On 2014-08-12 20:23 +0200, Ben Hutchings wrote:
> On Tue, 2014-08-12 at 09:05 -0700, Steve Langasek wrote:
> [...]
>> Matthias, could you please have a look at the below test case? We have a
>> regression in the latest nfs-kernel-server build, which appears to be caused
>> by a gcc-4.9 bug.
>>
>> Should I work around this in nfs-utils, or is a quick fix possible in
>> gcc-4.9?
>>
>> > char buf[100];
>> >
>> > void
>> > add_name(char *old)
>> > {
>> > char *cp = old;
>> >
>> > while (cp && *cp) {
>> > cp++;
>> > }
>> > __builtin_strncpy(buf, old, cp-old);
> [...]
>
> So far as I know (haven't checked the latest standard), pointer
> subtraction has undefined behaviour unless both operands point into (or
> one beyond) the same array. As this is not true of null pointers, the
> compiler may infer that old can't be null, so cp can't be null, so there
> is no need to check whether it is.
This is true in C, unfortunately. However…
> I.e. this is a bug in nfs-utils, not the compiler.
…Petr's example program crashes even when compiled with g++-4.9, and in
C++ subtracting two null pointers is valid, yielding zero.
Cheers,
Sven
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