Re: [Caml-list] OCaml C macros, what do they do?

[Marek Kubica <marek@xivilization.net>]

Hello Alain,

Thanks for your answer, I'm glad that I was not so far off and can see
clearer now.

On Wed, 10 Apr 2013 19:05:06 +0200
Alain Frisch <alain@frisch.fr> wrote:

> On 04/10/2013 06:15 PM, Marek Kubica wrote:
> >   - CAMLparam1(foo), notifies the GC that foo is used in this
> > function and not to be collected.
> 
> And also that the foo pointer must be updated if the corresponding
> block is moved by the GC.  So even if you can guarantee that foo will
> not be collected (because it is accessible otherwise), you must use
> CAMLparam if there is a possibility that the GC will be triggered in
> the function.

If I understand correctly, GC is only triggered by the caml_*
allocation functions, or is there some other possibility that I am
missing?

> >   - CAMLlocal1(foo), creates a local variable foo. This is only
> > required if I want to bind some "value" type to a variable name.
> > Not required if I immediately return it.
> 
> Same as above.  All local values holding OCaml values that can be
> blocks (not only "ints") must reside in variables marked as
> CAMLlocal/CAMLparam at the time a GC occurs.  In particular, it is
> generally unsafe to chain function calls without putting the
> intermediate result in a local variable marked as CAMLlocal.

Okay, I suspected so much but wasn't sure since when I tried chaining
without CAMLlocal it worked. But an edge case might require it. I'll
add CAMLlocals to my code to be on the safe side, thanks.

> AFAIK, CAMLprim is only used within the OCaml distribution itself to 
> collect (with sed) names of builtin primitives to be exposed by
> default by ocamlrun to bytecode programs.

Great, so it doesn't apply to my code at all. Will remove it.

regards,
Marek