Using Clang instead of GCC

You can use Clang instead of GCC as a C compiler for any package by overriding stdenv, which contains the compilation toolchain, with:

stdenv = pkgs.clangStdenv;

or to get a specific version of clang:

stdenv = pkgs.llvmPackages_9.stdenv;

Depending on the case you may want to set this value in different location, and using different mechanism.

Unlike gcc, you can not use the clang command to compile C++. You must use the clang++ command. Using clang to compile C++ will likely result in "fatal error: ... file not found" errors on standard library #include directives. This differs from Linux distributions like Debian and Arch Linux. If a project doesn't respect CXX and expects clang to compile C++, it will fail to build.

Globally, in a package repository tree

If you have a set of packages in a repository tree, you can set the stdenv value in the scope where the callPackage are called. Be carefull all the packages present in the scope will be built with Clang because the callPackage that resolves the package function inputs will use the pkgs.clangStdenv for all packages.

rec {
    stdenv = pkgs.clangStdenv;
    foo = callPackage ./foo { };
    bar = callPackage ./bar { };
}

or import nixpkgs with replaceStdenv.

import <nixpkgs> { config = { replaceStdenv = ({ pkgs }: pkgs.clangStdenv); }; }

For a specific package in a repository tree

If you a one specific package in your package repository that you want to build with Clang. You can either override stdenv in the callPackage or creating a package override.

Here only foo will be built with Clang, and only with Clang.

rec {
    foo = callPackage ./foo { stdenv = pkgs.clangStdenv; };
    bar = callPackage ./bar { };
}

But if you want both toolchains you can use:

rec {
    foo_gcc = callPackage ./foo { };
    foo_clang = callPackage ./foo { stdenv = pkgs.clangStdenv; };
    bar = callPackage ./bar { };
}

Using Nix CLI on existing packages

Directly inline with CLI just do:

nix-build -E "with import <nixpkgs> {}; pkgs.hello.override{ stdenv = pkgs.clangStdenv; }"

or, if you want a shell for development:

nix-shell -E "with import <nixpkgs> {}; pkgs.hello.override{ stdenv = pkgs.clangStdenv; }"

Using an external override definition

# in file ./hello_with_clan.nix
with import <nixpkgs> {};
hello.override {
    # use Clang instead of GCC
    stdenv = pkgs.clangStdenv;
}

nix-build ./hello_with_clan.nix

With nix-shell

To use clang in nix-shell instead of gcc:

# in file ./shell.nix
with import <nixpkgs> {};
clangStdenv.mkDerivation {
  name = "clang-nix-shell";
  buildInputs = [ /* add libraries here */ ];
}

Globally, in a package repository tree

For a specific package in a repository tree

Using Nix CLI on existing packages

Using an external override definition

With nix-shell

See also