difftime.c

   1 /*
   2 ** This file is in the public domain, so clarified as of
   3 ** 1996-06-05 by Arthur David Olson.
   4 */
   5
   6 /*LINTLIBRARY*/
   7
   8 #include "private.h"    /* for time_t, TYPE_INTEGRAL, and TYPE_SIGNED */
   9
  10 double
  11 difftime(time1, time0)
  12 const time_t    time1;
  13 const time_t    time0;
  14 {
  15         /*
  16         ** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
  17         ** (assuming that the larger type has more precision).
  18         ** This is the common real-world case circa 2004.
  19         */
  20         if (sizeof (double) > sizeof (time_t))
  21                 return (double) time1 - (double) time0;
  22         if (!TYPE_INTEGRAL(time_t)) {
  23                 /*
  24                 ** time_t is floating.
  25                 */
  26                 return time1 - time0;
  27         }
  28         if (!TYPE_SIGNED(time_t)) {
  29                 /*
  30                 ** time_t is integral and unsigned.
  31                 ** The difference of two unsigned values can't overflow
  32                 ** if the minuend is greater than or equal to the subtrahend.
  33                 */
  34                 if (time1 >= time0)
  35                         return time1 - time0;
  36                 else    return -((double) (time0 - time1));
  37         }
  38         /*
  39         ** time_t is integral and signed.
  40         ** Handle cases where both time1 and time0 have the same sign
  41         ** (meaning that their difference cannot overflow).
  42         */
  43         if ((time1 < 0) == (time0 < 0))
  44                 return time1 - time0;
  45         /*
  46         ** time1 and time0 have opposite signs.
  47         ** Punt if unsigned long is too narrow.
  48         */
  49         if (sizeof (unsigned long) < sizeof (time_t))
  50                 return (double) time1 - (double) time0;
  51         /*
  52         ** Stay calm...decent optimizers will eliminate the complexity below.
  53         */
  54         if (time1 >= 0 /* && time0 < 0 */)
  55                 return (unsigned long) time1 +
  56                         (unsigned long) (-(time0 + 1)) + 1;
  57         return -(double) ((unsigned long) time0 +
  58                 (unsigned long) (-(time1 + 1)) + 1);
  59 }