Passing a list of strings from Python to Rust

Question

I've been learning Rust for about two weeks now and today, I got into its FFI. I used Python to play with Rust, using ctypes and libc. I passed integers, strings and even learned to pass a list of integers (thanks to this wonderful answer).

Then, I tried to pass a list of strings (following the reasoning behind the that answer), but I failed, as I couldn't get a lead on it. In Python, I have something like this to pass the array of strings.

def testRust():
    lib = ctypes.cdll.LoadLibrary(rustLib)
    list_to_send = ['blah', 'blah', 'blah', 'blah']
    c_array = (ctypes.c_char_p * len(list_to_send))()
    lib.get_strings(c_array, len(list_to_send))

In Rust, I thought that there should be something (like a STRING_RECEIVER) to collect the incoming strings, but I can't find one.

#![feature(libc)]
extern crate libc;

use std::slice;
use libc::{size_t, STRING_RECEIVER};

#[no_mangle]
pub extern fn get_strings(array: *const STRING_RECEIVER, length: size_t) {
    let values = unsafe { slice::from_raw_parts(array, length as usize) };
    println!("{:?}", values);
}

Is there any alternative way to achieve this?

Answer 1

There is absolutely no difference with the case of array of numbers. C strings are zero-terminated arrays of bytes, so their representation in Rust will be *const c_char, which could then be converted to &CStr which then can be used to obtain &[u8] and then &str.

Python:

import ctypes

rustLib = "libtest.dylib"

def testRust():
    lib = ctypes.cdll.LoadLibrary(rustLib)
    list_to_send = ['blah', 'blah', 'blah', 'blah']
    c_array = (ctypes.c_char_p * len(list_to_send))(*list_to_send)
    lib.get_strings(c_array, len(list_to_send))

if __name__=="__main__":
    testRust()

Rust:

#![feature(libc)]
extern crate libc;

use std::slice;
use std::ffi::CStr;
use std::str;
use libc::{size_t, c_char};

#[no_mangle]
pub extern fn get_strings(array: *const *const c_char, length: size_t) {
    let values = unsafe { slice::from_raw_parts(array, length as usize) };
    let strs: Vec<&str> = values.iter()
        .map(|&p| unsafe { CStr::from_ptr(p) })  // iterator of &CStr
        .map(|cs| cs.to_bytes())                 // iterator of &[u8]
        .map(|bs| str::from_utf8(bs).unwrap())   // iterator of &str
        .collect();
    println!("{:?}", strs);
}

Running:

% rustc --crate-type=dylib test.rs
% python test.py
["blah", "blah", "blah", "blah"]

And again, you should be careful with lifetimes and ensure that Vec<&str> does not outlive the original value on the Python side.