How to get C++ headers included in NIX_CFLAGS_COMPILE?

danielbarter · September 13, 2022, 5:22am

Here is a simple shell.nix:

with (import <nixpkgs> {});

(mkShell.override {stdenv = llvmPackages_latest.stdenv;}) {
    buildInputs = [
      cmake
      gtest
    ];
}

which produces the following NIX_CFLAGS_COMPILE

-frandom-seed=8nzawrykna
-isystem /nix/store/b4zvm3271fldf2pn3yj03vichy0k28kz-compiler-rt-libc-13.0.1-dev/include
-isystem /nix/store/d1mprylqwl2lgjg7ng7svz3ryvf5m563-gtest-1.11.0-dev/include
-isystem /nix/store/b4zvm3271fldf2pn3yj03vichy0k28kz-compiler-rt-libc-13.0.1-dev/include
-isystem /nix/store/d1mprylqwl2lgjg7ng7svz3ryvf5m563-gtest-1.11.0-dev/include

It would be very convenient for developing cmake projects if there was an easy way to have

"-isystem ${gccForLibs}/include/c++/${gccForLibs.version}"
"-isystem ${gccForLibs}/include/c++/${gccForLibs.version}/${stdenv.targetPlatform.config}"

also included in NIX_CFLAGS_COMPILE. This functionality already exists in cc-wrapper.sh:

github.com

NixOS/nixpkgs/blob/98854cc86ccc85ce639f750dafcaa0eeef209c20/pkgs/build-support/cc-wrapper/cc-wrapper.sh#L123


      
                  if [[ "$p" = -m*=native ]]; then
                      skip "$p"
                  else
                      kept+=("$p")
                  fi
              done
              # Old bash empty array hack
              params=(${kept+"${kept[@]}"})
          fi
          
          
if [[ "$isCxx" = 1 ]]; then
              if [[ "$cxxInclude" = 1 ]]; then
                  NIX_CFLAGS_COMPILE_@suffixSalt@+=" $NIX_CXXSTDLIB_COMPILE_@suffixSalt@"
              fi
              if [[ "$cxxLibrary" = 1 ]]; then
                  NIX_CFLAGS_LINK_@suffixSalt@+=" $NIX_CXXSTDLIB_LINK_@suffixSalt@"
              fi
          fi
          
          
source @out@/nix-support/add-hardening.sh

github.com

NixOS/nixpkgs/blob/98854cc86ccc85ce639f750dafcaa0eeef209c20/pkgs/build-support/cc-wrapper/default.nix#L375


      
          ##
          ## General libc++ support
          ##
          
          
# We have a libc++ directly, we have one via "smuggled" GCC, or we have one
          # bundled with the C compiler because it is GCC
          + optionalString (libcxx != null || (useGccForLibs && gccForLibs.langCC or false) || (isGNU && cc.langCC or false)) ''
            touch "$out/nix-support/libcxx-cxxflags"
            touch "$out/nix-support/libcxx-ldflags"
          ''
          + optionalString (libcxx == null && (useGccForLibs && gccForLibs.langCC or false)) ''
            for dir in ${gccForLibs}/include/c++/*; do
              echo "-isystem $dir" >> $out/nix-support/libcxx-cxxflags
            done
            for dir in ${gccForLibs}/include/c++/*/${targetPlatform.config}; do
              echo "-isystem $dir" >> $out/nix-support/libcxx-cxxflags
            done
          ''
          + optionalString (libcxx.isLLVM or false) (''
            echo "-isystem ${lib.getDev libcxx}/include/c++/v1" >> $out/nix-support/libcxx-cxxflags
            echo "-stdlib=libc++" >> $out/nix-support/libcxx-ldflags

but as far as I can tell, the environment variable isCxx isn’t an attribute of stdenv which i can modify. Is there a way to trigger isCxx that i am missing?

danielbarter · September 18, 2022, 6:51pm

Sorted all of this out this weekend. Firstly, a PR to stop the flag duplication in NIX_CFLAGS_COMPILE: cc-wrapper/setup-hook.sh: remove duplicate flags in NIX_CFLAGS_COMPILE by danielbarter · Pull Request #191724 · NixOS/nixpkgs · GitHub.

Not super important, but it was bugging me. Secondly, reading the cc-wrapper.sh code more closely, all of the command line arguments are being written to files in nix-support. We can read those. For example, here is a shell.nix with a clang standard environment that exposes all the foundational compilation flags + flags for dependencies:

with (import <nixpkgs> {});
let llvm = llvmPackages_latest;
in (mkShell.override {stdenv = llvm.stdenv;}) {
    buildInputs = [
      cmake
      gtest
    ];

    CXXFLAGS = builtins.concatStringsSep " " [
      (lib.removeSuffix "\n" (builtins.readFile "${llvm.clang}/nix-support/cc-cflags"))
      (lib.removeSuffix "\n" (builtins.readFile "${llvm.clang}/nix-support/libc-cflags"))
      (lib.removeSuffix "\n" (builtins.readFile "${llvm.clang}/nix-support/libcxx-cxxflags"))
    ];


    shellHook = "export CXXFLAGS+=$NIX_CFLAGS_COMPILE";
}

CXXFLAGS is used by cmake specifically, but you could use any environment variable you want.