1 /* $NetBSD: difftime.c,v 1.7 2000/09/13 22:32:28 msaitoh 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 7.7"; 12 #else 13 __RCSID("$NetBSD: difftime.c,v 1.7 2000/09/13 22:32:28 msaitoh 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 if (/* LINTED */sizeof(time_t) < sizeof(double)) 42 return (double) time1 - (double) time0; 43 if (/* LINTED */sizeof(time_t) < sizeof(long_double)) 44 return (long_double) time1 - (long_double) time0; 45 if (time1 < time0) 46 return -difftime(time0, time1); 47 /* 48 ** As much as possible, avoid loss of precision 49 ** by computing the difference before converting to double. 50 */ 51 delta = time1 - time0; 52 if (delta >= 0) 53 return delta; 54 /* 55 ** Repair delta overflow. 56 */ 57 hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); 58 /* 59 ** The following expression rounds twice, which means 60 ** the result may not be the closest to the true answer. 61 ** For example, suppose time_t is 64-bit signed int, 62 ** long_double is IEEE 754 double with default rounding, 63 ** time1 = 9223372036854775807 and time0 = -1536. 64 ** Then the true difference is 9223372036854777343, 65 ** which rounds to 9223372036854777856 66 ** with a total error of 513. 67 ** But delta overflows to -9223372036854774273, 68 ** which rounds to -9223372036854774784, and correcting 69 ** this by subtracting 2 * (long_double) hibit 70 ** (i.e. by adding 2**64 = 18446744073709551616) 71 ** yields 9223372036854776832, which 72 ** rounds to 9223372036854775808 73 ** with a total error of 1535 instead. 74 ** This problem occurs only with very large differences. 75 ** It's too painful to fix this portably. 76 ** We are not alone in this problem; 77 ** some C compilers round twice when converting 78 ** large unsigned types to small floating types, 79 ** so if time_t is unsigned the "return delta" above 80 ** has the same double-rounding problem with those compilers. 81 */ 82 return delta - 2 * (long_double) hibit; 83 } 84