Using Rust outside Rust

Say you've got a bunch of really fast Rust code (remember, Rust is about the same as C, sometimes faster) and want to call it from a slower language like Python or Ruby, it's actually pretty easy to do. If you can run C code, you can also run Rust.

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Original Function

We're going to make a contrived example to check if a number is prime.

$ cat

pub fn is_prime(n: u64) -> bool {
    match n {
        0 | 1 => false,
        2 | 3 => true,
        n if n & 1 == 0 => false,   // even numbers
        n => {
            let limit = (n as f64).sqrt() as u64 + 1;
            for i in (4...limit).step_by(2) {
                if n % i == 0 {
                    return false;
            return true;

You need to explicitly tell the rust compiler not to mangle the function name, and export it as a C function. So the function signature needs to be changed a bit...

pub extern "C" fn is_prime(n: u64) -> bool {

And to make sure our library is callable from other code, it needs to be compiled as a cdylib (with optimisations, of course).

$ rustc -O --crate-type cdylib
$ ls

Aaaannnnddd we're done. How easy was that?

Because the library hasn't been installed into one of the standard locations (i.e. /usr/lib/), you'll also need to add an environment variable so things know where to find it. You'd need to do the exact same with a C/C++ library as well.

$ export LD_LIBRARY_PATH=.


Considering almost everything is descended from C and can interoperate with C, it makes sense to check out C first. In this case, calling Rust code from C is just like calling code from another library. Even easier if you take into account that you don't even need to write a header file.

#include <stdio.h>

extern int is_prime(unsigned int n);

int main() {
  unsigned int n = 1234567;

  printf("%d %s prime\n", n, is_prime(n) ? "is" : "is not");
  return 0;

And you link in the Rust library just like any other compiled DLL or *.so file:

$ gcc main.c -L. -lprime

Note: because of a compiler bug at the time and the std library isn't being used directly, it isn't linked in when rustc creates our shared object. Unfortunately, a couple things were indirectly needed from the std library so I was getting linker errors later on. The fix was to add a dummy function which uses something from the std library (e.g. a print statement).


Next is calling our Rust code from Lua. Lua is a super light-weight language (about 10,000 lines of C) often used for embedding into applications to make them scriptable. It's also one of the fastest scripting languages in use at the moment.

Here's the script I'm using:

ffi = require("ffi")

int is_prime(unsigned int n);

rust_lib = ffi.load("./")

n = 1234567

if rust_lib.is_prime(n) then
  print(n, "is prime")
  print(n, "is not prime")
$ luajit main.lua
1234567 is prime

That was easy enough, all you need to do is load() the *.so file and use it like pretty much any other library in Lua.


Next up is Python, unlike Lua, Python is well known for being slow. Particularly when it comes to computationally expensive programs.

Python's standard library comes with a useful module for working with C (or any other compiled language) called ctypes. You can use this pretty much the same way as in Lua by creating some sort of object out of the loaded *.so file and then calling it like normal.

import ctypes

primes = ctypes.CDLL('./')

n = 1234567

if primes.is_prime(n):
    print(n, "is prime")
    print(n, "is not a prime")

Python also has a really nice library called cffi. With ctypes you usually need to define each function, its input arguments, and what it returns in order to have type safety, but cffi uses a completely different approach. You just pass it the text for a typical C header file and it does the rest. For larger projects, or things which may still be evolving, cffi is a much nicer library to use.

from cffi import FFI

ffi = FFI()

# Load a header entry for the Rust function
ffi.cdef("bool is_prime(int n);")
lib = ffi.dlopen("./")

n = 1234567

if lib.is_prime(n):
    print(n, "is prime")
    print(n, "is not a prime")

# trying to use an invalid datatype should fail


Another language I've been interested in lately is Go. It's a compiled, statically typed, gabage collected language developed by the guys at Google and primarily oriented towards networks, concurrency, and microservices.

Go has a nice trick for allowing you to interact with C code. It's got special behaviour when you do import "C". Any comments above are passed to the underlying compiler, meaning you can # include any C library just like you would in native C. Then, using reflection and other forms of black magic, you're able to call anything in the imported namespace as an element of the C object. i.e. calling printf() is just a case of doing C.printf(...).

package main

#cgo LDFLAGS: -L. -lprime
#include "./prime.h"
import "C"
import "fmt"

func main() {
    var n C.uint = 1234567
    if C.is_prime(n) != 0 {
        fmt.Printf("%d is prime\n", n)
    } else {
        fmt.Printf("%d is not prime\n", n)