kumquat-buildroot/docs/manual/embedded-basics.txt

// -*- mode:doc; -*-

Embedded system basics
----------------------

When developing an embedded system, there are a number of choices to
address:

* the cross-toolchain: target architecture/C library/...
* the bootloader
* kernel options
* the device management
* the init system
* the package selection (busybox vs. "real" programs, ...)
* ...

Some of these may be influenced by the target hardware.

Some of the choices may also add some constraints when you develop the
final application for which your target is designed (e.g. some
functions may be provided by some C libraries and missing in some
others, ...). So, these choices should be carefully made.

Buildroot allows you to set most of these options to fit your needs.

Moreover, Buildroot provides an infrastructure for reproducing the
build process of your kernel, cross-toolchain, and embedded root
filesystem. Being able to reproduce the build process will be useful
when a component needs to be patched or updated or when another person
is supposed to take over the project.

[[cross-compilation-and-cross-toolchain]]
Cross-compilation & cross-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
http://www.gnu.org/software/libc/libc.html[GNU Libc],
http://www.uclibc.org/[uClibc] or
http://www.fefe.de/dietlibc/[dietlibc]).

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" footnote:[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)].

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.

Even if your embedded system uses an x86 processor, you might be
interested in Buildroot for two reasons:

* The compilation toolchain on your host certainly uses the GNU Libc
  which is a complete but huge C standard library. Instead of using
  GNU Libc on your target system, you can use uClibc which is a tiny C
  standard library. If you want to use this C library, then you need a
  compilation toolchain to generate binaries linked with it. Buildroot
  can do that for you.

* Buildroot automates the building of a root filesystem with all
  needed tools like busybox. That makes it much easier than doing it
  by hand.

You might wonder why such a tool is needed when you can compile +gcc+,
+binutils+, +uClibc+ and all the other tools by hand. Of course doing
so is possible, but dealing with all of the configure options and
problems of every +gcc+ or +binutils+ version is very time-consuming
and uninteresting.  Buildroot automates this process through the use
of Makefiles and has a collection of patches for each +gcc+ and
+binutils+ version to make them work on most architectures.

Buildroot offers a number of options and settings that can be tuned
when defining the cross-toolchain (refer to xref:toolchain-custom[]).

[[bootloader]]
Bootloader
~~~~~~~~~~

TODO

[[device-management]]
Device management
~~~~~~~~~~~~~~~~~

TODO

[[init-system]]
Init system
~~~~~~~~~~~

TODO