Re: mktime result may depend on previous calls

[Max Nikulin <manikulin@gmail.com>]

On 19/01/2023 11:34, Paul Eggert wrote:
> On 1/18/23 19:00, Max Nikulin wrote:
> 
>> Notice that in the case of ambiguous local time (around a transition 
>> with backward step) multiple mktime calls with the same argument may 
>> return both values. Such kind of uncertainty appears if tm_isdst is 
>> set to -1 or if local time changes keeping the same DST state.
> 
> The above wording isn't quite right, as it ignores the role of tm_gmtoff 
> in mktime. (Did I say it was complicated? :-)
...
> POSIX does allow that sort of thing as an extension, and glibc mktime 
> does so: it disambiguates via tm_gmtoff.

Has it been fixed recently?

https://www.gnu.org/software/libc/manual/html_node/Broken_002ddown-Time.html#index-mktime

> The mktime function *ignores* the specified contents of the tm_wday,
> tm_yday, *tm_gmtoff*, and tm_zone members of the broken-down time structure

 From the example below I would expect 1612128600 +0200 CAT gmtoff=7200 
on third line, but my glibc is dated

      1612119600 2021-01-31 22:00:00 +0300 EAT gmtoff=10800
      1612125000 2021-01-31 23:30:00 +0300 EAT gmtoff=10800
      1612125000 2021-01-31 23:30:00 +0300 EAT gmtoff=10800


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

void bt_init(struct tm *bt) {
	memset(bt, 0, sizeof(*bt));
	bt->tm_isdst = -1;
	bt->tm_year = 2021 - 1900;
	bt->tm_mon = 1 - 1;
	bt->tm_mday = 31;
	bt->tm_hour = 23;
	bt->tm_min = 30;
	bt->tm_sec = 0;
}

void bt_print(struct tm *bt) {
	char buf[64];
	time_t ts = mktime(bt);
	strftime(buf, sizeof(buf) - 1, "%F %T %z %Z", bt);
	printf("%15zd %s gmtoff=%ld\n", ts, buf, bt->tm_gmtoff);
}

int main() {
	struct tm bt;
	putenv("TZ=Africa/Juba");
	// Fix cached offset to the value before time jump
	bt_init(&bt);
	bt.tm_hour = 22;
	bt.tm_min = 0;
	// Default conversion
	bt_print(&bt);
	bt_init(&bt);
	bt_print(&bt);
	// Attempt to choose +0200 CAT instead of +0300 EAT
	bt_init(&bt);
	bt.tm_gmtoff = 2*3600;
	bt_print(&bt);
	return 0;
}