doc: remove 11 year old docs (#435857)
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Emily
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Setting up a cross compiler with Nix
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"Cross compilation" means compiling a program on one machine for another
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type of machine. A typical use of cross compilation is to compile programs
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for embedded devices. These devices often don't have the computing power
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and memory to compile programs natively.
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For a fully working cross compiler the following are needed:
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* cross binutils: assembler, archiver, linker, etcetera that understand
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the format of the target system
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* cross compiler: a compiler that can generate binary code and object files
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for the target platform
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* cross C library: a library to link object files with to create fully
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functional programs
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Cross compilers are difficult to set up. A lot of people report that they
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cannot succeed in building a cross toolchain successfully. The answers
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usually consist of "download this pre-built toolchain", which is equally
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unhelpful.
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A toolchain is set up in five steps:
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1. build binutils to that can run on the host platform, but generate code
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for the target platform
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2. build Linux kernel headers for the target platform
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3. build a minimal C only version of GCC, that can run on the host platform
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and generate code for the target platform
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4. build a C library for the target platform. This includes the dynamic
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linker, C library, etc.
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5. build a full GCC
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****
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NB:
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Keep in mind that many programs are not very well suited for cross
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compilation. Either they are not intended to run on other platforms,
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because the code is highly platform specific, or the configuration process
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is not written with cross compilation in mind.
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Nix will not solve these problems for you!
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***
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This document describes to set up a cross compiler to generate code for
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arm-linux with uClibc and runs on i686-linux. The "stdenv" used is the
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default from the standard Nix packages collection.
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Step 1: build binutils for arm-linux in the stdenv for i686-linux
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---
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{stdenv, fetchurl, noSysDirs}:
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stdenv.mkDerivation {
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name = "binutils-2.16.1-arm";
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builder = ./builder.sh;
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src = fetchurl {
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url = "http://ftp.nluug.nl/gnu/binutils/binutils-2.16.1.tar.bz2";
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hash = "sha256-14pv+YKrL3NyFwbnv9MoWsZHgEZk5+pHhuZtAfkcVsU=";
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};
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inherit noSysDirs;
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configureFlags = [ "--target=arm-linux" ];
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}
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---
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This will compile binutils that will run on i686-linux, but knows the
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format used by arm-linux.
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Step 2: build kernel headers for the target architecture
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default.nix for kernel-headers-arm:
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---
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{stdenv, fetchurl}:
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assert stdenv.buildPlatform.system == "i686-linux";
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stdenv.mkDerivation {
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name = "linux-headers-2.6.13.1-arm";
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builder = ./builder.sh;
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src = fetchurl {
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url = "http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.13.1.tar.bz2";
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hash = "sha256-qtICDjfiA1HxWBrHqtB5DCv9s9/HyznKV1C6IxCrHYs=";
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};
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}
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---
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builder.sh for kernel-headers-arm:
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---
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source $stdenv/setup
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buildPhase() {
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make include/linux/version.h
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}
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buildPhase=buildPhase
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installPhase() {
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mkdir $out
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mkdir $out/include
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#cd $out/include
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#ln -s asm-arm asm
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make include/asm ARCH=arm
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cp -prvd include/linux include/asm include/asm-arm include/asm-generic $out/include
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echo -n > $out/include/linux/autoconf.h
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}
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installPhase=installPhase
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genericBuild
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---
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Step 3: build a minimal GCC
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Extra/different parameters include the target platform and the kernel
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headers argument (this needs a major cleanup, as well as the name, it
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needs to be different!). Profiled compilers are disabled. The tarball
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used here is just gcc-core. For some reason it doesn't install nicely
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if the whole tarball is used (or is this some braino on my side? -- AH).
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Only C is used, because for other languages (such as C++) extra libraries
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need to be compiled, for which libraries compiled for the target system
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are needed.
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There is a bit of evilness going on. The cross compiled utilities need
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to be either copied to or be linked from the output tree of the compiler.
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(Is this really true? Back this up with arguments! -- AH)
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Symbolic links are not something we want inside the Nix store.
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---
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{ stdenv, fetchurl, noSysDirs
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, langC ? true, langCC ? true, langF77 ? false
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, profiledCompiler ? false
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, binutilsArm
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, kernelHeadersArm
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}:
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assert langC;
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stdenv.mkDerivation {
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name = "gcc-4.0.2-arm";
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builder = ./builder.sh;
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src = fetchurl {
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url = "ftp://ftp.nluug.nl/pub/gnu/gcc/gcc-4.0.2/gcc-core-4.0.2.tar.bz2";
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hash = "sha256-LANmXRS7/fN2zF5JUJVd8OjNA5aCDsGLQKhSpxWA3Qk=";
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};
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# !!! apply only if noSysDirs is set
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patches = [./no-sys-dirs.patch ./gcc-inhibit.patch];
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inherit noSysDirs langC langCC langF77 profiledCompiler;
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buildInputs = [binutilsArm];
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inherit kernelHeadersArm binutilsArm;
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platform = "arm-linux";
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}
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---
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The builder.sh for a cross-compiler. Note that the binutils are prefixed
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with the architecture name, so arm-linux-ld instead of ld, etc. This is
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necessary because when we cross-compile a lot of programs look for these
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tools with these specific names. The standard gcc-wrapper does not take this
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into account yet.
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---
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source $stdenv/setup
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export NIX_FIXINC_DUMMY=$NIX_BUILD_TOP/dummy
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mkdir $NIX_FIXINC_DUMMY
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if test "$noSysDirs" = "1"; then
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if test "$noSysDirs" = "1"; then
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# Figure out what extra flags to pass to the gcc compilers
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# being generated to make sure that they use our glibc.
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if test -e $NIX_CC/nix-support/orig-glibc; then
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glibc=$(cat $NIX_CC/nix-support/orig-glibc)
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# Ugh. Copied from gcc-wrapper/builder.sh. We can't just
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# source in $NIX_CC/nix-support/add-flags, since that
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# would cause *this* GCC to be linked against the
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# *previous* GCC. Need some more modularity there.
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extraCFlags="-B$glibc/lib -isystem $glibc/include"
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extraLDFlags="-B$glibc/lib -L$glibc/lib -Wl,-s \
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-Wl,-dynamic-linker,$glibc/lib/ld-linux.so.2"
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# Oh, what a hack. I should be shot for this.
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# In stage 1, we should link against the previous GCC, but
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# not afterwards. Otherwise we retain a dependency.
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# However, ld-wrapper, which adds the linker flags for the
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# previous GCC, is also used in stage 2/3. We can prevent
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# it from adding them by NIX_GLIBC_FLAGS_SET, but then
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# gcc-wrapper will also not add them, thereby causing
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# stage 1 to fail. So we use a trick to only set the
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# flags in gcc-wrapper.
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hook=$(pwd)/ld-wrapper-hook
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echo "NIX_GLIBC_FLAGS_SET=1" > $hook
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export NIX_LD_WRAPPER_START_HOOK=$hook
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fi
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export NIX_EXTRA_CFLAGS=$extraCFlags
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export NIX_EXTRA_LDFLAGS=$extraLDFlags
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export CFLAGS=$extraCFlags
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export CXXFLAGS=$extraCFlags
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export LDFLAGS=$extraLDFlags
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fi
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else
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patches=""
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fi
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preConfigure=preConfigure
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preConfigure() {
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# Determine the frontends to build.
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langs="c"
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if test -n "$langCC"; then
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langs="$langs,c++"
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fi
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if test -n "$langF77"; then
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langs="$langs,f77"
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fi
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# Cross compiler evilness
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mkdir -p $out
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mkdir -p $out/arm-linux
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mkdir -p $out/arm-linux/bin
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ln -s $binutilsArm/arm-linux/bin/as $out/arm-linux/bin/as
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ln -s $binutilsArm/arm-linux/bin/ld $out/arm-linux/bin/ld
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ln -s $binutilsArm/arm-linux/bin/ar $out/arm-linux/bin/ar
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ln -s $binutilsArm/arm-linux/bin/ranlib $out/arm-linux/bin/ranlib
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# Perform the build in a different directory.
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mkdir ../build
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cd ../build
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configureScript=../$sourceRoot/configure
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configureFlags="--enable-languages=$langs --target=$platform --disable-threads --disable-libmudflap --disable-shared --with-headers=$kernelHeadersArm/include --disable-multilib"
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}
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postInstall=postInstall
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postInstall() {
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# Remove precompiled headers for now. They are very big and
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# probably not very useful yet.
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find $out/include -name "*.gch" -exec rm -rf {} \; -prune
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# Remove `fixincl' to prevent a retained dependency on the
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# previous gcc.
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rm -rf $out/libexec/gcc/*/*/install-tools
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}
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#if test -z "$profiledCompiler"; then
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#makeFlags="bootstrap"
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#else
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#makeFlags="profiledbootstrap"
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#fi
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genericBuild
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---
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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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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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...
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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 process. 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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Obviously, 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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---
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After successfully completing the whole toolchain you can start building
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packages with the newly built tools. To make everything build correctly
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you will need a stdenv for your target platform. Setting up this platform
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will take some effort. Right now there is a very experimental setup for
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arm-linux, which needs to be cleaned up before it is production ready.
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Please note that many packages are not well suited for cross-compilation.
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Even though the package itself might be very well portable often the
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buildscripts are not. One thing that we have seen that causes frequent
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build failures is the use of the LD variable. This is often set to 'ld'
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and not $(CROSS)-ld.
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