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Merge pull request #113916 from siraben/xcompile-docs-patch-1

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stdenv: cross-compilation documentation enhancements
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Ryan Mulligan 2021-02-21 20:41:00 -08:00 committed by GitHub
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doc/stdenv/cross-compilation.chapter.md
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@ -16,7 +16,7 @@ Nixpkgs follows the [conventions of GNU autoconf](https://gcc.gnu.org/onlinedocs
In Nixpkgs, these three platforms are defined as attribute sets under the names `buildPlatform`, `hostPlatform`, and `targetPlatform`. They are always defined as attributes in the standard environment. That means one can access them like:
```nix
{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...
{ stdenv, fooDep, barDep, ... }: ...stdenv.buildPlatform...
```
`buildPlatform`
@ -99,15 +99,26 @@ Some examples will make this table clearer. Suppose there's some package that is
Some frequently encountered problems when packaging for cross-compilation should be answered here. Ideally, the information above is exhaustive, so this section cannot provide any new information, but it is ludicrous and cruel to expect everyone to spend effort working through the interaction of many features just to figure out the same answer to the same common problem. Feel free to add to this list!
#### My package fails to find a binutils command (`cc`/`ar`/`ld` etc.) {#cross-qa-fails-to-find-binutils}
Many packages assume that an unprefixed binutils (`cc`/`ar`/`ld` etc.) is available, but Nix doesn't provide one. It only provides a prefixed one, just as it only does for all the other binutils programs. It may be necessary to patch the package to fix the build system to use a prefix. For instance, instead of `cc`, use `${stdenv.cc.targetPrefix}cc`.
```nix
makeFlags = [ "CC=${stdenv.cc.targetPrefix}cc" ];
```
#### How do I avoid compiling a GCC cross-compiler from source? {#cross-qa-avoid-compiling-gcc-cross-compiler}
On less powerful machines, it can be inconvenient to cross-compile a package only to find out that GCC has to be compiled from source, which could take up to several hours. Nixpkgs maintains a limited [cross-related jobset on Hydra](https://hydra.nixos.org/jobset/nixpkgs/cross-trunk), which tests cross-compilation to various platforms from build platforms "x86\_64-darwin", "x86\_64-linux", and "aarch64-linux". See `pkgs/top-level/release-cross.nix` for the full list of target platforms and packages. For instance, the following invocation fetches the pre-built cross-compiled GCC for `armv6l-unknown-linux-gnueabihf` and builds GNU Hello from source.
```ShellSession
$ nix-build '<nixpkgs>' -A pkgsCross.raspberryPi.hello
```
#### What if my package's build system needs to build a C program to be run under the build environment? {#cross-qa-build-c-program-in-build-environment}
Add the following to your `mkDerivation` invocation.
```nix
depsBuildBuild = [ buildPackages.stdenv.cc ];
```
#### My package fails to find `ar`. {#cross-qa-fails-to-find-ar}
Many packages assume that an unprefixed `ar` is available, but Nix doesn't provide one. It only provides a prefixed one, just as it only does for all the other binutils programs. It may be necessary to patch the package to fix the build system to use a prefixed `ar`.
#### My package's testsuite needs to run host platform code. {#cross-testsuite-runs-host-code}
Add the following to your `mkDerivation` invocation.