What does "chunk's tail overwrite" overwrite mean?

It comes from malloc.c line 2074 (perl-5.8.5).  An XS module of mine 
keeps crashing with the message:

assertion botched (chunk's tail overwrite?): ((unsigned int 
*)((caddr_t)ovp + nbytes))[-1] == 0x55555555 (..\malloc.c:2074)

when run under a DEBUGGING perl with perl's malloc.  The software 
apparently runs OK under a release-mode perl, and under perls with the 
system malloc (both release and DEBUGGING), but I suspect that all is 
not really well in light of the above.

The Safefree() call is freeing up the SV's PV slot, which was earlier 
assigned to the SV (and realloc()'ed) via sv_usepvn().  I haven't made 
the mistake of attempting to free the space given to the SV as its PV 
slot separately, so I'm at a loss as to what the problem is.

Any ideas what this assertion failure might indicate?

- Steve



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