is c mktime different on Windows and GNU/Linux? - c

Question

the following code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
static const char * wday_abb_names[] =
{
"Mon",
"Tue",
"Wed",
"Thu",
"Fri",
"Sat",
"Sun",
};
static void mb_setenv(const char *name, const char *value)
{
#if !(defined _WIN32) || defined HAVE_SETENV
setenv(name, value, 1);
#else
int len = strlen(name)+1+strlen(value)+1;
char *str = malloc(len);
sprintf(str, "%s=%s", name, value);
putenv(str);
#endif
}
static void mb_unsetenv(const char *name)
{
#if !(defined _WIN32) || defined HAVE_SETENV
unsetenv(name);
#else
int len = strlen(name)+2;
char *str = malloc(len);
sprintf(str, "%s=", name);
putenv(str);
free(str);
#endif
}
time_t mb_timegm(struct tm *tm)
{
time_t ret;
char *tz;
tz = getenv("TZ");
mb_setenv("TZ", "");
tzset();
ret = mktime(tm);
if (tz)
{
mb_setenv("TZ", tz);
}
else
{
mb_unsetenv("TZ");
}
tzset();
return ret;
}
time_t get_test_time()
{
struct tm msg_time;
msg_time.tm_isdst = 0;
msg_time.tm_wday = 4;
msg_time.tm_mon = 5;
msg_time.tm_mday = 16;
msg_time.tm_hour = 4;
msg_time.tm_min = 53;
msg_time.tm_sec = 0;
msg_time.tm_year = 111; //2011 - 1900
time_t retval = mb_timegm(&msg_time);
printf("final msg_time = %ld\n", retval);
return retval;
}
void print_time(const char *msg, struct tm *t)
{
printf("%s %s, %02d.%02d.%2d %2d:%02d\n", msg,
wday_abb_names[t->tm_wday], t->tm_mday, t->tm_mon, t->tm_year,
t->tm_hour, t->tm_min);
}
int main()
{
printf( "=== ENVIRON ===\n");
printf("TZ = %s\n", getenv("TZ"));
time_t now;
struct tm l, g;
time(&now);
l = *localtime(&now);
g = *gmtime(&now);
print_time("Local time :", &l);
print_time("utc :", &g);
printf("=== END ENVIRON ===\n\n");
time_t tt = get_test_time();
printf("fix test (16.6.2011 04:53) --> %s\n", ctime(&tt));
printf("done.\n");
return 0;
}
running on GNU/Linux it produces:
=== ENVIRON ===
TZ = (null)
Local time : Sat, 24.05.111 14:20
utc : Sat, 24.05.111 12:20
=== END ENVIRON ===
final msg_time = 1308199980
fix test (16.6.2011 04:53) --> Thu Jun 16 06:53:00 2011
done.
running on Win7 it produces:
=== ENVIRON ===
TZ = (null)
Local time : Sat, 24.05.111 14:25
utc : Sat, 24.05.111 12:25
=== END ENVIRON ===
final msg_time = 1308196380
fix test (16.6.2011 04:53) --> Thu Jun 16 05:53:00 2011
done.
Both Systems have a Timezone of UTC+1 including DST (that makes UTC+2 in effect) and both systems are not having any time-problems at all - except for the difference displayed.
As you can see, the "final msg_time" is missing exactly 3600 seconds, so it is not a problem in ctime.
Can anybody explain to me why mktime seems to behave different on GNU/Linux and Windows - or how to correct that?
Edit:
Both systems (after calling tzset()) are reporting tzname[0] = CET, tzname[1] = CEST, daylight=1, timezone = -3600

Answer 1

My mb_timegm was based on the code stated in man 3 timegm and it stated
"set the TZ environment variable to UTC" to do this setenv("TZ", ""); is called.
However - this does not work on windows.
Using setenv("TZ", "UTC"); (or, in the above case mb_setenv) instead fixes the problem.

Answer 2

I'm assuming based on the info your provided where daylight savings time is in effect, that you would need to set msg_time.tm_isdst in get_test_time() to a value of 1 rather than 0. This may be the issue accounting for the missing hour. Either that, or you could set it to -1 and allow the system to attempt to figure out if you are in daylight savings time or not for the given input value.