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What should the "size" in "caml_alloc_custom" be?
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Date: -- (:)
From: Goswin von Brederlow <goswin-v-b@w...>
Subject: Re: [Caml-list] What should the "size" in "caml_alloc_custom" be?
lyn HONG <lynxiamen@gmail.com> writes:

> On Tue, May 18, 2010 at 2:58 AM, Goswin von Brederlow <goswin-v-b@web.de>
> wrote:
>
>     lyn HONG <lynxiamen@gmail.com> writes:
>
>     > Hi all,
>     >
>     > I have a question about "allocating custom blocks" in "iterfacing C with
>     object
>     > Ocaml". when we call function "caml_alloc_custom(ops, size, used, max)"
>     in the
>     > C side, if the structure we want to allocate has a pointer, is the
>     "size" 
>     > going to be size of the structure itself only, or should we also include
>     the
>     > memory block that pointer points to?
>     >
>     > Thanks,
>     > Lin
>
>     The size is the number of ocaml words in the structure itself. The
>     pointer then points outside the ocaml heap to some C memory allocated by
>     malloc(). You should account for that size in the used/max pair.
>
>
> Thank you so much for the reply.
> So when we free the memory, what do we do with the block that pointer points
> to? Free it in the 'finalization function' associated to 'ops'?
>
> Best,
> Lin
>  
>
>     MfG
>            Goswin

That depends on who allocted the memory and who is supposed to free
it.

In some cases a library will free the chunk when some cleanup function
is called and your binding for cleanup() should then invalidate the
pointer and other bindings should verify the pointer is still valid
before using it. In that case the 'finalization function' can either
call cleanup() when the pointer is still valid when ocaml declares the
block as unused, give a warning that cleanup() wasn't called and call it
or even give an error. I like the warning best.

In other cases the chunk is something you are supposed to free when you
are done with it. Then you just free() it.

I really depends on the situation. Make damn sure that neigther the
ocaml nor C code accesses the chunk after it has been freed, ever.
Also make sure it doesn't get leaked.

MfG
        Goswin