How to return Rust Vec of unknown size over FFI?


This Content is from Stack Overflow. Question asked by mwlon

I have a Rust library that returns Vecs of size that cannot be predicted by the caller, and I’m wondering if I can find an elegant way to do implement the FFI interface.
Since Vecs can’t be passed over FFI, I understand I need to first return the length, and then fill a buffer with the contents.

Here’s my current solution, which works by manually managing memory of the Vec and trusting the caller to

  1. Compute the FfiVec
  2. Create a buffer of appropriate size to copy into
  3. Copy from the FfiVec, cleaning it up in the process
pub struct FfiVec<T: Sized> {
  ptr: *mut Vec<T>,
  len: c_uint, // necessary for C process to know length of Rust Vec

impl<T: Sized> FfiVec<T> {
  pub fn from_vec(v: Vec<T>) -> Self {
    let len = v.len() as c_uint;
    FfiVec {
      ptr: Box::into_raw(Box::new(v)),

  pub fn drop(self) {
    unsafe {
      std::alloc::dealloc(self.ptr as *mut u8, Layout::new::<Vec<T>>())

pub extern compute_my_vec() -> FfiVec<u8> {...}

pub extern copy_from_my_vec(ffi_vec: FfiVec<u8>, buffer: *mut u8) -> {
  let len = c_vec.len as usize;
  let buffer_slice = unsafe { std::slice::from_raw_parts_mut(buffer, len) };
  for (i, &item) in c_vec.slice().iter().take(len).enumerate() {
    buffer_slice[i] = item;

Is there a simpler way to do this? Or a common library that can do this for me?


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