Re: [Caml-list] Auxiliary C function which manipulates the OCaml heap

[Jeremie Dimino <jdimino@janestreet.com>]

Hi,

On Tue, Mar 12, 2013 at 2:54 PM, Romain Bardou <romain.bardou@inria.fr> wrote:
> However, this function does not use CAMLreturn, as it does not return an
> OCaml value but a C value (actually void). So the compiler emits a warning:
> "unused variable 'caml__frame'" if I use CAMLparam in aux.
>
> Is it ok to call CAMLlocal in this way? The documentation says it cannot be
> called inside nested blocks. Is an auxiliary function considered a nested
> block?

No, you must use [CAMLreturn0] in this case.

> Note that the caller of "aux", or (in another case) the caller of the caller
> of "aux", has called CAMLparam. If this was not the case, would it make a
> difference?

No, CAMLparam macros are only for the current function.  They register
the C local variables as local roots.  In sub-function they are
different local variables (different position on the stack) so you
need to register them too.

> Similarly, if "aux" had some OCaml value arguments, should it call CAMLparam
> even though the caller of "aux" has declared them to the GC using CAMLparam
> or CAMLlocal?

Yes.

> Similarly, if "aux" did return an OCaml value, should it call CAMLreturn
> (and thus CAMLparam) to return the value, or should it use the normal
> "return"?

You must use [CAMLreturnT(C-type, expr)] in this case.

And for other cases you can also use Begin_roots and End_roots.