1 /*
2 * Copyright (c) 2018, Salvatore Sanfilippo <antirez at gmail dot com>
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions are met:
7 *
8 * * Redistributions of source code must retain the above copyright notice,
9 * this list of conditions and the following disclaimer.
10 * * Redistributions in binary form must reproduce the above copyright
11 * notice, this list of conditions and the following disclaimer in the
12 * documentation and/or other materials provided with the distribution.
13 * * Neither the name of Redis nor the names of its contributors may be used
14 * to endorse or promote products derived from this software without
15 * specific prior written permission.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
18 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
21 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
22 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
23 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
24 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
25 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
26 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
27 * POSSIBILITY OF SUCH DAMAGE.
28 */
29
30 #include <time.h>
31
32 /* This is a safe version of localtime() which contains no locks and is
33 * fork() friendly. Even the _r version of localtime() cannot be used safely
34 * in Redis. Another thread may be calling localtime() while the main thread
35 * forks(). Later when the child process calls localtime() again, for instance
36 * in order to log something to the Redis log, it may deadlock: in the copy
37 * of the address space of the forked process the lock will never be released.
38 *
39 * This function takes the timezone 'tz' as argument, and the 'dst' flag is
40 * used to check if daylight saving time is currently in effect. The caller
41 * of this function should obtain such information calling tzset() ASAP in the
42 * main() function to obtain the timezone offset from the 'timezone' global
43 * variable. To obtain the daylight information, if it is currently active or not,
44 * one trick is to call localtime() in main() ASAP as well, and get the
45 * information from the tm_isdst field of the tm structure. However the daylight
46 * time may switch in the future for long running processes, so this information
47 * should be refreshed at safe times.
48 *
49 * Note that this function does not work for dates < 1/1/1970, it is solely
50 * designed to work with what time(NULL) may return, and to support Redis
51 * logging of the dates, it's not really a complete implementation. */
is_leap_year(time_t year)52 static int is_leap_year(time_t year) {
53 if (year % 4) return 0; /* A year not divisible by 4 is not leap. */
54 else if (year % 100) return 1; /* If div by 4 and not 100 is surely leap. */
55 else if (year % 400) return 0; /* If div by 100 *and* 400 is not leap. */
56 else return 1; /* If div by 100 and not by 400 is leap. */
57 }
58
nolocks_localtime(struct tm * tmp,time_t t,time_t tz,int dst)59 void nolocks_localtime(struct tm *tmp, time_t t, time_t tz, int dst) {
60 const time_t secs_min = 60;
61 const time_t secs_hour = 3600;
62 const time_t secs_day = 3600*24;
63
64 t -= tz; /* Adjust for timezone. */
65 t += 3600*dst; /* Adjust for daylight time. */
66 time_t days = t / secs_day; /* Days passed since epoch. */
67 time_t seconds = t % secs_day; /* Remaining seconds. */
68
69 tmp->tm_isdst = dst;
70 tmp->tm_hour = seconds / secs_hour;
71 tmp->tm_min = (seconds % secs_hour) / secs_min;
72 tmp->tm_sec = (seconds % secs_hour) % secs_min;
73
74 /* 1/1/1970 was a Thursday, that is, day 4 from the POV of the tm structure
75 * where sunday = 0, so to calculate the day of the week we have to add 4
76 * and take the modulo by 7. */
77 tmp->tm_wday = (days+4)%7;
78
79 /* Calculate the current year. */
80 tmp->tm_year = 1970;
81 while(1) {
82 /* Leap years have one day more. */
83 time_t days_this_year = 365 + is_leap_year(tmp->tm_year);
84 if (days_this_year > days) break;
85 days -= days_this_year;
86 tmp->tm_year++;
87 }
88 tmp->tm_yday = days; /* Number of day of the current year. */
89
90 /* We need to calculate in which month and day of the month we are. To do
91 * so we need to skip days according to how many days there are in each
92 * month, and adjust for the leap year that has one more day in February. */
93 int mdays[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
94 mdays[1] += is_leap_year(tmp->tm_year);
95
96 tmp->tm_mon = 0;
97 while(days >= mdays[tmp->tm_mon]) {
98 days -= mdays[tmp->tm_mon];
99 tmp->tm_mon++;
100 }
101
102 tmp->tm_mday = days+1; /* Add 1 since our 'days' is zero-based. */
103 tmp->tm_year -= 1900; /* Surprisingly tm_year is year-1900. */
104 }
105
106 #ifdef LOCALTIME_TEST_MAIN
107 #include <stdio.h>
108
main(void)109 int main(void) {
110 /* Obtain timezone and daylight info. */
111 tzset(); /* Now 'timezome' global is populated. */
112 time_t t = time(NULL);
113 struct tm *aux = localtime(&t);
114 int daylight_active = aux->tm_isdst;
115
116 struct tm tm;
117 char buf[1024];
118
119 nolocks_localtime(&tm,t,timezone,daylight_active);
120 strftime(buf,sizeof(buf),"%d %b %H:%M:%S",&tm);
121 printf("[timezone: %d, dl: %d] %s\n", (int)timezone, (int)daylight_active, buf);
122 }
123 #endif
124