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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 tool chain (+gcc+, +binutils+ and
+uClibc+).
There is basically one Makefile per software package, and they are
named with the +.mk+ extension. Makefiles are split into three main
sections:
* *toolchain* (in 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+.
* *package* (in the +package/+ directory) contains the Makefiles and
associated files for all user-space tools that Buildroot can compile
and add to the target root filesystem. There is one sub-directory
per tool.
* *target* (in the +target+ directory) contains the Makefiles and
associated files for software related to the generation of the
target root filesystem image. Four types of filesystems are
supported: ext2, jffs2, cramfs and squashfs. For each of them there
is a sub-directory with the required files. There is also a
+default/+ directory that contains the target filesystem skeleton.
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+,
+stamps+, etc. in the output directory (+output/+ by default,
another value can be specified using +O=+)
* Generate all the targets listed in the +BASE_TARGETS+ variable. When
an internal toolchain is used, this means generating the
cross-compilation toolchain. When an external toolchain is used,
this means checking the features of the external toolchain and
importing it into the Buildroot environment.
* 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.
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