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