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difftime.c revision 1.1.1.4 
      1 /*
      2 ** This file is in the public domain, so clarified as of
      3 ** June 5, 1996 by Arthur David Olson (arthur_david_olson (at) nih.gov).
      4 */
      5 
      6 #ifndef lint
      7 #ifndef NOID
      8 static char	elsieid[] = "@(#)difftime.c	7.9";
      9 #endif /* !defined NOID */
     10 #endif /* !defined lint */
     11 
     12 /*LINTLIBRARY*/
     13 
     14 #include "private.h"
     15 
     16 /*
     17 ** Algorithm courtesy Paul Eggert (eggert (at) twinsun.com).
     18 */
     19 
     20 #ifdef HAVE_LONG_DOUBLE
     21 #define long_double	long double
     22 #endif /* defined HAVE_LONG_DOUBLE */
     23 #ifndef HAVE_LONG_DOUBLE
     24 #define long_double	double
     25 #endif /* !defined HAVE_LONG_DOUBLE */
     26 
     27 double
     28 difftime(time1, time0)
     29 const time_t	time1;
     30 const time_t	time0;
     31 {
     32 	time_t	delta;
     33 	time_t	hibit;
     34 
     35 	{
     36 		time_t		tt;
     37 		double		d;
     38 		long_double	ld;
     39 
     40 		if (sizeof tt < sizeof d)
     41 			return (double) time1 - (double) time0;
     42 		if (sizeof tt < sizeof ld)
     43 			return (long_double) time1 - (long_double) time0;
     44 	}
     45 	if (time1 < time0)
     46 		return -difftime(time0, time1);
     47 	/*
     48 	** As much as possible, avoid loss of precision
     49 	** by computing the difference before converting to double.
     50 	*/
     51 	delta = time1 - time0;
     52 	if (delta >= 0)
     53 		return delta;
     54 	/*
     55 	** Repair delta overflow.
     56 	*/
     57 	hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
     58 	/*
     59 	** The following expression rounds twice, which means
     60 	** the result may not be the closest to the true answer.
     61 	** For example, suppose time_t is 64-bit signed int,
     62 	** long_double is IEEE 754 double with default rounding,
     63 	** time1 = 9223372036854775807 and time0 = -1536.
     64 	** Then the true difference is 9223372036854777343,
     65 	** which rounds to 9223372036854777856
     66 	** with a total error of 513.
     67 	** But delta overflows to -9223372036854774273,
     68 	** which rounds to -9223372036854774784, and correcting
     69 	** this by subtracting 2 * (long_double) hibit
     70 	** (i.e. by adding 2**64 = 18446744073709551616)
     71 	** yields 9223372036854776832, which
     72 	** rounds to 9223372036854775808
     73 	** with a total error of 1535 instead.
     74 	** This problem occurs only with very large differences.
     75 	** It's too painful to fix this portably.
     76 	** We are not alone in this problem;
     77 	** some C compilers round twice when converting
     78 	** large unsigned types to small floating types,
     79 	** so if time_t is unsigned the "return delta" above
     80 	** has the same double-rounding problem with those compilers.
     81 	*/
     82 	return delta - 2 * (long_double) hibit;
     83 }
     84