1 /*
2 ** This file is in the public domain, so clarified as of
3 ** 1996-06-05 by Arthur David Olson.
4 */
5 
6 /*LINTLIBRARY*/
7 
8 #include "private.h"	/* for time_t and TYPE_SIGNED */
9 
10 /* Return -X as a double.  Using this avoids casting to 'double'.  */
11 static double
dminus(double x)12 dminus(double x)
13 {
14   return -x;
15 }
16 
17 double ATTRIBUTE_CONST
difftime(time_t time1,time_t time0)18 difftime(time_t time1, time_t time0)
19 {
20 	/*
21 	** If double is large enough, simply convert and subtract
22 	** (assuming that the larger type has more precision).
23 	*/
24 	if (sizeof (time_t) < sizeof (double)) {
25 	  double t1 = time1, t0 = time0;
26 	  return t1 - t0;
27 	}
28 
29 	/*
30 	** The difference of two unsigned values can't overflow
31 	** if the minuend is greater than or equal to the subtrahend.
32 	*/
33 	if (!TYPE_SIGNED(time_t))
34 	  return time0 <= time1 ? time1 - time0 : dminus(time0 - time1);
35 
36 	/* Use uintmax_t if wide enough.  */
37 	if (sizeof (time_t) <= sizeof (uintmax_t)) {
38 	  uintmax_t t1 = time1, t0 = time0;
39 	  return time0 <= time1 ? t1 - t0 : dminus(t0 - t1);
40 	}
41 
42 	/*
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 	/*
50 	** The values have opposite signs and uintmax_t is too narrow.
51 	** This suffers from double rounding; attempt to lessen that
52 	** by using long double temporaries.
53 	*/
54 	{
55 	  long double t1 = time1, t0 = time0;
56 	  return t1 - t0;
57 	}
58 }
59