and
tools, these changes will be lost.</li>
<li>Customize the target filesystem skeleton, available under
<code>target/default/target_skeleton/</code>. You can customize
configuration files or other stuff here. However, the full file hierarchy
is not yet present, because it's created during the compilation process.
So you can't do everything on this target filesystem skeleton, but
changes to it remains even you completely rebuild the cross-compilation
toolchain and the tools.<br />
You can also customize the <code>target/default/device_table.txt</code>
file which is used by the tools that generate the target filesystem image
to properly set permissions and create device nodes. The
<code>target/default/skel.tar.gz</code> file contains the main
directories of a root filesystem and there is no obvious reason for which
it should be changed. These main directories are in an tarball inside of
inside the skeleton because it contains symlinks that would be broken
otherwise.</li>
</ul>
<h2><a name="custom_busybox" id="custom_busybox"></a>Customizing the
Busybox configuration</h2>
<p>Busybox is very configurable, and you may want to customize it. You can
follow these simple steps to do it. It's not an optimal way, but it's
simple and it works.</p>
<ol>
<li>Make a first compilation of buildroot with busybox without trying to
customize it.</li>
<li>Go into <code>build_ARCH/busybox/</code> and run <code>make
menuconfig</code>. The nice configuration tool appears and you can
customize everything.</li>
<li>Copy the <code>.config</code> file to
<code>package/busybox/busybox.config</code> so that your customized
configuration will remains even if you remove the cross-compilation
toolchain.</li>
<li>Run the compilation of buildroot again.</li>
</ol>
<p>Otherwise, you can simply change the
<code>package/busybox/busybox.config</code> file if you know the options
you want to change without using the configuration tool.</p>
<h2><a name="custom_uclibc" id="custom_uclibc"></a>Customizing the uClibc
configuration</h2>
<p>Just like <a href="#custom_busybox">BusyBox</a>, <a
href="http://www.uclibc.org">uClibc</a> offers a lot of
configuration options. They allow to select various
functionalities, depending on your needs and limitations.</p>
<p>The easiest way to modify the configuration of uClibc is to
follow these steps :</p>
<ol>
<li>Make a first compilation of buildroot without trying to
customize uClibc.</li>
<li>Go into the directory
<code>toolchain_build_ARCH/uClibc/</code> and run <code>make
menuconfig</code>. The nice configuration assistant, similar to
the one used in the Linux Kernel or in Buildroot appears. Make
your configuration as appropriate.</li>
<li>Copy the <code>.config</code> file to
<code>toolchain/uClibc/uClibc.config</code> or
<code>toolchain/uClibc/uClibc.config-locale</code>. The former
is used if you haven't selected locale support in Buildroot
configuration, and the latter is used if you have selected
locale support.
<li>Run the compilation of Buildroot again</li>
</ol>
<p>Otherwise, you can simply change
<code>toolchain/uClibc/uClibc.config</code> or
<code>toolchain/uClibc/uClibc.config-locale</code> without running
the configuration assistant.</p>
<h2><a name="buildroot_innards" id="buildroot_innards"></a>How Buildroot
works</h2>
<p>As said above, Buildroot is basically a set of Makefiles that download,
configure and compiles software with the correct options. It also includes
some patches for various softwares, mainly the ones involved in the
cross-compilation tool chain (<code>gcc</code>, <code>binutils</code> and
uClibc).</p>
<p>There is basically one Makefile per software, and they are named with
the <code>.mk</code> extension. Makefiles are split into three
sections:</p>
<ul>
<li><b>package</b> (in the <code>package/</code> directory) contains the
Makefiles and associated files for all user-space tools that Buildroot
can compile and add to the target root filesystem. There is one
sub-directory per tool.</li>
<li><b>toolchain</b> (in the <code>toolchain/</code> directory) contains
the Makefiles and associated files for all softwares related to the
cross-compilation toolchain : <code>binutils</code>, <code>ccache</code>,
<code>gcc</code>, <code>gdb</code>, <code>kernel-headers</code> and
<code>uClibc</code>.</li>
<li><b>target</b> (in the <code>target</code> directory) contains the
Makefiles and associated files for softwares related to the generation of
the target root filesystem image. Four types of filesystems are supported
: ext2, jffs2, cramfs and squashfs. For each of them, there's a
sub-directory with the required files. There is also a
<code>default/</code> directory that contains the target filesystem
skeleton.</li>
</ul>
<p>Each directory contains at least 3 files :</p>
<ul>
<li><code>something.mk</code> is the Makefile that downloads, configures,
compiles and installs the software <code>something</code>.</li>
<li><code>Config.in</code> is a part of the configuration tool
description file. It describes the option related to the current
software.</li>
<li><code>Makefile.in</code> is a part of Makefile that sets various
variables according to the configuration given through the configuration
tool. For most tools it simply involves adding the name of the tool to
the <code>TARGETS</code> variable.</li>
</ul>
<p>The main Makefile do the job through the following steps (once the
configuration is done):</p>
<ol>
<li>Create the download directory (<code>dl/</code> by default). This is
where the tarballs will be downloaded. It is interesting to know that the
tarballs are in this directory because it may be useful to save them
somewhere to avoid further downloads.</li>
<li>Create the build directory (<code>build_ARCH/</code> by default,
where <code>ARCH</code> is your architecture). This is where all
user-space tools while be compiled.</li>
<li>Create the toolchain build directory
(<code>toolchain_build_ARCH/</code> by default, where <code>ARCH</code>
is your architecture). This is where the cross compilation toolchain will
be compiled.</li>
<li>Setup the staging directory (<code>build_ARCH/staging_dir/</code> by
default). This is where the cross-compilation toolchain will be
installed. If you want to use the same cross-compilation toolchain for
other purposes, such as compiling third-party applications, you can add
<code>build_ARCH/staging_dir/bin</code> to your PATH, and then use
<code>arch-linux-gcc</code> to compile your application. In order to
setup this staging directory, it first removes it, and then it creates
various subdirectories and symlinks inside it.</li>
<li>Create the target directory (<code>build_ARCH/root/</code> by
default) and the target filesystem skeleton. This directory will contain
the final root filesystem. To setup it up, it first deletes it, then it
uncompress the <code>target/default/skel.tar.gz</code> file to create the
main subdirectories and symlinks, copies the skeleton available in
<code>target/default/target_skeleton</code> and then removes useless
<code>CVS/</code> directories.</li>
<li>Make the <code>TARGETS</code> dependency. This is where all the job
is done : all <code>Makefile.in</code> files "subscribe" targets into
this global variable, so that the needed tools gets compiled.</li>
</ol>
<h2><a name="toolchain_standalone" id="toolchain_standalone"></a>Using the
uClibc toolchain without buildroot</h2>
<p>By default, the cross-compilation toolchain is generated inside
<code>build_ARCH/staging_dir/</code>. But sometimes, it may be useful to
install it somewhere else, so that it can be used to compile other programs
or by other users. Moving the <code>build_ARCH/staging_dir/</code>
directory elsewhere is <b>not possible</b>, because they are some hardcoded
paths in the toolchain configuration.</p>
<p>If you want to use the generated toolchain for other purposes,
you can configure Buildroot to generate it elsewhere using the
option of the configuration tool : <code>Build options ->
Toolchain and header file location</code>, which defaults to
<code>$(BUILD_DIR)/staging_dir/</code>.</p>
<h2><a name="downloaded_packages"
id="downloaded_packages"></a>Location of downloaded packages</h2>
<p>It might be useful to know that the various tarballs that are
downloaded by the <i>Makefiles</i> are all stored in the
<code>DL_DIR</code> which by default is the <code>dl</code>
directory. It's useful for example if you want to keep a complete
version of Buildroot which is know to be working with the
associated tarballs. This will allow you to regenerate the
toolchain and the target filesystem with exactly the same
versions.</p>
<h2><a name="add_software" id="add_software"></a>Extending Buildroot with
more software</h2>
<p>This section will only consider the case in which you want to
add user-space software.</p>
<h3>Package directory</h3>
<p>First of all, create a directory under the <code>package</code>
directory for your software, for example <code>foo</code>.</p>
<h3><code>Config.in</code> file</h3>
<p>Then, create a file named <code>Config.in</code>. This file
will contain the portion of options description related to our
<code>foo</code> software that will be used and displayed in the
configuration tool. It should basically contain :</p>
<pre>
config BR2_PACKAGE_FOO
bool "foo"
default n
help
This is a comment that explains what foo is.
</pre>
<p>Of course, you can add other options to configure particular
things in your software.</p>
<h3><code>Makefile.in</code> file</h3>
<p>Then, write a <code>Makefile.in</code> file. Basically, this is
a very short <i>Makefile</i> that adds the name of the software to
the list of <code>TARGETS</code> that Buildroot will generate. In
fact, the name of the software is the the identifier of the target
inside the real <i>Makefile</i> that will do everything (download,
compile, install), and that we study below. Back to
<code>Makefile.in</code>, here is an example :
<pre>
ifeq ($(strip $(BR2_PACKAGE_FOO)),y)
TARGETS+=foo
endif
</pre>
<p>As you can see, this short <i>Makefile</i> simply adds the
target <code>foo</code> to the list of targets handled by Buildroot
if software <i>foo</i> was selected using the configuration tool.</p>
<h3>The real <i>Makefile</i></h3>
<p>Finally, here's the hardest part. Create a file named
<code>foo.mk</code>. It will contain the <i>Makefile</i> rules that
are in charge of downloading, configuring, compiling and installing
the software. Below is an example that we will comment
afterwards.</p>
<pre>
1 #############################################################
2 #
3 # foo
4 #
5 #############################################################
6 FOO_VERSION:=1.0
7 FOO_SOURCE:=less-$(FOO_VERSION).tar.gz
8 FOO_SITE:=http://www.foosoftware.org/downloads
9 FOO_DIR:=$(BUILD_DIR)/less-$(FOO_VERSION)
10 FOO_BINARY:=foo
11 FOO_TARGET_BINARY:=usr/bin/foo
12
13 $(DL_DIR)/$(FOO_SOURCE):
14 $(WGET) -P $(DL_DIR) $(FOO_SITE)/$(FOO_SOURCE)
15
16 $(FOO_DIR)/.source: $(DL_DIR)/$(FOO_SOURCE)
17 zcat $(DL_DIR)/$(FOO_SOURCE) | tar -C $(BUILD_DIR) $(TAR_OPTIONS) -
18 touch $(FOO_DIR)/.source
19
20 $(FOO_DIR)/.configured: $(FOO_DIR)/.source
21 (cd $(FOO_DIR); \
22 $(TARGET_CONFIGURE_OPTS) \
23 CFLAGS="$(TARGET_CFLAGS)" \
24 ./configure \
25 --target=$(GNU_TARGET_NAME) \
26 --host=$(GNU_TARGET_NAME) \
27 --build=$(GNU_HOST_NAME) \
28 --prefix=/usr \
29 --sysconfdir=/etc \
30 );
31 touch $(FOO_DIR)/.configured;
32
33 $(FOO_DIR)/$(FOO_BINARY): $(FOO_DIR)/.configured
34 $(MAKE) CC=$(TARGET_CC) -C $(FOO_DIR)
35
36 $(TARGET_DIR)/$(FOO_TARGET_BINARY): $(FOO_DIR)/$(FOO_BINARY)
37 $(MAKE) prefix=$(TARGET_DIR)/usr -C $(FOO_DIR) install
38 rm -Rf $(TARGET_DIR)/usr/man
39
40 foo: uclibc ncurses $(TARGET_DIR)/$(FOO_TARGET_BINARY)
41
42 foo-source: $(DL_DIR)/$(FOO_SOURCE)
43
44 foo-clean:
45 $(MAKE) prefix=$(TARGET_DIR)/usr -C $(FOO_DIR) uninstall
46 -$(MAKE) -C $(FOO_DIR) clean
47
48 foo-dirclean:
49 rm -rf $(FOO_DIR)
50
</pre>
<p>First of all, this <i>Makefile</i> example works for a single
binary software. For other softwares such as libraries or more
complex stuff with multiple binaries, it should be adapted. Look at
the other <code>*.mk</code> files in the <code>package</code>
directory.</p>
<p>At lines 6-11, a couple of useful variables are defined :</p>
<ul>
<li><code>FOO_VERSION</code> : The version of <i>foo</i> that
should be downloaded.</li>
<li><code>FOO_SOURCE</code> : The name of the tarball of
<i>foo</i> on the download website of FTP site. As you can see
<code>FOO_VERSION</code> is used.</li>
<li><code>FOO_SITE</code> : The HTTP or FTP site from which
<i>foo</i> archive is downloaded. It must include the complete
path to the directory where <code>FOO_SOURCE</code> can be
found.</li>
<li><code>FOO_DIR</code> : The directory into which the software
will be configured and compiled. Basically, it's a subdirectory
of <code>BUILD_DIR</code> which is created upon decompression of
the tarball.</li>
<li><code>FOO_BINARY</code> : Software binary name. As said
previously, this is an example for a single binary software.</li>
<li><code>FOO_TARGET_BINARY</code> : The full path of the binary
inside the target filesystem.</li>
</ul>
<p>Lines 13-14 defines a target that downloads the tarball from
the remote site to the download directory
(<code>DL_DIR</code>).</p>
<p>Lines 16-18 defines a target and associated rules that
uncompress the downloaded tarball. As you can see, this target
depends on the tarball file, so that the previous target (line
13-14) is called before executing the rules of the current
target. Uncompressing is followed by <i>touching</i> a hidden file
to mark the software has having been uncompressed. This trick is
used everywhere in Buildroot <i>Makefile</i> to split steps
(download, uncompress, configure, compile, install) while still
having correct dependencies.</p>
<p>Lines 20-31 defines a target and associated rules that
configures the software. It depends on the previous target (the
hidden <code>.source</code> file) so that we are sure the software has
been uncompressed. In order to configure it, it basically runs the
well-known <code>./configure</code>script. As we may be doing
cross-compilation, <code>target</code>, <code>host</code> and
<code>build</code> arguments are given. The prefix is also set to
<code>/usr</code>, not because the software will be installed in
<code>/usr</code> on your host system, but in the target
filesystem. Finally it creates a <code>.configured</code> file to
mark the software as configured.</p>
<p>Lines 33-34 defines a target and a rule that compiles the
software. This target will create the binary file in the
compilation directory, and depends on the software being already
configured (hence the reference to the <code>.configured</code>
file). It basically runs <code>make</code> inside the source
directory.</p>
<p>Lines 36-38 defines a target and associated rules that install
the software inside the target filesystem. It depends on the
binary file in the source directory, to make sure the software has
been compiled. It uses the <code>install</code> target of the
software <code>Makefile</code> by passing a <code>prefix</code>
argument, so that the <code>Makefile</code> doesn't try to install
the software inside host <code>/usr</code> but inside target
<code>/usr</code>. After the installation, the
<code>/usr/man</code> directory inside the target filesystem is
removed to save space.</p>
<p>Line 40 defines the main target of the software, the one
referenced in the <code>Makefile.in</code> file. This targets
should first of all depends on the dependecies of the software (in
our example, <i>uclibc</i> and <i>ncurses</i>), and then to the
final binary. This last dependency will call all previous
dependencies in the right order. </p>
<p>Line 42 defines a simple target that only downloads the code
source. This is not used during normal operation of Buildroot, but
might be useful.</p>
<p>Lignes 44-46 define a simple target to clean the software build
by calling the <i>Makefiles</i> with the appropriate option.</p>
<p>Lines 48-49 define a simple target to completely remove the
directory in which the software was uncompressed, configured and
compiled.</p>
<h3>Conclusion</h3>
<p>As you can see, adding a software to buildroot is simply a
matter of writing a <i>Makefile</i> using an already existing
example and to modify it according to the compilation process of
the software.</p>
<p>If you package software that might be useful for other persons,
don't forget to send a patch to Buildroot developers !</p>
<h2><a name="links" id="links"></a>Ressources</h2>
<p>To learn more about Buildroot you can visit these
websites:</p>
<ul>
<li><a href="http://www.uclibc.org/">http://www.uclibc.org/</a></li>
<li><a href="http://www.busybox.net/">http://www.busybox.net/</a></li>
</ul>
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