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Heaps size problems with "caml_alloc_small" in foreign function interfaces
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Date: -- (:)
From: Sean Seefried <sean.seefried@n...>
Subject: Re: [Caml-list] Heaps size problems with "caml_alloc_small" in foreign function interfaces

On 11/07/2008, at 7:40 PM, Richard Jones wrote:

> On Fri, Jul 11, 2008 at 06:21:51PM +1000, Sean Seefried wrote:
>> I'm having a problem where sometimes a call to "caml_alloc_small"  
>> from
>> C results in a segmentation fault. If I increase the size of the  
>> stack
>> using OCAMLRUNPARAM=s=1000k then I don't get the crash anymore. It
>> seems strange that I have to increase the size of the heap manually
>> like this. Is this because I'm calling this function from C?
>
> Seems like you have confusion over heap & stack.
>
> First of all, caml_alloc_small is limited to small allocations, so
> number of words allocated must be <= Max_young_wosize.  You don't post
> any example code so we can't see whether that is true in your code.
>

caml_alloc_small(32,0);

is what seemed to cause the problem. I'm shying away from posting more  
code since it is auto-generated from CamlIDL and is very verbose. I  
didn't write it myself.

> Secondly (and much more likely to be the problem), the caml_alloc*
> functions allocate uninitialised memory.  If the garbage collector

What do you mean by "uninitialised memory"?

> gets to run before you've properly initialized all the fields then the
> GC will hit an uninitialised field and a segfault could be the result.
>
>  eg: This is a fail:
>
>  v = caml_alloc (2, 0);
>  vv = caml_alloc (3, 0);  /* GC could run here */
>  Store_field (v, 0, vv);
>

What do you do about this? Is there a way to stop the GC from running  
for a period of time?

> Changing the _stack_ size (or other tunables) probably just changes
> something about when the garbage collector runs, and thus moves the
> bug around.
>
>> If I want to increase the size of the heap in C how do I do this?
>> Could I write a "safe" caml_alloc_small which first checks to see if
>> there is enough memory and then increases the heap size if not?
>
> The "size of the heap in C" is (for most operating systems) extended
> automatically by malloc.  What you're saying here isn't necessary -
> you must have some other bug.
>

I wasn't very clear but I meant changing the OCaml heap size using C  
functions.

Sean