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.

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" [1].

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.

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 [full-rebuild].

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.

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 to build it either with Buildroot itself (see Build an external toolchain with Buildroot) or with crosstool-NG.

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 Buildroot or Crosstool-NG.

Build an external toolchain with Buildroot

The Buildroot internal toolchain option can be used to create an external toolchain. Here are a series of steps to build an internal toolchain and package it up for reuse by Buildroot itself (or other projects).

Create a new Buildroot configuration, with the following details:

  • Select the appropriate Target options for your target CPU architecture

  • In the Toolchain menu, keep the default of Buildroot toolchain for Toolchain type, and configure your toolchain as desired

  • In the System configuration menu, select None as the Init system and none as /bin/sh

  • In the Target packages menu, disable BusyBox

  • In the Filesystem images menu, disable tar the root filesystem

Then, we can trigger the build, and also ask Buildroot to generate a SDK. This will conveniently generate for us a tarball which contains our toolchain:

make sdk

This produces the SDK tarball in $(O)/images, with a name similar to arm-buildroot-linux-uclibcgnueabi_sdk-buildroot.tar.gz. Save this tarball, as it is now the toolchain that you can re-use as an external toolchain in other Buildroot projects.

In those other Buildroot projects, in the Toolchain menu:

  • Set Toolchain type to External toolchain

  • Set Toolchain to Custom toolchain

  • Set Toolchain origin to Toolchain to be downloaded and installed

  • Set Toolchain URL to file:///path/to/your/sdk/tarball.tar.gz

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

/dev management

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.adoc 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.adoc 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.adoc board/<yourcompany>/<yourproject>/device_table_dev.adoc. For more details about the format of the device table file, see [makedev-syntax].

  • 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 http://git.busybox.net/busybox/tree/docs/mdev.adoc.

  • 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 http://en.wikipedia.org/wiki/Udev.

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.

init system

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 http://git.busybox.net/busybox/tree/examples/inittab (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 http://www.freedesktop.org/wiki/Software/systemd.

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.

1. This terminology differs from what is used by GNU configure, where the host is the machine on which the application will run (which is usually the same as target)