1 #if defined(LIBC_SCCS) && !defined(lint) 2 static char rcsid[] = "$OpenBSD: difftime.c,v 1.3 1996/08/19 08:34:48 tholo Exp $"; 3 #endif /* LIBC_SCCS and not lint */ 4 5 /*LINTLIBRARY*/ 6 7 #include "private.h" 8 9 /* 10 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). 11 */ 12 13 #ifdef HAVE_LONG_DOUBLE 14 #define long_double long double 15 #endif /* defined HAVE_LONG_DOUBLE */ 16 #ifndef HAVE_LONG_DOUBLE 17 #define long_double double 18 #endif /* !defined HAVE_LONG_DOUBLE */ 19 20 double 21 difftime(time1, time0) 22 const time_t time1; 23 const time_t time0; 24 { 25 time_t delta; 26 time_t hibit; 27 28 if (sizeof(time_t) < sizeof(double)) 29 return (double) time1 - (double) time0; 30 if (sizeof(time_t) < sizeof(long_double)) 31 return (long_double) time1 - (long_double) time0; 32 if (time1 < time0) 33 return -difftime(time0, time1); 34 /* 35 ** As much as possible, avoid loss of precision 36 ** by computing the difference before converting to double. 37 */ 38 delta = time1 - time0; 39 if (delta >= 0) 40 return delta; 41 /* 42 ** Repair delta overflow. 43 */ 44 hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); 45 /* 46 ** The following expression rounds twice, which means 47 ** the result may not be the closest to the true answer. 48 ** For example, suppose time_t is 64-bit signed int, 49 ** long_double is IEEE 754 double with default rounding, 50 ** time1 = 9223372036854775807 and time0 = -1536. 51 ** Then the true difference is 9223372036854777343, 52 ** which rounds to 9223372036854777856 53 ** with a total error of 513. 54 ** But delta overflows to -9223372036854774273, 55 ** which rounds to -9223372036854774784, and correcting 56 ** this by subtracting 2 * (long_double) hibit 57 ** (i.e. by adding 2**64 = 18446744073709551616) 58 ** yields 9223372036854776832, which 59 ** rounds to 9223372036854775808 60 ** with a total error of 1535 instead. 61 ** This problem occurs only with very large differences. 62 ** It's too painful to fix this portably. 63 ** We are not alone in this problem; 64 ** some C compilers round twice when converting 65 ** large unsigned types to small floating types, 66 ** so if time_t is unsigned the "return delta" above 67 ** has the same double-rounding problem with those compilers. 68 */ 69 return delta - 2 * (long_double) hibit; 70 } 71