Re: [Caml-list] Troubles with C->Caml

[Joerg Czeranski <jc@joerch.org>]

On Sun, 21 Oct 2001 16:27:08 +0400 (MSD) malc <malc@pulsesoft.com> wrote:
>
> value func (value param)
> {
>   CAMLparam1 (param);
>   CAMLlocal2 (result, err);
>   char *p = String_val (param);
>   void *h = af (p);
>   err = yaf ();
>   if (some_condition) {
>     /* raise exception */
>     ...
>   } else {
>     if (Is_exception_result (err)) {
>       result = alloc_small (2, 1);
>       Field (result, 0) = (value)h;
>       Field (result, 1) = Extract_exception (err);
>       CAMLreturn (result);
>     }
>     result = alloc_small (1, 0);
>     Field (result, 0) = (value)h;
>     CAMLreturn (result);
>   }
> }
>
> The decorations are there ditto coredump.

Hi,

I don't remember whether somebody has answered this,
but I think you should never copy an O'Caml pointer into a
variable that's not in CAMLlocal...(), e.g.
"char *p = String_val (param);" copies the pointer param into
the local variable p.  When the GC moves param, p points to
garbage.  (Same for h.)

On the other hand you can't declare p with CAMLparam..., so
you probably have to write "String_val (param)" wherever you'd
use p.

I'm not sure whether the documentation mentions this, and
I think it's not completely obvious.

regards,
jörch
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