2 ** This file is in the public domain, so clarified as of
3 ** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov).
5 ** $FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.4.8.1 2001/03/05 11:37:21 obrien Exp $
6 ** $DragonFly: src/lib/libc/stdtime/difftime.c,v 1.4 2005/12/04 23:25:40 swildner Exp $
14 #include "namespace.h"
16 #include "un-namespace.h"
19 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com).
22 #ifdef HAVE_LONG_DOUBLE
23 #define long_double long double
24 #endif /* defined HAVE_LONG_DOUBLE */
25 #ifndef HAVE_LONG_DOUBLE
26 #define long_double double
27 #endif /* !defined HAVE_LONG_DOUBLE */
30 difftime(const time_t time1
, const time_t time0
)
35 if (sizeof(time_t) < sizeof(double))
36 return (double) time1
- (double) time0
;
37 if (sizeof(time_t) < sizeof(long_double
))
38 return (long_double
) time1
- (long_double
) time0
;
40 return -difftime(time0
, time1
);
42 ** As much as possible, avoid loss of precision
43 ** by computing the difference before converting to double.
45 delta
= time1
- time0
;
49 ** Repair delta overflow.
51 hibit
= (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
53 ** The following expression rounds twice, which means
54 ** the result may not be the closest to the true answer.
55 ** For example, suppose time_t is 64-bit signed int,
56 ** long_double is IEEE 754 double with default rounding,
57 ** time1 = 9223372036854775807 and time0 = -1536.
58 ** Then the true difference is 9223372036854777343,
59 ** which rounds to 9223372036854777856
60 ** with a total error of 513.
61 ** But delta overflows to -9223372036854774273,
62 ** which rounds to -9223372036854774784, and correcting
63 ** this by subtracting 2 * (long_double) hibit
64 ** (i.e. by adding 2**64 = 18446744073709551616)
65 ** yields 9223372036854776832, which
66 ** rounds to 9223372036854775808
67 ** with a total error of 1535 instead.
68 ** This problem occurs only with very large differences.
69 ** It's too painful to fix this portably.
70 ** We are not alone in this problem;
71 ** some C compilers round twice when converting
72 ** large unsigned types to small floating types,
73 ** so if time_t is unsigned the "return delta" above
74 ** has the same double-rounding problem with those compilers.
76 return delta
- 2 * (long_double
) hibit
;