Buildroot

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

Last modification : $Id: buildroot-documentation.html,v 1.2 2004/12/28 19:15:20 andersen Exp $

• About Buildroot
• Obtaining Buildroot
• Using Buildroot
• Customizing the target filesystem
• Customizing the Busybox configuration
• Customizing the uClibc configuration
• How Buildroot works
• Using the uClibc toolchain
• Using the uClibc toolchain outside of Buildroot
• Location of downloaded packages
• Extending Buildroot with more Software
• Ressources

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:

• 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.
• Buildroot automates the building of a root filesystem with all needed tools like busybox. It makes it much easier than doing it by hand.

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 SVN snapshots or directly using SVN.

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

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

 $ svn co svn://uclibc.org/trunk/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:

• Customize the target filesystem directly, and rebuild the image. The target filesystem is available under build_ARCH/root/ where ARCH 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.
• Customize the target filesystem skeleton, available under target/generic/target_skeleton/. 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 remain even if you completely rebuild the cross-compilation toolchain and the tools.
  You can also customize the target/generic/device_table.txt file which is used by the tools that generate the target filesystem image to properly set permissions and create device nodes. The target/generic/skel.tar.gz 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.
  These customizations are deployed into build_ARCH/root/ just before the actual image is made. So simply rebuilding the image by running make should propogate any new changes to the image.

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 software, 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:

• package (in the package/ 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.
• toolchain (in the toolchain/ directory) contains the Makefiles and associated files for all software related to the cross-compilation toolchain : binutils, ccache, gcc, gdb, kernel-headers and uClibc.
• target (in the target directory) contains the Makefiles and associated files for software 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 default/ directory that contains the target filesystem skeleton.

Each directory contains at least 2 files :

• something.mk is the Makefile that downloads, configures, compiles and installs the software something.
• Config.in is a part of the configuration tool description file. It describes the option related to the current software.

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/generic/skel.tar.gz file to create the main subdirectories and symlinks, copies the skeleton available in target/generic/target_skeleton and then removes useless .svn/ directories.
6. Add the TARGETS dependency. This should generally check if the configuration option for this package is enabled, and if so then "subscribe" this package to be compiled by adding it to the TARGETS global variable.

Using the uClibc toolchain

You may want to compile 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 by default is located in build_ARCH/staging_dir/. The simplest way to use it is to add build_ARCH/staging_dir/bin/ to your PATH environnement variable, and then to use arch-linux-gcc, arch-linux-objdump, arch-linux-ld, etc.

For example, you may add the following to your .bashrc (considering you're building for the MIPS architecture and that Buildroot is located in ~/buildroot/) :

export PATH=$PATH:~/buildroot/build_mips/staging_dir/bin/

Then you can simply do :

mips-linux-gcc -o foo foo.c

Important : do not try to move the toolchain to an other directory, it won't work. There are some hard-coded paths in the gcc configuration. If the default toolchain directory doesn't suit your needs, please refer to the Using the uClibc toolchain outside of buildroot section.

Using the uClibc toolchain outside of 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.

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 $(FOO_DIR)
    50
    51 #############################################################
    52 #
    53 # Toplevel Makefile options
    54 #
    55 #############################################################
    56 ifeq ($(strip $(BR2_PACKAGE_FOO)),y)
    57 TARGETS+=foo
    58 endif

First of all, this Makefile example works for a single binary software. For other software 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 :

• FOO_VERSION : The version of foo that should be downloaded.
• FOO_SOURCE : The name of the tarball of foo on the download website of FTP site. As you can see FOO_VERSION is used.
• FOO_SITE : The HTTP or FTP site from which foo archive is downloaded. It must include the complete path to the directory where FOO_SOURCE can be found.
• FOO_DIR : The directory into which the software will be configured and compiled. Basically, it's a subdirectory of BUILD_DIR which is created upon decompression of the tarball.
• FOO_BINARY : Software binary name. As said previously, this is an example for a single binary software.
• FOO_TARGET_BINARY : The full path of the binary inside the target filesystem.

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 that will be eventually be used by the top level Makefile to download, compile, and then install this package. This target should first of all depends on all needed dependecies of the software (in our example, uclibc and ncurses), and also depend on the final binary. This last dependency will call all previous dependencies in the correct 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.

Lines 51-58 adds the target foo to the list of targets to be compiled by Buildroot by first checking if the configuration option for this package has been enabled using the configuration tool, and if so then "subscribes" this package to be compiled by adding it to the TARGETS global variable. The name added to the TARGETS global variable is the name of this package's target, as defined on line 40, which is used by Buildroot to download, compile, and then install this package.

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:

• http://www.uclibc.org/
• http://www.busybox.net/