File.........: overview.txt
Content......: Overview of how ct-ng works.
Copyrigth....: (C) 2006 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
License......: see COPYING in the root of this package
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Introduction  /
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crosstool-NG aims at building toolchains. Toolchains are an essential component
in a software development project. It will compile, assemble and link the code
that is being developped. Some pieces of the toolchain will eventually end up
in the resulting binary/ies: static libraries are but an example.

So, a toolchain is a very sensitive piece of software, as any bug in one of the
components, or a poorly configured component, can lead to execution problems,
ranging from poor performance, to applications ending unexpectedly, to
mis-behaving software (which more than often is hard to detect), to hardware
damage, or even to human risks (which is more than regretable).

Toolchains are made of different piece of software, each being quite complex
and requiring specially crafted options to build and work seamlessly. This
is usually not that easy, even in the not-so-trivial case of native toolchains.
The work reaches a higher degree of complexity when it comes to cross-
compilation, where it can become quite a nightmare...

Some cross-toolchains exist on the internet, and can be used for general
development, but they have a number of limitations:
  - they can be general purpose, in that they are configured for the majority:
    no optimisation for your specific target,
  - they can be prepared for a specific target and thus are not easy to use,
    nor optimised for, or even supporting your target,
  - they often are using ageing components (compiler, C library, etc...) not
    supporting special features of your shiny new processor;
On the other side, these toolchain offer some advantages:
  - they are ready to use and quite easy to install and setup,
  - they are proven if used by a wide community.

But once you want to get all the juice out of your specific hardware, you will
want to build your own toolchain. This is where crosstool-ng comes into play.

There are also a number of tools that builds toolchains for specific needs,
which is not really scalable. Examples are:
  - buildroot (buildroot.uclibc.org) whose main puprpose is to build root file
    systems, hence the name. But once you have your toolchain with buildroot,
    part of it is installed in the root-to-be, so if you want to build a whole
    new root, you either have to save the existing one as a template and
    restore it later, or restart again from scratch. This is not convenient,
  - ptxdist (www.pengutronix.de/software/ptxdist), whose purpose is very
    similar to buildroot,
  - other projects (openembeded.org for example), which is again used to
    build root file systems.

crosstool-NG is really targetted at building toolchains, and only toolchains.
It is then up to you to use it the way you want.

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History  /
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crosstool was first 'conceived' by Dan Kegel, which offered it to the community,
as a set of scripts, a repository of patches, and some pre-configured, general
purpose setup files to be used to configure crosstool. This is available at
www.kegel.com/crosstool, and the subversion repository is hosted on google at
http://code.google.com/p/crosstool/.

At the time of writing, crosstool only supports building with one C library,
namely glibc, and one C compiler, gcc; it is cripled with historical support
for legacy components, and is some kind of a mess to upgrade.

I once managed to add support for uClibc-based toolchains, but it did not make
into mainline, mostly because I don't have time to port the patch forward to
the new versions, due in part to the big effort it was taking.

So I decided to clean up crosstool in the state it was, re-order the things
in place, and add appropriate support for what I needed, that is uClibc
support.

The only option left to me was rewrite crosstool from scratch. I decided to go
this way, and name the new implementation ct-ng, standing for crosstool Next
Generation, as many other comunity projects do, and as a wink at the TV series
"Star Trek: The Next Generation". ;-)

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Operation  /
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ct-ng is configured by a configurator presenting a menu-stuctured set of
options. These options let you specify the way you want your toolchain built,
where you want it installed, what architecture and specific processor it
will support, the version of the components you want to use, etc... The
value for those options are then stored in a configuration file.

You then simply run make. It will use this configuration file to retrieve,
extract and patch the components, build, install and test your newly built
toolchain.

You are then free to add the toolchain /bin directory in your PATH to use
it at will.


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Toolchain types  /
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There are four kinds of toolchains you could encounter.

First off, you must understand the following: when it comes to compilers there
are up to four machines involved:
  1) the machine configuring the toolchain components: the config machine
  2) the machine building the toolchain components:    the build machine
  3) the machine running the toolchain:                the host machine
  4) the machine the toolchain is building for:        the target machine

We can most of the time assume that the config machine and the build machine
are the same. Most of the time, this will be true. The only time it isn't
is if you're using distributed compilation (such as distcc). Let's forget
this for the sake of simplicity.

So we're left with three machines:
 - build
 - host
 - target

Any toolchain will involve those three machines. You can be as pretty sure of
this as "2 and 2 are 4". Here is how they come into play:

1) build == host == target
    This is a plain native toolchain, targetting the exact same machine as the
    one it is built on, and running again on this exact same machine. You have
    to build such a toolchain when you want to use an updated component, such
    as a newer gcc for example.
    ct-ng calls it "native".

2) build == host != target
    This is a classic cross-toolchain, which is expected to be run on the same
    machine it is compiled on, and generate code to run on a second machine,
    the target.
    ct-ng calls it "cross".

3) build != host == target
    Such a toolchain is also a native toolchain, as it targets the same machine
    as it runs on. But it is build on another machine. You want such a
    toolchain when porting to a new architecture, or if the build machine is
    much faster than the host machine.
    ct-ng calls it "cross-native".

4) build != host != target
    This one is called a canadian-toolchain (*), is is tricky. The three
    machines in play are different. You might want such a toolchain if you
    have a fast build machine, but the users will use it on another machine,
    and will produce code to run on a third machine.
    ct-ng calls it "canadian".

ct-ng can build all these kinds of toolchains (or is aiming at it, anyway!)

(*) The term Canadian Cross came about because at the time that these issues
    were all being hashed out, Canada had three national political parties.
    http://en.wikipedia.org/wiki/Cross_compiler

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Internals  /
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<To be completed>