Comment by cross

The first attempt appears to try and transfer ownership of the allocated memory from the Vec to C, so my first question is, why not allocate the returned memory using libc::malloc?

But I do recognize that the code in the post was a simplified example, and it's possible that the flexibility of `Vec` is actually used; perhaps elements are pushed into the `Vec` dynamically or something, and it would be inconvenient to simulate that with `libc::malloc` et al. But even then, in an environment that's not inherently memory starved, a viable approach might be to build up the data in a `Vec`, and then allocate a properly-sized region using `libc::malloc` and copy the data into it.

Another option might be to maintain something like a BTreeMap indexed by pointer on the Rust side, keeping track of the capacity there so it can be recovered on free.