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Re: FPE in localtime.cc




On 2018-07-10 07:35, cyg Simple wrote:
> On 7/9/2018 3:07 PM, Corinna Vinschen wrote:
>> On Jul  9 11:16, Brian Inglis wrote:
>>> On 2018-07-09 10:49, Corinna Vinschen wrote:
>>>> On Jul  9 15:47, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
>>>>> 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?
>> You may want to ask the original author why he used FP arithmetic in
>> this place.  Maybe it's a way to avoid integer overflow.  I'm reluctant
>> to change this given that this code is still used in BSD as well.
> I don't see a fetestexcept() being used.  Setting all the bits is
> definitely going to raise an exception.
>>>> 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 ;^>
>> That's what I meant.
> Yes, see:
> https://en.cppreference.com/w/c/numeric/fenv/FE_exceptions

Nasty: I didn't realize decimal fraction to binary conversions would raise
FE_INEXACT!

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

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