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From: Malcolm Matalka <mmatalka@gmail.com>
To: Jeremy Yallop <yallop@gmail.com>
Cc: "David Sheets" <sheets@alum.mit.edu>,
	"Jeremie Dimino" <jdimino@janestreet.com>,
	"Christoph Höger" <christoph.hoeger@tu-berlin.de>,
	"caml users" <caml-list@inria.fr>
Subject: Re: [Caml-list] Save callbacks from OCaml to C
Date: Wed, 03 Feb 2016 20:15:10 +0000	[thread overview]
Message-ID: <86mvrhr1o1.fsf@gmail.com> (raw)
In-Reply-To: <CAAxsn=G4oRTdE_Peh8FWvfjTuSu+9jSw8q-dvZ+QNmGyWpWw4Q@mail.gmail.com> (Jeremy Yallop's message of "Wed, 3 Feb 2016 10:02:06 -0800")

Jeremy Yallop <yallop@gmail.com> writes:

> On 3 February 2016 at 05:44, David Sheets <sheets@alum.mit.edu> wrote:
>> On Wed, Feb 3, 2016 at 12:26 PM, Malcolm Matalka <mmatalka@gmail.com> wrote:
>>> Jeremie Dimino <jdimino@janestreet.com> writes:
>>>> You need to register [ml_t], [ml_x] and [ml_g
>>>> ] as GC roots. Otherwise if the GC runs in caml_ba_alloc for instance,
>>>> [ml_t] might ends up containing garbage even before reaching
>>>> [caml_callback3]. You can use the normal macros for that:
>>>>
>>> If one is using ctypes, is all of this taken care of?  I have a library
>>> that registers a bunch of Ocaml functions in C code, which the C code
>>> calls.  I haven't experienced anything bad happening yet, but that
>>> doesn't mean much...
>>
>> If you use ctypes and pass OCaml closures to C, you *must* retain a
>> reference to the closure to avoid it being GCed. If you do not, you
>> may experience the exception CallToExpiredClosure sporadically.
>
> Besides David's caveat, the answer is yes: ctypes will take care of
> registering arguments as GC roots as necessary.

Can you clarify this a bit?  I'm not that familiar with how the C FFI
works.  If I pass in a closure to a C function and it is registered as a
GC root, doesn't that mean it won't be GCd if my Ocaml program forgets
about it or?

Also, David and I were talking about how to solve this on IRC.  In my
specific case, callbacks are one-shot, which means I know they need to
be remembered until they are called then they can (possibly) be freed.
Is there a nice solution here?  I'd prefer not to store them in some
other data structure and remove them later just to keep a reference
alive, if possible.  That is overhead I'd prefer to avoid, if possible.
I plan on having possibly hundreds of thousands of these callbacks alive
at any point in time.

Thanks

  reply	other threads:[~2016-02-03 20:15 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-02-03 10:54 Christoph Höger
2016-02-03 11:48 ` Jeremie Dimino
2016-02-03 12:26   ` Malcolm Matalka
2016-02-03 13:44     ` David Sheets
2016-02-03 18:02       ` Jeremy Yallop
2016-02-03 20:15         ` Malcolm Matalka [this message]
2016-02-04  0:14           ` Jeremy Yallop
2016-02-04  7:26             ` Malcolm Matalka
2016-02-04 19:29               ` Jeremy Yallop
     [not found]   ` <56B1EC33.2090303@tu-berlin.de>
2016-02-03 13:49     ` Jeremie Dimino
2016-02-03 14:38       ` Christoph Höger

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