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  /

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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 |

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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 |

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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 |

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To Be Written later...