From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: FPE in localtime.cc
Date: Mon, 09 Jul 2018 19:07:00 -0000 [thread overview]
Message-ID: <20180709190729.GF27673@calimero.vinschen.de> (raw)
In-Reply-To: <cf7088aa-9cb5-bc43-1bf2-6acc0603b57b@SystematicSw.ab.ca>
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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:
> >> 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?
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.
> > 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.
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
Red Hat
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next prev parent reply other threads:[~2018-07-09 19:07 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-09 15:47 Lavrentiev, Anton (NIH/NLM/NCBI) [C]
2018-07-09 16:49 ` Corinna Vinschen
2018-07-09 17:17 ` Brian Inglis
2018-07-09 19:07 ` Corinna Vinschen [this message]
2018-07-10 13:35 ` cyg Simple
2018-07-11 1:30 ` Brian Inglis
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