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difftime.c revision 1.6 
      1 /*	$NetBSD: difftime.c,v 1.6 1998/11/15 17:11:06 christos Exp $	*/
      2 
      3 /*
      4 ** This file is in the public domain, so clarified as of
      5 ** June 5, 1996 by Arthur David Olson (arthur_david_olson (at) nih.gov).
      6 */
      7 
      8 #include <sys/cdefs.h>
      9 #ifndef lint
     10 #ifndef NOID
     11 #if 0
     12 static char	elsieid[] = "@(#)difftime.c	7.7";
     13 #else
     14 __RCSID("$NetBSD: difftime.c,v 1.6 1998/11/15 17:11:06 christos Exp $");
     15 #endif
     16 #endif /* !defined NOID */
     17 #endif /* !defined lint */
     18 
     19 /*LINTLIBRARY*/
     20 
     21 #include "private.h"
     22 
     23 /*
     24 ** Algorithm courtesy Paul Eggert (eggert (at) twinsun.com).
     25 */
     26 
     27 #ifdef HAVE_LONG_DOUBLE
     28 #define long_double	long double
     29 #endif /* defined HAVE_LONG_DOUBLE */
     30 #ifndef HAVE_LONG_DOUBLE
     31 #define long_double	double
     32 #endif /* !defined HAVE_LONG_DOUBLE */
     33 
     34 double
     35 difftime(time1, time0)
     36 const time_t	time1;
     37 const time_t	time0;
     38 {
     39 	time_t	delta;
     40 	time_t	hibit;
     41 
     42 	if (/* LINTED */sizeof(time_t) < sizeof(double))
     43 		return (double) time1 - (double) time0;
     44 	if (/* LINTED */sizeof(time_t) < sizeof(long_double))
     45 		return (long_double) time1 - (long_double) time0;
     46 	if (time1 < time0)
     47 		return -difftime(time0, time1);
     48 	/*
     49 	** As much as possible, avoid loss of precision
     50 	** by computing the difference before converting to double.
     51 	*/
     52 	delta = time1 - time0;
     53 	if (delta >= 0)
     54 		return delta;
     55 	/*
     56 	** Repair delta overflow.
     57 	*/
     58 	hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
     59 	/*
     60 	** The following expression rounds twice, which means
     61 	** the result may not be the closest to the true answer.
     62 	** For example, suppose time_t is 64-bit signed int,
     63 	** long_double is IEEE 754 double with default rounding,
     64 	** time1 = 9223372036854775807 and time0 = -1536.
     65 	** Then the true difference is 9223372036854777343,
     66 	** which rounds to 9223372036854777856
     67 	** with a total error of 513.
     68 	** But delta overflows to -9223372036854774273,
     69 	** which rounds to -9223372036854774784, and correcting
     70 	** this by subtracting 2 * (long_double) hibit
     71 	** (i.e. by adding 2**64 = 18446744073709551616)
     72 	** yields 9223372036854776832, which
     73 	** rounds to 9223372036854775808
     74 	** with a total error of 1535 instead.
     75 	** This problem occurs only with very large differences.
     76 	** It's too painful to fix this portably.
     77 	** We are not alone in this problem;
     78 	** some C compilers round twice when converting
     79 	** large unsigned types to small floating types,
     80 	** so if time_t is unsigned the "return delta" above
     81 	** has the same double-rounding problem with those compilers.
     82 	*/
     83 	return delta - 2 * (long_double) hibit;
     84 }
     85