2wice.c

#include <stdio.h>
#include <stdint.h>

extern int32_t twice(int32_t, int32_t(*)(int32_t));

int32_t dbl(int32_t x) {
    /* x = arbitrary number here */
    return x+x; 
}

int main() {
   printf("%d\n", twice(10, dbl));
   return 0;
}

2wice.rs

#[no_mangle]
pub extern "C" fn twice(x: i32, f: extern "C" fn(i32) -> i32) -> i32 {
  // x = 10 here
  f(x) + f(x)
}

extern "C" fn dbl(x: i32) -> i32 { x+x }

#[allow(dead_code)]
fn main() {
  println!("{}", twice(10, dbl));
}

Makefile

all: 2wice 2wice-c

clean:
	rm -f 2wice 2wice-c lib2wice.a

2wice: 2wice.rs
	rustc $<

lib2wice.a: 2wice.rs
	rustc --crate-type=staticlib $<

2wice-c: 2wice.c lib2wice.a
	gcc -Wall 2wice.c lib2wice.a -ldl -lpthread -lgcc_s -lpthread -lc -lm -o 2wice-c