Buildroot

Usage and documentation by Thomas Petazzoni. Contributions from Karsten Kruse, Ned Ludd, Martin Herren.

Last modification : $Date: 2004/12/27 19:01:52 $

About Buildroot

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 (http://www.uclibc.org/), a tiny C standard library.

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.

A compilation toolchain is the set of tools that allows 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, uClibc or dietlibc). 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.

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.

Even if your embedded system uses a x86 processor, you might interested in Buildroot, for two reasons:

You might wonder why such a tool is needed when you can compile gcc, binutils, uClibc and all the tools by hand. Of course, doing so is possible. But dealing with all configure options, with all problems of every gcc or binutils version it very time-consuming and uninteresting. Buildroot automates this process through the use of Makefiles, and has a collection of patches for each gcc and binutils version to make them work on most architectures.

Obtaining Buildroot

Buildroot is available as daily CVS snapshots or directly using CVS.

The latest snapshot is always available at http://uclibc.org/downloads/snapshots/buildroot-snapshot.tar.bz2, and previous snapshots are also available at http://uclibc.org/downloads/snapshots/.

To download Buildroot using CVS, you can simply follow the rules described on the "Accessing CVS"-page (http://www.uclibc.org/cvs_anon.html) of the uClibc website (http://www.uclibc.org), and download the buildroot CVS module. For the impatient, here's a quick recipe:

 $ cvs -d:pserver:anonymous@uclibc.org:/var/cvs login
 $ cvs -z3 -d:pserver:anonymous@uclibc.org:/var/cvs co buildroot

Using Buildroot

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.org/). 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:

 $ make menuconfig

For each entry of the configuration tool, you can find associated help that describes the purpose of the entry.

Once everything is configured, the configuration tool has generated a .config file that contains the description of your configuration. It will be used by the Makefiles to do what's needed.

Let's go:

 $ make

This command will download, configure and compile all the selected tools, and finally generate a target filesystem. The target filesystem will be named root_fs_ARCH.EXT where ARCH is your architecture and EXT depends on the type of target filesystem selected in the Target options section of the configuration tool.

Customizing the target filesystem

There are two ways to customize the resulting target filesystem:

Customizing the Busybox configuration

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.

  1. Make a first compilation of buildroot with busybox without trying to customize it.
  2. Go into build_ARCH/busybox/ and run make menuconfig. The nice configuration tool appears and you can customize everything.
  3. Copy the .config file to package/busybox/busybox.config so that your customized configuration will remains even if you remove the cross-compilation toolchain.
  4. Run the compilation of buildroot again.

Otherwise, you can simply change the package/busybox/busybox.config file if you know the options you want to change without using the configuration tool.

Customizing the uClibc configuration

Just like BusyBox, uClibc offers a lot of configuration options. They allow to select various functionalities, depending on your needs and limitations.

The easiest way to modify the configuration of uClibc is to follow these steps :

  1. Make a first compilation of buildroot without trying to customize uClibc.
  2. Go into the directory toolchain_build_ARCH/uClibc/ and run make menuconfig. The nice configuration assistant, similar to the one used in the Linux Kernel or in Buildroot appears. Make your configuration as appropriate.
  3. Copy the .config file to toolchain/uClibc/uClibc.config or toolchain/uClibc/uClibc.config-locale. 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.
  4. Run the compilation of Buildroot again

Otherwise, you can simply change toolchain/uClibc/uClibc.config or toolchain/uClibc/uClibc.config-locale without running the configuration assistant.

How Buildroot works

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 (gcc, binutils and uClibc).

There is basically one Makefile per software, and they are named with the .mk extension. Makefiles are split into three sections:

Each directory contains at least 3 files :

The main Makefile do the job through the following steps (once the configuration is done):

  1. Create the download directory (dl/ 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.
  2. Create the build directory (build_ARCH/ by default, where ARCH is your architecture). This is where all user-space tools while be compiled.
  3. Create the toolchain build directory (toolchain_build_ARCH/ by default, where ARCH is your architecture). This is where the cross compilation toolchain will be compiled.
  4. Setup the staging directory (build_ARCH/staging_dir/ 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 build_ARCH/staging_dir/bin to your PATH, and then use arch-linux-gcc 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.
  5. Create the target directory (build_ARCH/root/ 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 target/default/skel.tar.gz file to create the main subdirectories and symlinks, copies the skeleton available in target/default/target_skeleton and then removes useless CVS/ directories.
  6. Make the TARGETS dependency. This is where all the job is done : all Makefile.in files "subscribe" targets into this global variable, so that the needed tools gets compiled.

Using the uClibc toolchain without buildroot

By default, the cross-compilation toolchain is generated inside build_ARCH/staging_dir/. 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 build_ARCH/staging_dir/ directory elsewhere is not possible, because they are some hardcoded paths in the toolchain configuration.

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 : Build options -> Toolchain and header file location, which defaults to $(BUILD_DIR)/staging_dir/.

Location of downloaded packages

It might be useful to know that the various tarballs that are downloaded by the Makefiles are all stored in the DL_DIR which by default is the dl 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.

Extending Buildroot with more software

This section will only consider the case in which you want to add user-space software.

Package directory

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

Config.in file

Then, create a file named Config.in. This file will contain the portion of options description related to our foo software that will be used and displayed in the configuration tool. It should basically contain :

config BR2_PACKAGE_FOO
        bool "foo"
        default n
        help
	     This is a comment that explains what foo is.

Of course, you can add other options to configure particular things in your software.

Makefile.in file

Then, write a Makefile.in file. Basically, this is a very short Makefile that adds the name of the software to the list of TARGETS that Buildroot will generate. In fact, the name of the software is the the identifier of the target inside the real Makefile that will do everything (download, compile, install), and that we study below. Back to Makefile.in, here is an example : 

ifeq ($(strip $(BR2_PACKAGE_FOO)),y)
TARGETS+=foo
endif

As you can see, this short Makefile simply adds the target foo to the list of targets handled by Buildroot if software foo was selected using the configuration tool.

The real Makefile

Finally, here's the hardest part. Create a file named foo.mk. It will contain the Makefile rules that are in charge of downloading, configuring, compiling and installing the software. Below is an example that we will comment afterwards.

     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

First of all, this Makefile 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 *.mk files in the package directory.

At lines 6-11, a couple of useful variables are defined :

Lines 13-14 defines a target that downloads the tarball from the remote site to the download directory (DL_DIR).

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 touching a hidden file to mark the software has having been uncompressed. This trick is used everywhere in Buildroot Makefile to split steps (download, uncompress, configure, compile, install) while still having correct dependencies.

Lines 20-31 defines a target and associated rules that configures the software. It depends on the previous target (the hidden .source file) so that we are sure the software has been uncompressed. In order to configure it, it basically runs the well-known ./configurescript. As we may be doing cross-compilation, target, host and build arguments are given. The prefix is also set to /usr, not because the software will be installed in /usr on your host system, but in the target filesystem. Finally it creates a .configured file to mark the software as configured.

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 .configured file). It basically runs make inside the source directory.

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 install target of the software Makefile by passing a prefix argument, so that the Makefile doesn't try to install the software inside host /usr but inside target /usr. After the installation, the /usr/man directory inside the target filesystem is removed to save space.

Line 40 defines the main target of the software, the one referenced in the Makefile.in file. This targets should first of all depends on the dependecies of the software (in our example, uclibc and ncurses), and then to the final binary. This last dependency will call all previous dependencies in the right order.

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.

Lignes 44-46 define a simple target to clean the software build by calling the Makefiles with the appropriate option.

Lines 48-49 define a simple target to completely remove the directory in which the software was uncompressed, configured and compiled.

Conclusion

As you can see, adding a software to buildroot is simply a matter of writing a Makefile using an already existing example and to modify it according to the compilation process of the software.

If you package software that might be useful for other persons, don't forget to send a patch to Buildroot developers !

Ressources

To learn more about Buildroot you can visit these websites: