Date: Wed, 20 Jan 2010 22:13:25 GMT From: Paul Guyot <paulguyot@ieee.org> To: freebsd-gnats-submit@FreeBSD.org Subject: kern/143037: strtod doesn't always round to the nearest Message-ID: <201001202213.o0KMDPY5030953@www.freebsd.org> Resent-Message-ID: <201001202220.o0KMK6hg077066@freefall.freebsd.org>
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>Number: 143037 >Category: kern >Synopsis: strtod doesn't always round to the nearest >Confidential: no >Severity: non-critical >Priority: low >Responsible: freebsd-bugs >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Wed Jan 20 22:20:06 UTC 2010 >Closed-Date: >Last-Modified: >Originator: Paul Guyot >Release: 7.1 >Organization: >Environment: FreeBSD papaya.local 7.1-RELEASE-p4 FreeBSD 7.1-RELEASE-p4 #0: Fri Apr 17 05:48:23 UTC 2009 root@papaya.local:/usr/obj/usr/src/sys/GENERIC amd64 >Description: Calling strtod with "2.225073858507201e-308" yields 2^-1022 (0,16,0,0,0,0,0,0), while it should yield 2^-1022 - 2^-1074 (0,15,255,255,255,255,255,255), which is the nearest representable value. gcc on the very same FreeBSD box rounds this to the proper value (see sample code below). >How-To-Repeat: #include <stdio.h> #include <stdlib.h> #include <fenv.h> int main() { if (fegetround() == FE_TONEAREST) { printf("rounding to nearest\n"); } else { printf("not rounding to nearest\n"); } const char* fp_str = "2.225073858507201e-308"; printf("string = %s\n", fp_str); double fp_gcc = 2.225073858507201e-308; printf("gcc = %.20g\n", fp_gcc); double parsed = strtod(fp_str, NULL); printf("parsed = %.20g\n", parsed); double fp = 0.0; char* fp_p = (char*) &fp; fp_p[0] = 255; fp_p[1] = 255; fp_p[2] = 255; fp_p[3] = 255; fp_p[4] = 255; fp_p[5] = 255; fp_p[6] = 15; fp_p[7] = 0; printf("smaller = %.20g\n", fp); if (fp == parsed) { printf("actually rounded to nearest (and towards zero)\n"); } fp_p[0] = 0; fp_p[1] = 0; fp_p[2] = 0; fp_p[3] = 0; fp_p[4] = 0; fp_p[5] = 0; fp_p[6] = 16; fp_p[7] = 0; printf("larger = %.20g\n", fp); if (fp == parsed) { printf("actually rounded towards +infinity\n"); } return 0; } >Fix: >Release-Note: >Audit-Trail: >Unformatted:
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