[ocaml-ctypes] Pointer to pointer and null check

[Andre Nathan]

Thanks a lot Jeremy!

I went with the ptr_opt way, thought I'm a bit surprised it works:

>   let foo_bar x =
>     let ret = allocate foo_opt (Some x) in
>     let r = foo_bar ret (Some x) in
>     (r, !@ret)

Intuitively I thought `ret` could never be null, because calling
`allocate` would be akin to calling malloc() + memset() in C. I also
believed that if the C library would set `*ret = NULL`, this would cause
a leak of the memory allocated in `allocate`.

So I guess my understanding is incorrect :)

In the actual code I used

  let ret = allocate foo_opt None in
  ...

since there's no meaningful initial value to be set on `ret`. It seems
to be working fine :)

Thanks again,
Andre


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