Re: [PATCH] [BZ 20628] make mallinfo saturating

[Paul Eggert <eggert at cs dot ucla dot edu>]

DJ Delorie wrote:
> Paul Eggert <eggert@cs.ucla.edu> writes:
> > These don't look right, as INTERNAL_SIZE_T might not be wider than int,
>
> In which case, the values we're adding up won't overflow because memory
> is limited to an int-sized address space.

I don't see how that follows. If INTERNAL_SIZE_T and int are both 32 bits, then 
(INTERNAL_SIZE_T)(SUM) + (INTERNAL_SIZE_T)(ADD) is a 32-bit unsigned addition. 
This addition can overflow and wrap around, but the code doesn't check for that 
overflow.

> > Also, if SUM is INT_MIN
>
> "Assumes ADD and SUM are positive."

That assumption is incorrect, since SUM can be INT_MIN after a wraparound overflow.