zig c++: Doesn't support c++ modules

[kassane]

LLVM 16 currently adds support for c++ modules to clang/++, in addition to clang modules.
However, zig cc/c++ does not have optimal support based on the references below.

Test

zig - version 0.11.0-dev.2868+1a455b2dd

# download fork
$> git clone https://github.com/kassane/fmt
$> cd fmt/
$> zig build # add -DTests to build all tests (default libfmt only)
$> zig cc -x c++ -std=c++2b -Iinclude test/module-test.cc -o module -lc++ -Lzig-out/lib -lfmt
test/module-test.cc:47:8: fatal error: module 'fmt' not found
import fmt;
~~~~~~~^~~
1 error generated.

$>  zig cc -x c++-module -std=c++2b -Iinclude test/module-test.cc -o module -lc++ -Lzig-out/lib -lfmt
error: language not recognized: 'c++-module'

cc: @Vexu => (please, add tag)

References

• C++ Module: zig c++ does not support  kassane/fmt#1
• zig run/cc: recognize "-x language" #13544
• [LLD] C++20 Modules - unclosed quote llvm/llvm-project#61198

[aviplayer]

Also tried with build.zig

const std = @import("std");
pub fn build(b: *std.build.Builder) void {
  const flags: []const []const u8 = &.{
      "-std=c++20"
  };

    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    const exe = b.addExecutable(.{
        .name = "run",
        .target = target,
        .optimize = optimize,
    });

     exe.addCSourceFiles(&.{
        "./src/main.cpp",
        "./src/helloworld.cpp"
    }, flags);

    exe.linkLibCpp();

    b.installArtifact(exe);
}

getting fatal error: module 'helloworld' not found

with g++-13 everything is Ok.

[the-argus]

Adding -x c++-module to the end of flags yields the following:

clang failed with stderr: zig: warning: '-x c++-module' after last input file has no effect [-Wunused-command-line-argument]

And this seems to only err because of -Wall.

So I think to implement this, the zig cc invocation just needs to have -x c++-module right at the front, before everything else? Not sure what version of clang we're using, though.

documentation for clang 18 c++ modules: https://clang.llvm.org/docs/StandardCPlusPlusModules.html#how-to-enable-standard-c-modules

[Deins]

Bumped into same issue trying to build amd ROCm stuff with -x hip

 error: error(compilation): clang failed with stderr: zig: warning: '-x hip' after last input file has no effect [-Wunused-command-line-argument]

[kevyuu]

Hi, I want to know whether zig will ever support c++ module in the future before migrating my project from cmake to zig.

[allocgator]

Hi @kassane - I would like to work on this issue. The references look quite old and I wanted to check whether they're still valid. If they're not can you let me know how I can get started?

[kassane]

The problem in question is still the same as that cited in the references.

However, I don't know if the current progress of the Zig project will give relevance to this topic.

cc: @alexrp - would it be planned to improve support for clang modules?

[alexrp]

Depends on how hard it would be to add, I suppose.

Do you have some relevant links for how this stuff works with the vanilla clang driver?

[allocgator]

Relevant discussion on Ziggit using vanilla clang: https://ziggit.dev/t/how-to-use-zig-build-for-importing-c-23s-std/9866/5

---

Edit 2: The last thread on that post was done on Fedora 42

[alexrp]

I'm more curious about how the CLI works; CMake obscures too much.

[kassane]

Unfortunately the topic is very fragmented between LLVM-repo and LLVM-forum.

C++/clang modules support:

• Disambiguate Clang modules and C++20 modules llvm/llvm-project#55891

Impl:

• PR for llvm/llvm-project#60650 llvm/llvm-project-release-prs#293 - clangScanDeps patches for CMake support (since LLVM16)
• [clang][driver][clang-cl] Support --precompile and -fmodule-* options in Clang-CL llvm/llvm-project#98761
• [clang][driver] Support -x for all languages in CL mode llvm/llvm-project#89772

[JoMazM]

Is anyone working on this issue?

I'm trying following the instructions provided in the clang documentation using clang++-20 and zig c++ (0.15.0-dev.635+7dbd21bd5).

CPP Modules Example

// Hello.cppm
module;
#include <iostream>
export module Hello;
export void hello() {
  std::cout << "Hello World!\n";

// use.cpp
import Hello;
int main() {
  hello();
  return 0;
}

I tried to build with different commands and fails in different ways with zig, however clang-20 is successful:

clang++-20 is successful.

clang++-20 --verbose -H -std=c++20 Hello.cppm --precompile -o Hello.pcm > clang20_hello.log 2>&1
clang20_use.log

Zig using target x86_64-linux-gnu

I used triple x
zig c++ -target x86_64-linux-gnu --verbose -H -std=c++20 --stdlib=libc++ Hello.cppm --precompile -o Hello.pcm > zig_hello.log 2>&1

zig_hello.log

Error snippet:

In file included from Hello.cppm:3:
In file included from /usr/include/c++/v1/iostream:45:
In file included from /usr/include/c++/v1/ios:223:
In file included from /usr/include/c++/v1/__locale:17:
In file included from /usr/include/c++/v1/__locale_dir/locale_base_api.h:140:
In file included from /usr/include/c++/v1/__locale_dir/locale_base_api/bsd_locale_fallbacks.h:16:
In file included from /usr/include/locale.h:28:
/usr/include/c++/v1/stddef.h:38:17: fatal error: 'stddef.h' file not found
   38 | #  include_next <stddef.h>
      |                 ^~~~~~~~~~

Zig using libc++ libraries provided by clang-20

zig c++ -target x86_64-linux-gnu --verbose -H -std=c++20 --stdlib=libc++ -resource-dir /usr/lib/llvm-20/lib/clang/20 -isystem /usr/lib/llvm-20/bin/../include/c++/v1 -isystem /usr/lib/llvm-20/lib/clang/20/include Hello.cppm --precompile -o Hello.pcm > zig_hello.log 2>&1

The previous command is successful, however next step fails:
zig c++ -target x86_64-linux-gnu --verbose -H -std=c++20 --stdlib=libc++ -resource-dir /usr/lib/llvm-20/lib/clang/20 -isystem /usr/lib/llvm-20/bin/../include/c++/v1 -isystem /usr/lib/llvm-20/lib/clang/20/include use.cpp -fmodule-file=Hello=Hello.pcm -o Hello.out

Error snippet:

error: stack protector mode differs in AST file 'Hello.pcm' vs. current file
error: AST file 'Hello.pcm' was compiled for the target 'x86_64-unknown-linux-gnu' but the current translation unit is being compiled for target 'x86_64-unknown-linux5.10.0-gnu2.31.0'
error: module file Hello.pcm cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]
use.cpp:4:3: error: use of undeclared identifier 'hello'
    4 |   hello();
      |   ^
4 errors generated.

What are the possible solutions here?
What are the files/directories of interest to look into the zig compiler to enable modules?
Should zig package libc++ library as well to have an standalone compiler instead of relying on the system library?