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Re: xalloc.h (x2nrealloc): Don't always double the buffer size.
From: |
Paul Eggert |
Subject: |
Re: xalloc.h (x2nrealloc): Don't always double the buffer size. |
Date: |
Sat, 03 Feb 2007 18:25:18 -0800 |
User-agent: |
Gnus/5.1008 (Gnus v5.10.8) Emacs/21.4 (gnu/linux) |
Jim Meyering <address@hidden> writes:
> - if (((size_t) -1) / 2 / s < n)
> + if ((2 * (((size_t) -1 - 1) / 3)) / s < n)
That's not quite right. As an extreme case, suppose S is
SIZE_MAX/4 + 1 and N is 2. Then (2 * (((size_t) -1 - 1) / 3)) / S
evaluates to 2 and N will appear to be in range here, but:
> + n = n + n / 2 + 1;
will cause N to become 4, and N * S will then overflow.
I installed this:
2007-02-03 Paul Eggert <address@hidden>
* lib/xalloc.h (x2nrealloc): Fix an unlikely bug in the overflow
checking code. Set N = ceil (1.5 * N) rather than to a slightly
larger value.
--- lib/xalloc.h 1 Feb 2007 23:53:04 -0000 1.33
+++ lib/xalloc.h 4 Feb 2007 02:20:07 -0000
@@ -141,7 +141,7 @@ xnrealloc (void *p, size_t n, size_t s)
In the following implementation, nonzero sizes are increased by a
factor of approximately 1.5 so that repeated reallocations have
- O(N log N) overall cost rather than O(N**2) cost, but the
+ O(N) overall cost rather than O(N**2) cost, but the
specification for this function does not guarantee that rate.
Here is an example of use:
@@ -204,9 +204,13 @@ x2nrealloc (void *p, size_t *pn, size_t
}
else
{
- if ((2 * (((size_t) -1 - 1) / 3)) / s < n)
+ /* Set N = ceil (1.5 * N) so that progress is made if N == 1.
+ Check for overflow, so that N * S stays in size_t range.
+ The check is slightly conservative, but an exact check isn't
+ worth the trouble. */
+ if ((size_t) -1 / 3 * 2 / s <= n)
xalloc_die ();
- n = n + n / 2 + 1;
+ n += (n + 1) / 2;
}
*pn = n;