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/* $NetBSD: difftime.c,v 1.5 1997/07/13 20:26:48 christos Exp $ */ /* ** This file is in the public domain, so clarified as of ** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov). */ #include <sys/cdefs.h> #ifndef lint #ifndef NOID #if 0 static char elsieid[] = "@(#)difftime.c 7.7"; #else __RCSID("$NetBSD: difftime.c,v 1.5 1997/07/13 20:26:48 christos Exp $"); #endif #endif /* !defined NOID */ #endif /* !defined lint */ /*LINTLIBRARY*/ #include "private.h" /* ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). */ #ifdef HAVE_LONG_DOUBLE #define long_double long double #endif /* defined HAVE_LONG_DOUBLE */ #ifndef HAVE_LONG_DOUBLE #define long_double double #endif /* !defined HAVE_LONG_DOUBLE */ double difftime(time1, time0) const time_t time1; const time_t time0; { time_t delta; time_t hibit; if (sizeof(time_t) < sizeof(double)) return (double) time1 - (double) time0; if (sizeof(time_t) < sizeof(long_double)) return (long_double) time1 - (long_double) time0; if (time1 < time0) return -difftime(time0, time1); /* ** As much as possible, avoid loss of precision ** by computing the difference before converting to double. */ delta = time1 - time0; if (delta >= 0) return delta; /* ** Repair delta overflow. */ hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); /* ** The following expression rounds twice, which means ** the result may not be the closest to the true answer. ** For example, suppose time_t is 64-bit signed int, ** long_double is IEEE 754 double with default rounding, ** time1 = 9223372036854775807 and time0 = -1536. ** Then the true difference is 9223372036854777343, ** which rounds to 9223372036854777856 ** with a total error of 513. ** But delta overflows to -9223372036854774273, ** which rounds to -9223372036854774784, and correcting ** this by subtracting 2 * (long_double) hibit ** (i.e. by adding 2**64 = 18446744073709551616) ** yields 9223372036854776832, which ** rounds to 9223372036854775808 ** with a total error of 1535 instead. ** This problem occurs only with very large differences. ** It's too painful to fix this portably. ** We are not alone in this problem; ** some C compilers round twice when converting ** large unsigned types to small floating types, ** so if time_t is unsigned the "return delta" above ** has the same double-rounding problem with those compilers. */ return delta - 2 * (long_double) hibit; }