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difftime.c revision 1.1.1.5 
      1 /*
      2 ** This file is in the public domain, so clarified as of
      3 ** 1996-06-05 by Arthur David Olson.
      4 */
      5 
      6 #ifndef lint
      7 #ifndef NOID
      8 static char	elsieid[] = "@(#)difftime.c	8.1";
      9 #endif /* !defined NOID */
     10 #endif /* !defined lint */
     11 
     12 /*LINTLIBRARY*/
     13 
     14 #include "private.h"	/* for time_t, TYPE_INTEGRAL, and TYPE_SIGNED */
     15 
     16 double
     17 difftime(time1, time0)
     18 const time_t	time1;
     19 const time_t	time0;
     20 {
     21 	/*
     22 	** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
     23 	** (assuming that the larger type has more precision).
     24 	** This is the common real-world case circa 2004.
     25 	*/
     26 	if (sizeof (double) > sizeof (time_t))
     27 		return (double) time1 - (double) time0;
     28 	if (!TYPE_INTEGRAL(time_t)) {
     29 		/*
     30 		** time_t is floating.
     31 		*/
     32 		return time1 - time0;
     33 	}
     34 	if (!TYPE_SIGNED(time_t)) {
     35 		/*
     36 		** time_t is integral and unsigned.
     37 		** The difference of two unsigned values can't overflow
     38 		** if the minuend is greater than or equal to the subtrahend.
     39 		*/
     40 		if (time1 >= time0)
     41 			return time1 - time0;
     42 		else	return -((double) (time0 - time1));
     43 	}
     44 	/*
     45 	** time_t is integral and signed.
     46 	** Handle cases where both time1 and time0 have the same sign
     47 	** (meaning that their difference cannot overflow).
     48 	*/
     49 	if ((time1 < 0) == (time0 < 0))
     50 		return time1 - time0;
     51 	/*
     52 	** time1 and time0 have opposite signs.
     53 	** Punt if unsigned long is too narrow.
     54 	*/
     55 	if (sizeof (unsigned long) < sizeof (time_t))
     56 		return (double) time1 - (double) time0;
     57 	/*
     58 	** Stay calm...decent optimizers will eliminate the complexity below.
     59 	*/
     60 	if (time1 >= 0 /* && time0 < 0 */)
     61 		return (unsigned long) time1 +
     62 			(unsigned long) (-(time0 + 1)) + 1;
     63 	return -(double) ((unsigned long) time0 +
     64 		(unsigned long) (-(time1 + 1)) + 1);
     65 }
     66