1 /* 2 ** This file is in the public domain, so clarified as of 3 ** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov). 4 */ 5 6 #if defined(LIBC_SCCS) && !defined(lint) && !defined(NOID) 7 static char elsieid[] = "@(#)difftime.c 7.9"; 8 static char rcsid[] = "$OpenBSD: difftime.c,v 1.6 2002/04/04 19:12:09 millert Exp $"; 9 #endif /* LIBC_SCCS and not lint */ 10 11 /*LINTLIBRARY*/ 12 13 #include "private.h" 14 15 /* 16 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). 17 */ 18 19 #ifdef HAVE_LONG_DOUBLE 20 #define long_double long double 21 #endif /* defined HAVE_LONG_DOUBLE */ 22 #ifndef HAVE_LONG_DOUBLE 23 #define long_double double 24 #endif /* !defined HAVE_LONG_DOUBLE */ 25 26 double 27 difftime(time1, time0) 28 const time_t time1; 29 const time_t time0; 30 { 31 time_t delta; 32 time_t hibit; 33 34 { 35 time_t tt; 36 double d; 37 long_double ld; 38 39 if (sizeof tt < sizeof d) 40 return (double) time1 - (double) time0; 41 if (sizeof tt < sizeof ld) 42 return (long_double) time1 - (long_double) time0; 43 } 44 if (time1 < time0) 45 return -difftime(time0, time1); 46 /* 47 ** As much as possible, avoid loss of precision 48 ** by computing the difference before converting to double. 49 */ 50 delta = time1 - time0; 51 if (delta >= 0) 52 return delta; 53 /* 54 ** Repair delta overflow. 55 */ 56 hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); 57 /* 58 ** The following expression rounds twice, which means 59 ** the result may not be the closest to the true answer. 60 ** For example, suppose time_t is 64-bit signed int, 61 ** long_double is IEEE 754 double with default rounding, 62 ** time1 = 9223372036854775807 and time0 = -1536. 63 ** Then the true difference is 9223372036854777343, 64 ** which rounds to 9223372036854777856 65 ** with a total error of 513. 66 ** But delta overflows to -9223372036854774273, 67 ** which rounds to -9223372036854774784, and correcting 68 ** this by subtracting 2 * (long_double) hibit 69 ** (i.e. by adding 2**64 = 18446744073709551616) 70 ** yields 9223372036854776832, which 71 ** rounds to 9223372036854775808 72 ** with a total error of 1535 instead. 73 ** This problem occurs only with very large differences. 74 ** It's too painful to fix this portably. 75 ** We are not alone in this problem; 76 ** some C compilers round twice when converting 77 ** large unsigned types to small floating types, 78 ** so if time_t is unsigned the "return delta" above 79 ** has the same double-rounding problem with those compilers. 80 */ 81 return delta - 2 * (long_double) hibit; 82 } 83