Re: FPE in localtime.cc

[Brian Inglis <Brian.Inglis@SystematicSw.ab.ca>]

On 2018-07-09 10:49, Corinna Vinschen wrote:
> On Jul  9 15:47, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
>> Hello,
>>
>> the following sample coredumps with FPE at localhost.cc:1962 with the latest snapshot (6/29/2018):
...
> You can simplify your testcase by not calling any time functions:
> 
>   #define _GNU_SOURCE
>   #include <fenv.h>
>   #include <stdio.h>
>   #include <stdlib.h>
> 
>   #define SECSPERDAY 86400
> 
>   int main(int argc, char **argv)
>   {
>     feenableexcept(FE_ALL_EXCEPT);
>     long tdays = argc > 1 ? strtol (argv[1], NULL, 10) : 189;
>     long seconds = tdays * SECSPERDAY + 0.5;
>     printf ("%ld\n", seconds);
>   }
> 
> This generates a SIGFPE on Linux as well.
> 
> The line computing seconds is the same line as used by the localtime
> function.  Cygwin shares the entire localtime code with the various
> BSDs, so I guess they would have the same problem.
What is that line meant to do? Am I missing something?
It should be the equivalent of (tdays*SECSPERDAY*2 + 1)/2!
It converts an integer value to double, adds 1/2, and throws it away on
conversion back, unless the intermediate has insufficient mantissa bits, in
which case, it loses the low bits?

> Bottom line is, don't bulk enable FP exceptions, but only if you really
> need it for certain parts of your code.  Don't expect library functions
> to be SIGFPE clean under all circumstances.

Maybe selectively enable specific FPEs to check for where needed.
Or be careful what you wish for, as you just might get a lot more than you
bargained for ;^>

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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