C

This article gives practical advices when working C/C++ projects with Nix. It is written to be read from top to bottom.

Differences between nixpkgs and the rest

The way nixpkgs and its stdenv handles compiling and linking is very different from other linux distributions. Usually header files are put into well known paths i.e. /usr/include, where the compiler will look for them. Same is true when linking against libraries, which are put in a few places, where the build-time linker. Dynamically linked libraries will have a run-time linker (also known as ld.so) set as an interpreter. This linker reads /etc/ld.so.conf to figure out where to find libraries. In nixpkgs in contrast this information is provided by environment variables that will be set based on the build inputs that are given when building a package or when loading a nix expression into a `nix-shell`. Therefore it is not sufficient to just install libraries with nix-env into the profile since the compiler will not look in those paths when compiling.

The compiler wrapper

When expecting the compiler or linker executable one will notice that those are not actual binaries but shell scripts:

$ nix-shell -p hello --command 'which $CC'
/nix/store/isg8rxaxkijl9x3hr2gzsf8pqfnqxg3k-gcc-wrapper-7.4.0/bin/gcc
$ nix-shell -p hello --command 'which $LD'
/nix/store/isg8rxaxkijl9x3hr2gzsf8pqfnqxg3k-gcc-wrapper-7.4.0/bin/ld
$ file /nix/store/isg8rxaxkijl9x3hr2gzsf8pqfnqxg3k-gcc-wrapper-7.4.0/bin/gcc /nix/store/isg8rxaxkijl9x3hr2gzsf8pqfnqxg3k-gcc-wrapper-7.4.0/bin/ld
/nix/store/isg8rxaxkijl9x3hr2gzsf8pqfnqxg3k-gcc-wrapper-7.4.0/bin/gcc: a /nix/store/vs6d2fjkl4kb3jb7rwibsd76k9v2n4xy-bash-4.4-p23/bin/bash script, ASCII text executable
/nix/store/isg8rxaxkijl9x3hr2gzsf8pqfnqxg3k-gcc-wrapper-7.4.0/bin/ld:  symbolic link to /nix/store/lwdkm354f3zzsvkf7pqmnc8w6r164b42-binutils-wrapper-2.30/bin/ld

These shell-scripts wrapper around the actual compiler and additional compiler flags depending on environment variables. In particular the wrapper around the c compiler, will also look for a NIX_CFLAGS_COMPILE variable and prepend the content to command line arguments passed to the underlying compiler. A different variable is called NIX_LDFLAGS, which will be provided as input to the build time linker. Nixpkgs use these variables to influence what kind of header files and libraries are visible to the build tools when running. For example when we add zlib to buildInputs of a stdenv.mkDerivation call and load the resulting file in a nix-shell, we can see the effect on both NIX_CFLAGS_COMPILE and NIX_LDFLAGS

$ cat > shell.nix <<EOF
with import <nixpkgs> {};
stdenv.mkDerivation {
  name = "myenv";
  buildInputs = [ zlib ];
};
EOF
[nix-shell:~] echo $NIX_CFLAGS_COMPILE
-isystem /nix/store/bjl5kk674rmdzzpmcsvmw73hvf35jwh8-zlib-1.2.11-dev/include -isystem /nix/store/bjl5kk674rmdzzpmcsvmw73hvf35jwh8-zlib-1.2.11-dev/include
[nix-shell:~] echo $NIX_LDFLAGS
-rpath /nix/store/d5dzr90q2wy2nlw0z7s0pgxkjfjv1jrj-myenv/lib64 -rpath /nix/store/d5dzr90q2wy2nlw0z7s0pgxkjfjv1jrj-myenv/lib -L/nix/store/5dphwv1xs46n0qbhynny2lbhmx4xh1fc-zlib-1.2.11/lib -L/nix/store/5dphwv1xs46n0qbhynny2lbhmx4xh1fc-zlib-1.2.11/lib

In $NIX_CFLAGS_COMPILE we see that the include search path by appending new directories using the -isystem</code flag. For $NIX_LDFLAGS see that the library link path is extended using the -L flag. We also notice that in addition to library paths the linker gets instructed to extend the RPATH of the program using the -rpath flag. This is needed when the executable is executed since the runtime linker will read the RPATH from the elf header to figure out where to find shared libraries. We can print the RPATH of executable using the patchelf command.

nix-shell -p hello --command 'patchelf --print-rpath $(which hello)'
/nix/store/fivq0nbggp4y8mhy3ixprqd7qyn1hy2j-glibc-2.27/lib