Home | History | Annotate | Line # | Download | only in time
difftime.c revision 1.8 
      1 /*	$NetBSD: difftime.c,v 1.8 2002/01/29 12:40:33 kleink 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 #if defined(LIBC_SCCS) && !defined(lint)
     10 #if 0
     11 static char	elsieid[] = "@(#)difftime.c	7.9";
     12 #else
     13 __RCSID("$NetBSD: difftime.c,v 1.8 2002/01/29 12:40:33 kleink Exp $");
     14 #endif
     15 #endif /* LIBC_SCCS and not lint */
     16 
     17 
     18 /*LINTLIBRARY*/
     19 
     20 #include "private.h"
     21 
     22 /*
     23 ** Algorithm courtesy Paul Eggert (eggert (at) twinsun.com).
     24 */
     25 
     26 #ifdef HAVE_LONG_DOUBLE
     27 #define long_double	long double
     28 #endif /* defined HAVE_LONG_DOUBLE */
     29 #ifndef HAVE_LONG_DOUBLE
     30 #define long_double	double
     31 #endif /* !defined HAVE_LONG_DOUBLE */
     32 
     33 double
     34 difftime(time1, time0)
     35 const time_t	time1;
     36 const time_t	time0;
     37 {
     38 	time_t	delta;
     39 	time_t	hibit;
     40 
     41 	{
     42 		time_t		tt;
     43 		double		d;
     44 		long_double	ld;
     45 
     46 		if (sizeof tt < sizeof d)
     47 			return (double) time1 - (double) time0;
     48 		if (sizeof tt < sizeof ld)
     49 			return (long_double) time1 - (long_double) time0;
     50 	}
     51 	if (time1 < time0)
     52 		return -difftime(time0, time1);
     53 	/*
     54 	** As much as possible, avoid loss of precision
     55 	** by computing the difference before converting to double.
     56 	*/
     57 	delta = time1 - time0;
     58 	if (delta >= 0)
     59 		return delta;
     60 	/*
     61 	** Repair delta overflow.
     62 	*/
     63 	hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
     64 	/*
     65 	** The following expression rounds twice, which means
     66 	** the result may not be the closest to the true answer.
     67 	** For example, suppose time_t is 64-bit signed int,
     68 	** long_double is IEEE 754 double with default rounding,
     69 	** time1 = 9223372036854775807 and time0 = -1536.
     70 	** Then the true difference is 9223372036854777343,
     71 	** which rounds to 9223372036854777856
     72 	** with a total error of 513.
     73 	** But delta overflows to -9223372036854774273,
     74 	** which rounds to -9223372036854774784, and correcting
     75 	** this by subtracting 2 * (long_double) hibit
     76 	** (i.e. by adding 2**64 = 18446744073709551616)
     77 	** yields 9223372036854776832, which
     78 	** rounds to 9223372036854775808
     79 	** with a total error of 1535 instead.
     80 	** This problem occurs only with very large differences.
     81 	** It's too painful to fix this portably.
     82 	** We are not alone in this problem;
     83 	** some C compilers round twice when converting
     84 	** large unsigned types to small floating types,
     85 	** so if time_t is unsigned the "return delta" above
     86 	** has the same double-rounding problem with those compilers.
     87 	*/
     88 	return delta - 2 * (long_double) hibit;
     89 }
     90