lib/libc/stdtime/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 ** $FreeBSD: head/contrib/tzcode/stdtime/difftime.c 192625 2009-05-23 06:31:50Z edwin $
   6 */
   7 /*LINTLIBRARY*/
   8
   9 #include "namespace.h"
  10 #include "private.h"    /* for time_t and TYPE_SIGNED */
  11 #include "un-namespace.h"
  12
  13 double
  14 difftime(time_t time1, time_t time0)
  15 {
  16         /*
  17         ** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
  18         ** (assuming that the larger type has more precision).
  19         */
  20         if (sizeof (double) > sizeof (time_t))
  21                 return (double) time1 - (double) time0;
  22         if (!TYPE_SIGNED(time_t)) {
  23                 /*
  24                 ** The difference of two unsigned values can't overflow
  25                 ** if the minuend is greater than or equal to the subtrahend.
  26                 */
  27                 if (time1 >= time0)
  28                         return            time1 - time0;
  29                 else    return -(double) (time0 - time1);
  30         }
  31         /*
  32         ** Handle cases where both time1 and time0 have the same sign
  33         ** (meaning that their difference cannot overflow).
  34         */
  35         if ((time1 < 0) == (time0 < 0))
  36                 return time1 - time0;
  37         /*
  38         ** time1 and time0 have opposite signs.
  39         ** Punt if uintmax_t is too narrow.
  40         ** This suffers from double rounding; attempt to lessen that
  41         ** by using long double temporaries.
  42         */
  43         if (sizeof (uintmax_t) < sizeof (time_t))
  44                 return (long double) time1 - (long double) time0;
  45         /*
  46         ** Stay calm...decent optimizers will eliminate the complexity below.
  47         */
  48         if (time1 >= 0 /* && time0 < 0 */)
  49                 return    (uintmax_t) time1 + (uintmax_t) (-1 - time0) + 1;
  50         return -(double) ((uintmax_t) time0 + (uintmax_t) (-1 - time1) + 1);
  51 }