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