On 02/03/2018 16:43, Jonathan Wakely wrote:
> On 2 March 2018 at 15:00, Mason wrote:
>
>> On 02/03/2018 15:38, Jonathan Wakely wrote:
>>
>>> On 2 March 2018 at 14:27, Mason wrote:
>>>
>>>> I am confused by both of these warnings.
>>>> I am aware that adding two 60-char strings might overflow an 80-char buffer,
>>>> which is why I'm using snprintf.
>>>
>>> In which case the output would be truncated instead of overflowing,
>>> and that's what the warning says.
>>>
>>>> I'm not sure these warnings are very helpful. What am I missing?
>>>> (I may have oversimplified the testcase.)
>>>
>>> I think the point is that although you avoid undefined behaviour, you
>>> might still not get the output you expected.
>>
>> To provide some context, I am using the following struct:
>>
>> struct expr
>> {
>> int val;
>> char txt[60];
>> };
>>
>> to store symbolic arithmetic expressions.
>>
>> I start with simple symbolic constants "a", "b", "c", etc
>> and build up more complex expressions, such as "(a + b)" and "((a + b) * c)"
>>
>> Actual code, for example,
>>
>> struct expr bak = tab[i];
>> snprintf(tab[i].txt, 60, "(%s + %s)", bak.txt, tab[j].txt);
>>
>> Is there a different way to write this code to avoid the warning(s) or should
>> I just disable Wformat-truncation?
>> (I hate disabling warnings, but this one looks irrelevant, in my situation.)
>
> I would have hoped that either assert(strlen(p->txt) < 40) or if
> (strlen(p->txt) >= 40) __builtin_unreachable(); would help, at least
> with optimizations enabled, but apparently not (and of course they can
> incur run-time cost).
According to perf, my program is spending most of its run-time in
stdio functions. I need to work on that :-)
45.98% a.out libc-2.17.so [.] vfprintf
16.69% a.out libc-2.17.so [.] _IO_default_xsputn
12.40% a.out a.out [.] step
7.85% a.out libc-2.17.so [.] __strchrnul
3.63% a.out libc-2.17.so [.] __vsnprintf_chk
3.31% a.out libc-2.17.so [.] _IO_str_init_static_internal
2.83% a.out libc-2.17.so [.] _IO_old_init
2.51% a.out libc-2.17.so [.] __GI___mempcpy
1.21% a.out libc-2.17.so [.] __snprintf_chk
1.18% a.out libc-2.17.so [.] _IO_no_init
1.13% a.out libc-2.17.so [.] _IO_setb
0.80% a.out libc-2.17.so [.] free
0.38% a.out libc-2.17.so [.] @plt
0.10% a.out a.out [.] __snprintf_chk@plt
Regards.
