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difftime.c revision 1.16 
      1 /*	$NetBSD: difftime.c,v 1.16 2015/08/13 11:21:18 christos Exp $	*/
      2 
      3 /*
      4 ** This file is in the public domain, so clarified as of
      5 ** 1996-06-05 by Arthur David Olson.
      6 */
      7 
      8 #include <sys/cdefs.h>
      9 #if defined(LIBC_SCCS) && !defined(lint)
     10 #if 0
     11 static char	elsieid[] = "@(#)difftime.c	8.1";
     12 #else
     13 __RCSID("$NetBSD: difftime.c,v 1.16 2015/08/13 11:21:18 christos Exp $");
     14 #endif
     15 #endif /* LIBC_SCCS and not lint */
     16 
     17 /*LINTLIBRARY*/
     18 
     19 #include "private.h"	/* for time_t and TYPE_SIGNED */
     20 
     21 /* Return -X as a double.  Using this avoids casting to 'double'.  */
     22 static double
     23 dminus(double x)
     24 {
     25 	return -x;
     26 }
     27 
     28 double ATTRIBUTE_CONST
     29 difftime(time_t time1, time_t time0)
     30 {
     31 	/*
     32 	** If double is large enough, simply convert and subtract
     33 	** (assuming that the larger type has more precision).
     34 	*/
     35 	if (sizeof (time_t) < sizeof (double)) {
     36 		double t1 = time1, t0 = time0;
     37 		return t1 - t0;
     38  	}
     39 
     40 	/*
     41 	** The difference of two unsigned values can't overflow
     42 	** if the minuend is greater than or equal to the subtrahend.
     43 	*/
     44 	if (!TYPE_SIGNED(time_t))
     45 		return time0 <= time1 ? time1 - time0 : dminus(time0 - time1);
     46 
     47 	/* Use uintmax_t if wide enough.  */
     48 	if (sizeof (time_t) <= sizeof (uintmax_t)) {
     49 		uintmax_t t1 = time1, t0 = time0;
     50 		return time0 <= time1 ? t1 - t0 : dminus(t0 - t1);
     51 	}
     52 
     53 	/*
     54 	** Handle cases where both time1 and time0 have the same sign
     55 	** (meaning that their difference cannot overflow).
     56 	*/
     57 	if ((time1 < 0) == (time0 < 0))
     58 		return time1 - time0;
     59 
     60 	/*
     61 	** The values have opposite signs and uintmax_t is too narrow.
     62 	** This suffers from double rounding; attempt to lessen that
     63 	** by using long double temporaries.
     64 	*/
     65 	{
     66 		long double t1 = time1, t0 = time0;
     67 		return t1 - t0;
     68 	}
     69 }
     70