call-rs.c

#include <stdio.h>

void test_rs_lib(void (*cb)(int, void*), void*);

void my_cb(int a, void* ctx)
{
  *((int*)ctx) = a;
}

int main()
{
  int saved = 0;
  test_rs_lib(&my_cb, &saved);
  printf("%d\n", saved);
}

ccb.rs

use std::ffi::c_void;

struct CCb(unsafe extern "C" fn(i32, *mut c_void), *mut c_void);

impl CCb {
    unsafe fn call(self, arg: i32) {
        self.0(arg, self.1)
    }
}

#[no_mangle]
extern "C" fn test_rs_lib(cb: unsafe extern "C" fn(i32, *mut c_void), ctx: *mut c_void) {
    let ccb = CCb(cb, ctx);
    unsafe {
        ccb.call(42);
    }
}