[PATCH][BZ #1349] Return requested size for malloc_usable_size when MALLOC_CHECK_ > 0

[Siddhesh Poyarekar]

On Fri, 31 Aug 2012 14:38:22 +0200, Andreas wrote:
> > +  memset (p, 0, malloc_usable_size (p));
> > +  free (p);
> > +  return 0;
> 
> In this case malloc_usable_size () should return >= 7, so I suggest
> to check for that value as well. The memset/free is a good idea!

When MALLOC_CHECK_ is exported as a positive value (which I am doing
for this test), malloc_usable_size should return exactly 7, since the
malloc_check code adds a magic number at p[7] if p is the mem pointer.
Anything greater than that and the user may risk writing over the magic
value and tripping an assertion during free.

But I get your point; I will add a check to ensure that usable_size ==
7, lest it returns a value less than 7 due to some future bug. Updated
patch attached.

Regards,
Siddhesh
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