How do I return an vector of dynamic length in a pub extern "C" fn?

Question

I want to return a vector in a pub extern "C" fn. Since a vector has an arbitrary length, I guess I need to return a struct with

1. the pointer to the vector, and

2. the number of elements in the vector

My current code is:

extern crate libc;
use self::libc::{size_t, int32_t, int64_t};

// struct to represent an array and its size
#[repr(C)]
pub struct array_and_size {
    values: int64_t, // this is probably not how you denote a pointer, right?
    size: int32_t,
}

// The vector I want to return the address of is already in a Boxed struct, 
// which I have a pointer to, so I guess the vector is on the heap already. 
// Dunno if this changes/simplifies anything?
#[no_mangle]
pub extern "C" fn rle_show_values(ptr: *mut Rle) -> array_and_size {
    let rle = unsafe {
        assert!(!ptr.is_null());
        &mut *ptr
    };

    // this is the Vec<i32> I want to return 
    // the address and length of
    let values = rle.values; 
    let length = values.len();

    array_and_size {
       values: Box::into_raw(Box::new(values)),
       size: length as i32,
       }
}

#[derive(Debug, PartialEq)]
pub struct Rle {
    pub values: Vec<i32>,
}

The error I get is

$ cargo test
   Compiling ranges v0.1.0 (file:///Users/users/havpryd/code/rust-ranges)
error[E0308]: mismatched types
  --> src/rle.rs:52:17
   |
52 |         values: Box::into_raw(Box::new(values)),
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected i64, found *-ptr
   |
   = note: expected type `i64`
   = note:    found type `*mut std::vec::Vec<i32>`

error: aborting due to previous error

error: Could not compile `ranges`.

To learn more, run the command again with --verbose.
-> exit code: 101

I posted the whole thing because I could not find an example of returning arrays/vectors in the eminently useful Rust FFI Omnibus.

Is this the best way to return a vector of unknown size from Rust? How do I fix my remaining compile error? Thanks!

Bonus q: if the fact that my vector is in a struct changes the answer, perhaps you could also show how to do this if the vector was not in a Boxed struct already (which I think means the vector it owns is on the heap too)? I guess many people looking up this q will not have their vectors boxed already.

Bonus q2: I only return the vector to view its values (in Python), but I do not want to let the calling code change the vector. But I guess there is no way to make the memory read-only and ensure the calling code does not fudge with the vector? const is just for showing intent, right?

Ps: I do not know C or Rust well, so my attempt might be completely WTF.

Answer 1

pub struct array_and_size {
    values: int64_t, // this is probably not how you denote a pointer, right?
    size: int32_t,
}

First of all, you're correct. The type you want for values is *mut int32_t.

In general, and note that there are a variety of C coding styles, C often doesn't "like" returning ad-hoc sized array structs like this. The more common C API would be

int32_t rle_values_size(RLE *rle);
int32_t *rle_values(RLE *rle);

(Note: many internal programs do in fact use sized array structs, but this is by far the most common for user-facing libraries because it's automatically compatible with the most basic way of representing arrays in C).

In Rust, this would translate to:

extern "C" fn rle_values_size(rle: *mut RLE) -> int32_t
extern "C" fn rle_values(rle: *mut RLE) -> *mut int32_t

The size function is straightforward, to return the array, simply do

extern "C" fn rle_values(rle: *mut RLE) -> *mut int32_t {
    unsafe { &mut (*rle).values[0] }
}

This gives a raw pointer to the first element of the Vec's underlying buffer, which is all C-style arrays really are.

If, instead of giving C a reference to your data you want to give C the data, the most common option would be to allow the user to pass in a buffer that you clone the data into:

extern "C" fn rle_values_buf(rle: *mut RLE, buf: *mut int32_t, len: int32_t) {
    use std::{slice,ptr}
    unsafe {
        // Make sure we don't overrun our buffer's length
        if len > (*rle).values.len() {
           len = (*rle).values.len()
        }
        ptr::copy_nonoverlapping(&(*rle).values[0], buf, len as usize);
    }
}

Which, from C, looks like

void rle_values_buf(RLE *rle, int32_t *buf, int32_t len);

This (shallowly) copies your data into the presumably C-allocated buffer, which the C user is then responsible for destroying. It also prevents multiple mutable copies of your array from floating around at the same time (assuming you don't implement the version that returns a pointer).

Note that you could sort of "move" the array into C as well, but it's not particularly recommended and involves the use mem::forget and expecting the C user to explicitly call a destruction function, as well as requiring both you and the user to obey some discipline that may be difficult to structure the program around.

If you want to receive an array from C, you essentially just ask for both a *mut i32 and i32 corresponding to the buffer start and length. You can assemble this into a slice using the from_raw_parts function, and then use the to_vec function to create an owned Vector containing the values allocated from the Rust side. If you don't plan on needing to own the values, you can simply pass around the slice you produced via from_raw_parts.

However, it is imperative that all values be initialized from either side, typically to zero. Otherwise you invoke legitimately undefined behavior which often results in segmentation faults (which tend to frustratingly disappear when inspected with GDB).

Answer 2

There are multiple ways to pass an array to C.

---

First of all, while C has the concept of fixed-size arrays (int a[5] has type int[5] and sizeof(a) will return 5 * sizeof(int)), it is not possible to directly pass an array to a function or return an array from it.

On the other hand, it is possible to wrap a fixed size array in a struct and return that struct.

Furthermore, when using an array, all elements must be initialized, otherwise a memcpy technically has undefined behavior (as it is reading from undefined values) and valgrind will definitely report the issue.

---

Using a dynamic array

A dynamic array is an array whose length is unknown at compile-time.

One may chose to return a dynamic array if no reasonable upper-bound is known, or this bound is deemed too large for passing by value.

There are two ways to handle this situation:

• ask C to pass a suitably sized buffer
• allocate a buffer and return it to C

They differ in who allocates the memory: the former is simpler, but may require to either have a way to hint at a suitable size or to be able to "rewind" if the size proves unsuitable.

Ask C to pass a suitable sized buffer

// file.h
int rust_func(int32_t* buffer, size_t buffer_length);

// file.rs
#[no_mangle]
pub extern fn rust_func(buffer: *mut libc::int32_t, buffer_length: libc::size_t) -> libc::c_int {
    // your code here
}

Note the existence of std::slice::from_raw_parts_mut to transform this pointer + length into a mutable slice (do initialize it with 0s before making it a slice or ask the client to).

Allocate a buffer and return it to C

// file.h
struct DynArray {
    int32_t* array;
    size_t length;
}

DynArray rust_alloc();
void rust_free(DynArray);

// file.rs
#[repr(C)]
struct DynArray {
    array: *mut libc::int32_t,
    length: libc::size_t,
}

#[no_mangle]
pub extern fn rust_alloc() -> DynArray {
    let mut v: Vec<i32> = vec!(...);

    let result = DynArray {
        array: v.as_mut_ptr(),
        length: v.len() as _,
    };

    std::mem::forget(v);

    result
}

#[no_mangle]
pub extern fn rust_free(array: DynArray) {
    if !array.array.is_null() {
        unsafe { Box::from_raw(array.array); }
    }
}

---

Using a fixed-size array

Similarly, a struct containing a fixed size array can be used. Note that both in Rust and C all elements should be initialized, even if unused; zeroing them works well.

Similarly to the dynamic case, it can be either passed by mutable pointer or returned by value.

// file.h
struct FixedArray {
    int32_t array[32];
};

// file.rs
#[repr(C)]
struct FixedArray {
    array: [libc::int32_t; 32],
}