File.........: overview.txt

 Content......: Overview of how ct-ng works.

 Copyrigth....: (C) 2007 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  /

 ________/

 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. Also, submited

 patches take a looong 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.

 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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 Configuring crosstool-NG  /

 _________________________/

 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:

   make menuconfig

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

 String and number options can refer to environment variables. In such a case,

 you  must use the shell syntax: ${VAR}. No such option is ever needed by make.

 You need to neither single- nor double-quote the string options.

 There are three environment variablea 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 sits. 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 copy of crosstool-NG, 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 help 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  /

 _____________________/

 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.

 To build the toolchain, simply type:

   make

 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:

   make help

 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:

   make 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:

   make RESTART=some_step

 The list of steps is, in order of appearence in the build process:

   - libc_check_config

   - kernel_check_config

   - kernel_headers

   - binutils

   - cc_core_pass_1

   - libc_headers

   - libc_start_files

   - cc_core_pass_2

   - libfloat

   - libc

   - cc

   - libc_finish

   - tools

   - debug

 Alternatively, you can call make with the name of a step to just do that step:

   make libc_headers

 is equivalent to:

   make RESTART=libs_headers STOP=libc_headers

 The shortcuts -step_name and step_name- allow to respectively stop or restart

 at that step. Thus:

   make -libc_headers        and:    make libc_headers-

 are equivalent to:

   make STOP=libc_headers    and:    make RESTART=libc_headers

 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:

   make 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 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 (*), 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.

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

 Makefile-based.

 Makefile front-end |

 -------------------*

 The Makefile defines a set of rules to call each action. You can get the

 list, along with some terse description, by typing "make help" in your

 favourite command line.

 The Makefile sets the version variable from the version file in ${CT_TOP_DIR}

 which is then available to others in the CT_VERSION environment variable.

 The kconfig language is a hacked version, vampirised from the toybox project

 by Rob LANDLEY (http://www.landley.net/code/toybox/), adapted to my needs.