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Date: Mon, 9 Jul 2018 18:49:47 +0200
From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: FPE in localtime.cc
Message-ID: <20180709164947.GE27673@calimero.vinschen.de>
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On Jul  9 15:47, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
> Hello,
>=20
> the following sample coredumps with FPE at localhost.cc:1962 with the lat=
est snapshot (6/29/2018):
>=20
> #define _GNU_SOURCE
> #include <fenv.h>
> #include <time.h>
> #include <stdio.h>
>=20
> static time_t s_Time;
>=20
> static void fun(void)
> {
>   char buf[40];
>   strftime(buf, sizeof(buf), "%m/%d/%y %H:%M:%S", localtime(&s_Time));
>   printf("%s\n", buf);
> }
>=20
> int main()
> {
>   feenableexcept(FE_ALL_EXCEPT);
>   time(&s_Time);
>   fun();
> }
>=20
> $ cat a.exe.stackdump
> Exception: STATUS_FLOAT_INEXACT_RESULT at rip=3D001800BBA24
> rax=3D000000000000016D rbx=3D000000005B43498E rcx=3D00000000000001EC
> rdx=3D1845C8A0CE512957 rsi=3D0000000051EB851F rdi=3D2CC3D8D4A245F203
> r8 =3D0000000000F92B80 r9 =3D00000000FFFFFFED r10=3D00000000000007E2
> r11=3D000000000000000C r12=3D00000000FFFFFFFF r13=3D0000000080000000
> r14=3D00000000000000BD r15=3D00000000000007E2
> rbp=3D000000007FFFFFFF rsp=3D00000000FFFFCA70
> program=3DC:\Cygwin64\...\a.exe, pid 14232, thread main
> cs=3D0033 ds=3D002B es=3D002B fs=3D0053 gs=3D002B ss=3D002B
>=20
> Removing the first line of the main() function lets the program run succe=
ssfully.
> The same code runs fine unmodified on Linux.
>=20
> $ gcc -Wall sample.c -lm
> $ ./a.out
> 07/09/18 11:44:26
>=20
> 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 =3D argc > 1 ? strtol (argv[1], NULL, 10) : 189;
    long seconds =3D 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

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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