[Caml-list] GC, Anonymous Functions, and C

[Caml-list] GC, Anonymous Functions, and C

[Jonathan Roewen]

Hi,

When passing an anonymous ocaml function to a C function, is it safe
to store that value in an array, and then call it later at an abitrary
time, or will the GC reclaim it? And if so, what should be done to
stop the GC from doing so?

Basically, we're trying to do interrupt handling from OCaml, and need
to store the anonymous functions somewhere for the IDT to jump into,
to provide a little context for the problem.

Regards,

Jonathan Roewen

Re: [Caml-list] GC, Anonymous Functions, and C

[Richard Jones]

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On Sat, Dec 18, 2004 at 10:55:23AM +1300, Jonathan Roewen wrote:
> Hi,
> 
> When passing an anonymous ocaml function to a C function, is it safe
> to store that value in an array, and then call it later at an abitrary
> time, or will the GC reclaim it? And if so, what should be done to
> stop the GC from doing so?
> 
> Basically, we're trying to do interrupt handling from OCaml, and need
> to store the anonymous functions somewhere for the IDT to jump into,
> to provide a little context for the problem.

The GC might reclaim the value (function, or whatever) unless you
register it as a global root.  To do this you need to call
caml_register_global_root on each value which you put in the array.
Furthermore you ought to call caml_remove_global_root when you remove
the value from the array and no longer need it.

Rich.

-- 
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Re: [Caml-list] GC, Anonymous Functions, and C

[David Brown]

On Sat, Dec 18, 2004 at 10:55:23AM +1300, Jonathan Roewen wrote:

> When passing an anonymous ocaml function to a C function, is it safe
> to store that value in an array, and then call it later at an abitrary
> time, or will the GC reclaim it? And if so, what should be done to
> stop the GC from doing so?

You'll need to register the value as a global root, using
register_global_root.  Please see the Interfacing C with Objective Caml,
where there is brief mention of this function.  It is also important to
dereference the value each time you use it.

static value hook = Val_int (0);

  ...
  hook = passed_argument;
  register_global_root (&hook);

  ...
  callback (hook, arg);

You can also use remove_global_root() later to remove the reference.

Dave

Re: [Caml-list] GC, Anonymous Functions, and C

[Damien Doligez]

On 17 déc. 2004, at 22:55, Jonathan Roewen wrote:

> Basically, we're trying to do interrupt handling from OCaml, and need
> to store the anonymous functions somewhere for the IDT to jump into,
> to provide a little context for the problem.

This is going to be extremely tricky, if it ever works, because
OCaml code can allocate memory by calling malloc.  And malloc
is not reentrant, so calling it from a signal handler cannot be
correct unless your main program never calls malloc, which rules
out a large number of functions of the C stdlib.  And you don't
even know which ones exactly...

-- Damien