1 /* Copyright (C) 1991-2022 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
8
9 The GNU C Library is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
17
18
19 #include <time.h>
20
21 #include <limits.h>
22 #include <float.h>
23 #include <stdint.h>
24
25 #define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
26 #define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
27 #define TYPE_SIGNED(type) ((type) -1 < 0)
28
29 /* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
30 time_t is known to be an integer type. */
31
32 static double
subtract(__time64_t time1,__time64_t time0)33 subtract (__time64_t time1, __time64_t time0)
34 {
35 if (! TYPE_SIGNED (__time64_t))
36 return time1 - time0;
37 else
38 {
39 /* Optimize the common special cases where time_t
40 can be converted to uintmax_t without losing information. */
41 uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
42 double delta = dt;
43
44 if (UINTMAX_MAX / 2 < INTMAX_MAX)
45 {
46 /* This is a rare host where uintmax_t has padding bits, and possibly
47 information was lost when converting time_t to uintmax_t.
48 Check for overflow by comparing dt/2 to (time1/2 - time0/2).
49 Overflow occurred if they differ by more than a small slop.
50 Thanks to Clive D.W. Feather for detailed technical advice about
51 hosts with padding bits.
52
53 In the following code the "h" prefix means half. By range
54 analysis, we have:
55
56 -0.5 <= ht1 - 0.5*time1 <= 0.5
57 -0.5 <= ht0 - 0.5*time0 <= 0.5
58 -1.0 <= dht - 0.5*(time1 - time0) <= 1.0
59
60 If overflow has not occurred, we also have:
61
62 -0.5 <= hdt - 0.5*(time1 - time0) <= 0
63 -1.0 <= dht - hdt <= 1.5
64
65 and since dht - hdt is an integer, we also have:
66
67 -1 <= dht - hdt <= 1
68
69 or equivalently:
70
71 0 <= dht - hdt + 1 <= 2
72
73 In the above analysis, all the operators have their exact
74 mathematical semantics, not C semantics. However, dht - hdt +
75 1 is unsigned in C, so it need not be compared to zero. */
76
77 uintmax_t hdt = dt / 2;
78 __time64_t ht1 = time1 / 2;
79 __time64_t ht0 = time0 / 2;
80 __time64_t dht = ht1 - ht0;
81
82 if (2 < dht - hdt + 1)
83 {
84 /* Repair delta overflow.
85
86 The following expression contains a second rounding,
87 so the result may not be the closest to the true answer.
88 This problem occurs only with very large differences.
89 It's too painful to fix this portably. */
90
91 delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
92 }
93 }
94
95 return delta;
96 }
97 }
98
99 /* Return the difference between TIME1 and TIME0. */
100 double
__difftime64(__time64_t time1,__time64_t time0)101 __difftime64 (__time64_t time1, __time64_t time0)
102 {
103 /* Convert to double and then subtract if no double-rounding error could
104 result. */
105
106 if (TYPE_BITS (__time64_t) <= DBL_MANT_DIG
107 || (TYPE_FLOATING (__time64_t) && sizeof (__time64_t) < sizeof (long double)))
108 return (double) time1 - (double) time0;
109
110 /* Likewise for long double. */
111
112 if (TYPE_BITS (__time64_t) <= LDBL_MANT_DIG || TYPE_FLOATING (__time64_t))
113 return (long double) time1 - (long double) time0;
114
115 /* Subtract the smaller integer from the larger, convert the difference to
116 double, and then negate if needed. */
117
118 return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
119 }
120
121 /* Provide a 32-bit wrapper if needed. */
122
123 #if __TIMESIZE != 64
124
libc_hidden_def(__difftime64)125 libc_hidden_def (__difftime64)
126
127 double
128 __difftime (time_t time1, time_t time0)
129 {
130 return __difftime64 (time1, time0);
131 }
132
133 #endif
134
135 strong_alias (__difftime, difftime)
136