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Re: [PATCH] string: Add tests for zero length string inputs

Richard Earnshaw <> writes:

> On 22/09/14 18:48, Paul Eggert wrote:
>> On 09/22/2014 09:09 AM, Richard Earnshaw wrote:
>>> Valid pointers is more than just non-NULL.  In particular, it implies
>>> that is safe to dereference the addressed byte in a source operand even
>>> when the length parameter is zero.
>> I just reread C99 7.1.4 clause 1 and 7.21.2 clause 2, and I don't see 
>> that implication.  For example, the following program appears to be 
>> strictly conforming:
>>     #include <string.h>
>>     char src[1];
>>     char dst[1];
>>     int
>>     main (void)
>>     {
>>       memcpy (dst, src + 1, 0);
>>       return 0;
>>     }
>> Here, src + 1 is a valid pointer even though one cannot safely 
>> dereference it.  So it appears to be reasonable to check that memcpy 
>> doesn't dereference the source when the size is zero.
> Read clause 1 of 7.1.4 again.  "If an argument to a function has an
> invalid value ... or a pointer outside of the address space of the
> program... the behaviour is undefined."
> Ergo, if src+1 can point outside of the address space of the program,
> it's undefined behaviour.

src+1 is _not_ outside of the address space.  It is a valid pointer
(which you must not dereference).


Andreas Schwab, SUSE Labs,
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"And now for something completely different."

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