On Mär 17 2022, Jeff Law via Gcc-patches wrote:
> On Thu, Mar 17, 2022 at 9:32 AM Marek Polacek via Gcc-patches <
> gcc-patches@gcc.gnu.org> wrote:
>> I think I agree, I've tried
>>
>> #include <string.h>
>> char a[] = "abc";
>> char b[] = "abcd";
>>
>> int f (void)
>> {
>> return strncmp (a, b, 8);
>> }
>>
>> where I get
>>
>> t.c:7:10: warning: ‘strncmp’ specified bound 8 exceeds source size 5
>> [-Wstringop-overread]
>> 7 | return strncmp (a, b, 8); // -Wstringop-overread
>> | ^~~~~~~~~~~~~~~~~
>>
>> even without -Wall. strncmp sees that a[3] is '\0' so it stops comparing
>> and there's no UB.
>>
> This one is a clear case where warning is bad. Both arguments are
> constant and we can determine they are NUL terminated and an overread will
> never occur. No deep analysis really needed here.
Both a and b are modifiable, thus the compiler cannot assume anything.
--
Andreas Schwab, sch...@linux-m68k.org
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