buildroot/docs/manual/how-buildroot-works.txt

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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.