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 7.7"; 8 static char rcsid[] = "$OpenBSD: difftime.c,v 1.5 1998/01/18 23:24:51 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 if (sizeof(time_t) < sizeof(double)) 35 return (double) time1 - (double) time0; 36 if (sizeof(time_t) < sizeof(long_double)) 37 return (long_double) time1 - (long_double) time0; 38 if (time1 < time0) 39 return -difftime(time0, time1); 40 /* 41 ** As much as possible, avoid loss of precision 42 ** by computing the difference before converting to double. 43 */ 44 delta = time1 - time0; 45 if (delta >= 0) 46 return delta; 47 /* 48 ** Repair delta overflow. 49 */ 50 hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); 51 /* 52 ** The following expression rounds twice, which means 53 ** the result may not be the closest to the true answer. 54 ** For example, suppose time_t is 64-bit signed int, 55 ** long_double is IEEE 754 double with default rounding, 56 ** time1 = 9223372036854775807 and time0 = -1536. 57 ** Then the true difference is 9223372036854777343, 58 ** which rounds to 9223372036854777856 59 ** with a total error of 513. 60 ** But delta overflows to -9223372036854774273, 61 ** which rounds to -9223372036854774784, and correcting 62 ** this by subtracting 2 * (long_double) hibit 63 ** (i.e. by adding 2**64 = 18446744073709551616) 64 ** yields 9223372036854776832, which 65 ** rounds to 9223372036854775808 66 ** with a total error of 1535 instead. 67 ** This problem occurs only with very large differences. 68 ** It's too painful to fix this portably. 69 ** We are not alone in this problem; 70 ** some C compilers round twice when converting 71 ** large unsigned types to small floating types, 72 ** so if time_t is unsigned the "return delta" above 73 ** has the same double-rounding problem with those compilers. 74 */ 75 return delta - 2 * (long_double) hibit; 76 } 77