Re: FPE in localtime.cc

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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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):
> 
> #define _GNU_SOURCE
> #include <fenv.h>
> #include <time.h>
> #include <stdio.h>
> 
> static time_t s_Time;
> 
> static void fun(void)
> {
>   char buf[40];
>   strftime(buf, sizeof(buf), "%m/%d/%y %H:%M:%S", localtime(&s_Time));
>   printf("%s\n", buf);
> }
> 
> int main()
> {
>   feenableexcept(FE_ALL_EXCEPT);
>   time(&s_Time);
>   fun();
> }
> 
> $ cat a.exe.stackdump
> Exception: STATUS_FLOAT_INEXACT_RESULT at rip=001800BBA24
> rax=000000000000016D rbx=000000005B43498E rcx=00000000000001EC
> rdx=1845C8A0CE512957 rsi=0000000051EB851F rdi=2CC3D8D4A245F203
> r8 =0000000000F92B80 r9 =00000000FFFFFFED r10=00000000000007E2
> r11=000000000000000C r12=00000000FFFFFFFF r13=0000000080000000
> r14=00000000000000BD r15=00000000000007E2
> rbp=000000007FFFFFFF rsp=00000000FFFFCA70
> program=C:\Cygwin64\...\a.exe, pid 14232, thread main
> cs=0033 ds=002B es=002B fs=0053 gs=002B ss=002B
> 
> Removing the first line of the main() function lets the program run successfully.
> The same code runs fine unmodified on Linux.
> 
> $ gcc -Wall sample.c -lm
> $ ./a.out
> 07/09/18 11:44:26
> 
> Any ideas?

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.

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.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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