crosstool-ng/docs/overview.txt

File.........: overview.txt
Content......: Overview of how crosstool-NG works.
Copyrigth....: (C) 2007 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
License......: Creative Commons Attribution Share Alike (CC-by-sa), v2.5

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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
http://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. Also, submited
patches take a loooong time before they are integrated mainline.

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. That was a disaster, as inclusion into mainline is slow as hell,
and the changes were so numerous.

The only option left to me was rewrite crosstool from scratch. I decided to go
this way, and name the new implementation crosstool-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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Configuring crosstool-NG  /
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crosstool-NG is configured the same way you configure your Linux kernel: by
using a curses-based menu. It is assumed you now how to handle this.

To enter the menu, type:
  ct-ng menuconfig

Almost every config item has a help entry. Read them carefully.

String and number options can refer to environment variables. In such a case,
you must use the shell syntax: ${VAR}. You shall neither single- nor double-
quote the string options.

There are three environment variables that are computed by crosstool-NG, and
that you can use:

CT_TARGET:
  It represents the target triplet you are building for. You can use it for
  example in the installation/prefix directory, such as:
    /opt/x-tools/${CT_TARGET}

CT_TOP_DIR:
  The top directory where crosstool-NG is running. You shouldn't need it in
  most cases. There is one case where you may need it: if you have local
  patches and you store them in your running directory, you can refer to them
  by using CT_TOP_DIR, such as:
    ${CT_TOP_DIR}/patches.myproject

CT_VERSION:
  The version of crosstool-NG you are using. Not much use for you, but it's
  there if you need it.


Interesting config options |
---------------------------*

CT_LOCAL_TARBALLS_DIR:
  If you already have sone tarballs in a direcotry, enter it here. That will
  speed up the retrieving phase, where crosstool-NG would otherwise download
  those tarballs.

CT_PREFIX_DIR:
  This is where the toolchain will be installed in (and for now, where it
  will run from).

CT_TARGET_VENDOR:
  An identifier for your toolchain, will take place in the vendor part of the
  target triplet. It shall *not* contain spaces or dashes. Usually, keep it
  to a one-word string, or use underscores to separate words if you need.
  Avoid dots, commas, and special characters.

CT_TARGET_ALIAS:
  An alias for the toolchian. It will be used as a prefix to the toolchain
  tools. For example, you will have ${CT_TARGET_ALIAS}-gcc

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Running crosstool-NG  /
_____________________/

crosstool-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.

To build the toolchain, simply type:
  ct-ng build

This will use the above configuration to retrieve, extract and patch the
components, build, install and eventually test your newly built toolchain.

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

In any case, you can get some terse help. Just type:
  ct-ng help
or:
  man 1 ct-ng


Stoping and restarting a build |
-------------------------------*

If you want to stop the build after a step you are debugging, you can pass the
variable STOP to make:
  ct-ng STOP=some_step

Conversely, if you want to restart a build at a specific step you are
debugging, you can pass the RESTART variable to make:
  ct-ng RESTART=some_step

Alternatively, you can call make with the name of a step to just do that step:
  ct-ng libc_headers
is equivalent to:
  ct-ng RESTART=libs_headers STOP=libc_headers

The shortcuts -step_name and step_name- allow to respectively stop or restart
at that step. Thus:
  ct-ng -libc_headers        and:    ct-ng libc_headers-
are equivalent to:
  ct-ng STOP=libc_headers    and:    ct-ng RESTART=libc_headers

To obtain the list of acceptable steps, please call:
  ct-ng liststeps

Note that in order to restart a build, you'll have to say 'Y' to the config
option CT_DEBUG_CT_SAVE_STEPS, and that the previous build effectively went
that far.


Testing all toolchains at once |
-------------------------------*

You can test-build all samples; simply call:
  ct-ng regtest

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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 generating code 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.
    crosstool-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.
    crosstool-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.
    crosstool-NG calls it "cross-native".

4) build != host != target
    This one is called a canadian-toolchain (*), and 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.
    crosstool-NG calls it "canadian".

crosstool-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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Internally, crosstool-NG is script-based. To ease usage, the frontend is
Makefile-based.

Makefile front-end |
-------------------*

The entry point to crosstool-NG is the Makefile script "ct-ng". Calling this
script with an action will act exactly as if the Makefile was in the current
working directory and make was called with the action as rule. Thus:
  ct-ng menuconfig
is equivalent to having the Makefile in CWD, and calling:
  make menuconfig

Having ct-ng as it is avoids copying the Makefile everywhere, and acts as a
traditional command.

ct-ng loads sub- Makefiles from the library directory $(CT_LIB_DIR), as set up
at configuration time with ./configure.

ct-ng also search for config files, sub-tools, samples, scripts and patches in
that library directory.

Kconfig parser |
---------------*

The kconfig language is a hacked version, vampirised from the toybox project
by Rob LANDLEY (http://www.landley.net/code/toybox/), itself coming from the
Linux kernel (http://www.linux.org/ http://www.kernel.org/), and (heavily)
adapted to my needs.

The kconfig parsers (conf and mconf) are not installed pre-built, but as
source files. Thus you can have the directory where crosstool-NG is installed,
exported (via NFS or whatever) and have clients with different architectures
use the same crosstool-NG installation, and most notably, the same set of
patches.

Build scripts |
--------------*

To Be Written later...