The Buildroot user manual


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Chapter 6. Buildroot configuration

All the configuration options in make *config have a help text providing details about the option.

The make *config commands also offer a search tool. Read the help message in the different frontend menus to know how to use it:

  • in menuconfig, the search tool is called by pressing /;
  • in xconfig, the search tool is called by pressing Ctrl + f.

The result of the search shows the help message of the matching items. In menuconfig, numbers in the left column provide a shortcut to the corresponding entry. Just type this number to directly jump to the entry, or to the containing menu in case the entry is not selectable due to a missing dependency.

Although the menu structure and the help text of the entries should be sufficiently self-explanatory, a number of topics require additional explanation that cannot easily be covered in the help text and are therefore covered in the following sections.

6.1. Cross-compilation toolchain

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-ng).

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" [3].

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.

Buildroot provides two solutions for the cross-compilation toolchain:

  • The internal toolchain backend, called Buildroot toolchain in the configuration interface.
  • The external toolchain backend, called External toolchain in the configuration interface.

The choice between these two solutions is done using the Toolchain Type option in the Toolchain menu. Once one solution has been chosen, a number of configuration options appear, they are detailed in the following sections.

6.1.1. Internal toolchain backend

The internal toolchain backend is the backend where Buildroot builds by itself a cross-compilation toolchain, before building the userspace applications and libraries for your target embedded system.

This backend supports several C libraries: uClibc-ng, glibc and musl.

Once you have selected this backend, a number of options appear. The most important ones allow to:

  • Change the version of the Linux kernel headers used to build the toolchain. This item deserves a few explanations. In the process of building a cross-compilation toolchain, the C library is being built. This library provides the interface between userspace applications and the Linux kernel. In order to know how to "talk" to the Linux kernel, the C library needs to have access to the Linux kernel headers (i.e. the .h files from the kernel), which define the interface between userspace and the kernel (system calls, data structures, etc.). Since this interface is backward compatible, the version of the Linux kernel headers used to build your toolchain do not need to match exactly the version of the Linux kernel you intend to run on your embedded system. They only need to have a version equal or older to the version of the Linux kernel you intend to run. If you use kernel headers that are more recent than the Linux kernel you run on your embedded system, then the C library might be using interfaces that are not provided by your Linux kernel.
  • Change the version of the GCC compiler, binutils and the C library.
  • Select a number of toolchain options (uClibc only): whether the toolchain should have RPC support (used mainly for NFS), wide-char support, locale support (for internationalization), C++ support or thread support. Depending on which options you choose, the number of userspace applications and libraries visible in Buildroot menus will change: many applications and libraries require certain toolchain options to be enabled. Most packages show a comment when a certain toolchain option is required to be able to enable those packages. If needed, you can further refine the uClibc configuration by running make uclibc-menuconfig. Note however that all packages in Buildroot are tested against the default uClibc configuration bundled in Buildroot: if you deviate from this configuration by removing features from uClibc, some packages may no longer build.

It is worth noting that whenever one of those options is modified, then the entire toolchain and system must be rebuilt. See Section 8.2, “Understanding when a full rebuild is necessary”.

Advantages of this backend:

  • Well integrated with Buildroot
  • Fast, only builds what’s necessary

Drawbacks of this backend:

  • Rebuilding the toolchain is needed when doing make clean, which takes time. If you’re trying to reduce your build time, consider using the External toolchain backend.

6.1.2. External toolchain backend

The external toolchain backend allows to use existing pre-built cross-compilation toolchains. Buildroot knows about a number of well-known cross-compilation toolchains (from Linaro for ARM, Sourcery CodeBench for ARM, x86-64, PowerPC, and MIPS, and is capable of downloading them automatically, or it can be pointed to a custom toolchain, either available for download or installed locally.

Then, you have three solutions to use an external toolchain:

  • Use a predefined external toolchain profile, and let Buildroot download, extract and install the toolchain. Buildroot already knows about a few CodeSourcery and Linaro toolchains. Just select the toolchain profile in Toolchain from the available ones. This is definitely the easiest solution.
  • Use a predefined external toolchain profile, but instead of having Buildroot download and extract the toolchain, you can tell Buildroot where your toolchain is already installed on your system. Just select the toolchain profile in Toolchain through the available ones, unselect Download toolchain automatically, and fill the Toolchain path text entry with the path to your cross-compiling toolchain.
  • Use a completely custom external toolchain. This is particularly useful for toolchains generated using crosstool-NG or with Buildroot itself. To do this, select the Custom toolchain solution in the Toolchain list. You need to fill the Toolchain path, Toolchain prefix and External toolchain C library options. Then, you have to tell Buildroot what your external toolchain supports. If your external toolchain uses the glibc library, you only have to tell whether your toolchain supports C++ or not and whether it has built-in RPC support. If your external toolchain uses the uClibc library, then you have to tell Buildroot if it supports RPC, wide-char, locale, program invocation, threads and C++. At the beginning of the execution, Buildroot will tell you if the selected options do not match the toolchain configuration.

Our external toolchain support has been tested with toolchains from CodeSourcery and Linaro, 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.

We do not support toolchains or SDK generated by OpenEmbedded or Yocto, because these toolchains are not pure toolchains (i.e. just the compiler, binutils, the C and C++ libraries). Instead these toolchains come with a very large set of pre-compiled libraries and programs. Therefore, Buildroot cannot import the sysroot of the toolchain, as it would contain hundreds of megabytes of pre-compiled libraries that are normally built by Buildroot.

We also do not support using the distribution toolchain (i.e. the gcc/binutils/C library installed by your distribution) as the toolchain to build software for the target. This is because your distribution toolchain is not a "pure" toolchain (i.e. only with the C/C++ library), so we cannot import it properly into the Buildroot build environment. So even if you are building a system for a x86 or x86_64 target, you have to generate a cross-compilation toolchain with Buildroot or crosstool-NG.

If you want to generate a custom toolchain for your project, that can be used as an external toolchain in Buildroot, our recommendation is definitely to build it with crosstool-NG. We recommend to build the toolchain separately from Buildroot, and then import it in Buildroot using the external toolchain backend.

Advantages of this backend:

  • Allows to use well-known and well-tested cross-compilation toolchains.
  • Avoids the build time of the cross-compilation toolchain, which is often very significant in the overall build time of an embedded Linux system.

Drawbacks of this backend:

  • If your pre-built external toolchain has a bug, may be hard to get a fix from the toolchain vendor, unless you build your external toolchain by yourself using Crosstool-NG.

External toolchain wrapper

When using an external toolchain, Buildroot generates a wrapper program, that transparently passes the appropriate options (according to the configuration) to the external toolchain programs. In case you need to debug this wrapper to check exactly what arguments are passed, you can set the environment variable BR2_DEBUG_WRAPPER to either one of:

  • 0, empty or not set: no debug
  • 1: trace all arguments on a single line
  • 2: trace one argument per line

On a Linux system, the /dev directory contains special files, called device files, that allow userspace applications to access the hardware devices managed by the Linux kernel. Without these device files, your userspace applications would not be able to use the hardware devices, even if they are properly recognized by the Linux kernel.

Under System configuration, /dev management, Buildroot offers four different solutions to handle the /dev directory :

  • The first solution is Static using device table. This is the old classical way of handling device files in Linux. With this method, the device files are persistently stored in the root filesystem (i.e. they persist across reboots), and there is nothing that will automatically create and remove those device files when hardware devices are added or removed from the system. Buildroot therefore creates a standard set of device files using a device table, the default one being stored in system/device_table_dev.txt in the Buildroot source code. This file is processed when Buildroot generates the final root filesystem image, and the device files are therefore not visible in the output/target directory. The BR2_ROOTFS_STATIC_DEVICE_TABLE option allows to change the default device table used by Buildroot, or to add an additional device table, so that additional device files are created by Buildroot during the build. So, if you use this method, and a device file is missing in your system, you can for example create a board/<yourcompany>/<yourproject>/device_table_dev.txt file that contains the description of your additional device files, and then you can set BR2_ROOTFS_STATIC_DEVICE_TABLE to system/device_table_dev.txt board/<yourcompany>/<yourproject>/device_table_dev.txt. For more details about the format of the device table file, see Chapter 23, Makedev syntax documentation.
  • The second solution is Dynamic using devtmpfs only. devtmpfs is a virtual filesystem inside the Linux kernel that has been introduced in kernel 2.6.32 (if you use an older kernel, it is not possible to use this option). When mounted in /dev, this virtual filesystem will automatically make device files appear and disappear as hardware devices are added and removed from the system. This filesystem is not persistent across reboots: it is filled dynamically by the kernel. Using devtmpfs requires the following kernel configuration options to be enabled: CONFIG_DEVTMPFS and CONFIG_DEVTMPFS_MOUNT. When Buildroot is in charge of building the Linux kernel for your embedded device, it makes sure that those two options are enabled. However, if you build your Linux kernel outside of Buildroot, then it is your responsibility to enable those two options (if you fail to do so, your Buildroot system will not boot).
  • The third solution is Dynamic using devtmpfs + mdev. This method also relies on the devtmpfs virtual filesystem detailed above (so the requirement to have CONFIG_DEVTMPFS and CONFIG_DEVTMPFS_MOUNT enabled in the kernel configuration still apply), but adds the mdev userspace utility on top of it. mdev is a program part of BusyBox that the kernel will call every time a device is added or removed. Thanks to the /etc/mdev.conf configuration file, mdev can be configured to for example, set specific permissions or ownership on a device file, call a script or application whenever a device appears or disappear, etc. Basically, it allows userspace to react on device addition and removal events. mdev can for example be used to automatically load kernel modules when devices appear on the system. mdev is also important if you have devices that require a firmware, as it will be responsible for pushing the firmware contents to the kernel. mdev is a lightweight implementation (with fewer features) of udev. For more details about mdev and the syntax of its configuration file, see
  • The fourth solution is Dynamic using devtmpfs + eudev. This method also relies on the devtmpfs virtual filesystem detailed above, but adds the eudev userspace daemon on top of it. eudev is a daemon that runs in the background, and gets called by the kernel when a device gets added or removed from the system. It is a more heavyweight solution than mdev, but provides higher flexibility. eudev is a standalone version of udev, the original userspace daemon used in most desktop Linux distributions, which is now part of Systemd. For more details, see

The Buildroot developers recommendation is to start with the Dynamic using devtmpfs only solution, until you have the need for userspace to be notified when devices are added/removed, or if firmwares are needed, in which case Dynamic using devtmpfs + mdev is usually a good solution.

Note that if systemd is chosen as init system, /dev management will be performed by the udev program provided by systemd.

The init program is the first userspace program started by the kernel (it carries the PID number 1), and is responsible for starting the userspace services and programs (for example: web server, graphical applications, other network servers, etc.).

Buildroot allows to use three different types of init systems, which can be chosen from System configuration, Init system:

  • The first solution is BusyBox. Amongst many programs, BusyBox has an implementation of a basic init program, which is sufficient for most embedded systems. Enabling the BR2_INIT_BUSYBOX will ensure BusyBox will build and install its init program. This is the default solution in Buildroot. The BusyBox init program will read the /etc/inittab file at boot to know what to do. The syntax of this file can be found in (note that BusyBox inittab syntax is special: do not use a random inittab documentation from the Internet to learn about BusyBox inittab). The default inittab in Buildroot is stored in system/skeleton/etc/inittab. Apart from mounting a few important filesystems, the main job the default inittab does is to start the /etc/init.d/rcS shell script, and start a getty program (which provides a login prompt).
  • The second solution is systemV. This solution uses the old traditional sysvinit program, packed in Buildroot in package/sysvinit. This was the solution used in most desktop Linux distributions, until they switched to more recent alternatives such as Upstart or Systemd. sysvinit also works with an inittab file (which has a slightly different syntax than the one from BusyBox). The default inittab installed with this init solution is located in package/sysvinit/inittab.
  • The third solution is systemd. systemd is the new generation init system for Linux. It does far more than traditional init programs: aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux control groups, supports snapshotting and restoring of the system state, etc. systemd will be useful on relatively complex embedded systems, for example the ones requiring D-Bus and services communicating between each other. It is worth noting that systemd brings a fairly big number of large dependencies: dbus, udev and more. For more details about systemd, see

The solution recommended by Buildroot developers is to use the BusyBox init as it is sufficient for most embedded systems. systemd can be used for more complex situations.

Chapter 8. General Buildroot usage

This is a collection of tips that help you make the most of Buildroot.

Display all commands executed by make: 

 $ make V=1 <target>

Display the list of boards with a defconfig: 

 $ make list-defconfigs

Display all available targets: 

 $ make help

Not all targets are always available, some settings in the .config file may hide some targets:

  • busybox-menuconfig only works when busybox is enabled;
  • linux-menuconfig and linux-savedefconfig only work when linux is enabled;
  • uclibc-menuconfig is only available when the uClibc C library is selected in the internal toolchain backend;
  • barebox-menuconfig and barebox-savedefconfig only work when the barebox bootloader is enabled.
  • uboot-menuconfig and uboot-savedefconfig only work when the U-Boot bootloader is enabled.

Cleaning: Explicit cleaning is required when any of the architecture or toolchain configuration options are changed.

To delete all build products (including build directories, host, staging and target trees, the images and the toolchain):

 $ make clean

Generating the manual: The present manual sources are located in the docs/manual directory. To generate the manual:

 $ make manual-clean
 $ make manual

The manual outputs will be generated in output/docs/manual.

Resetting Buildroot for a new target: To delete all build products as well as the configuration:

 $ make distclean

Notes. If ccache is enabled, running make clean or distclean does not empty the compiler cache used by Buildroot. To delete it, refer to Section 8.12.3, “Using ccache in Buildroot”.

Dumping the internal make variables: One can dump all the variables known to make, along with their values:

 $ make -s printvars

It is possible to tweak the output using some variables:

  • VARS will limit the listing to variables which names match the specified make-pattern
  • QUOTED_VARS, if set to YES, will single-quote the value
  • RAW_VARS, if set to YES, will print the unexpanded value

For example:

 $ make -s printvars VARS=BUSYBOX_%DEPENDENCIES
 BUSYBOX_DEPENDENCIES=skeleton toolchain
 BUSYBOX_RDEPENDENCIES=ncurses util-linux
 BUSYBOX_DEPENDENCIES='skeleton toolchain'
 BUSYBOX_FINAL_DEPENDENCIES='skeleton toolchain'
 BUSYBOX_RDEPENDENCIES='ncurses util-linux'
 BUSYBOX_DEPENDENCIES=skeleton toolchain
 BUSYBOX_RDEPENDENCIES=ncurses util-linux

The output of quoted variables can be reused in shell scripts, for example:

 skeleton toolchain

8.2. Understanding when a full rebuild is necessary

Buildroot does not attempt to detect what parts of the system should be rebuilt when the system configuration is changed through make menuconfig, make xconfig or one of the other configuration tools. In some cases, Buildroot should rebuild the entire system, in some cases, only a specific subset of packages. But detecting this in a completely reliable manner is very difficult, and therefore the Buildroot developers have decided to simply not attempt to do this.

Instead, it is the responsibility of the user to know when a full rebuild is necessary. As a hint, here are a few rules of thumb that can help you understand how to work with Buildroot:

  • When the target architecture configuration is changed, a complete rebuild is needed. Changing the architecture variant, the binary format or the floating point strategy for example has an impact on the entire system.
  • When the toolchain configuration is changed, a complete rebuild generally is needed. Changing the toolchain configuration often involves changing the compiler version, the type of C library or its configuration, or some other fundamental configuration item, and these changes have an impact on the entire system.
  • When an additional package is added to the configuration, a full rebuild is not necessarily needed. Buildroot will detect that this package has never been built, and will build it. However, if this package is a library that can optionally be used by packages that have already been built, Buildroot will not automatically rebuild those. Either you know which packages should be rebuilt, and you can rebuild them manually, or you should do a full rebuild. For example, let’s suppose you have built a system with the ctorrent package, but without openssl. Your system works, but you realize you would like to have SSL support in ctorrent, so you enable the openssl package in Buildroot configuration and restart the build. Buildroot will detect that openssl should be built and will be build it, but it will not detect that ctorrent should be rebuilt to benefit from openssl to add OpenSSL support. You will either have to do a full rebuild, or rebuild ctorrent itself.
  • When a package is removed from the configuration, Buildroot does not do anything special. It does not remove the files installed by this package from the target root filesystem or from the toolchain sysroot. A full rebuild is needed to get rid of this package. However, generally you don’t necessarily need this package to be removed right now: you can wait for the next lunch break to restart the build from scratch.
  • When the sub-options of a package are changed, the package is not automatically rebuilt. After making such changes, rebuilding only this package is often sufficient, unless enabling the package sub-option adds some features to the package that are useful for another package which has already been built. Again, Buildroot does not track when a package should be rebuilt: once a package has been built, it is never rebuilt unless explicitly told to do so.
  • When a change to the root filesystem skeleton is made, a full rebuild is needed. However, when changes to the root filesystem overlay, a post-build script or a post-image script are made, there is no need for a full rebuild: a simple make invocation will take the changes into account.

Generally speaking, when you’re facing a build error and you’re unsure of the potential consequences of the configuration changes you’ve made, do a full rebuild. If you get the same build error, then you are sure that the error is not related to partial rebuilds of packages, and if this error occurs with packages from the official Buildroot, do not hesitate to report the problem! As your experience with Buildroot progresses, you will progressively learn when a full rebuild is really necessary, and you will save more and more time.

For reference, a full rebuild is achieved by running:

$ make clean all

8.3. 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 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, or which package would be compiled differently depending on the availability of another package.

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. You can ask buildroot to do this with the make <package>-dirclean command.

On the other hand, if you only want to restart the build process of a package from its compilation step, you can run make <package>-rebuild, followed by make or make <package>. It will restart the compilation and installation of the package, but not from scratch: it basically re-executes make and make install inside the package, so it will only rebuild files that changed.

If you want to restart the build process of a package from its configuration step, you can run make <package>-reconfigure, followed by make or make <package>. It will restart the configuration, compilation and installation of the package.

Internally, Buildroot creates so-called stamp files to keep track of which build steps have been completed for each package. They are stored in the package build directory, output/build/<package>-<version>/ and are named .stamp_<step-name>. The commands detailed above simply manipulate these stamp files to force Buildroot to restart a specific set of steps of a package build process.

Further details about package special make targets are explained in Section 8.12.5, “Package-specific make targets”.

If you intend to do an offline build and just want to download all sources that you previously selected in the configurator (menuconfig, nconfig, xconfig or gconfig), then issue:

 $ make source

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

8.5. Building out-of-tree

As default, everything built by Buildroot is stored in the directory output in the Buildroot tree.

Buildroot also 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


 $ cd /tmp/build; make O=$PWD -C path/to/buildroot

All the output files will be located under /tmp/build. If the O path does not exist, Buildroot will create it.

Note: the O path can be either an absolute or a relative path, but if it’s passed as a relative path, it is important to note that it is interpreted relative to the main Buildroot source directory, not the current working directory.

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>

8.6. Environment variables

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

  • HOSTCXX, the host C++ compiler to use
  • HOSTCC, the host C compiler to use
  • UCLIBC_CONFIG_FILE=<path/to/.config>, path to the uClibc configuration file, used to compile uClibc, if an internal toolchain is being built. Note that the uClibc configuration file can also be set from the configuration interface, so through the Buildroot .config file; this is the recommended way of setting it.
  • BUSYBOX_CONFIG_FILE=<path/to/.config>, path to the BusyBox configuration file. Note that the BusyBox configuration file can also be set from the configuration interface, so through the Buildroot .config file; this is the recommended way of setting it.
  • BR2_CCACHE_DIR to override the directory where Buildroot stores the cached files when using ccache.
  • BR2_DL_DIR to override the directory in which Buildroot stores/retrieves downloaded files Note that the Buildroot download directory can also be set from the configuration interface, so through the Buildroot .config file. See Section 8.12.4, “Location of downloaded packages” for more details on how you can set the download directory.
  • BR2_GRAPH_ALT, if set and non-empty, to use an alternate color-scheme in build-time graphs
  • BR2_GRAPH_OUT to set the filetype of generated graphs, either pdf (the default), or png.
  • BR2_GRAPH_DEPS_OPTS to pass extra options to the dependency graph; see Section 8.8, “Graphing the dependencies between packages” for the accepted options
  • BR2_GRAPH_DOT_OPTS is passed verbatim as options to the dot utility to draw the dependency graph.

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

8.7. Dealing efficiently with filesystem images

Filesystem images can get pretty big, depending on the filesystem you choose, the number of packages, whether you provisioned free space… Yet, some locations in the filesystems images may just be empty (e.g. a long run of zeroes); such a file is called a sparse file.

Most tools can handle sparse files efficiently, and will only store or write those parts of a sparse file that are not empty.

For example:

  • tar accepts the -S option to tell it to only store non-zero blocks of sparse files:

    • tar cf archive.tar -S [files…] will efficiently store sparse files in a tarball
    • tar xf archive.tar -S will efficiently store sparse files extracted from a tarball
  • cp accepts the --sparse=WHEN option (WHEN is one of auto, never or always):

    • cp --sparse=always source.file dest.file will make dest.file a sparse file if source.file has long runs of zeroes

Other tools may have similar options. Please consult their respective man pages.

You can use sparse files if you need to store the filesystem images (e.g. to transfer from one machine to another), or if you need to send them (e.g. to the Q&A team).

Note however that flashing a filesystem image to a device while using the sparse mode of dd may result in a broken filesystem (e.g. the block bitmap of an ext2 filesystem may be corrupted; or, if you have sparse files in your filesystem, those parts may not be all-zeroes when read back). You should only use sparse files when handling files on the build machine, not when transferring them to an actual device that will be used on the target.

8.8. Graphing the dependencies between packages

One of Buildroot’s jobs is to know the dependencies between packages, and make sure they are built in the right order. These dependencies can sometimes be quite complicated, and for a given system, it is often not easy to understand why such or such package was brought into the build by Buildroot.

In order to help understanding the dependencies, and therefore better understand what is the role of the different components in your embedded Linux system, Buildroot is capable of generating dependency graphs.

To generate a dependency graph of the full system you have compiled, simply run:

make graph-depends

You will find the generated graph in output/graphs/graph-depends.pdf.

If your system is quite large, the dependency graph may be too complex and difficult to read. It is therefore possible to generate the dependency graph just for a given package:

make <pkg>-graph-depends

You will find the generated graph in output/graph/<pkg>-graph-depends.pdf.

Note that the dependency graphs are generated using the dot tool from the Graphviz project, which you must have installed on your system to use this feature. In most distributions, it is available as the graphviz package.

By default, the dependency graphs are generated in the PDF format. However, by passing the BR2_GRAPH_OUT environment variable, you can switch to other output formats, such as PNG, PostScript or SVG. All formats supported by the -T option of the dot tool are supported.

BR2_GRAPH_OUT=svg make graph-depends

The graph-depends behaviour can be controlled by setting options in the BR2_GRAPH_DEPS_OPTS environment variable. The accepted options are:

  • --depth N, -d N, to limit the dependency depth to N levels. The default, 0, means no limit.
  • --stop-on PKG, -s PKG, to stop the graph on the package PKG. PKG can be an actual package name, a glob, the keyword virtual (to stop on virtual packages), or the keyword host (to stop on host packages). The package is still present on the graph, but its dependencies are not.
  • --exclude PKG, -x PKG, like --stop-on, but also omits PKG from the graph.
  • --transitive, --no-transitive, to draw (or not) the transitive dependencies. The default is to not draw transitive dependencies.
  • --colours R,T,H, the comma-separated list of colours to draw the root package (R), the target packages (T) and the host packages (H). Defaults to: lightblue,grey,gainsboro
BR2_GRAPH_DEPS_OPTS='-d 3 --no-transitive --colours=red,green,blue' make graph-depends

8.9. Graphing the build duration

When the build of a system takes a long time, it is sometimes useful to be able to understand which packages are the longest to build, to see if anything can be done to speed up the build. In order to help such build time analysis, Buildroot collects the build time of each step of each package, and allows to generate graphs from this data.

To generate the build time graph after a build, run:

make graph-build

This will generate a set of files in output/graphs :

  • build.hist-build.pdf, a histogram of the build time for each package, ordered in the build order.
  • build.hist-duration.pdf, a histogram of the build time for each package, ordered by duration (longest first)
  • build.hist-name.pdf, a histogram of the build time for each package, order by package name.
  • build.pie-packages.pdf, a pie chart of the build time per package
  • build.pie-steps.pdf, a pie chart of the global time spent in each step of the packages build process.

This graph-build target requires the Python Matplotlib and Numpy libraries to be installed (python-matplotlib and python-numpy on most distributions), and also the argparse module if you’re using a Python version older than 2.7 (python-argparse on most distributions).

By default, the output format for the graph is PDF, but a different format can be selected using the BR2_GRAPH_OUT environment variable. The only other format supported is PNG:

BR2_GRAPH_OUT=png make graph-build

8.10. Graphing the filesystem size contribution of packages

When your target system grows, it is sometimes useful to understand how much each Buildroot package is contributing to the overall root filesystem size. To help with such an analysis, Buildroot collects data about files installed by each package and using this data, generates a graph and CSV files detailing the size contribution of the different packages.

To generate these data after a build, run:

make graph-size

This will generate:

  • output/graphs/graph-size.pdf, a pie chart of the contribution of each package to the overall root filesystem size
  • output/graphs/package-size-stats.csv, a CSV file giving the size contribution of each package to the overall root filesystem size
  • output/graphs/file-size-stats.csv, a CSV file giving the size contribution of each installed file to the package it belongs, and to the overall filesystem size.

This graph-size target requires the Python Matplotlib library to be installed (python-matplotlib on most distributions), and also the argparse module if you’re using a Python version older than 2.7 (python-argparse on most distributions).

Just like for the duration graph, a BR2_GRAPH_OUT environment is supported to adjust the output file format. See Section 8.8, “Graphing the dependencies between packages” for details about this environment variable.

Note. The collected filesystem size data is only meaningful after a complete clean rebuild. Be sure to run make clean all before using make graph-size.

To compare the root filesystem size of two different Buildroot compilations, for example after adjusting the configuration or when switching to another Buildroot release, use the size-stats-compare script. It takes two file-size-stats.csv files (produced by make graph-size) as input. Refer to the help text of this script for more details:

utils/size-stats-compare -h

8.11. Integration with Eclipse

While a part of the embedded Linux developers like classical text editors like Vim or Emacs, and command-line based interfaces, a number of other embedded Linux developers like richer graphical interfaces to do their development work. Eclipse being one of the most popular Integrated Development Environment, Buildroot integrates with Eclipse in order to ease the development work of Eclipse users.

Our integration with Eclipse simplifies the compilation, remote execution and remote debugging of applications and libraries that are built on top of a Buildroot system. It does not integrate the Buildroot configuration and build processes themselves with Eclipse. Therefore, the typical usage model of our Eclipse integration would be:

  • Configure your Buildroot system with make menuconfig, make xconfig or any other configuration interface provided with Buildroot.
  • Build your Buildroot system by running make.
  • Start Eclipse to develop, execute and debug your own custom applications and libraries, that will rely on the libraries built and installed by Buildroot.

The Buildroot Eclipse integration installation process and usage is described in detail at

8.12.1. 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/host/. The simplest way to use it is to add output/host/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, this allows to distribute the toolchain to other developers to build applications for your target. To achieve this:

  • run make sdk, which prepares the toolchain to be relocatable;
  • tarball the contents of the output/host directory;
  • distribute the resulting tarball.

Once the toolchain is installed to the new location, the user must run the script to make sure all paths are updated with the new location.

8.12.2. Using gdb in Buildroot

Buildroot allows to do cross-debugging, where the debugger runs on the build machine and communicates with gdbserver on the target to control the execution of the program.

To achieve this:

  • If you are using an internal toolchain (built by Buildroot), you must enable BR2_PACKAGE_HOST_GDB, BR2_PACKAGE_GDB and BR2_PACKAGE_GDB_SERVER. This ensures that both the cross gdb and gdbserver get built, and that gdbserver gets installed to your target.
  • If you are using an external toolchain, you should enable BR2_TOOLCHAIN_EXTERNAL_GDB_SERVER_COPY, which will copy the gdbserver included with the external toolchain to the target. If your external toolchain does not have a cross gdb or gdbserver, it is also possible to let Buildroot build them, by enabling the same options as for the internal toolchain backend.

Now, to start debugging a program called foo, you should run on the target:

gdbserver :2345 foo

This will cause gdbserver to listen on TCP port 2345 for a connection from the cross gdb.

Then, on the host, you should start the cross gdb using the following command line:

<buildroot>/output/host/bin/<tuple>-gdb -x <buildroot>/output/staging/usr/share/buildroot/gdbinit foo

Of course, foo must be available in the current directory, built with debugging symbols. Typically you start this command from the directory where foo is built (and not from output/target/ as the binaries in that directory are stripped).

The <buildroot>/output/staging/usr/share/buildroot/gdbinit file will tell the cross gdb where to find the libraries of the target.

Finally, to connect to the target from the cross gdb:

(gdb) target remote <target ip address>:2345

8.12.3. Using ccache in Buildroot

ccache is a compiler cache. It stores the object files resulting from each compilation process, and is able to skip future compilation of the same source file (with same compiler and same arguments) by using the pre-existing object files. When doing almost identical builds from scratch a number of times, it can nicely speed up the build process.

ccache support is integrated in Buildroot. You just have to enable Enable compiler cache in Build options. This will automatically build ccache and use it for every host and target compilation.

The cache is located in $HOME/.buildroot-ccache. It is stored outside of Buildroot output directory so that it can be shared by separate Buildroot builds. If you want to get rid of the cache, simply remove this directory.

You can get statistics on the cache (its size, number of hits, misses, etc.) by running make ccache-stats.

The make target ccache-options and the CCACHE_OPTIONS variable provide more generic access to the ccache. For example

# set cache limit size
make CCACHE_OPTIONS="--max-size=5G" ccache-options

# zero statistics counters
make CCACHE_OPTIONS="--zero-stats" ccache-options

ccache makes a hash of the source files and of the compiler options. If a compiler option is different, the cached object file will not be used. Many compiler options, however, contain an absolute path to the staging directory. Because of this, building in a different output directory would lead to many cache misses.

To avoid this issue, buildroot has the Use relative paths option (BR2_CCACHE_USE_BASEDIR). This will rewrite all absolute paths that point inside the output directory into relative paths. Thus, changing the output directory no longer leads to cache misses.

A disadvantage of the relative paths is that they also end up to be relative paths in the object file. Therefore, for example, the debugger will no longer find the file, unless you cd to the output directory first.

See the ccache manual’s section on "Compiling in different directories" for more details about this rewriting of absolute paths.

8.12.4. Location of downloaded packages

The various tarballs that are downloaded by Buildroot are all stored in BR2_DL_DIR, which by default is the dl directory. If you want to keep a complete version of Buildroot which is known to be working with the associated tarballs, you can make a copy of this directory. 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 achieved by pointing the BR2_DL_DIR environment variable to a directory. If this is set, then the value of BR2_DL_DIR in the Buildroot configuration is overridden. The following line should be added to <~/.bashrc>.

 export BR2_DL_DIR=<shared download location>

The download location can also be set in the .config file, with the BR2_DL_DIR option. Unlike most options in the .config file, this value is overridden by the BR2_DL_DIR environment variable.

8.12.5. Package-specific make targets

Running make <package> builds and installs that particular package and its dependencies.

For packages relying on the Buildroot infrastructure, there are numerous special make targets that can be called independently like this:

make <package>-<target>

The package build targets are (in the order they are executed):

command/target Description


Fetch the source (download the tarball, clone the source repository, etc)


Build and install all dependencies required to build the package


Put the source in the package build directory (extract the tarball, copy the source, etc)


Apply the patches, if any


Run the configure commands, if any


Run the compilation commands


target package: Run the installation of the package in the staging directory, if necessary


target package: Run the installation of the package in the target directory, if necessary


target package: Run the 2 previous installation commands

host package: Run the installation of the package in the host directory

Additionally, there are some other useful make targets:

command/target Description


Displays the dependencies required to build the package


Generate a dependency graph of the package, in the context of the current Buildroot configuration. See this section Section 8.8, “Graphing the dependencies between packages” for more details about dependency graphs.


Remove the whole package build directory


Re-run the install commands


Re-run the compilation commands - this only makes sense when using the OVERRIDE_SRCDIR feature or when you modified a file directly in the build directory


Re-run the configure commands, then rebuild - this only makes sense when using the OVERRIDE_SRCDIR feature or when you modified a file directly in the build directory

8.12.6. Using Buildroot during development

The normal operation of Buildroot is to download a tarball, extract it, configure, compile and install the software component found inside this tarball. The source code is extracted in output/build/<package>-<version>, which is a temporary directory: whenever make clean is used, this directory is entirely removed, and re-created at the next make invocation. Even when a Git or Subversion repository is used as the input for the package source code, Buildroot creates a tarball out of it, and then behaves as it normally does with tarballs.

This behavior is well-suited when Buildroot is used mainly as an integration tool, to build and integrate all the components of an embedded Linux system. However, if one uses Buildroot during the development of certain components of the system, this behavior is not very convenient: one would instead like to make a small change to the source code of one package, and be able to quickly rebuild the system with Buildroot.

Making changes directly in output/build/<package>-<version> is not an appropriate solution, because this directory is removed on make clean.

Therefore, Buildroot provides a specific mechanism for this use case: the <pkg>_OVERRIDE_SRCDIR mechanism. Buildroot reads an override file, which allows the user to tell Buildroot the location of the source for certain packages. By default this override file is named and located in the top directory of the Buildroot source tree, but a different location can be specified through the BR2_PACKAGE_OVERRIDE_FILE configuration option.

In this override file, Buildroot expects to find lines of the form:

<pkg1>_OVERRIDE_SRCDIR = /path/to/pkg1/sources
<pkg2>_OVERRIDE_SRCDIR = /path/to/pkg2/sources

For example:

LINUX_OVERRIDE_SRCDIR = /home/bob/linux/
BUSYBOX_OVERRIDE_SRCDIR = /home/bob/busybox/

When Buildroot finds that for a given package, an <pkg>_OVERRIDE_SRCDIR has been defined, it will no longer attempt to download, extract and patch the package. Instead, it will directly use the source code available in the specified directory and make clean will not touch this directory. This allows to point Buildroot to your own directories, that can be managed by Git, Subversion, or any other version control system. To achieve this, Buildroot will use rsync to copy the source code of the component from the specified <pkg>_OVERRIDE_SRCDIR to output/build/<package>-custom/.

This mechanism is best used in conjunction with the make <pkg>-rebuild and make <pkg>-reconfigure targets. A make <pkg>-rebuild all sequence will rsync the source code from <pkg>_OVERRIDE_SRCDIR to output/build/<package>-custom (thanks to rsync, only the modified files are copied), and restart the build process of just this package.

In the example of the linux package above, the developer can then make a source code change in /home/bob/linux and then run:

make linux-rebuild all

and in a matter of seconds gets the updated Linux kernel image in output/images. Similarly, a change can be made to the BusyBox source code in /home/bob/busybox, and after:

make busybox-rebuild all

the root filesystem image in output/images contains the updated BusyBox.

Chapter 9. Project-specific customization

Typical actions you may need to perform for a given project are:

  • configuring Buildroot (including build options and toolchain, bootloader, kernel, package and filesystem image type selection)
  • configuring other components, like the Linux kernel and BusyBox
  • customizing the generated target filesystem

    • adding or overwriting files on the target filesystem (using BR2_ROOTFS_OVERLAY)
    • modifying or deleting files on the target filesystem (using BR2_ROOTFS_POST_BUILD_SCRIPT)
    • running arbitrary commands prior to generating the filesystem image (using BR2_ROOTFS_POST_BUILD_SCRIPT)
    • setting file permissions and ownership (using BR2_ROOTFS_DEVICE_TABLE)
    • adding custom devices nodes (using BR2_ROOTFS_STATIC_DEVICE_TABLE)
  • adding custom user accounts (using BR2_ROOTFS_USERS_TABLES)
  • running arbitrary commands after generating the filesystem image (using BR2_ROOTFS_POST_IMAGE_SCRIPT)
  • adding project-specific patches to some packages (using BR2_GLOBAL_PATCH_DIR)
  • adding project-specific packages

An important note regarding such project-specific customizations: please carefully consider which changes are indeed project-specific and which changes are also useful to developers outside your project. The Buildroot community highly recommends and encourages the upstreaming of improvements, packages and board support to the official Buildroot project. Of course, it is sometimes not possible or desirable to upstream because the changes are highly specific or proprietary.

This chapter describes how to make such project-specific customizations in Buildroot and how to store them in a way that you can build the same image in a reproducible way, even after running make clean. By following the recommended strategy, you can even use the same Buildroot tree to build multiple distinct projects!

9.1. Recommended directory structure

When customizing Buildroot for your project, you will be creating one or more project-specific files that need to be stored somewhere. While most of these files could be placed in any location as their path is to be specified in the Buildroot configuration, the Buildroot developers recommend a specific directory structure which is described in this section.

Orthogonal to this directory structure, you can choose where you place this structure itself: either inside the Buildroot tree, or outside of it using a br2-external tree. Both options are valid, the choice is up to you.

+-- board/
|   +-- <company>/
|       +-- <boardname>/
|           +-- linux.config
|           +-- busybox.config
|           +-- <other configuration files>
|           +--
|           +--
|           +-- rootfs_overlay/
|           |   +-- etc/
|           |   +-- <some file>
|           +-- patches/
|               +-- foo/
|               |   +-- <some patch>
|               +-- libbar/
|                   +-- <some other patches>
+-- configs/
|   +-- <boardname>_defconfig
+-- package/
|   +-- <company>/
|       +-- (if not using a br2-external tree)
|       +-- <company>.mk (if not using a br2-external tree)
|       +-- package1/
|       |    +--
|       |    +--
|       +-- package2/
|           +--
|           +--
+-- (if using a br2-external tree)
+-- (if using a br2-external tree)

Details on the files shown above are given further in this chapter.

Note: if you choose to place this structure outside of the Buildroot tree but in a br2-external tree, the <company> and possibly <boardname> components may be superfluous and can be left out.

9.1.1. Implementing layered customizations

It is quite common for a user to have several related projects that partly need the same customizations. Instead of duplicating these customizations for each project, it is recommended to use a layered customization approach, as explained in this section.

Almost all of the customization methods available in Buildroot, like post-build scripts and root filesystem overlays, accept a space-separated list of items. The specified items are always treated in order, from left to right. By creating more than one such item, one for the common customizations and another one for the really project-specific customizations, you can avoid unnecessary duplication. Each layer is typically embodied by a separate directory inside board/<company>/. Depending on your projects, you could even introduce more than two layers.

An example directory structure for where a user has two customization layers common and fooboard is:

+-- board/
    +-- <company>/
        +-- common/
        |   +--
        |   +-- rootfs_overlay/
        |   |   +-- ...
        |   +-- patches/
        |       +-- ...
        +-- fooboard/
            +-- linux.config
            +-- busybox.config
            +-- <other configuration files>
            +-- rootfs_overlay/
            |   +-- ...
            +-- patches/
                +-- ...

For example, if the user has the BR2_GLOBAL_PATCH_DIR configuration option set as:

BR2_GLOBAL_PATCH_DIR="board/<company>/common/patches board/<company>/fooboard/patches"

then first the patches from the common layer would be applied, followed by the patches from the fooboard layer.

9.2. Keeping customizations outside of Buildroot

As already briefly mentioned in Section 9.1, “Recommended directory structure”, you can place project-specific customizations in two locations:

  • directly within the Buildroot tree, typically maintaining them using branches in a version control system so that upgrading to a newer Buildroot release is easy.
  • outside of the Buildroot tree, using the br2-external mechanism. This mechanism allows to keep package recipes, board support and configuration files outside of the Buildroot tree, while still having them nicely integrated in the build logic. We call this location a br2-external tree. This section explains how to use the br2-external mechanism and what to provide in a br2-external tree.

One can tell Buildroot to use one or more br2-external trees by setting the BR2_EXTERNAL make variable set to the path(s) of the br2-external tree(s) to use. It can be passed to any Buildroot make invocation. It is automatically saved in the hidden file in the output directory. Thanks to this, there is no need to pass BR2_EXTERNAL at every make invocation. It can however be changed at any time by passing a new value, and can be removed by passing an empty value.

Note. The path to a br2-external tree can be either absolute or relative. If it is passed as a relative path, it is important to note that it is interpreted relative to the main Buildroot source directory, not to the Buildroot output directory.

Note: If using an br2-external tree from before Buildroot 2016.11, you need to convert it before you can use it with Buildroot 2016.11 onward. See Section 25.1, “Migrating to 2016.11” for help on doing so.

Some examples:

buildroot/ $ make BR2_EXTERNAL=/path/to/foo menuconfig

From now on, definitions from the /path/to/foo br2-external tree will be used:

buildroot/ $ make
buildroot/ $ make legal-info

We can switch to another br2-external tree at any time:

buildroot/ $ make BR2_EXTERNAL=/where/we/have/bar xconfig

We can also use multiple br2-external trees:

buildroot/ $ make BR2_EXTERNAL=/path/to/foo:/where/we/have/bar menuconfig

Or disable the usage of any br2-external tree:

buildroot/ $ make BR2_EXTERNAL= xconfig

9.2.1. Layout of a br2-external tree

A br2-external tree must contain at least those three files, described in the following chapters:

  • external.desc

Apart from those mandatory files, there may be additional and optional content that may be present in a br2-external tree, like the configs/ directory. They are described in the following chapters as well.

A complete example br2-external tree layout is also described later.

That file describes the br2-external tree: the name and description for that br2-external tree.

The format for this file is line based, with each line starting by a keyword, followed by a colon and one or more spaces, followed by the value assigned to that keyword. There are two keywords currently recognised:

  • name, mandatory, defines the name for that br2-external tree. That name must only use ASCII characters in the set [A-Za-z0-9_]; any other character is forbidden. Buildroot sets the variable BR2_EXTERNAL_$(NAME)_PATH to the absolute path of the br2-external tree, so that you can use it to refer to your br2-external tree. This variable is available both in Kconfig, so you can use it to source your Kconfig files (see below) and in the Makefile, so that you can use it to include other Makefiles (see below) or refer to other files (like data files) from your br2-external tree.

    Note: Since it is possible to use multiple br2-external trees at once, this name is used by Buildroot to generate variables for each of those trees. That name is used to identify your br2-external tree, so try to come up with a name that really describes your br2-external tree, in order for it to be relatively unique, so that it does not clash with another name from another br2-external tree, especially if you are planning on somehow sharing your br2-external tree with third parties or using br2-external trees from third parties.

  • desc, optional, provides a short description for that br2-external tree. It shall fit on a single line, is mostly free-form (see below), and is used when displaying information about a br2-external tree (e.g. above the list of defconfig files, or as the prompt in the menuconfig); as such, it should relatively brief (40 chars is probably a good upper limit). The description is available in the BR2_EXTERNAL_$(NAME)_DESC variable.

Examples of names and the corresponding BR2_EXTERNAL_$(NAME)_PATH variables:


In the following examples, it is assumed the name to be set to BAR_42.

Note: Both BR2_EXTERNAL_$(NAME)_PATH and BR2_EXTERNAL_$(NAME)_DESC are available in the Kconfig files and the Makefiles. They are also exported in the environment so are available in post-build, post-image and in-fakeroot scripts.

The and files

Those files (which may each be empty) can be used to define package recipes (i.e. foo/ and foo/ like for packages bundled in Buildroot itself) or other custom configuration options or make logic.

Buildroot automatically includes the from each br2-external tree to make it appear in the top-level configuration menu, and includes the from each br2-external tree with the rest of the makefile logic.

The main usage of this is to store package recipes. The recommended way to do this is to write a file that looks like:

source "$BR2_EXTERNAL_BAR_42_PATH/package/package1/"
source "$BR2_EXTERNAL_BAR_42_PATH/package/package2/"

Then, have an file that looks like:

include $(sort $(wildcard $(BR2_EXTERNAL_BAR_42_PATH)/package/*/*.mk))

And then in $(BR2_EXTERNAL_BAR_42_PATH)/package/package1 and $(BR2_EXTERNAL_BAR_42_PATH)/package/package2 create normal Buildroot package recipes, as explained in Chapter 17, Adding new packages to Buildroot. If you prefer, you can also group the packages in subdirectories called <boardname> and adapt the above paths accordingly.

You can also define custom configuration options in and custom make logic in

One can store Buildroot defconfigs in the configs subdirectory of the br2-external tree. Buildroot will automatically show them in the output of make list-defconfigs and allow them to be loaded with the normal make <name>_defconfig command. They will be visible in the make list-defconfigs output, below an External configs label that contains the name of the br2-external tree they are defined in.

Note: If a defconfig file is present in more than one br2-external tree, then the one from the last br2-external tree is used. It is thus possible to override a defconfig bundled in Buildroot or another br2-external tree.

One can store all the board-specific configuration files there, such as the kernel configuration, the root filesystem overlay, or any other configuration file for which Buildroot allows to set the location (by using the BR2_EXTERNAL_$(NAME)_PATH variable). For example, you could set the paths to a global patch directory, to a rootfs overlay and to the kernel configuration file as follows (e.g. by running make menuconfig and filling in these options):


Here is an example layout using all features of br2-external (the sample content is shown for the file above it, when it is relevant to explain the br2-external tree; this is all entirely made up just for the sake of illustration, of course):

  |- external.desc
  |     |name: BAR_42
  |     |desc: Example br2-external tree
  |     `----
  |     |source "$BR2_EXTERNAL_BAR_42_PATH/package/pkg-1/"
  |     |source "$BR2_EXTERNAL_BAR_42_PATH/package/pkg-2/"
  |     |
  |     |config BAR_42_FLASH_ADDR
  |     |    hex "my-board flash address"
  |     |    default 0x10AD
  |     `----
  |     |include $(sort $(wildcard $(BR2_EXTERNAL_BAR_42_PATH)/package/*/*.mk))
  |     |
  |     |flash-my-board:
  |     |    $(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/flash-image \
  |     |        --image $(BINARIES_DIR)/image.bin \
  |     |        --address $(BAR_42_FLASH_ADDR)
  |     `----
  |- package/pkg-1/
  |     |config BR2_PACKAGE_PKG_1
  |     |    bool "pkg-1"
  |     |    help
  |     |      Some help about pkg-1
  |     `----
  |- package/pkg-1/pkg-1.hash
  |- package/pkg-1/
  |     |PKG_1_VERSION = 1.2.3
  |     |PKG_1_SITE = /some/where/to/get/pkg-1
  |     |PKG_1_LICENSE = blabla
  |     |
  |     |define PKG_1_INSTALL_INIT_SYSV
  |     |    $(INSTALL) -D -m 0755 $(PKG_1_PKGDIR)/S99my-daemon \
  |     |                          $(TARGET_DIR)/etc/init.d/S99my-daemon
  |     |endef
  |     |
  |     |$(eval $(autotools-package))
  |     `----
  |- package/pkg-1/S99my-daemon
  |- package/pkg-2/
  |- package/pkg-2/pkg-2.hash
  |- package/pkg-2/
  |- configs/my-board_defconfig
  |     |BR2_GLOBAL_PATCH_DIR="$(BR2_EXTERNAL_BAR_42_PATH)/patches/"
  |     |BR2_ROOTFS_OVERLAY="$(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/overlay/"
  |     |BR2_ROOTFS_POST_IMAGE_SCRIPT="$(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/"
  |     |BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="$(BR2_EXTERNAL_BAR_42_PATH)/board/my-board/kernel.config"
  |     `----
  |- patches/linux/0001-some-change.patch
  |- patches/linux/0002-some-other-change.patch
  |- patches/busybox/0001-fix-something.patch
  |- board/my-board/kernel.config
  |- board/my-board/overlay/var/www/index.html
  |- board/my-board/overlay/var/www/my.css
  |- board/my-board/flash-image
  `- board/my-board/
        |generate-my-binary-image \
        |    --root ${BINARIES_DIR}/rootfs.tar \
        |    --kernel ${BINARIES_DIR}/zImage \
        |    --dtb ${BINARIES_DIR}/my-board.dtb \
        |    --output ${BINARIES_DIR}/image.bin

The br2-external tree will then be visible in the menuconfig (with the layout expanded):

External options  --->
    *** Example br2-external tree (in /path/to/br2-ext-tree/)
    [ ] pkg-1
    [ ] pkg-2
    (0x10AD) my-board flash address

If you are using more than one br2-external tree, it would look like (with the layout expanded and the second one with name FOO_27 but no desc: field in external.desc):

External options  --->
    Example br2-external tree  --->
        *** Example br2-external tree (in /path/to/br2-ext-tree)
        [ ] pkg-1
        [ ] pkg-2
        (0x10AD) my-board flash address
    FOO_27  --->
        *** FOO_27 (in /path/to/another-br2-ext)
        [ ] foo
        [ ] bar

9.3. Storing the Buildroot configuration

The Buildroot configuration can be stored using the command make savedefconfig.

This strips the Buildroot configuration down by removing configuration options that are at their default value. The result is stored in a file called defconfig. If you want to save it in another place, change the BR2_DEFCONFIG option in the Buildroot configuration itself, or call make with make savedefconfig BR2_DEFCONFIG=<path-to-defconfig>.

The recommended place to store this defconfig is configs/<boardname>_defconfig. If you follow this recommendation, the configuration will be listed in make help and can be set again by running make <boardname>_defconfig.

Alternatively, you can copy the file to any other place and rebuild with make defconfig BR2_DEFCONFIG=<path-to-defconfig-file>.

9.4. Storing the configuration of other components

The configuration files for BusyBox, the Linux kernel, Barebox, U-Boot and uClibc should be stored as well if changed. For each of these components, a Buildroot configuration option exists to point to an input configuration file, e.g. BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE. To store their configuration, set these configuration options to a path where you want to save the configuration files, and then use the helper targets described below to actually store the configuration.

As explained in Section 9.1, “Recommended directory structure”, the recommended path to store these configuration files is board/<company>/<boardname>/foo.config.

Make sure that you create a configuration file before changing the BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE etc. options. Otherwise, Buildroot will try to access this config file, which doesn’t exist yet, and will fail. You can create the configuration file by running make linux-menuconfig etc.

Buildroot provides a few helper targets to make the saving of configuration files easier.

  • make linux-update-defconfig saves the linux configuration to the path specified by BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE. It simplifies the config file by removing default values. However, this only works with kernels starting from 2.6.33. For earlier kernels, use make linux-update-config.
  • make busybox-update-config saves the busybox configuration to the path specified by BR2_PACKAGE_BUSYBOX_CONFIG.
  • make uclibc-update-config saves the uClibc configuration to the path specified by BR2_UCLIBC_CONFIG.
  • make barebox-update-defconfig saves the barebox configuration to the path specified by BR2_TARGET_BAREBOX_CUSTOM_CONFIG_FILE.
  • make uboot-update-defconfig saves the U-Boot configuration to the path specified by BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE.
  • For at91bootstrap3, no helper exists so you have to copy the config file manually to BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_CONFIG_FILE.

9.5. Customizing the generated target filesystem

Besides changing the configuration through make *config, there are a few other ways to customize the resulting target filesystem.

The two recommended methods, which can co-exist, are root filesystem overlay(s) and post build script(s).

Root filesystem overlays (BR2_ROOTFS_OVERLAY)

A filesystem overlay is a tree of files that is copied directly over the target filesystem after it has been built. To enable this feature, set config option BR2_ROOTFS_OVERLAY (in the System configuration menu) to the root of the overlay. You can even specify multiple overlays, space-separated. If you specify a relative path, it will be relative to the root of the Buildroot tree. Hidden directories of version control systems, like .git, .svn, .hg, etc., files called .empty and files ending in ~ are excluded from the copy.

As shown in Section 9.1, “Recommended directory structure”, the recommended path for this overlay is board/<company>/<boardname>/rootfs-overlay.

Post-build scripts (BR2_ROOTFS_POST_BUILD_SCRIPT)

Post-build scripts are shell scripts called after Buildroot builds all the selected software, but before the rootfs images are assembled. To enable this feature, specify a space-separated list of post-build scripts in config option BR2_ROOTFS_POST_BUILD_SCRIPT (in the System configuration menu). If you specify a relative path, it will be relative to the root of the Buildroot tree.

Using post-build scripts, you can remove or modify any file in your target filesystem. You should, however, use this feature with care. Whenever you find that a certain package generates wrong or unneeded files, you should fix that package rather than work around it with some post-build cleanup scripts.

As shown in Section 9.1, “Recommended directory structure”, the recommended path for this script is board/<company>/<boardname>/

The post-build scripts are run with the main Buildroot tree as current working directory. The path to the target filesystem is passed as the first argument to each script. If the config option BR2_ROOTFS_POST_SCRIPT_ARGS is not empty, these arguments will be passed to the script too. All the scripts will be passed the exact same set of arguments, it is not possible to pass different sets of arguments to each script.

In addition, you may also use these environment variables:

  • BR2_CONFIG: the path to the Buildroot .config file
  • HOST_DIR, STAGING_DIR, TARGET_DIR: see Section 17.5.2, “generic-package reference”
  • BUILD_DIR: the directory where packages are extracted and built
  • BINARIES_DIR: the place where all binary files (aka images) are stored
  • BASE_DIR: the base output directory

Below three more methods of customizing the target filesystem are described, but they are not recommended.

Direct modification of the target filesystem

For temporary modifications, you can modify the target filesystem directly and rebuild the image. The target filesystem is available under output/target/. After making your changes, run make to rebuild the target filesystem image.

This method allows you to do anything to the target filesystem, but if you need to clean your Buildroot tree using make clean, these changes will be lost. Such cleaning is necessary in several cases, refer to Section 8.2, “Understanding when a full rebuild is necessary” for details. This solution is therefore only useful for quick tests: changes do not survive the make clean command. Once you have validated your changes, you should make sure that they will persist after a make clean, using a root filesystem overlay or a post-build script.

Custom target skeleton (BR2_ROOTFS_SKELETON_CUSTOM)

The root filesystem image is created from a target skeleton, on top of which all packages install their files. The skeleton is copied to the target directory output/target before any package is built and installed. The default target skeleton provides the standard Unix filesystem layout and some basic init scripts and configuration files.

If the default skeleton (available under system/skeleton) does not match your needs, you would typically use a root filesystem overlay or post-build script to adapt it. However, if the default skeleton is entirely different than what you need, using a custom skeleton may be more suitable.

To enable this feature, enable config option BR2_ROOTFS_SKELETON_CUSTOM and set BR2_ROOTFS_SKELETON_CUSTOM_PATH to the path of your custom skeleton. Both options are available in the System configuration menu. If you specify a relative path, it will be relative to the root of the Buildroot tree.

This method is not recommended because it duplicates the entire skeleton, which prevents taking advantage of the fixes or improvements brought to the default skeleton in later Buildroot releases.

Post-fakeroot scripts (BR2_ROOTFS_POST_FAKEROOT_SCRIPT)

When aggregating the final images, some parts of the process requires root rights: creating device nodes in /dev, setting permissions or ownership to files and directories… To avoid requiring actual root rights, Buildroot uses fakeroot to simulate root rights. This is not a complete substitute for actually being root, but is enough for what Buildroot needs.

Post-fakeroot scripts are shell scripts that are called at the end of the fakeroot phase, right before the filesystem image generator is called. As such, they are called in the fakeroot context.

Post-fakeroot scripts can be useful in case you need to tweak the filesystem to do modifications that are usually only available to the root user.

Note: It is recommended to use the existing mechanisms to set file permissions or create entries in /dev (see Section 9.5.1, “Setting file permissions and ownership and adding custom devices nodes”) or to create users (see Section 9.6, “Adding custom user accounts”)

Note: The difference between post-build scripts (above) and fakeroot scripts, is that post-build scripts are not called in the fakeroot context.

Note;. Using fakeroot is not an absolute substitute for actually being root. fakeroot only ever fakes the file access rights and types (regular, block-or-char device…) and uid/gid; these are emulated in-memory.

9.5.1. Setting file permissions and ownership and adding custom devices nodes

Sometimes it is needed to set specific permissions or ownership on files or device nodes. For example, certain files may need to be owned by root. Since the post-build scripts are not run as root, you cannot do such changes from there unless you use an explicit fakeroot from the post-build script.

Instead, Buildroot provides support for so-called permission tables. To use this feature, set config option BR2_ROOTFS_DEVICE_TABLE to a space-separated list of permission tables, regular text files following the makedev syntax Chapter 23, Makedev syntax documentation.

If you are using a static device table (i.e. not using devtmpfs, mdev, or (e)udev) then you can add device nodes using the same syntax, in so-called device tables. To use this feature, set config option BR2_ROOTFS_STATIC_DEVICE_TABLE to a space-separated list of device tables.

As shown in Section 9.1, “Recommended directory structure”, the recommended location for such files is board/<company>/<boardname>/.

It should be noted that if the specific permissions or device nodes are related to a specific application, you should set variables FOO_PERMISSIONS and FOO_DEVICES in the package’s .mk file instead (see Section 17.5.2, “generic-package reference”).

9.7. Customization after the images have been created

While post-build scripts (Section 9.5, “Customizing the generated target filesystem”) are run before building the filesystem image, kernel and bootloader, post-image scripts can be used to perform some specific actions after all images have been created.

Post-image scripts can for example be used to automatically extract your root filesystem tarball in a location exported by your NFS server, or to create a special firmware image that bundles your root filesystem and kernel image, or any other custom action required for your project.

To enable this feature, specify a space-separated list of post-image scripts in config option BR2_ROOTFS_POST_IMAGE_SCRIPT (in the System configuration menu). If you specify a relative path, it will be relative to the root of the Buildroot tree.

Just like post-build scripts, post-image scripts are run with the main Buildroot tree as current working directory. The path to the images output directory is passed as the first argument to each script. If the config option BR2_ROOTFS_POST_SCRIPT_ARGS is not empty, these arguments will be passed to the script too. All the scripts will be passed the exact same set of arguments, it is not possible to pass different sets of arguments to each script.

Again just like for the post-build scripts, the scripts have access to the environment variables BR2_CONFIG, HOST_DIR, STAGING_DIR, TARGET_DIR, BUILD_DIR, BINARIES_DIR and BASE_DIR.

The post-image scripts will be executed as the user that executes Buildroot, which should normally not be the root user. Therefore, any action requiring root permissions in one of these scripts will require special handling (usage of fakeroot or sudo), which is left to the script developer.

9.8. Adding project-specific patches

It is sometimes useful to apply extra patches to packages - on top of those provided in Buildroot. This might be used to support custom features in a project, for example, or when working on a new architecture.

The BR2_GLOBAL_PATCH_DIR configuration option can be used to specify a space separated list of one or more directories containing package patches.

For a specific version <packageversion> of a specific package <packagename>, patches are applied from BR2_GLOBAL_PATCH_DIR as follows:

  1. For every directory - <global-patch-dir> - that exists in BR2_GLOBAL_PATCH_DIR, a <package-patch-dir> will be determined as follows:

    • <global-patch-dir>/<packagename>/<packageversion>/ if the directory exists.
    • Otherwise, <global-patch-dir>/<packagename> if the directory exists.
  2. Patches will then be applied from a <package-patch-dir> as follows:

    • If a series file exists in the package directory, then patches are applied according to the series file;
    • Otherwise, patch files matching *.patch are applied in alphabetical order. So, to ensure they are applied in the right order, it is highly recommended to name the patch files like this: <number>-<description>.patch, where <number> refers to the apply order.

For information about how patches are applied for a package, see Section 18.2, “How patches are applied”

The BR2_GLOBAL_PATCH_DIR option is the preferred method for specifying a custom patch directory for packages. It can be used to specify a patch directory for any package in buildroot. It should also be used in place of the custom patch directory options that are available for packages such as U-Boot and Barebox. By doing this, it will allow a user to manage their patches from one top-level directory.

The exception to BR2_GLOBAL_PATCH_DIR being the preferred method for specifying custom patches is BR2_LINUX_KERNEL_PATCH. BR2_LINUX_KERNEL_PATCH should be used to specify kernel patches that are available at an URL. Note: BR2_LINUX_KERNEL_PATCH specifies kernel patches that are applied after patches available in BR2_GLOBAL_PATCH_DIR, as it is done from a post-patch hook of the Linux package.

9.9. Adding project-specific packages

In general, any new package should be added directly in the package directory and submitted to the Buildroot upstream project. How to add packages to Buildroot in general is explained in full detail in Chapter 17, Adding new packages to Buildroot and will not be repeated here. However, your project may need some proprietary packages that cannot be upstreamed. This section will explain how you can keep such project-specific packages in a project-specific directory.

As shown in Section 9.1, “Recommended directory structure”, the recommended location for project-specific packages is package/<company>/. If you are using the br2-external tree feature (see Section 9.2, “Keeping customizations outside of Buildroot”) the recommended location is to put them in a sub-directory named package/ in your br2-external tree.

However, Buildroot will not be aware of the packages in this location, unless we perform some additional steps. As explained in Chapter 17, Adding new packages to Buildroot, a package in Buildroot basically consists of two files: a .mk file (describing how to build the package) and a file (describing the configuration options for this package).

Buildroot will automatically include the .mk files in first-level subdirectories of the package directory (using the pattern package/*/*.mk). If we want Buildroot to include .mk files from deeper subdirectories (like package/<company>/package1/) then we simply have to add a .mk file in a first-level subdirectory that includes these additional .mk files. Therefore, create a file package/<company>/<company>.mk with following contents (assuming you have only one extra directory level below package/<company>/):

include $(sort $(wildcard package/<company>/*/*.mk))

For the files, create a file package/<company>/ that includes the files of all your packages. An exhaustive list has to be provided since wildcards are not supported in the source command of kconfig. For example:

source "package/<company>/package1/"
source "package/<company>/package2/"

Include this new file package/<company>/ from package/, preferably in a company-specific menu to make merges with future Buildroot versions easier.

If using a br2-external tree, refer to Section 9.2, “Keeping customizations outside of Buildroot” for how to fill in those files.

9.10. Quick guide to storing your project-specific customizations

Earlier in this chapter, the different methods for making project-specific customizations have been described. This section will now summarize all this by providing step-by-step instructions to storing your project-specific customizations. Clearly, the steps that are not relevant to your project can be skipped.

  1. make menuconfig to configure toolchain, packages and kernel.
  2. make linux-menuconfig to update the kernel config, similar for other configuration like busybox, uclibc, …
  3. mkdir -p board/<manufacturer>/<boardname>
  4. Set the following options to board/<manufacturer>/<boardname>/<package>.config (as far as they are relevant):

  5. Write the configuration files:

    • make linux-update-defconfig
    • make busybox-update-config
    • make uclibc-update-config
    • cp <output>/build/at91bootstrap3-*/.config board/<manufacturer>/<boardname>/at91bootstrap3.config
    • make barebox-update-defconfig
    • make uboot-update-defconfig
  6. Create board/<manufacturer>/<boardname>/rootfs-overlay/ and fill it with additional files you need on your rootfs, e.g. board/<manufacturer>/<boardname>/rootfs-overlay/etc/inittab. Set BR2_ROOTFS_OVERLAY to board/<manufacturer>/<boardname>/rootfs-overlay.
  7. Create a post-build script board/<manufacturer>/<boardname>/ Set BR2_ROOTFS_POST_BUILD_SCRIPT to board/<manufacturer>/<boardname>/
  8. If additional setuid permissions have to be set or device nodes have to be created, create board/<manufacturer>/<boardname>/device_table.txt and add that path to BR2_ROOTFS_DEVICE_TABLE.
  9. If additional user accounts have to be created, create board/<manufacturer>/<boardname>/users_table.txt and add that path to BR2_ROOTFS_USERS_TABLES.
  10. To add custom patches to certain packages, set BR2_GLOBAL_PATCH_DIR to board/<manufacturer>/<boardname>/patches/ and add your patches for each package in a subdirectory named after the package. Each patch should be called <packagename>-<num>-<description>.patch.
  11. Specifically for the Linux kernel, there also exists the option BR2_LINUX_KERNEL_PATCH with as main advantage that it can also download patches from a URL. If you do not need this, BR2_GLOBAL_PATCH_DIR is preferred. U-Boot, Barebox, at91bootstrap and at91bootstrap3 also have separate options, but these do not provide any advantage over BR2_GLOBAL_PATCH_DIR and will likely be removed in the future.
  12. If you need to add project-specific packages, create package/<manufacturer>/ and place your packages in that directory. Create an overall <manufacturer>.mk file that includes the .mk files of all your packages. Create an overall file that sources the files of all your packages. Include this file from Buildroot’s package/ file.
  13. make savedefconfig to save the buildroot configuration.
  14. cp defconfig configs/<boardname>_defconfig

Chapter 10. Frequently Asked Questions & Troubleshooting

10.1. The boot hangs after Starting network…

If the boot process seems to hang after the following messages (messages not necessarily 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 into the Buildroot configuration, in System configuration, modify Run a getty (login prompt) after boot and set the appropriate port and baud rate in the getty options submenu. This will automatically tune the /etc/inittab file of the generated system so that a shell starts on the correct serial port.

10.2. Why is there no compiler on the target?

It has been decided that support for the native compiler on the target would be stopped from the Buildroot-2012.11 release because:

  • this feature was neither maintained nor tested, and often broken;
  • this feature was only available for Buildroot toolchains;
  • Buildroot mostly targets small or very small target hardware with limited resource onboard (CPU, ram, mass-storage), for which compiling on the target does not make much sense;
  • Buildroot aims at easing the cross-compilation, making native compilation on the target unnecessary.

If you need a compiler on your target anyway, then Buildroot is not suitable for your purpose. In such case, you need a real distribution and you should opt for something like:

10.3. Why are there no development files on the target?

Since there is no compiler available on the target (see Section 10.2, “Why is there no compiler on the target?”), it does not make sense to waste space with headers or static libraries.

Therefore, those files are always removed from the target since the Buildroot-2012.11 release.

10.4. Why is there no documentation on the target?

Because Buildroot mostly targets small or very small target hardware with limited resource onboard (CPU, ram, mass-storage), it does not make sense to waste space with the documentation data.

If you need documentation data on your target anyway, then Buildroot is not suitable for your purpose, and you should look for a real distribution (see: Section 10.2, “Why is there no compiler on the target?”).

10.5. Why are some packages not visible in the Buildroot config menu?

If a package exists in the Buildroot tree and does not appear in the config menu, this most likely means that some of the package’s dependencies are not met.

To know more about the dependencies of a package, search for the package symbol in the config menu (see Section 8.1, “make tips”).

Then, you may have to recursively enable several options (which correspond to the unmet dependencies) to finally be able to select the package.

If the package is not visible due to some unmet toolchain options, then you should certainly run a full rebuild (see Section 8.1, “make tips” for more explanations).

10.6. Why not use the target directory as a chroot directory?

There are plenty of reasons to not use the target directory a chroot one, among these:

  • file ownerships, modes and permissions are not correctly set in the target directory;
  • device nodes are not created in the target directory.

For these reasons, commands run through chroot, using the target directory as the new root, will most likely fail.

If you want to run the target filesystem inside a chroot, or as an NFS root, then use the tarball image generated in images/ and extract it as root.

10.7. Why doesn’t Buildroot generate binary packages (.deb, .ipkg…)?

One feature that is often discussed on the Buildroot list is the general topic of "package management". To summarize, the idea would be to add some tracking of which Buildroot package installs what files, with the goals of:

  • being able to remove files installed by a package when this package gets unselected from the menuconfig;
  • being able to generate binary packages (ipk or other format) that can be installed on the target without re-generating a new root filesystem image.

In general, most people think it is easy to do: just track which package installed what and remove it when the package is unselected. However, it is much more complicated than that:

  • It is not only about the target/ directory, but also the sysroot in host/<tuple>/sysroot and the host/ directory itself. All files installed in those directories by various packages must be tracked.
  • When a package is unselected from the configuration, it is not sufficient to remove just the files it installed. One must also remove all its reverse dependencies (i.e. packages relying on it) and rebuild all those packages. For example, package A depends optionally on the OpenSSL library. Both are selected, and Buildroot is built. Package A is built with crypto support using OpenSSL. Later on, OpenSSL gets unselected from the configuration, but package A remains (since OpenSSL is an optional dependency, this is possible.) If only OpenSSL files are removed, then the files installed by package A are broken: they use a library that is no longer present on the target. Although this is technically doable, it adds a lot of complexity to Buildroot, which goes against the simplicity we try to stick to.
  • In addition to the previous problem, there is the case where the optional dependency is not even known to Buildroot. For example, package A in version 1.0 never used OpenSSL, but in version 2.0 it automatically uses OpenSSL if available. If the Buildroot .mk file hasn’t been updated to take this into account, then package A will not be part of the reverse dependencies of OpenSSL and will not be removed and rebuilt when OpenSSL is removed. For sure, the .mk file of package A should be fixed to mention this optional dependency, but in the mean time, you can have non-reproducible behaviors.
  • The request is to also allow changes in the menuconfig to be applied on the output directory without having to rebuild everything from scratch. However, this is very difficult to achieve in a reliable way: what happens when the suboptions of a package are changed (we would have to detect this, and rebuild the package from scratch and potentially all its reverse dependencies), what happens if toolchain options are changed, etc. At the moment, what Buildroot does is clear and simple so its behaviour is very reliable and it is easy to support users. If configuration changes done in menuconfig are applied after the next make, then it has to work correctly and properly in all situations, and not have some bizarre corner cases. The risk is to get bug reports like "I have enabled package A, B and C, then ran make, then disabled package C and enabled package D and ran make, then re-enabled package C and enabled package E and then there is a build failure". Or worse "I did some configuration, then built, then did some changes, built, some more changes, built, some more changes, built, and now it fails, but I don’t remember all the changes I did and in which order". This will be impossible to support.

For all these reasons, the conclusion is that adding tracking of installed files to remove them when the package is unselected, or to generate a repository of binary packages, is something that is very hard to achieve reliably and will add a lot of complexity.

On this matter, the Buildroot developers make this position statement:

  • Buildroot strives to make it easy to generate a root filesystem (hence the name, by the way.) That is what we want to make Buildroot good at: building root filesystems.
  • Buildroot is not meant to be a distribution (or rather, a distribution generator.) It is the opinion of most Buildroot developers that this is not a goal we should pursue. We believe that there are other tools better suited to generate a distro than Buildroot is. For example, Open Embedded, or openWRT, are such tools.
  • We prefer to push Buildroot in a direction that makes it easy (or even easier) to generate complete root filesystems. This is what makes Buildroot stands out in the crowd (among other things, of course!)
  • We believe that for most embedded Linux systems, binary packages are not necessary, and potentially harmful. When binary packages are used, it means that the system can be partially upgraded, which creates an enormous number of possible combinations of package versions that should be tested before doing the upgrade on the embedded device. On the other hand, by doing complete system upgrades by upgrading the entire root filesystem image at once, the image deployed to the embedded system is guaranteed to really be the one that has been tested and validated.

10.8. How to speed-up the build process?

Since Buildroot often involves doing full rebuilds of the entire system that can be quite long, we provide below a number of tips to help reduce the build time:

  • Use a pre-built external toolchain instead of the default Buildroot internal toolchain. By using a pre-built Linaro toolchain (on ARM) or a Sourcery CodeBench toolchain (for ARM, x86, x86-64, MIPS, etc.), you will save the build time of the toolchain at each complete rebuild, approximately 15 to 20 minutes. Note that temporarily using an external toolchain does not prevent you to switch back to an internal toolchain (that may provide a higher level of customization) once the rest of your system is working;
  • Use the ccache compiler cache (see: Section 8.12.3, “Using ccache in Buildroot”);
  • Learn about rebuilding only the few packages you actually care about (see Section 8.3, “Understanding how to rebuild packages”), but beware that sometimes full rebuilds are anyway necessary (see Section 8.2, “Understanding when a full rebuild is necessary”);
  • Make sure you are not using a virtual machine for the Linux system used to run Buildroot. Most of the virtual machine technologies are known to cause a significant performance impact on I/O, which is really important for building source code;
  • Make sure that you’re using only local files: do not attempt to do a build over NFS, which significantly slows down the build. Having the Buildroot download folder available locally also helps a bit.
  • Buy new hardware. SSDs and lots of RAM are key to speeding up the builds.

Chapter 12. Legal notice and licensing

12.1. Complying with open source licenses

All of the end products of Buildroot (toolchain, root filesystem, kernel, bootloaders) contain open source software, released under various licenses.

Using open source software gives you the freedom to build rich embedded systems, choosing from a wide range of packages, but also imposes some obligations that you must know and honour. Some licenses require you to publish the license text in the documentation of your product. Others require you to redistribute the source code of the software to those that receive your product.

The exact requirements of each license are documented in each package, and it is your responsibility (or that of your legal office) to comply with those requirements. To make this easier for you, Buildroot can collect for you some material you will probably need. To produce this material, after you have configured Buildroot with make menuconfig, make xconfig or make gconfig, run:

make legal-info

Buildroot will collect legally-relevant material in your output directory, under the legal-info/ subdirectory. There you will find:

  • A README file, that summarizes the produced material and contains warnings about material that Buildroot could not produce.
  • buildroot.config: this is the Buildroot configuration file that is usually produced with make menuconfig, and which is necessary to reproduce the build.
  • The source code for all packages; this is saved in the sources/ and host-sources/ subdirectories for target and host packages respectively. The source code for packages that set <PKG>_REDISTRIBUTE = NO will not be saved. Patches that were applied are also saved, along with a file named series that lists the patches in the order they were applied. Patches are under the same license as the files that they modify. Note: Buildroot applies additional patches to Libtool scripts of autotools-based packages. These patches can be found under support/libtool in the Buildroot source and, due to technical limitations, are not saved with the package sources. You may need to collect them manually.
  • A manifest file (one for host and one for target packages) listing the configured packages, their version, license and related information. Some of this information might not be defined in Buildroot; such items are marked as "unknown".
  • The license texts of all packages, in the licenses/ and host-licenses/ subdirectories for target and host packages respectively. If the license file(s) are not defined in Buildroot, the file is not produced and a warning in the README indicates this.

Please note that the aim of the legal-info feature of Buildroot is to produce all the material that is somehow relevant for legal compliance with the package licenses. Buildroot does not try to produce the exact material that you must somehow make public. Certainly, more material is produced than is needed for a strict legal compliance. For example, it produces the source code for packages released under BSD-like licenses, that you are not required to redistribute in source form.

Moreover, due to technical limitations, Buildroot does not produce some material that you will or may need, such as the toolchain source code and the Buildroot source code itself (including patches to packages for which source distribution is required). When you run make legal-info, Buildroot produces warnings in the README file to inform you of relevant material that could not be saved.

Finally, keep in mind that the output of make legal-info is based on declarative statements in each of the packages recipes. The Buildroot developers try to do their best to keep those declarative statements as accurate as possible, to the best of their knowledge. However, it is very well possible that those declarative statements are not all fully accurate nor exhaustive. You (or your legal department) have to check the output of make legal-info before using it as your own compliance delivery. See the NO WARRANTY clauses (clauses 11 and 12) in the COPYING file at the root of the Buildroot distribution.

12.2. Complying with the Buildroot license

Buildroot itself is an open source software, released under the GNU General Public License, version 2 or (at your option) any later version, with the exception of the package patches detailed below. However, being a build system, it is not normally part of the end product: if you develop the root filesystem, kernel, bootloader or toolchain for a device, the code of Buildroot is only present on the development machine, not in the device storage.

Nevertheless, the general view of the Buildroot developers is that you should release the Buildroot source code along with the source code of other packages when releasing a product that contains GPL-licensed software. This is because the GNU GPL defines the "complete source code" for an executable work as "all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable". Buildroot is part of the scripts used to control compilation and installation of the executable, and as such it is considered part of the material that must be redistributed.

Keep in mind that this is only the Buildroot developers' opinion, and you should consult your legal department or lawyer in case of any doubt.

12.2.1. Patches to packages

Buildroot also bundles patch files, which are applied to the sources of the various packages. Those patches are not covered by the license of Buildroot. Instead, they are covered by the license of the software to which the patches are applied. When said software is available under multiple licenses, the Buildroot patches are only provided under the publicly accessible licenses.

See Chapter 18, Patching a package for the technical details.