1 /* $NetBSD: difftime.c,v 1.2 1995/03/09 23:41:15 jtc Exp $ */ 2 3 #ifndef lint 4 #ifndef NOID 5 static char elsieid[] = "@(#)difftime.c 7.5"; 6 #endif /* !defined NOID */ 7 #endif /* !defined lint */ 8 9 /*LINTLIBRARY*/ 10 11 #include "private.h" 12 13 /* 14 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). 15 */ 16 17 #ifdef HAVE_LONG_DOUBLE 18 #define long_double long double 19 #endif /* defined HAVE_LONG_DOUBLE */ 20 #ifndef HAVE_LONG_DOUBLE 21 #define long_double double 22 #endif /* !defined HAVE_LONG_DOUBLE */ 23 24 double 25 difftime(time1, time0) 26 const time_t time1; 27 const time_t time0; 28 { 29 time_t delta; 30 time_t hibit; 31 32 if (sizeof(time_t) < sizeof(double)) 33 return (double) time1 - (double) time0; 34 if (sizeof(time_t) < sizeof(long_double)) 35 return (long_double) time1 - (long_double) time0; 36 if (time1 < time0) 37 return -difftime(time0, time1); 38 /* 39 ** As much as possible, avoid loss of precision 40 ** by computing the difference before converting to double. 41 */ 42 delta = time1 - time0; 43 if (delta >= 0) 44 return delta; 45 /* 46 ** Repair delta overflow. 47 */ 48 hibit = 1; 49 while ((hibit <<= 1) > 0) 50 continue; 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 ** many 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. 74 */ 75 return delta - 2 * (long_double) hibit; 76 } 77