[PATCH] add memrchr(3)
Wed May 9 19:02:00 GMT 2012
On Wed, May 09, 2012 at 08:23:52AM -0600, Eric Blake wrote:
>On 05/09/2012 03:01 AM, Corinna Vinschen wrote:
>>> _CONST _PTR src_end = (const unsigned char *) src + length - sizeof (unsigned char);
>sizeof(unsigned char) is defined by C to be exactly 1; I always question
>code that spells it out longhand instead of using 1.
>>> _CONST _PTR last = NULL;
>>> while ((src = memchr (src, c, length)))
>>> if (src > src_end)
>Also, src will never be > src_end - memchr returns NULL rather than
>reading beyond the bounds of length.
>>> last = src;
>>> src = (const unsigned char *) src + sizeof (unsigned char);
>>> return (_PTR) last;
>> The patch has a problem. The loop misses to change the length variable
>> while iterating over the memchr results. That means, any subsequent
>> call after the first one will potentiall read beyond the requested
>> length. In border cases this will result in reading beyond the
>> available memory or beyond the allocated virtual memory. The call to
>> memchr must never read beyond src + length - 1.
>Additionally, I think that searching forwards through the array via one
>function call per occurrence of the byte in question is wasteful - since
>we already know the array bounds, we might as well search in reverse by
>doing a single C loop that iterates backwards over a word at a time.
>strrchr must search forwards, because it is also searching for the
>terminating NUL and doesn't know the length in advance, but memrchr
>should be faster.
Why not just grab the source from FreeBSD?
(or actually from OpenBSD since that's where the above originated)
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