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Re: scm_must_malloc() vs. malloc()

From: Dirk Herrmann
Subject: Re: scm_must_malloc() vs. malloc()
Date: Tue, 13 Mar 2001 17:48:39 +0100 (MET)

On Tue, 13 Mar 2001, Martin Grabmueller wrote:

> is there any reason why fports.c uses malloc() instead of
> scm_must_malloc()?

Hmmm.... It seems that in this case, scm_must_malloc should be
allright.  Maybe Gary Houston should also take a look at it.

> Additionally, gh_data.c contains a lot of code like this:
>       if (m == 0)
>       m = (short *) malloc (n * sizeof (short));
>       for (i = 0; i < n; ++i)
>       m[i] = SCM_INUM (SCM_VELTS (obj)[i]);
> I think these should get replaced by calls to scm_must_malloc too.  We
> don't want segfaults, do we?

These should, IMO, _not_ be replaced by scm_must_malloc.  Let me try to
give an explanation about when scm_must_malloc should be used:  If you
call scm_must_malloc, then you say "This memory is under garbage
collection control".  For example, you allocate memory for some smob.  If
the smob get's garbage collected, the corresponding memory should be freed
by the smob's free function.  In other words, the memory that is allocated
might be freed by garbage collection.  Guile records the amount of memory
that is under garbage collection control.  If it becomes too much, it
starts a garbage collection, because by collecting some unused objects,
the corresponding memory will be freed.

The counterexample is, if you just allocate some C level array.  No matter
how often guile's garbage collector will be called, it won't be able to
free that array.  Thus, that array is not under the control of the garbage
collector.  If you allocate this array with scm_must_malloc, however, you
fool the garbage collector.  The garbage collector might think "Hey,
there is a lot of malloc'd memory that I might be able to collect."  Thus,
the garbage collection would be called in situations, where there is
actually no chance that it could free the desired amount of memory.

Now, looking at the code in gh_data.c:  Some functions convert scheme
vectors into a corresponding C level array.  The C level array is created
with malloc.  But, these arrays are not under control of the garbage
collector.  It completely depends on the calling function what it will do
with them.  Thus, in these cases you should not use scm_must_malloc.

Best regards,
Dirk Herrmann

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