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 ** $FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.4.8.1 2001/03/05 11:37:21 obrien Exp $ 6 ** $DragonFly: src/lib/libc/stdtime/difftime.c,v 1.3 2005/01/31 22:29:44 dillon Exp $ 7 */ 8 9 /* 10 * @(#)difftime.c 7.7 11 */ 12 /*LINTLIBRARY*/ 13 14 #include "namespace.h" 15 #include "private.h" 16 #include "un-namespace.h" 17 18 /* 19 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). 20 */ 21 22 #ifdef HAVE_LONG_DOUBLE 23 #define long_double long double 24 #endif /* defined HAVE_LONG_DOUBLE */ 25 #ifndef HAVE_LONG_DOUBLE 26 #define long_double double 27 #endif /* !defined HAVE_LONG_DOUBLE */ 28 29 double 30 difftime(time1, time0) 31 const time_t time1; 32 const time_t time0; 33 { 34 time_t delta; 35 time_t hibit; 36 37 if (sizeof(time_t) < sizeof(double)) 38 return (double) time1 - (double) time0; 39 if (sizeof(time_t) < sizeof(long_double)) 40 return (long_double) time1 - (long_double) time0; 41 if (time1 < time0) 42 return -difftime(time0, time1); 43 /* 44 ** As much as possible, avoid loss of precision 45 ** by computing the difference before converting to double. 46 */ 47 delta = time1 - time0; 48 if (delta >= 0) 49 return delta; 50 /* 51 ** Repair delta overflow. 52 */ 53 hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); 54 /* 55 ** The following expression rounds twice, which means 56 ** the result may not be the closest to the true answer. 57 ** For example, suppose time_t is 64-bit signed int, 58 ** long_double is IEEE 754 double with default rounding, 59 ** time1 = 9223372036854775807 and time0 = -1536. 60 ** Then the true difference is 9223372036854777343, 61 ** which rounds to 9223372036854777856 62 ** with a total error of 513. 63 ** But delta overflows to -9223372036854774273, 64 ** which rounds to -9223372036854774784, and correcting 65 ** this by subtracting 2 * (long_double) hibit 66 ** (i.e. by adding 2**64 = 18446744073709551616) 67 ** yields 9223372036854776832, which 68 ** rounds to 9223372036854775808 69 ** with a total error of 1535 instead. 70 ** This problem occurs only with very large differences. 71 ** It's too painful to fix this portably. 72 ** We are not alone in this problem; 73 ** some C compilers round twice when converting 74 ** large unsigned types to small floating types, 75 ** so if time_t is unsigned the "return delta" above 76 ** has the same double-rounding problem with those compilers. 77 */ 78 return delta - 2 * (long_double) hibit; 79 } 80