Source of how-buildroot-works.txt - buildroot

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== How Buildroot works

As mentioned above, Buildroot is basically a set of Makefiles that

download, configure, and compile software with the correct options. It

also includes patches for various software packages - mainly the ones

involved in the cross-compilation toolchain (+gcc+, +binutils+ and

+uClibc+).

There is basically one Makefile per software package, and they are

named with the +.mk+ extension. Makefiles are split into many different

parts.

* The +toolchain/+ directory contains the Makefiles

  and associated files for all software related to the

  cross-compilation toolchain: +binutils+, +gcc+, +gdb+,

  +kernel-headers+ and +uClibc+.

* The +arch/+ directory contains the definitions for all the processor

  architectures that are supported by Buildroot.

* The +package/+ directory contains the Makefiles and

  associated files for all user-space tools and libraries that Buildroot

  can compile and add to the target root filesystem. There is one

  sub-directory per package.

* The +linux/+ directory contains the Makefiles and associated files for

  the Linux kernel.

* The +boot/+ directory contains the Makefiles and associated files for

  the bootloaders supported by Buildroot.

* The +system/+ directory contains support for system integration, e.g.

  the target filesystem skeleton and the selection of an init system.

* The +fs/+ directory contains the Makefiles and

  associated files for software related to the generation of the

  target root filesystem image.

Each directory contains at least 2 files:

* +something.mk+ is the Makefile that downloads, configures,

  compiles and installs the package +something+.

* +Config.in+ is a part of the configuration tool

  description file. It describes the options related to the

  package.

The main Makefile performs the following steps (once the

configuration is done):

* Create all the output directories: +staging+, +target+, +build+,

  etc. in the output directory (+output/+ by default,

  another value can be specified using +O=+)

* Generate the toolchain target. When an internal toolchain is used, this

  means generating the cross-compilation toolchain. When an external

  toolchain is used, this means checking the features of the external

  toolchain and importing it into the Buildroot environment.

* Generate all the targets listed in the +TARGETS+ variable. This

  variable is filled by all the individual components'

  Makefiles. Generating these targets will trigger the compilation of

  the userspace packages (libraries, programs), the kernel, the

  bootloader and the generation of the root filesystem images,

  depending on the configuration.