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 ** @(#)difftime.c       7.7
   6 ** $FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.4.8.1 2001/03/05 11:37:21 obrien Exp $
   7 ** $DragonFly: src/lib/libc/stdtime/difftime.c,v 1.5 2008/10/19 20:15:58 swildner Exp $
   8 */
   9 /*LINTLIBRARY*/
  10
  11 #include "namespace.h"
  12 #include "private.h"
  13 #include "un-namespace.h"
  14
  15 double
  16 difftime(const time_t time1, const time_t time0)
  17 {
  18         /*
  19         ** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
  20         ** (assuming that the larger type has more precision).
  21         ** This is the common real-world case circa 2004.
  22         */
  23         if (sizeof (double) > sizeof (time_t))
  24                 return (double) time1 - (double) time0;
  25         if (!TYPE_INTEGRAL(time_t)) {
  26                 /*
  27                 ** time_t is floating.
  28                 */
  29                 return time1 - time0;
  30         }
  31         if (!TYPE_SIGNED(time_t)) {
  32                 /*
  33                 ** time_t is integral and unsigned.
  34                 ** The difference of two unsigned values can't overflow
  35                 ** if the minuend is greater than or equal to the subtrahend.
  36                 */
  37                 if (time1 >= time0)
  38                         return time1 - time0;
  39                 else    return -((double) (time0 - time1));
  40         }
  41         /*
  42         ** time_t is integral and signed.
  43         ** Handle cases where both time1 and time0 have the same sign
  44         ** (meaning that their difference cannot overflow).
  45         */
  46         if ((time1 < 0) == (time0 < 0))
  47                 return time1 - time0;
  48         /*
  49         ** time1 and time0 have opposite signs.
  50         ** Punt if unsigned long is too narrow.
  51         */
  52         if (sizeof (unsigned long) < sizeof (time_t))
  53                 return (double) time1 - (double) time0;
  54         /*
  55         ** Stay calm...decent optimizers will eliminate the complexity below.
  56         */
  57         if (time1 >= 0 /* && time0 < 0 */)
  58                 return (unsigned long) time1 +
  59                         (unsigned long) (-(time0 + 1)) + 1;
  60         return -(double) ((unsigned long) time0 +
  61                 (unsigned long) (-(time1 + 1)) + 1);
  62 }