AW: [Caml-list] Segmentation fault right after caml_alloc_custom

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am 01.08.2012 23:08:12 schrieb(en) Lin Hong:
> Hi Gerd,
> 
> thank you for the reply.
> 
> how do we know if some value is registered as local root or not?

Easy: It is your task as developer of a C external to register values,  
so if you don't do it, it is not done.

> the code is long so I will just paste some of it first
> 
> in that c file, the value we tried to access but failed was also  
> allocated by caml_alloc_custom(ops, size, used, max).
> 
> caml_alloc_custom (&sankoff_custom_operations_eltarr,sizeof (struct  
> elt_arr)+sz, 1,alloc_custom_max);
> 
> the ops was custom_operations with all standard options as follow:
> 
> static struct custom_operations sankoff_custom_operations_eltarr = {
>     "http://www.amnh.org/poy/",
>     custom_finalize_default,
>     custom_compare_default,
>     custom_hash_default,
>     custom_serialize_default,
>     custom_deserialize_default
> };
> 
> 
> the size in caml_alloc_custom is set to the full size , that is the  
> data structure itself, plus everything its pointers point to.

This is probably all ok. Did you use the CAMLparam and CAMLlocal macros?

Gerd


> 
> we  also debug with valgrind,  the feed back suggest the memory we  
> tried to access is indeed freed by caml gc
> 
> ==64563== Invalid read of size 4
> ==64563==    at 0x38477B: sankoff_median_3 (in ../poy.native)
> ==64563==    by 0x386125: sankoff_CAML_median_3 (in ../poy.native)
> ==64563==    by 0x13177D: camlSankCS__median_3_196 (in ../poy.native)
> ==64563==  Address 0x8bb0c04 is 81,108 bytes inside a block of size  
> 512,016 free'd
> ==64563==    at 0xAFD7BF: free (vg_replace_malloc.c:325)
> ==64563==    by 0x3E5AC2: caml_free_for_heap (in ../poy.native)
> ==64563==    by 0x3E5C31: caml_shrink_heap (in ../poy.native)
> ==64563==    by 0x3F16FC: caml_compact_heap (in ../poy.native)
> ==64563==    by 0x3F1A89: caml_compact_heap_maybe (in ../poy.native)
> ==64563==    by 0x3E4EBC: caml_major_collection_slice (in  
> ../poy.native)
> ==64563==    by 0x3E5692: caml_minor_collection (in ../poy.native)
> ==64563==    by 0x3E57E0: caml_check_urgent_gc (in ../poy.native)
> ==64563==    by 0x3F209C: caml_alloc_custom (in ../poy.native)
> 
> 
> 
> Cheers,
> Lin Hong
> American Museum of Natural History
> POY website :
> https://code.google.com/p/poy/
> http://research.amnh.org/scicomp/scripts/download.php
> 
> ________________________________________
> From: Gerd Stolpmann [gerd@edgespring.com]
> Sent: Wednesday, August 01, 2012 4:13 PM
> To: Lin Hong
> Cc: Mailing OCaML
> Subject: Re: [Caml-list] Segmentation fault right after  
> caml_alloc_custom
> 
> You did not send code, so we can only speculate. A reason for this
> behavior could be that a value is not registered as local root, so the
> GC thinks it is unused.
> 
> That you see the problem only on some platforms can be explained by  
> the
> different implementations of malloc, and how quickly freed memory is
> really deallocated (i.e. given back to the kernel). I wouldn't give
> much on this observation.
> 
> Gerd
> 
> Am 01.08.2012 22:05:19 schrieb(en) Lin Hong:
> > Hi all,
> >
> > we have a weird problem here. In a c file, right after one call of
> > 'caml_alloc_custom', some old information we have in memory is gone,
> > then we have the segmentation fault because the code try to access
> > those informations later.
> >
> >  turn on debug message with OCAMLRUNPARAM, get this right before it
> > crash.
> >
> > ......
> > $Starting new major GC cycle
> > Marking 9223372036854775807 words
> > Subphase = 10
> > Sweeping 9223372036854775807 words
> > Compacting heap...
> > Shrinking heap to 37696k bytes
> > Shrinking heap to 36704k bytes
> > Shrinking heap to 35712k bytes
> > Shrinking heap to 34720k bytes
> > Shrinking heap to 33728k bytes
> > Shrinking heap to 32736k bytes
> > Shrinking heap to 31744k bytes
> > Shrinking heap to 30752k bytes
> > done.
> >
> > is it possible that the ocaml garbage collection free memory that is
> > still in use?
> >
> > the data cause this is kind of big, it doesn't happen right away,and
> > it doesn't happen on every machine (linux with ocaml4 is fine,
> > ocaml3.xx is not. mac with ocaml4 still shows the problem though).
> >
> > any suggestion is appreciated.
> >
> >
> > Thanks,
> > Lin
> >
> >
> > =======================
> > American Museum of Natural History
> > POY website :
> > https://code.google.com/p/poy/
> > http://research.amnh.org/scicomp/scripts/download.php
> >
> > --
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> >
> >
> 
> 
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> 
> 

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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
Creator of GODI and camlcity.org.
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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