From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 23819 invoked by alias); 10 Jul 2018 13:35:20 -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 23685 invoked by uid 89); 10 Jul 2018 13:35:19 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,SPF_PASS autolearn=ham version=3.3.2 spammy= X-HELO: mail-it0-f41.google.com Received: from mail-it0-f41.google.com (HELO mail-it0-f41.google.com) (209.85.214.41) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Tue, 10 Jul 2018 13:35:17 +0000 Received: by mail-it0-f41.google.com with SMTP id g4-v6so14464589iti.1 for ; Tue, 10 Jul 2018 06:35:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:references:from:openpgp:autocrypt:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=P1IPd+POg9hSo7gCDKhfHMdkJGr1Z2cLVMs18leLZek=; b=G0qEGt3NWH3PXRXllQ3c+N2kdbQgdZppjoS0dMVjJY2bbwP/xaW40/Mg2WSj6uVu61 V03xovfgwGLQM3p7Nurb3iJQSdXCofb2tDynI8yvpeyg6w36VvCzCs3kvzZWJm6TyOBh 8ysFn6Sy80Te20h15hpY90xRzeAQcAzVA7W385f0Xtub0YAeXErCcMhD+BIIzpz55dCW 9+vEwIYXruIoG1DdXOXm6x5W5JOPPrxQDhqrZ/uHmg56IqXH7frkZpuP7Is/aOKHeR65 elEoT4XERjKM+DgKCH2POJm0fnUuc9He6gABYgS9lkqmYGzqB+3ilnHggcgvBrmiVXLv +09w== Return-Path: Received: from [192.168.0.3] ([69.47.131.109]) by smtp.gmail.com with ESMTPSA id j6-v6sm2885923iog.39.2018.07.10.06.35.14 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 10 Jul 2018 06:35:14 -0700 (PDT) Subject: Re: FPE in localtime.cc To: cygwin@cygwin.com References: <20180709164947.GE27673@calimero.vinschen.de> <20180709190729.GF27673@calimero.vinschen.de> From: cyg Simple Openpgp: preference=signencrypt Message-ID: Date: Tue, 10 Jul 2018 13:35:00 -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: <20180709190729.GF27673@calimero.vinschen.de> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-IsSubscribed: yes X-SW-Source: 2018-07/txt/msg00092.txt.bz2 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: >>>> 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 >>> #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 -- cyg Simple -- 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