1 /* $NetBSD: difftime.c,v 1.9 2002/01/29 12:58:32 kleink Exp $ */ 2 3 /* 4 ** This file is in the public domain, so clarified as of 5 ** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov). 6 */ 7 8 #include <sys/cdefs.h> 9 #if defined(LIBC_SCCS) && !defined(lint) 10 #if 0 11 static char elsieid[] = "@(#)difftime.c 7.9"; 12 #else 13 __RCSID("$NetBSD: difftime.c,v 1.9 2002/01/29 12:58:32 kleink Exp $"); 14 #endif 15 #endif /* LIBC_SCCS and not lint */ 16 17 18 /*LINTLIBRARY*/ 19 20 #include "private.h" 21 22 /* 23 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). 24 */ 25 26 #ifdef HAVE_LONG_DOUBLE 27 #define long_double long double 28 #endif /* defined HAVE_LONG_DOUBLE */ 29 #ifndef HAVE_LONG_DOUBLE 30 #define long_double double 31 #endif /* !defined HAVE_LONG_DOUBLE */ 32 33 double 34 difftime(time1, time0) 35 const time_t time1; 36 const time_t time0; 37 { 38 time_t delta; 39 time_t hibit; 40 41 { 42 time_t tt; 43 double d; 44 long_double ld; 45 46 #ifdef __lint__ 47 /* LINTED unused warning bug */&tt; 48 /* LINTED unused warning bug */&d; 49 /* LINTED unused warning bug */&ld; 50 #endif 51 52 if (/* LINTED constant */sizeof tt < sizeof d) 53 return (double) time1 - (double) time0; 54 if (/* LINTED constant */sizeof tt < sizeof ld) 55 return (long_double) time1 - (long_double) time0; 56 } 57 if (time1 < time0) 58 return -difftime(time0, time1); 59 /* 60 ** As much as possible, avoid loss of precision 61 ** by computing the difference before converting to double. 62 */ 63 delta = time1 - time0; 64 if (delta >= 0) 65 return delta; 66 /* 67 ** Repair delta overflow. 68 */ 69 hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); 70 /* 71 ** The following expression rounds twice, which means 72 ** the result may not be the closest to the true answer. 73 ** For example, suppose time_t is 64-bit signed int, 74 ** long_double is IEEE 754 double with default rounding, 75 ** time1 = 9223372036854775807 and time0 = -1536. 76 ** Then the true difference is 9223372036854777343, 77 ** which rounds to 9223372036854777856 78 ** with a total error of 513. 79 ** But delta overflows to -9223372036854774273, 80 ** which rounds to -9223372036854774784, and correcting 81 ** this by subtracting 2 * (long_double) hibit 82 ** (i.e. by adding 2**64 = 18446744073709551616) 83 ** yields 9223372036854776832, which 84 ** rounds to 9223372036854775808 85 ** with a total error of 1535 instead. 86 ** This problem occurs only with very large differences. 87 ** It's too painful to fix this portably. 88 ** We are not alone in this problem; 89 ** some C compilers round twice when converting 90 ** large unsigned types to small floating types, 91 ** so if time_t is unsigned the "return delta" above 92 ** has the same double-rounding problem with those compilers. 93 */ 94 return delta - 2 * (long_double) hibit; 95 } 96