Transfer rust vec to c array strings.

cstr_array.rs

use std::ffi::CString;
use std::os::raw::{c_char, c_int};
use std::{ptr, mem};

#[no_mangle]
unsafe extern "C" fn get_strings(outlen: *mut c_int) -> *mut *mut c_char {
    let mut v = vec![];

    // Let's fill a vector with null-terminated strings
    v.push(CString::new("Hello").unwrap());
    v.push(CString::new("World").unwrap());
    v.push(CString::new("!").unwrap());

    // Turning each null-terminated string into a pointer.
    // `into_raw` takes ownershop, gives us the pointer and does NOT drop the data.
    let mut out = v
        .into_iter()
        .map(|s| s.into_raw())
        .collect::<Vec<_>>();

    // Make sure we're not wasting space.
    out.shrink_to_fit();
    assert!(out.len() == out.capacity());

    // Get the pointer to our vector.
    let len = out.len();
    let ptr = out.as_mut_ptr();
    mem::forget(out);

    // Let's write back the length the caller can expect
    ptr::write(outlen, len as c_int);
    
    // Finally return the data
    ptr
}

#[no_mangle]
unsafe extern "C" fn free_string_array(ptr: *mut *mut c_char, len: c_int) {
    let len = len as usize;

    // Get back our vector.
    // Previously we shrank to fit, so capacity == length.
    let v = Vec::from_raw_parts(ptr, len, len);

    // Now drop one string at a time.
    for elem in v {
        let s = CString::from_raw(elem);
        mem::drop(s);
    }

    // Afterwards the vector will be dropped and thus freed.
}


/*
// The C code calling into our Rust part:

#include <stdio.h>

char** get_strings(int* outlen);
void free_string_array(char **ptr, int len);


int main(int argc, char** argv)
{
  int len;
  char** s = get_strings(&len);

  for (int i=0; i<len; i++) {
    printf("String %d: %s\n", i, s[i]);
  }

  free_string_array(s, len);

  return 0;
}

*/

Are you sure, that manually dropping the strings (line 48) is necessary? From the docs:
"(CString::from_raw()) Retakes ownership of a CString that was transferred to C via CString::into_raw."
This means, that the loop can be written as

    for elem in v {
        CString::from_raw(elem);
    }

Or do I miss something here?

OK, looks like the returned value must be used. But this should work:

    for elem in v {
        let _ = CString::from_raw(elem);
    }