[ocaml-ctypes] Lifetime of Ctypes allocation

[Markus Müller]

Dear Jeremy,

thanks a lot for your elaborate reply. This program works as expected.

open Ctypes
open Foreign
let f = foreign "f" ((ptr (ptr int)) @-> returning void)
let v = ref (from_voidp (ptr int) null)
let a = ref (from_voidp int null)
let b = ref (from_voidp int null)
let _ =
  v := allocate_n (ptr int) ~count:2 ;
  a := allocate int 0 ;
  b := allocate int 1 ;
  (!v) <-@ (!a);
  ((!v) +@ 1) <-@ (!b) ;
  f (!v) ; Gc.full_major () ; f (!v)

and the following program is fine too.

open Ctypes
open Foreign
let f = foreign "f" ((ptr (ptr int)) @-> returning void)
let v = allocate_n (ptr int) ~count:2
let a = allocate int 0
let b = allocate int 1
let _ =
  v <-@ a;
  (v +@ 1) <-@ b ;
  f v ; Gc.full_major () ; f v

However, having v, a, b in a local scope does not work.

open Ctypes
open Foreign
let f = foreign "f" ((ptr (ptr int)) @-> returning void)
let _ =
  let v = ref (from_voidp (ptr int) null) in
  let a = ref (from_voidp int null) in
  let b = ref (from_voidp int null) in
  v := allocate_n (ptr int) ~count:2 ;
  a := allocate int 0 ;
  b := allocate int 1 ;
  (!v) <-@ (!a);
  ((!v) +@ 1) <-@ (!b) ;
  f (!v) ;
  Gc.full_major () ;
  f (!v)

I fail to see why it does not work in this case.

>>     v <-@ (allocate int 0);
>
> This allocates an piece of C-memory large enough to hold an integer,
> initialized with 0.  The address of the allocated memory is written to
> the C-memory referenced by 'v'.
>
> At this point there are no references to the original pointer returned
> by 'allocate', so the allocated memory may be freed.

Good point: (allocate int 0) is not bound to a variable and thus can be
freed. However, in the broken example above, a reference exists and
still it does not work. Can you explain why? Thank you.

Best regards,
Markus