Re: [m-users.] MR_GC_free ?

Am Mittwoch, dem 08.11.2023 um 12:15 +1100 schrieb Peter Wang:
> On Tue, 07 Nov 2023 20:17:06 +0100 Volker Wysk <[email protected]> wrote:
> > Hi
> > 
> > Am Mittwoch, dem 08.11.2023 um 05:53 +1100 schrieb Zoltan Somogyi:
> > > On 2023-11-08 05:48 +11:00 AEDT, "Volker Wysk" <[email protected]> wrote:
> > > > Hi.
> > > > 
> > > > In the ODBC library, there's space allocated by MR_GC_NEW and explicitly
> > > > freed with MR_GC_free. This confuses me. Shouldn't space allocated by
> > > > MR_CG_NEW be deallocated by the garbage collector? If it is to be
> > > > deallocated explicitly, shouldn't the space be allocated by MR_NEW and be
> > > > deallocated by MR_free?
> > > 
> > > Follow the definition of MR_GC_free in runtime/mercury_memory.h
> > > to boehm_gc/include/gc.h.
> > 
> > This leads to this (I couldn't understand the definition of this function):
> > 
> > /* Explicitly deallocate an object.  Dangerous if used incorrectly.     */
> > /* Requires a pointer to the base of an object.                         */
> > /* An object should not be enabled for finalization (and it should not  */
> > /* contain registered disappearing links of any kind) when it is        */
> > /* explicitly deallocated.                                              */
> > /* GC_free(0) is a no-op, as required by ANSI C for free.               */
> > GC_API void GC_CALL GC_free(void *);
> > 
> > So the gc explicitly frees the object and remembers that it has done so, so
> > it won't be garbage collected later.
> > 
> > Is it bad style to use MR_GC_NEW and MR_GC_free, since it should rather be
> > done with MR_NEW/MR_free? (That's what those are there for, aren't they?)
> 
> They do different things.
> 
> MR_GC_NEW allocates memory that will be garbage collected when there are
> no more references to it (as far as the GC can see). MR_GC_free
> deallocates the memory immediately, before a GC cycle, even if there
> might actually be references to it hanging around. Rarely, there MAY be
> performance advantages to doing so, e.g. if you allocate a large object
> and definitely don't need it any more, and don't want to wait for one or
> more GC cycles for that memory to be reclaimed.
> 
> MR_NEW is just a wrapper around the C library's malloc routine.
> The GC doesn't see inside inside malloc-allocated memory. You should
> generally not use MR_NEW or any variant of malloc to allocate memory for
> an object that may contain pointers to GC-allocated memory. If the last
> reference(s) to a GC-allocated object only exist within memory allocated
> by malloc then, as far as the GC is concerned, that GC-allocated object
> is no longer referenced and may be reclaimed.
> 
> In a Mercury program, you'll want to allocate GC-able memory
> unless there is a specific reason not to.

Thank you for the explanation. There's one thing which I don't get yet. Can
the garbage collector see into C structures which are allocated in a foreign
procedure? Or can it only see inside Mercury-land data types? If it can,
how?

Cheers,
Volker
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