stdtime/difftime.c

   1 /*
   2 ** This file is in the public domain, so clarified as of
   3 ** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov).
   4 **
   5 ** $FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.4.8.1 2001/03/05 11:37:21 obrien Exp $
   6 */
   7
   8 #ifndef lint
   9 #ifndef NOID
  10 static char     elsieid[] = "@(#)difftime.c     7.7";
  11 #endif /* !defined NOID */
  12 #endif /* !defined lint */
  13
  14 /*LINTLIBRARY*/
  15
  16 #include "private.h"
  17
  18 /*
  19 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com).
  20 */
  21
  22 #ifdef HAVE_LONG_DOUBLE
  23 #define long_double     long double
  24 #endif /* defined HAVE_LONG_DOUBLE */
  25 #ifndef HAVE_LONG_DOUBLE
  26 #define long_double     double
  27 #endif /* !defined HAVE_LONG_DOUBLE */
  28
  29 double
  30 difftime(time1, time0)
  31 const time_t    time1;
  32 const time_t    time0;
  33 {
  34         time_t  delta;
  35         time_t  hibit;
  36
  37         if (sizeof(time_t) < sizeof(double))
  38                 return (double) time1 - (double) time0;
  39         if (sizeof(time_t) < sizeof(long_double))
  40                 return (long_double) time1 - (long_double) time0;
  41         if (time1 < time0)
  42                 return -difftime(time0, time1);
  43         /*
  44         ** As much as possible, avoid loss of precision
  45         ** by computing the difference before converting to double.
  46         */
  47         delta = time1 - time0;
  48         if (delta >= 0)
  49                 return delta;
  50         /*
  51         ** Repair delta overflow.
  52         */
  53         hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
  54         /*
  55         ** The following expression rounds twice, which means
  56         ** the result may not be the closest to the true answer.
  57         ** For example, suppose time_t is 64-bit signed int,
  58         ** long_double is IEEE 754 double with default rounding,
  59         ** time1 = 9223372036854775807 and time0 = -1536.
  60         ** Then the true difference is 9223372036854777343,
  61         ** which rounds to 9223372036854777856
  62         ** with a total error of 513.
  63         ** But delta overflows to -9223372036854774273,
  64         ** which rounds to -9223372036854774784, and correcting
  65         ** this by subtracting 2 * (long_double) hibit
  66         ** (i.e. by adding 2**64 = 18446744073709551616)
  67         ** yields 9223372036854776832, which
  68         ** rounds to 9223372036854775808
  69         ** with a total error of 1535 instead.
  70         ** This problem occurs only with very large differences.
  71         ** It's too painful to fix this portably.
  72         ** We are not alone in this problem;
  73         ** some C compilers round twice when converting
  74         ** large unsigned types to small floating types,
  75         ** so if time_t is unsigned the "return delta" above
  76         ** has the same double-rounding problem with those compilers.
  77         */
  78         return delta - 2 * (long_double) hibit;
  79 }