apt-get install gcc-4.7-arm-linux-gnueabihf

Hi! As you may know, I'm working on multiarch cross-toolchains as part of the Google summer of Code (see my previous post for details), and although there is still a lot to do, the project is doing fairly well, and a cross-gcc is already installable and working. I'll proceed with detailed instructions on how to install the cross-compiler, along with some explanations on multiarch itself, and how to build cross-toolchains from source.

Prerequisites

First, you need specific versions apt (>= 0.9.6) and dpkg (>= 1.16.5).
This is because the cross-toolchains packages I'm working on depend on specific cross-arch dependencies, such as libc6:armhf. Support for such dependencies has been added in the aforementioned versions of apt and dpkg. This means you should be running unstable, or at least use dpkg from unstable.

Adding a foreign architecture

Adding a foreign architecture is straightforward, but I've seen users do overcomplicated, or just plain wrong things. So, let's do this step by step (well, there are only two of them), with some explanations.

Tell dpkg to handle an additional architecture by typing dpkg --add-architecture armhf. There isn't much to say. It will only add an architecture to the list of architectures dpkg would accept to handle.
Update APT's database. You need to run apt-get update after adding or removing an architecture to get package lists for all wanted architectures and rebuild APT's database, including the implicit cross-arch dependencies/conflicts brought by multiarch.

And that's it for the common case. Now, a word on architecture restrictions: a common misconception about multiarch seems to be that you have to tell APT which architectures you want from the mirrors by modifying your sources.list file(s). This is not true, as, by default, APT gets the list of accepted architectures from dpkg, and gets the correct package lists from the mirrors configured in the sources.list. You only need to use arch restrictions (“deb [arch=arch1,arch2,...] ...”) when you want to get packages from a mirror only for a subset of your accepted architectures. This is very unlikely, and in the general you do not need to modify your sources.list, at all.

Getting the cross-compilers from my repository

I provide compiled versions of gcc-4.7-arm-linux-gnueabihf for i386 and amd64 in my repository. If you have followed the previous paragraphs, nothing should be easier than adding a “deb http://emdebian.org/~thibg/repo/ sid main” line to your sources.list and doing the following:

# apt-get update
[...]
# apt-get --no-install-recommends install gcc-4.7-arm-linux-gnueabihf
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  binutils-arm-linux-gnueabihf cpp-4.7-arm-linux-gnueabihf gcc-4.7-arm-linux-gnueabihf-base gcc-4.7-base:armhf libc6:armhf libgcc1:armhf
  libgmp10 libgomp1:armhf libmpc2 libmpfr4
Suggested packages:
  binutils-doc gcc-4.7-locales libmudflap0-4.7-dev:armhf gcc-4.7-doc libgcc1-dbg:armhf libgomp1-dbg:armhf libitm1-dbg:armhf
  libquadmath-dbg:armhf libmudflap0-dbg:armhf binutils-gold glibc-doc:armhf locales:armhf
Recommended packages:
  libc6-dev:armhf
The following NEW packages will be installed:
  binutils-arm-linux-gnueabihf cpp-4.7-arm-linux-gnueabihf gcc-4.7-arm-linux-gnueabihf gcc-4.7-arm-linux-gnueabihf-base gcc-4.7-base:armhf
  libc6:armhf libgcc1:armhf libgmp10 libgomp1:armhf libmpc2 libmpfr4
0 upgraded, 11 newly installed, 0 to remove and 0 not upgraded.
Need to get 23.5 MB of archives.
After this operation, 57.2 MB of additional disk space will be used.
Do you want to continue [Y/n]?

I've disabled the recommends since, libc6-dev:armhf would recommend gcc:armhf itself, which is not co-installable with your native gcc if you have it installed.

Building the cross-toolchain yourselves

If you don't trust the repository above, or simply want to rebuild the toolchain yourself, it is rather simple: get my gcc-4.7 patches and:

$ apt-get source binutils
$ cd binutils-*/
$ TARGET=arm-linux-gnueabihf dpkg-buildpackage -uc -us -b
$ cd ../
# dpkg -i binutils*arm-linux-gnueabihf*.deb
$ apt-get source gcc-4.7
$ cd gcc-4.7/
$ for patch in path/to/the/patches/*.patch; do patch -p1 < $patch ; done
$ GCC_TARGET=armhf debian/rules control
$ GCC_TARGET=armhf dpkg-buildpackage -uc -us -b

Using the cross-compiler

It's easy, but there is a trick, and dpkg-buildpackage -aarmhf won't work out of the box yet, as gcc-4.7-arm-linux-gnueabihf provides the “arm-linux-gnueabihf-gcc-4.7” executable, but no “arm-linux-gnueabihf-gcc” symlink.

So, for the moment:

$ cat > test.c << __EOF__
#include <stdio.h>
int main(void)
{
  printf("Hello world\\n");
  return 0;
}
__EOF__
$ arm-linux-gnueabihf-gcc-4.7 test.c
$ qemu-arm ./a.out 
Hello world

Conclusion

The toolchain is not complete yet (there is some more work to do on binutils and libstdc++, and equivalents to the “gcc”, “cpp”, and “build-essential” packages to provide), but it should still be useful to cross-compile C programs. Multi-arch cross-toolchains for wheezy can definitely be hosted at emdebian, and I hope to clean up the build process so that it could get included in the archive for wheezy+1.