From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 98239 invoked by alias); 11 Jul 2018 01:30:58 -0000 Mailing-List: contact cygwin-help@cygwin.com; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner@cygwin.com Mail-Followup-To: cygwin@cygwin.com Received: (qmail 97948 invoked by uid 89); 11 Jul 2018 01:30:32 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-1.8 required=5.0 tests=AWL,BAYES_00,KAM_LAZY_DOMAIN_SECURITY,RCVD_IN_DNSWL_LOW autolearn=no version=3.3.2 spammy=fraction X-HELO: smtp-out-so.shaw.ca Received: from smtp-out-so.shaw.ca (HELO smtp-out-so.shaw.ca) (64.59.136.139) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Wed, 11 Jul 2018 01:30:29 +0000 Received: from [192.168.1.100] ([24.64.240.204]) by shaw.ca with ESMTP id d3xHfTJ865Hxqd3xIfU9wV; Tue, 10 Jul 2018 19:30:08 -0600 Reply-To: Brian.Inglis@SystematicSw.ab.ca Subject: Re: FPE in localtime.cc To: cygwin@cygwin.com References: <20180709164947.GE27673@calimero.vinschen.de> <20180709190729.GF27673@calimero.vinschen.de> From: Brian Inglis Openpgp: preference=signencrypt Message-ID: Date: Wed, 11 Jul 2018 01:30:00 -0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-IsSubscribed: yes X-SW-Source: 2018-07/txt/msg00098.txt.bz2 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 >>>> #include >>>> #include >>>> #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 -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple