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04277e02 | 1 | /* Copyright (C) 1991-2019 Free Software Foundation, Inc. |
6bc31da0 | 2 | This file is part of the GNU C Library. |
28f540f4 | 3 | |
6bc31da0 | 4 | The GNU C Library is free software; you can redistribute it and/or |
41bdb6e2 AJ |
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. | |
28f540f4 | 8 | |
6bc31da0 UD |
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 | |
41bdb6e2 | 12 | Lesser General Public License for more details. |
28f540f4 | 13 | |
41bdb6e2 | 14 | You should have received a copy of the GNU Lesser General Public |
59ba27a6 | 15 | License along with the GNU C Library; if not, see |
5a82c748 | 16 | <https://www.gnu.org/licenses/>. */ |
28f540f4 | 17 | |
29311370 RM |
18 | /* Written by Paul Eggert <eggert@cs.ucla.edu>. */ |
19 | ||
28f540f4 RM |
20 | #include <time.h> |
21 | ||
29311370 RM |
22 | #include <limits.h> |
23 | #include <float.h> | |
24 | #include <stdint.h> | |
25 | ||
26 | #define TYPE_BITS(type) (sizeof (type) * CHAR_BIT) | |
27 | #define TYPE_FLOATING(type) ((type) 0.5 == 0.5) | |
28 | #define TYPE_SIGNED(type) ((type) -1 < 0) | |
29 | ||
30 | /* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1. | |
31 | time_t is known to be an integer type. */ | |
32 | ||
33 | static double | |
ac253355 | 34 | subtract (__time64_t time1, __time64_t time0) |
29311370 | 35 | { |
ac253355 | 36 | if (! TYPE_SIGNED (__time64_t)) |
29311370 RM |
37 | return time1 - time0; |
38 | else | |
39 | { | |
40 | /* Optimize the common special cases where time_t | |
41 | can be converted to uintmax_t without losing information. */ | |
42 | uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0; | |
43 | double delta = dt; | |
44 | ||
45 | if (UINTMAX_MAX / 2 < INTMAX_MAX) | |
46 | { | |
47 | /* This is a rare host where uintmax_t has padding bits, and possibly | |
48 | information was lost when converting time_t to uintmax_t. | |
49 | Check for overflow by comparing dt/2 to (time1/2 - time0/2). | |
50 | Overflow occurred if they differ by more than a small slop. | |
51 | Thanks to Clive D.W. Feather for detailed technical advice about | |
52 | hosts with padding bits. | |
53 | ||
54 | In the following code the "h" prefix means half. By range | |
55 | analysis, we have: | |
56 | ||
57 | -0.5 <= ht1 - 0.5*time1 <= 0.5 | |
58 | -0.5 <= ht0 - 0.5*time0 <= 0.5 | |
59 | -1.0 <= dht - 0.5*(time1 - time0) <= 1.0 | |
60 | ||
61 | If overflow has not occurred, we also have: | |
62 | ||
63 | -0.5 <= hdt - 0.5*(time1 - time0) <= 0 | |
64 | -1.0 <= dht - hdt <= 1.5 | |
65 | ||
66 | and since dht - hdt is an integer, we also have: | |
67 | ||
68 | -1 <= dht - hdt <= 1 | |
69 | ||
70 | or equivalently: | |
71 | ||
72 | 0 <= dht - hdt + 1 <= 2 | |
73 | ||
74 | In the above analysis, all the operators have their exact | |
75 | mathematical semantics, not C semantics. However, dht - hdt + | |
76 | 1 is unsigned in C, so it need not be compared to zero. */ | |
77 | ||
78 | uintmax_t hdt = dt / 2; | |
ac253355 AA |
79 | __time64_t ht1 = time1 / 2; |
80 | __time64_t ht0 = time0 / 2; | |
81 | __time64_t dht = ht1 - ht0; | |
29311370 RM |
82 | |
83 | if (2 < dht - hdt + 1) | |
84 | { | |
85 | /* Repair delta overflow. | |
86 | ||
87 | The following expression contains a second rounding, | |
88 | so the result may not be the closest to the true answer. | |
89 | This problem occurs only with very large differences. | |
90 | It's too painful to fix this portably. */ | |
91 | ||
92 | delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2); | |
93 | } | |
94 | } | |
95 | ||
96 | return delta; | |
97 | } | |
98 | } | |
28f540f4 RM |
99 | |
100 | /* Return the difference between TIME1 and TIME0. */ | |
101 | double | |
ac253355 | 102 | __difftime64 (__time64_t time1, __time64_t time0) |
28f540f4 | 103 | { |
29311370 RM |
104 | /* Convert to double and then subtract if no double-rounding error could |
105 | result. */ | |
28f540f4 | 106 | |
ac253355 AA |
107 | if (TYPE_BITS (__time64_t) <= DBL_MANT_DIG |
108 | || (TYPE_FLOATING (__time64_t) && sizeof (__time64_t) < sizeof (long double))) | |
28f540f4 | 109 | return (double) time1 - (double) time0; |
29311370 RM |
110 | |
111 | /* Likewise for long double. */ | |
112 | ||
ac253355 | 113 | if (TYPE_BITS (__time64_t) <= LDBL_MANT_DIG || TYPE_FLOATING (__time64_t)) |
6bc31da0 | 114 | return (long double) time1 - (long double) time0; |
28f540f4 | 115 | |
29311370 RM |
116 | /* Subtract the smaller integer from the larger, convert the difference to |
117 | double, and then negate if needed. */ | |
118 | ||
119 | return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0); | |
28f540f4 | 120 | } |
ac253355 AA |
121 | |
122 | /* Provide a 32-bit wrapper if needed. */ | |
123 | ||
124 | #if __TIMESIZE != 64 | |
125 | ||
126 | libc_hidden_def (__difftime64) | |
127 | ||
128 | double | |
129 | __difftime (time_t time1, time_t time0) | |
130 | { | |
131 | return __difftime64 (time1, time0); | |
132 | } | |
133 | ||
134 | #endif | |
135 | ||
47eb6e76 | 136 | strong_alias (__difftime, difftime) |