Re: [Caml-list] Interfacing with C question...

["Remi Vanicat" <remi.vanicat@gmail.com>]

2007/1/24, David Allsopp <dra-news@metastack.com>:
> Sorry if this an RADBOTFM case. Rule 2 in Chapter 18 of the manual states
> that all local variables of type value must be declared using CAMLlocal
> macros. However, later on when demonstrating caml_callback we get the
> statements:
>
> value* format_result_closure = caml_named_value("format_result");
> return strdup(String_val(caml_callback(*format_result_closure, Val_int(n))))
>
> (I've "simplified" the opening lines for clarity here - naturally it should
> be static and once only!).
>
> Two questions arise:
>
> 1. Presumably it's OK to cache values returned by caml_named_value without
> declaring them in a CAMLlocal "call" or by using register_global_root?

No, any C pointer to a caml value must be known to the caml GC,
because the caml GC might move the caml value. But you might no
declare such a pointer if you are sur that the GC won't be triger
between your affectation of the value to the C variable, and the use
of the C variable.*

In the given exemple, this is the case: nothing is done between the
affectation and the use.

By the way, when in doubt, or when you are a begginer not nowing well
how the GC work, you should always use the CAMLlocal call.

> 2. The result of caml_callback is passed straight to String_val. Therefore,
> if I expand the line to:
>
> value result = caml_callback(*format_result_closure, Val_int(n)));
> return strdup(String_val(result));
>
> then does that work ok without using CAMLlocal1(result);

Yes, for the same reason: you do nothing between the affectation of
the result function, and the use of the variable.