Link error with static library in C++20 module

Question

I'm learning the C++20 modules feature, but whenever I try to include a header file inside a module and then try to link it, I get a link error. I got the error when working with the GLFW library, but I made a minimum reproducible example so you can help me more easily.

Here are the steps to reproduce this issue:

1. Get a .a file with a function definition:

// lib.h
void libraryFunction();

// lib.cpp
#include "lib.h"

void libraryFunction() {}

clang++ -c -o lib.o lib.cpp
ar rc lib.a lib.o

2. Now that you have the lib.a file, write and compile a module that uses it:

// test.cpp
export module test;
#include "lib.h"

export void testFunction() {
    libraryFunction();
}

clang++ -std=c++20 -c -fmodules-ts -Xclang -emit-module-interface -o test.pcm test.cpp

3. Write a main.cpp file and try to compile and link it:

clang++ -std=c++20 -fmodules-ts -fprebuilt-module-path=. -o main main.cpp test.cpp -l:lib.a -L.

4. You should get this error:

/usr/bin/ld: /tmp/test-25d7c8.o: in function `testFunction()':
test.cpp:(.text+0x5): undefined reference to `_ZW4testE15libraryFunctionv'
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

How do I fix this? I don't get the error when I #include "lib.h" and call libraryFunction() in main.cpp, but what I want is to call the library API from a module. Is this possible?

Answer 1

You've put #include below module, which probably declared libraryFunction as a module function. It makes sense, since linker looks for something like test-libraryFunction.

MS sources on modules (what a time to be alive to use MS as credible source) say you can include "global" headers but it should be done before 1st module declaration.

Try switching the order:

// test.cpp
#include "lib.h"

export module test;

export void testFunction() {
    libraryFunction();
}

Should help.