in main.rs:

```rs
#[link(name = "test")]
extern "C" {
    pub fn multiply(a: u32, b: u32) -> u32;
}
```

in test.c

```c
#include <stdint.h>

uint32_t multiply(uint32_t a, uint32_t b)
{
    return a * b;
}
```

In `build.rs`:

```rs
fn main() {
    // println!("cargo::rustc-link-lib=test")
    println!("cargo:rustc-link-search=c_src");
}

```

archive created with:

`arm-none-eabi-gcc.exe -mthumb -mcpu=cortex-m4 -std=c99 -c -g3 .\test.c`
`arm-none-eabi-ar.exe -rcs libtest.a test.o`