lib/libc/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 ** $DragonFly: src/lib/libc/stdtime/difftime.c,v 1.4 2005/12/04 23:25:40 swildner Exp $
   7 */
   8
   9 /*
  10  * @(#)difftime.c       7.7
  11  */
  12 /*LINTLIBRARY*/
  13
  14 #include "namespace.h"
  15 #include "private.h"
  16 #include "un-namespace.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(const time_t time1, const time_t time0)
  31 {
  32         time_t  delta;
  33         time_t  hibit;
  34
  35         if (sizeof(time_t) < sizeof(double))
  36                 return (double) time1 - (double) time0;
  37         if (sizeof(time_t) < sizeof(long_double))
  38                 return (long_double) time1 - (long_double) time0;
  39         if (time1 < time0)
  40                 return -difftime(time0, time1);
  41         /*
  42         ** As much as possible, avoid loss of precision
  43         ** by computing the difference before converting to double.
  44         */
  45         delta = time1 - time0;
  46         if (delta >= 0)
  47                 return delta;
  48         /*
  49         ** Repair delta overflow.
  50         */
  51         hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
  52         /*
  53         ** The following expression rounds twice, which means
  54         ** the result may not be the closest to the true answer.
  55         ** For example, suppose time_t is 64-bit signed int,
  56         ** long_double is IEEE 754 double with default rounding,
  57         ** time1 = 9223372036854775807 and time0 = -1536.
  58         ** Then the true difference is 9223372036854777343,
  59         ** which rounds to 9223372036854777856
  60         ** with a total error of 513.
  61         ** But delta overflows to -9223372036854774273,
  62         ** which rounds to -9223372036854774784, and correcting
  63         ** this by subtracting 2 * (long_double) hibit
  64         ** (i.e. by adding 2**64 = 18446744073709551616)
  65         ** yields 9223372036854776832, which
  66         ** rounds to 9223372036854775808
  67         ** with a total error of 1535 instead.
  68         ** This problem occurs only with very large differences.
  69         ** It's too painful to fix this portably.
  70         ** We are not alone in this problem;
  71         ** some C compilers round twice when converting
  72         ** large unsigned types to small floating types,
  73         ** so if time_t is unsigned the "return delta" above
  74         ** has the same double-rounding problem with those compilers.
  75         */
  76         return delta - 2 * (long_double) hibit;
  77 }