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Diffstat (limited to 'src/localtime.c')
| -rw-r--r-- | src/localtime.c | 123 |
1 files changed, 123 insertions, 0 deletions
diff --git a/src/localtime.c b/src/localtime.c new file mode 100644 index 0000000..1cefdfa --- /dev/null +++ b/src/localtime.c @@ -0,0 +1,123 @@ +/* + * Copyright (c) 2018, Salvatore Sanfilippo <antirez at gmail dot com> + * All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions are met: + * + * * Redistributions of source code must retain the above copyright notice, + * this list of conditions and the following disclaimer. + * * Redistributions in binary form must reproduce the above copyright + * notice, this list of conditions and the following disclaimer in the + * documentation and/or other materials provided with the distribution. + * * Neither the name of Redis nor the names of its contributors may be used + * to endorse or promote products derived from this software without + * specific prior written permission. + * + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" + * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE + * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE + * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR + * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF + * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS + * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN + * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) + * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE + * POSSIBILITY OF SUCH DAMAGE. + */ + +#include <time.h> + +/* This is a safe version of localtime() which contains no locks and is + * fork() friendly. Even the _r version of localtime() cannot be used safely + * in Redis. Another thread may be calling localtime() while the main thread + * forks(). Later when the child process calls localtime() again, for instance + * in order to log something to the Redis log, it may deadlock: in the copy + * of the address space of the forked process the lock will never be released. + * + * This function takes the timezone 'tz' as argument, and the 'dst' flag is + * used to check if daylight saving time is currently in effect. The caller + * of this function should obtain such information calling tzset() ASAP in the + * main() function to obtain the timezone offset from the 'timezone' global + * variable. To obtain the daylight information, if it is currently active or not, + * one trick is to call localtime() in main() ASAP as well, and get the + * information from the tm_isdst field of the tm structure. However the daylight + * time may switch in the future for long running processes, so this information + * should be refreshed at safe times. + * + * Note that this function does not work for dates < 1/1/1970, it is solely + * designed to work with what time(NULL) may return, and to support Redis + * logging of the dates, it's not really a complete implementation. */ +static int is_leap_year(time_t year) { + if (year % 4) return 0; /* A year not divisible by 4 is not leap. */ + else if (year % 100) return 1; /* If div by 4 and not 100 is surely leap. */ + else if (year % 400) return 0; /* If div by 100 *and* not by 400 is not leap. */ + else return 1; /* If div by 100 and 400 is leap. */ +} + +void nolocks_localtime(struct tm *tmp, time_t t, time_t tz, int dst) { + const time_t secs_min = 60; + const time_t secs_hour = 3600; + const time_t secs_day = 3600*24; + + t -= tz; /* Adjust for timezone. */ + t += 3600*dst; /* Adjust for daylight time. */ + time_t days = t / secs_day; /* Days passed since epoch. */ + time_t seconds = t % secs_day; /* Remaining seconds. */ + + tmp->tm_isdst = dst; + tmp->tm_hour = seconds / secs_hour; + tmp->tm_min = (seconds % secs_hour) / secs_min; + tmp->tm_sec = (seconds % secs_hour) % secs_min; + + /* 1/1/1970 was a Thursday, that is, day 4 from the POV of the tm structure + * where sunday = 0, so to calculate the day of the week we have to add 4 + * and take the modulo by 7. */ + tmp->tm_wday = (days+4)%7; + + /* Calculate the current year. */ + tmp->tm_year = 1970; + while(1) { + /* Leap years have one day more. */ + time_t days_this_year = 365 + is_leap_year(tmp->tm_year); + if (days_this_year > days) break; + days -= days_this_year; + tmp->tm_year++; + } + tmp->tm_yday = days; /* Number of day of the current year. */ + + /* We need to calculate in which month and day of the month we are. To do + * so we need to skip days according to how many days there are in each + * month, and adjust for the leap year that has one more day in February. */ + int mdays[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; + mdays[1] += is_leap_year(tmp->tm_year); + + tmp->tm_mon = 0; + while(days >= mdays[tmp->tm_mon]) { + days -= mdays[tmp->tm_mon]; + tmp->tm_mon++; + } + + tmp->tm_mday = days+1; /* Add 1 since our 'days' is zero-based. */ + tmp->tm_year -= 1900; /* Surprisingly tm_year is year-1900. */ +} + +#ifdef LOCALTIME_TEST_MAIN +#include <stdio.h> + +int main(void) { + /* Obtain timezone and daylight info. */ + tzset(); /* Now 'timezone' global is populated. */ + time_t t = time(NULL); + struct tm *aux = localtime(&t); + int daylight_active = aux->tm_isdst; + + struct tm tm; + char buf[1024]; + + nolocks_localtime(&tm,t,timezone,daylight_active); + strftime(buf,sizeof(buf),"%d %b %H:%M:%S",&tm); + printf("[timezone: %d, dl: %d] %s\n", (int)timezone, (int)daylight_active, buf); +} +#endif |