Link C library with Rust project

[MarcAntoine-Arnaud]

Describe the Bug

I have a part of the project in C, so I use emscripten to compile it (the complex Makefile is already present, that's why I don't want to use cc).
With emscripten I generate a lib.a file.

On a second time I have a Rust project to integrate the process.
For that I have defined the C API to call FFI from Rust and in the build.rs I add the link to the lib.a.

Using wasm-pack build everything is builded - not tested yet in web environnement.
But in my case I need to target the Web Worker, so I try to compile with wasm-pack build --no-module which provides me that error:

error: cannot import from modules (`env`) with `--no-modules`

Does I missed something ?
Is it linked to a configuration on Emscripten side ?
Does I need to use Wasm in Wasm ? So generate a first wasm with Emscripten, then include bytes in the Rust code ?

Additional Context

Emscripten is used on Ubuntu (docker), Rust on Mac OSX.

[MarcAntoine-Arnaud]

I have discovered it appears when I return a pointer from a function.
For example, that works:

• C part

int wasm_c_add(int a, int b) {
	return a + b;
}

• Rust side

#[link(name = "wasm_c_add")]
extern {
  pub fn wasm_c_add(a: c_int, b: c_int) -> c_int;
}

#[wasm_bindgen]
pub fn add() {
  let sum = unsafe {
    wasm_c_add(3, 6)
  };
  log::debug!("Addition = {}", sum);
}

But if I wrote:

• C part

struct Something {
	const char* value;
} Something;

struct Something* wasm_c_something() {
	struct Something* something = malloc(sizeof(Something));
	return something;
}

• Rust part

#[repr(C)]
pub struct Something {
  pub value: *const c_char,
}

#[link(name = "wasm_c_add")]
extern {
  pub fn wasm_c_something() -> *mut Something;
}


#[wasm_bindgen]
pub fn generate() {
  let _something = unsafe {
    wasm_c_something()
  };
}

it return the error:

error: cannot import from modules (`env`) with `--no-modules`

I don't know if it can have a link with #190. But I have founded some support here

[lights0123]

That means that you're attempting to link to an undefined function. Upload your wasm file here, and look for functions at the top marked as import "env".

[chrysn]

Linking between C and wasm-bindgen Rust seems to still have several pitfalls discussed in #2209; could you be running into these?