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 allows you to easily generate a cross-compilation toolchain, a root filesystem and a Linux kernel image for your target. Buildroot can be used for 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 having in his PC. They can be PowerPC processors, MIPS processors, ARM processors, etc.

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, uClibc or dietlibc). 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". 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.

Even if your embedded system uses an x86 processor, you might be 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 other tools by hand. Of course doing so is possible but, dealing with all of the configure options and problems of every gcc or binutils version is 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 kernel, cross-toolchain, and 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). 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 (and should) 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

or

 $ make gconfig

to run the Qt3 or GTK-based configurators.

All of these "make" commands will need to build a configuration utility, so you may need to install "development" packages for relevant libraries used by the configuration utilities. On Debian-like systems, the libncurses5-dev package is required to use the menuconfig interface, libqt3-mt-dev is required to use the xconfig interface, and libglib2.0-dev, libgtk2.0-dev and libglade2-dev are needed to use the gconfig interface.

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

Once everything is configured, the configuration tool generates 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

You should never use make -jN with Buildroot: it does not support top-level parallel make. Instead, use the BR2_JLEVEL option to tell Buildroot to run each package compilation with 

make
    -jN
.

This command will generally perform the following steps:

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, xconfig or gconfig), 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:

 $ 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.

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>

Environment variables

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

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

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 so. This method isn't optimal, but it's simple, and it works:

  1. Do an initial 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 you 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. Do an initial 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 Buildroot, appears. Make your configuration changes 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 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 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, implementing clean package removal is on the TODO-list of Buildroot developers.

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.

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 remain to be done, Buildroot maintains stamp files (empty files that just tell whether this or that action has been done). The problem is that these stamp files are not uniformly named and handled by the different packages, so some understanding of the particular package is needed.

For packages relying on Buildroot packages infrastructures (see this section for details), the following stamp files are relevant:

For other packages, an analysis of the specific package.mk file is needed. For example, the zlib Makefile used to look like this (before it was converted to the generic package infrastructure):

$(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 $@

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

Note that most packages, if not all, will progressively be ported over to the generic or autotools infrastructure, making it much easier to rebuild individual packages.

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 tool chain (gcc, binutils and uClibc).

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

Each directory contains at least 2 files:

The main Makefile performs 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, 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.
  3. 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.

Creating your own board support

Creating your own board support in Buildroot allows you to have a convenient place to store your project's target filesystem skeleton and configuration files for Buildroot, Busybox, uClibc, and the kernel.

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 of your company's projects 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, you should 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. Create the target/device/yourcompany/project-foobar/Makefile.in file. It is recommended that you 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 more 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 skeleton is stored in the board specific directory.
  7. In the target/device/yourcompany/project-foobar/ directory you can store configuration files for the kernel, 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, 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/staging/. The simplest way to use it is to add output/staging/usr/bin/ to your PATH environment variable and then to use ARCH-linux-gcc, ARCH-linux-objdump, ARCH-linux-ld, etc.

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.

It is also possible to generate the Buildroot toolchain in a directory other than output/staging by using the Build options -> Toolchain and header file location options. 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 known 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:

 $ 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

Using an already existing toolchain is useful for different reasons:

Buildroot supports using existing toolchains through a mechanism called external toolchain.

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

Our external toolchain support has been tested with toolchains from CodeSourcery, toolchains generated by Crosstool-NG, 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.

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.

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: multimedia, java, x11r7, and games. If your package fits in one of these categories, then create your package directory in these.

Config.in file

Then, 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.

	  http://foosoftware.org/libfoo/

Of course, you can add other options 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://lxr.free-electrons.com/source/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"

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:

Makefile for generic packages : tutorial

01: #############################################################
02: #
03: # libfoo
04: #
05: #############################################################
06: LIBFOO_VERSION = 1.0
07: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
08: LIBFOO_SITE = http://www.foosoftware.org/download
09: LIBFOO_INSTALL_STAGING = YES
10: LIBFOO_DEPENDENCIES = host-libaaa libbbb
11:
12: define LIBFOO_BUILD_CMDS
13: 	$(MAKE) CC=$(TARGET_CC) LD=$(TARGET_LD) -C $(@D) all
14: endef
15:
16: define LIBFOO_INSTALL_STAGING_CMDS
17: 	$(INSTALL) -D -m 0755 $(@D)/libfoo.a $(STAGING_DIR)/usr/lib/libfoo.a
18: 	$(INSTALL) -D -m 0644 $(@D)/foo.h $(STAGING_DIR)/usr/include/foo.h
19: 	$(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(STAGING_DIR)/usr/lib
20: endef
21:
22: define LIBFOO_INSTALL_TARGET_CMDS
23: 	$(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(TARGET_DIR)/usr/lib
24: 	$(INSTALL) -d -m 0755 $(TARGET_DIR)/etc/foo.d
25: endef
26:
27: $(eval $(call GENTARGETS,package,libfoo))

The Makefile begins on line 6 to 8 with metadata information: the version of the package (LIBFOO_VERSION), the name of the tarball containing the package (LIBFOO_SOURCE) and the Internet location at which the tarball can be downloaded (LIBFOO_SITE). 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 9, 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 10, 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 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.

Finally, on line 27, we call the GENTARGETS which generates, according to the variables defined previously, all the Makefile code necessary to make your package working.

Makefile for generic packages : reference

The GENTARGETS macro takes three arguments:

For a given package, in a single .mk file, it is possible to call GENTARGETS twice, once to create the rules to generate a target package and once to create the rules to generate a host package: 

$(eval $(call GENTARGETS,package,libfoo))
$(eval $(call GENTARGETS,package,libfoo,host))

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 GENTARGETS macro must be at the end of the .mk file, after all variable definitions.

For the target package, the GENTARGETS uses the variables defined by the .mk file and prefixed by the uppercased package name: LIBFOO_*. For the host package, it uses the HOST_LIBFOO_*. 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) :

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.

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:

The last feature of the generic infrastructure is the ability to add 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 hooks are more useful for packages using the autotools infrastructure described below. However, since they are provided by the generic infrastructure, they are documented here. The exception is LIBFOO_POST_PATCH_HOOKS. Patching the package is not user definable, so LIBFOO_POST_PATCH_HOOKS will be userful for generic packages.

The following hook points are available:

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

Makefile for autotools-based packages : 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: LIBFOO_VERSION = 1.0
07: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
08: LIBFOO_SITE = http://www.foosoftware.org/download
09: LIBFOO_INSTALL_STAGING = YES
10: LIBFOO_INSTALL_TARGET = YES
11: LIBFOO_CONF_OPT = --enable-shared
12: LIBFOO_DEPENDENCIES = libglib2 host-pkg-config
13:
14: $(eval $(call AUTOTARGETS,package,libfoo))

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 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 10, we tell Buildroot to also install the package to the target directory. This directory contains what will become the root filesystem running on the target. Usually, we try not to install header files and to install stripped versions of the binary. By default, target installation is enabled, so in fact, this line is not strictly necessary. Also by default, packages are installed in this location using the make install 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 14, we invoke the AUTOTARGETS macro that generates all the Makefile rules that actually allows the package to be built.

Makefile for autotools packages : reference

The main macro of the autotools package infrastructure is AUTOTARGETS. It has the same number of arguments and the same semantic as the GENTARGETS macro, which is the main macro of the generic package infrastructure. For autotools packages, the ability to have target and host packages is also available (and is actually widely used).

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

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:

Manual Makefile : tutorial

NOTE: new manual makefiles should not be created, and existing manual makefiles should be converted either to the generic infrastructure or the autotools infrastructure. This section is only kept to document the existing manual makefiles and to help understand how they work.

01: #############################################################
02: #
03: # libfoo
04: #
05: #############################################################
06: LIBFOO_VERSION:=1.0
07: LIBFOO_SOURCE:=libfoo-$(LIBFOO_VERSION).tar.gz
08: LIBFOO_SITE:=http://www.foosoftware.org/downloads
09: LIBFOO_DIR:=$(BUILD_DIR)/foo-$(FOO_VERSION)
10: LIBFOO_BINARY:=foo
11: LIBFOO_TARGET_BINARY:=usr/bin/foo
12:
13: $(DL_DIR)/$(LIBFOO_SOURCE):
14: 	$(call DOWNLOAD,$(LIBFOO_SITE),$(LIBFOO_SOURCE))
15:
16: $(LIBFOO_DIR)/.source: $(DL_DIR)/$(LIBFOO_SOURCE)
17: 	$(ZCAT) $(DL_DIR)/$(LIBFOO_SOURCE) | tar -C $(BUILD_DIR) $(TAR_OPTIONS) -
18: 	touch $@
19:
20: $(LIBFOO_DIR)/.configured: $(LIBFOO_DIR)/.source
21: 	(cd $(LIBFOO_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: $(LIBFOO_DIR)/$(LIBFOO_BINARY): $(LIBFOO_DIR)/.configured
34: 	$(MAKE) CC=$(TARGET_CC) -C $(LIBFOO_DIR)
35:
36: $(TARGET_DIR)/$(LIBFOO_TARGET_BINARY): $(LIBFOO_DIR)/$(LIBFOO_BINARY)
37: 	$(MAKE) DESTDIR=$(TARGET_DIR) -C $(LIBFOO_DIR) install-strip
38: 	rm -Rf $(TARGET_DIR)/usr/man
39:
40: libfoo: uclibc ncurses $(TARGET_DIR)/$(LIBFOO_TARGET_BINARY)
41:
42: libfoo-source: $(DL_DIR)/$(LIBFOO_SOURCE)
43:
44: libfoo-clean:
45: 	$(MAKE) prefix=$(TARGET_DIR)/usr -C $(LIBFOO_DIR) uninstall
46: 	-$(MAKE) -C $(LIBFOO_DIR) clean
47:
48: libfoo-dirclean:
49: 	rm -rf $(LIBFOO_DIR)
50:
51: #############################################################
52: #
53: # Toplevel Makefile options
54: #
55: #############################################################
56: ifeq ($(BR2_PACKAGE_LIBFOO),y)
57: TARGETS+=libfoo
58: endif

First of all, this Makefile example works for a package which comprises a single binary executable. For other software, such as libraries or more complex stuff with multiple binaries, it must be adapted. For examples look at the other *.mk files in the package directory.

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

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

Lines 16-18 define 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 (lines 13-14) is called before executing the rules of the current target. Uncompressing is followed by touching a hidden file to mark the software as having been uncompressed. This trick is used everywhere in a Buildroot Makefile to split steps (download, uncompress, configure, compile, install) while still having correct dependencies.

Lines 20-31 define a target and associated rules that configure 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 the package, 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 because the software will be installed in /usr on the target filesystem. Finally it creates a .configured file to mark the software as configured.

Lines 33-34 define a target and a rule that compile 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 define a target and associated rules that install the software inside the target filesystem. They depend on the binary file in the source directory to make sure the software has been compiled. They use the install-strip target of the software Makefile by passing a DESTDIR argument so that the Makefile doesn't try to install the software in the host /usr but rather in the 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 eventually be used by the top level Makefile to download, compile, and then install this package. This target should first of all depend on all needed dependencies 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 libfoo-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 Makefile with the appropriate options. 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 add the target libfoo 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. If so, it then "subscribes" this package to be compiled by adding the package 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.

Gettext integration and interaction with packages

Many packages that support internationalization use the gettext library. Dependencies for this library are fairly complicated and therefore, deserves some explanation.

The uClibc C library doesn't implement gettext functionality, therefore with this C library, a separate gettext must be compiled. On the other hand, the glibc C library does integrate its own gettext, and in this case, the separate gettext library should not be compiled, because it creates various kinds of build failures.

Additionally, some packages (such as libglib2) do require gettext unconditionally, while other packages (those who support --disable-nls in general) only require gettext when locale support is enabled.

Therefore, Buildroot defines two configuration options:

Therefore, packages that unconditionally need gettext should:

  1. Use select BR2_PACKAGE_GETTEXT if BR2_NEEDS_GETTEXT and possibly select BR2_PACKAGE_LIBINTL if BR2_NEEDS_GETTEXT, if libintl is also needed
  2. Use $(if $(BR2_NEEDS_GETTEXT),gettext) in the package DEPENDENCIES variable

Packages that need gettext only when locale support is enabled should:

  1. Use select BR2_PACKAGE_GETTEXT if BR2_NEEDS_GETTEXT_IF_LOCALE and possibly select BR2_PACKAGE_LIBINTL if BR2_NEEDS_GETTEXT_IF_LOCALE, if libintl is also needed
  2. Use $(if $(BR2_NEEDS_GETTEXT_IF_LOCALE),gettext) in the package DEPENDENCIES variable

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 Buildroot developers!

Frequently asked questions

The boot hangs after Starting network...

If the boot process seems to hang after the following messages (messages not necessarly 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 in the Buildroot configuration, Target options, enable Generic serial port config, and select the serial port and speed you would like to use for the shell. This will automatically tune the /etc/inittab file of the generated system so that a shell starts on the correct serial port.

To learn more about Buildroot you can visit these websites: