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What happens if I push all of nixpkgs
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"Cross compilation" means compiling a program on one machine for anothertype of machine. A typical use of cross compilation is to compile programsfor embedded devices. These devices often don't have the computing powerand memory to compile programs natively.
For a fully working cross compiler the following are needed:
* cross binutils: assembler, archiver, linker, etcetera that understandthe format of the target system
* cross compiler: a compiler that can generate binary code and object filesfor the target platform
* cross C library: a library to link object files with to create fullyfunctional programs
Cross compilers are difficult to set up. A lot of people report that theycannot succeed in building a cross toolchain successfully. The answersusually consist of "download this pre-built toolchain", which is equallyunhelpful.
A toolchain is set up in five steps:
1. build binutils to that can run on the host platform, but generate codefor the target platform
2. build Linux kernel headers for the target platform
3. build a minimal C only version of GCC, that can run on the host platformand generate code for the target platform
4. build a C library for the target platform. This includes the dynamiclinker, C library, etc.
5. build a full GCC
****NB:
Keep in mind that many programs are not very well suited for crosscompilation. Either they are not intended to run on other platforms,because the code is highly platform specific, or the configuration processis not written with cross compilation in mind.
Nix will not solve these problems for you!***
This document describes to set up a cross compiler to generate code forarm-linux with uClibc and runs on i686-linux. The "stdenv" used is thedefault from the standard Nix packages collection.
Step 1: build binutils for arm-linux in the stdenv for i686-linux
---{stdenv, fetchurl, noSysDirs}:
stdenv.mkDerivation { name = "binutils-2.16.1-arm"; builder = ./builder.sh; src = fetchurl { url = "http://ftp.nluug.nl/gnu/binutils/binutils-2.16.1.tar.bz2"; hash = "sha256-14pv+YKrL3NyFwbnv9MoWsZHgEZk5+pHhuZtAfkcVsU="; }; inherit noSysDirs; configureFlags = [ "--target=arm-linux" ];}---
This will compile binutils that will run on i686-linux, but knows theformat used by arm-linux.
Step 2: build kernel headers for the target architecture
default.nix for kernel-headers-arm:
---{stdenv, fetchurl}:
assert stdenv.buildPlatform.system == "i686-linux";
stdenv.mkDerivation { name = "linux-headers-2.6.13.1-arm"; builder = ./builder.sh; src = fetchurl { url = "http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.13.1.tar.bz2"; hash = "sha256-qtICDjfiA1HxWBrHqtB5DCv9s9/HyznKV1C6IxCrHYs="; };}---
builder.sh for kernel-headers-arm:
---source $stdenv/setup
buildPhase() { make include/linux/version.h}
buildPhase=buildPhase
installPhase() { mkdir $out mkdir $out/include #cd $out/include #ln -s asm-arm asm make include/asm ARCH=arm cp -prvd include/linux include/asm include/asm-arm include/asm-generic $out/include echo -n > $out/include/linux/autoconf.h}
installPhase=installPhase
genericBuild---
Step 3: build a minimal GCC
Extra/different parameters include the target platform and the kernelheaders argument (this needs a major cleanup, as well as the name, itneeds to be different!). Profiled compilers are disabled. The tarballused here is just gcc-core. For some reason it doesn't install nicelyif the whole tarball is used (or is this some braino on my side? -- AH).
Only C is used, because for other languages (such as C++) extra librariesneed to be compiled, for which libraries compiled for the target systemare needed.
There is a bit of evilness going on. The cross compiled utilities needto be either copied to or be linked from the output tree of the compiler.(Is this really true? Back this up with arguments! -- AH)
Symbolic links are not something we want inside the Nix store.
---{ stdenv, fetchurl, noSysDirs, langC ? true, langCC ? true, langF77 ? false, profiledCompiler ? false, binutilsArm, kernelHeadersArm}:
assert langC;
stdenv.mkDerivation { name = "gcc-4.0.2-arm"; builder = ./builder.sh; src = fetchurl { url = "ftp://ftp.nluug.nl/pub/gnu/gcc/gcc-4.0.2/gcc-core-4.0.2.tar.bz2"; hash = "sha256-LANmXRS7/fN2zF5JUJVd8OjNA5aCDsGLQKhSpxWA3Qk="; }; # !!! apply only if noSysDirs is set patches = [./no-sys-dirs.patch ./gcc-inhibit.patch]; inherit noSysDirs langC langCC langF77 profiledCompiler; buildInputs = [binutilsArm]; inherit kernelHeadersArm binutilsArm; platform = "arm-linux";}---
The builder.sh for a cross-compiler. Note that the binutils are prefixedwith the architecture name, so arm-linux-ld instead of ld, etc. This isnecessary because when we cross-compile a lot of programs look for thesetools with these specific names. The standard gcc-wrapper does not take thisinto account yet.
---source $stdenv/setup
export NIX_FIXINC_DUMMY=$NIX_BUILD_TOP/dummymkdir $NIX_FIXINC_DUMMY
if test "$noSysDirs" = "1"; then
if test "$noSysDirs" = "1"; then # Figure out what extra flags to pass to the gcc compilers # being generated to make sure that they use our glibc. if test -e $NIX_CC/nix-support/orig-glibc; then glibc=$(cat $NIX_CC/nix-support/orig-glibc) # Ugh. Copied from gcc-wrapper/builder.sh. We can't just # source in $NIX_CC/nix-support/add-flags, since that # would cause *this* GCC to be linked against the # *previous* GCC. Need some more modularity there. extraCFlags="-B$glibc/lib -isystem $glibc/include" extraLDFlags="-B$glibc/lib -L$glibc/lib -Wl,-s \ -Wl,-dynamic-linker,$glibc/lib/ld-linux.so.2"
# Oh, what a hack. I should be shot for this. # In stage 1, we should link against the previous GCC, but # not afterwards. Otherwise we retain a dependency. # However, ld-wrapper, which adds the linker flags for the # previous GCC, is also used in stage 2/3. We can prevent # it from adding them by NIX_GLIBC_FLAGS_SET, but then # gcc-wrapper will also not add them, thereby causing # stage 1 to fail. So we use a trick to only set the # flags in gcc-wrapper. hook=$(pwd)/ld-wrapper-hook echo "NIX_GLIBC_FLAGS_SET=1" > $hook export NIX_LD_WRAPPER_START_HOOK=$hook fi
export NIX_EXTRA_CFLAGS=$extraCFlags export NIX_EXTRA_LDFLAGS=$extraLDFlags export CFLAGS=$extraCFlags export CXXFLAGS=$extraCFlags export LDFLAGS=$extraLDFlags fi
else patches=""fi
preConfigure=preConfigurepreConfigure() {
# Determine the frontends to build. langs="c" if test -n "$langCC"; then langs="$langs,c++" fi if test -n "$langF77"; then langs="$langs,f77" fi
# Cross compiler evilness mkdir -p $out mkdir -p $out/arm-linux mkdir -p $out/arm-linux/bin ln -s $binutilsArm/arm-linux/bin/as $out/arm-linux/bin/as ln -s $binutilsArm/arm-linux/bin/ld $out/arm-linux/bin/ld ln -s $binutilsArm/arm-linux/bin/ar $out/arm-linux/bin/ar ln -s $binutilsArm/arm-linux/bin/ranlib $out/arm-linux/bin/ranlib
# Perform the build in a different directory. mkdir ../build cd ../build
configureScript=../$sourceRoot/configure configureFlags="--enable-languages=$langs --target=$platform --disable-threads --disable-libmudflap --disable-shared --with-headers=$kernelHeadersArm/include --disable-multilib"}
postInstall=postInstallpostInstall() { # Remove precompiled headers for now. They are very big and # probably not very useful yet. find $out/include -name "*.gch" -exec rm -rf {} \; -prune
# Remove `fixincl' to prevent a retained dependency on the # previous gcc. rm -rf $out/libexec/gcc/*/*/install-tools}
#if test -z "$profiledCompiler"; then #makeFlags="bootstrap"#else #makeFlags="profiledbootstrap"#fi
genericBuild---
Step 4: build a C library for the target platform.
The previous steps are enough to compile a C library. In our case we takeuClibc. It's intended to be a small sized replacement for glibc. It is widelyused 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 allthe right tools and the right C library. Successfully compiled programs withthis compiler and verified to be working on a HP Jornada 820 running Linuxare "patch", "make" and "wget".
If we want to build C++ programs it gets a lot more difficult. GCC has athree step compilation process. In the first step a simple compiler, calledxgcc, that can compile only C programs is built. With that compiler itcompiles itself two more times: one time to build a full compiler, and anothertime to build a full compiler once again with the freshly built compiler fromstep 2. In the second and third step support for C++ is compiled, if thisis 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 becompiled 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, glibcin the case of i686-linux using the default Nix packages collection.
Obviously, since we need to compile libstc++ for arm-linux with uClibc linkingwill not be done correctly: you can't link object files built for arm-linuxwith a glibc built for i686-linux.
Setting X_CFLAGS to use the uClibc libraries and dynamic linker will failtoo. Earlier on in the build process these flags are used to compile importantfiles like libgcc.a by the host system gcc, which does need to be linkedto glibc. To make this work correctly you will need to carefully jugglewith compilation flags. This is still work in progress for Nix.
---
After successfully completing the whole toolchain you can start buildingpackages with the newly built tools. To make everything build correctlyyou will need a stdenv for your target platform. Setting up this platformwill take some effort. Right now there is a very experimental setup forarm-linux, which needs to be cleaned up before it is production ready.
Please note that many packages are not well suited for cross-compilation.Even though the package itself might be very well portable often thebuildscripts are not. One thing that we have seen that causes frequentbuild failures is the use of the LD variable. This is often set to 'ld'and not $(CROSS)-ld.