Buildroot

Buildroot usage and documentation by Thomas Petazzoni. Contributions from Karsten Kruse, Ned Ludd, Martin Herren and others.

About Buildroot

Buildroot is a set of Makefiles and patches that allow to easily generate a cross-compilation toolchain, a root filesystem and a Linux kernel image for your target. Buildroot can be used for either one, two or all of these options, independently.

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.

Moreover, Buildroot provides an infrastructure for reproducing the build process of your embedded root filesystem. Being able to reproduce the build process will be useful when a component needs to be patched or updated, or when another person is supposed to take over the project.

Obtaining Buildroot

Buildroot releases are made approximately every 3 months. Direct Git access and daily snapshots are also available if you want more bleeding edge.

Releases are available at http://buildroot.net/downloads/.

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

To download Buildroot using Git, you can simply follow the rules described on the "Accessing Git"-page (http://buildroot.net/git.html) of the Buildroot website (http://buildroot.net), and download buildroot from Git. For the impatient, here's a quick recipe:

 $ git clone git://git.buildroot.net/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 build 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

to run the curses-based configurator, or

 $ make xconfig

to run the Qt3-based configurator. On Debian-like systems, the libncurses5-dev package is required to use the menuconfig interface, and the libqt3-mt-dev is required to use the xconfig interface.

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 toolchain, a root filesystem image and a kernel image (or only one of these elements, depending on the configuration).

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

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 or xconfig) then issue:

 $ make source

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

Building out-of-tree

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

 $ make O=/tmp/build

And all the output files will be located under /tmp/build.

Environment variables

Buildroot optionally honors some environment variables that are passed to make :

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

If you want the result of your build to be copied to another directory like /tftpboot for downloading to a board using tftp, then you can use BUILDROOT_COPYTO to specify your location

Typically, this is set in your ~/.bashrc file 

$ export BUILDROOT_COPYTO=/tftpboot

Customizing the generated target filesystem

There are a few 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. Invoke make busybox-menuconfig. The nice configuration tool appears and you can customize everything.
  3. Run the compilation of buildroot again.

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

If you want to use an existing config file for busybox, then see section environment variables.

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. Invoke make uclibc-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.

If you want to use an existing config file for uclibc, then see section environment variables.

Customizing the Linux kernel configuration

The Linux kernel configuration can be customized just like BusyBox and uClibc using make linux26-menuconfig. Make sure you have enabled the kernel build in make menuconfig first. Once done, run make to (re)build everything.

If you want to use an existing config file for Linux, then see section environment variables.

Understanding how to rebuild packages

One of the most common question and issue about Buildroot encountered by users is how to rebuild a given package or how to remove a package without rebuilding everything from scratch.

Removing a package is currently 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. However, implement clean package removal is on the TODO-list of Buildroot developers.

To rebuild a single package from scratch, the easiest way 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.

However, if you don't want to rebuild the package completely from scratch, a better understanding of the Buildroot internals is needed. Internally, to keep track of which steps have been done and which steps remains to be done, Buildroot maintains stamps files (i.e, empty files that just tell whether this or this action has been done). The problem is that these stamps files are not uniformely named and handled by the different packages, so some understanding of the particular package is needed.

For packages relying on the autotools Buildroot infrastructure (see this section for details), the following stamps files are interesting:

For other packages, an analysis of the specific package.mk file is needed. For example, the zlib Makefile looks like:

$(ZLIB_DIR)/.configured: $(ZLIB_DIR)/.patched
        (cd $(ZLIB_DIR); rm -rf config.cache; \
                        [...]
        )
        touch $@

$(ZLIB_DIR)/libz.a: $(ZLIB_DIR)/.configured
        $(MAKE) -C $(ZLIB_DIR) all libz.a
        touch -c $@

So, if you want to trigger the reconfiguration, you need to remove output/build/zlib-version/.configured and if you want to trigger only the recompilation, you need to remove output/build/zlib-version/libz.a.

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 four sections:

Each directory contains at least 2 files :

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

  1. Create all the output directories: staging, target, build, stamps, etc. in the output directory (output/ by default, another value can be specified using O=)
  2. Generate all the targets listed in the BASE_TARGETS variable. When an internal toolchain is used, it means generating the cross-compilation toolchain. When an external toolchain is used, it means checking the features of the external toolchain and importing it into the Buildroot environment.
  3. Generate all the targets listed in the TARGETS variable. This variable is filled by all the individual components Makefiles. So, generating all 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.

Creating your own board support

Creating your own board support in Buildroot allows you to have a convenient place to store the Busybox, uClibc, kernel configurations, your target filesystem skeleton, and a Buildroot configuration that match your project.

Follow these steps to integrate your board in Buildroot:

  1. Create a new directory in target/device/, named after your company or organization
  2. Add a line source "target/device/yourcompany/Config.in" in target/device/Config.in so that your board appears in the configuration system
  3. In target/device/yourcompany/, create a directory for your project. This way, you'll be able to store several projects of your company/organization inside Buildroot.
  4. Create a target/device/yourcompany/Config.in file that looks like the following: 
    menuconfig BR2_TARGET_COMPANY
       bool "Company projects"

if BR2_TARGET_COMPANY

config BR2_TARGET_COMPANY_PROJECT_FOOBAR
       bool "Support for Company project Foobar"
       help
         This option enables support for Company project Foobar

endif
    Of course, customize the different values to match your company/organization and your project. This file will create a menu entry that contains the different projects of your company/organization.
  5. Create a target/device/yourcompany/Makefile.in file that looks like the following: 
    ifeq ($(BR2_TARGET_COMPANY_PROJECT_FOOBAR),y)
include target/device/yourcompany/project-foobar/Makefile.in
endif
  6. Now, create the target/device/yourcompany/project-foobar/Makefile.in file. It is first recommended to define a BOARD_PATH variable set to target/device/yourcompany/project-foobar, as it will simplify further definitions. Then, the file might define one or several of the following variables:
    • TARGET_SKELETON to a directory that contains the target skeleton for your project. If this variable is defined, this target skeleton will be used instead of the default one. If defined, the convention is to define it to $(BOARD_PATH)/target_skeleton, so that the target skeletonn is stored in the board specific directory.
    • TARGET_DEVICE_TABLE to a file that contains the target device table, i.e the list of device files (in /dev/) created by the root filesystem building procedure. If this variable is defined, the given device table will be used instead of the default one. If defined, the convention is to define it to $(BOARD_PATH)/target_device_table.txt. See target/generic/device_table.txt for an example file.
  7. Then, in the target/device/yourcompany/project-foobar/ directory, you can store configuration files for the kernel, for Busybox or uClibc. You can furthermore create one or more preconfigured configuration files, referencing those files. These config files are named something_defconfig and are stored in the toplevel configs/ directory. Your users will then be able to run make something_defconfig and get the right configuration for your project

Using the generated toolchain outside Buildroot

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 output/staging/. The simplest way to use it is to add output/staging/usr/bin/ to your PATH environnement variable, and then to use ARCH-linux-gcc, ARCH-linux-objdump, ARCH-linux-ld, etc.

The easiest way is of course to add the output/staging/usr/bin/ directory to your PATH environment variable.

Important : do not try to move a gcc-3.x toolchain to an other directory, it won't work. There are some hardcoded paths in the gcc configuration. If you are using a current gcc-4.x, it is possible to relocate the toolchain, but then --sysroot must be passed every time the compiler is called to tell where the libraries and header files are, which might be cumbersome.

It is also possible to generate the Buildroot toolchain in another directory than output/staging using the Build options -> Toolchain and header file location option. This could be useful if the toolchain must be shared with other users.

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.

If you maintain several buildroot trees, it might be better to have a shared download location. This can be accessed by creating a symbolic link from the dl directory to the shared download location.

I.E:

ln -s <shared download location> dl

Another way of accessing a shared download location is to create the BUILDROOT_DL_DIR environment variable. If this is set, then the value of DL_DIR in the project is overridden. The following line should be added to "~/.bashrc". 

export BUILDROOT_DL_DIR <shared download location>

Using an external toolchain

It might be useful not to use the toolchain generated by Buildroot, for example if you already have a toolchain that is known to work for your specific CPU, or if the toolchain generation feature of Buildroot is not sufficiently flexible for you (for example if you need to generate a system with glibc instead of uClibc). Buildroot supports using an external toolchain.

To enable the use of an external toolchain, go in the Toolchain menu, and :

If you are using an external toolchain based on uClibc, the Core C library from the external toolchain and Libraries to copy from the external toolchain options should already have correct values. However, if your external toolchain is based on glibc, you'll have to change these values according to your cross-compiling toolchain.

To generate external toolchains, we recommend using Crosstool-NG. It allows to generate toolchains based on uClibc, glibc and eglibc for a wide range of architectures, and has good community support.

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"
        help
	  This is a comment that explains what foo is.

	  http://foosoftware.org/foo/

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

Finally you have to add your new foo/Config.in to package/Config.in. 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/procps/Config.in"

Note:
Generally all packages should live directly in the package directory to make it easier to find them.

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.

Two types of Makefiles can be written :

First, let's see how to write a Makefile for an autotools-based package, with an example :

     1  #############################################################
     2  #
     3  # foo
     4  #
     5  #############################################################
     6  FOO_VERSION:=1.0
     7  FOO_SOURCE:=foo-$(FOO_VERSION).tar.gz
     8  FOO_SITE:=http://www.foosoftware.org/downloads
     9  FOO_INSTALL_STAGING = YES
     10  FOO_INSTALL_TARGET = YES
     11  FOO_CONF_OPT =  --enable-shared
     12  FOO_DEPENDENCIES = libglib2 host-pkgconfig
     13  $(eval $(call AUTOTARGETS,package,foo))

On line 6, we declare the version of the package. On line 7 and 8, we declare the name of the tarball and the location of the tarball on the Web. Buildroot will automatically download the tarball from this location.

On line 9, we tell Buildroot to install the application to the staging directory. The staging directory, located in output/staging/ is the directory where all the packages are installed, including their documentation, etc. By default, packages are installed in this location using the make install command.

On line 10, we tell Buildroot to also install the application to the target directory. This directory contains what will become the root filesystem running on the target. Usually, we try not to install the documentation, and to install stripped versions of the binary. By default, packages are installed in this location using the make install-strip command.

On line 11, 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 12, we declare our dependencies, so that they are built before the build process of our package starts.

Finally, on line line 13, we invoke the package/Makefile.autotools.in magic to get things working.

For more details about the available variables and options, see the comment at the top of package/Makefile.autotools.in and the examples in all the available packages.

The second solution, suitable for every type of package, looks like this :

     1  #############################################################
     2  #
     3  # foo
     4  #
     5  #############################################################
     6  FOO_VERSION:=1.0
     7  FOO_SOURCE:=foo-$(FOO_VERSION).tar.gz
     8  FOO_SITE:=http://www.foosoftware.org/downloads
     9  FOO_DIR:=$(BUILD_DIR)/foo-$(FOO_VERSION)
    10  FOO_BINARY:=foo
    11  FOO_TARGET_BINARY:=usr/bin/foo
    12
    13  $(DL_DIR)/$(FOO_SOURCE):
    14          $(call DOWNLOAD,$(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 $@
    19
    20  $(FOO_DIR)/.configured: $(FOO_DIR)/.source
    21          (cd $(FOO_DIR); rm -rf config.cache; \
    22                  $(TARGET_CONFIGURE_OPTS) \
    23                  $(TARGET_CONFIGURE_ARGS) \
    24                  ./configure \
    25                  --target=$(GNU_TARGET_NAME) \
    26                  --host=$(GNU_TARGET_NAME) \
    27                  --build=$(GNU_HOST_NAME) \
    28                  --prefix=/usr \
    29                  --sysconfdir=/etc \
    30          )
    31          touch $@
    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          $(MAKE) DESTDIR=$(TARGET_DIR) -C $(FOO_DIR) install-strip
    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 $(FOO_DIR)
    50
    51 #############################################################
    52 #
    53 # Toplevel Makefile options
    54 #
    55 #############################################################
    56 ifeq ($(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 :

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 ./configure script. 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-strip target of the software Makefile by passing a DESTDIR 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 is needed if you intend to download all required sources at once for later offline build. Note that if you add a new package providing a foo-source target is mandatory to support users that wish to do offline-builds. Furthermore it eases checking if all package-sources are downloadable.

Lines 44-46 define a simple target to clean the software build by calling the Makefiles with the appropriate option. The -clean target should run make clean on $(BUILD_DIR)/package-version and MUST uninstall all files of the package from $(STAGING_DIR) and from $(TARGET_DIR).

Lines 48-49 define a simple target to completely remove the directory in which the software was uncompressed, configured and compiled. The -dirclean target MUST completely rm $(BUILD_DIR)/ package-version.

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 !

Resources

To learn more about Buildroot you can visit these websites: