WASM build instructions for Nix

[bouk]

Hi there, I was trying to build this package for a mixed wasm/non-wasm build and landed on adding the following for ensuring clang is available but only used for the wasm build:

export CC_wasm32_unknown_unknown="$(nix build --no-link --print-out-paths nixpkgs#clang)/bin/clang"
export AR_wasm32_unknown_unknown="$(nix build --no-link --print-out-paths nixpkgs#llvmPackages.bintools-unwrapped)/bin/llvm-ar"

This works for me on macOS when using nix

Perhaps this can be added to the wiki, I couldn't find a way to submit an edit

[insipx]

To add to this, if the resulting wasm package has errors that look like "Can't resolve 'env' in ../pkg' or similar, the wasm.js tries to "import 'env'", you must disable zerousdcallregs and stackprotector, i.e

hardeningDisable = [ "zerocallusedregs" "stackprotector" ];

My full arguments:

CC_wasm32_unknown_unknown = "${llvmPackages.libcxxClang}/bin/clang";
AR_wasm32_unknown_unknown = "${llvmPackages.bintools-unwrapped}/bin/llvm-ar";
hardeningDisable = [ "zerocallusedregs" "stackprotector" ];

ensure libcxxClang is used, as that provides some standard C headers that zstd needs to compile. otherwise you will run into compile errors like "limits.h" not found. Of course, zstd must be in the buildInputs section.

[bouk]

Right, I'm actually using this now:

export CC_wasm32_unknown_unknown="$(nix build --no-link --print-out-paths nixpkgs#llvmPackages_19.clang-unwrapped)/bin/clang"
export AR_wasm32_unknown_unknown="$(nix build --no-link --print-out-paths nixpkgs#llvmPackages_19.bintools-unwrapped)/bin/llvm-ar"
export CFLAGS_wasm32_unknown_unknown="-I $(nix build --no-link --print-out-paths nixpkgs#llvmPackages_19.clang-unwrapped.lib)/lib/clang/19/include"

This ensures the stdlib is available and the hardening flags that are normally set on the stdenv are not used.