Re: [Caml-list] Question about register_global_root

[Jerome Simeon <simeon@research.bell-labs.com>]

Thanks Maas,
We managed to pin down that *!@#! bug after all. You mail was very
helpful.
- Jerome

On Wed, 2003-05-28 at 16:33, Maas-Maarten Zeeman wrote:
> Hello,
> 
> I'm by far no Caml expert, but it works pretty well for me in 
> OcamlExpat, an OCaml wrapper for Expat I wrote a while back. I use a 
> malloced value to store a pointer to a tuple, which is used to store 
> event handlers (ocaml functions).
> 
> This is what I did:
> 
>   /* Malloc a value for a tuple which will contain the callback
>    * handlers and register it as global root.
>    */
>   handlers = (value *) malloc(sizeof(value));
>   register_global_root(handlers);
> 
>   *handlers = alloc_tuple(NUM_HANDLERS);
>   for(i = 0; i < NUM_HANDLERS; i++) {
>     Field(*handlers, i) = Val_unit;
>   }
> 
> One thing to keep in mind is that you have to be really careful, and use 
> the right CAMLxxxx macro in the right place, otherwise things start 
> crashing at seemingly random times. The reason for this is that the 
> global root will correctly stay at the same memory location, but any 
> ocaml values stored in the root can move during a garbage collection 
> cycle. If you do not use the CAMLxxxx macro's correctly the values will 
> change, and move in the middle of your c-function, which almost always 
> results in a crashing program.
> 
> It is very easy to make mistakes here, but there is a way to (sort of) 
> test if your c-extension behaves correctly. If you call "Gc.full_major 
> ()" in your tests you will know almost instantly if there is a problem 
> related to garbage collection, and possible incorrect use of CAMLxxxx 
> macros (at least that is the idea).
> 
> Regards,
> 
> Maas
> 
> > Hi Caml experts,
> > We are using register_global_root to register addresses in the malloc'd
> > C heap that contain Caml values.  We noticed that register_global_root
> > (comment below) expects a global C variable, but we assumed that it 
> > should
> > work with malloc'd addresses as well.  Is there any reason that it 
> > would not?
> >
> > /* [register_global_root] registers a global C variable as a memory root
> >    for the duration of the program, or until [remove_global_root] is
> >    called. */
> >
> > We are getting (somewhat random) core dumps after calling the same Caml
> > function repeatedly on the same Caml value (which is referenced
> > from the C heap).
> >
> > Thanks,
> > Mary
> 
> 
-- 
Jerome Simeon <simeon@research.bell-labs.com>
Lucent Technologies

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