function_arg.c

int apply(int (*func)(int), int x) {
	return func(x);
}

function_arg.rs

#![crate_type = "lib"]
#![crate_name = "foo"]

extern crate libc;


/*
int apply()
	int (*)(int) func [int (*)(int)]
	int x
*/
#[link(name="cxx")]
extern "C" {
	pub fn apply(func: extern fn(libc::c_int) -> libc::c_int, x: libc::c_int) -> libc::c_int;
}

function_arg_run.rs

extern crate libc;
extern crate foo;

extern fn double(x: libc::c_int) -> libc::c_int { 2*x }

fn main() {
	unsafe { println!("{}", foo::apply(double, 2)) }
//	assert!(foo::apply(double, 2) == 4)
}

gistfile1.txt

gcc -c function_arg.c -o /tmp/libcxx.a

rustc -o /tmp/libfoo.rlib function_arg.rs

rustc -L /tmp/ function_arg_run.rs -o /tmp/main