[Bug malloc/12140] mallopt(M_PERTURB) free() anomaly

["siddhesh at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=12140

--- Comment #8 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2012-10-17 14:33:13 UTC ---
(In reply to comment #7)
> However, that begs the question: why do the values in the first 2* sizeof(void
> *) not look like pointers. (Okay, the zero could be a NULL pointer, but that
> seems unlikely.) I think imagine that the reason is this: this particular block
> of memory is in a FASTBIN, and IIRC, pointers are used there, just bitmaps of
> free and in use slots. Sound reasonable?

They're NULL pointers as you guessed, since the fastbins are empty in this
case.  In a more elaborate usage, you'll find actual pointers there, especially
if a block is used long enough after it has been freed.

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