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add some stuff about the difficulties we've encountered so far with building cross compilers, especially when building a C++ compiler.

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svn path=/nixpkgs/trunk/; revision=4346
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Armijn Hemel 2005-12-06 21:47:41 +00:00
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commit e6f931a14b
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maintainers/docs/cross.txt
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@ -271,3 +271,41 @@ Step 4: build a C library for the target platform.
The previous steps are enough to compile a C library. In our case we take The previous steps are enough to compile a C library. In our case we take
uClibc. It's intended to be a small sized replacement for glibc. It is widely uClibc. It's intended to be a small sized replacement for glibc. It is widely
used in embedded environments. used in embedded environments.
...
Step 5: Build a compiler to link with the newly built C library.
...
If we restrict the compiler to just C programs it is relatively easy,
since we only need to wrap the GCC we built in the previous step with all
the right tools and the right C library. Successfully compiled programs with
this compiler and verified to be working on a HP Jornada 820 running Linux
are "patch", "make" and "wget".
If we want to build C++ programs it gets a lot more difficult. GCC has a
three step compilation proces. In the first step a simple compiler, called
xgcc, that can compile only C programs is built. With that compiler it
compiles itself two more times: one time to build a full compiler, and another
time to build a full compiler once again with the freshly built compiler from
step 2. In the second and third step support for C++ is compiled, if this
is configured.
One of the libraries that has to be built for C++ support step is libstdc++.
This library uses xgcc, even when cross compiling, since libstdc++ has to be
compiled for arm-linux.
One of the compiler flags that GCC uses for this compiler is called X_CFLAGS.
This is used by the Nix build process to set the dynamic linker, glibc
in the case of i686-linux using the default Nix packages collection.
Obiously, since we need to compile libstc++ for arm-linux with uClibc linking
will not be done correctly: you can't link object files built for arm-linux
with a glibc built for i686-linux.
Setting X_CFLAGS to use the uClibc libraries and dynamic linker will fail
too. Earlier on in the build process these flags are used to compile important
files like libgcc.a by the host system gcc, which does need to be linked
to glibc. To make this work correctly you will need to carefully juggle
with compilation flags. This is still work in progress for Nix.