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    <p>The problem with gmtime, localtime and their variants is the reliance on the TZ environment variable. The time functions first call tzset(void), which reads TZ to determine offsets DST, etc. If TZ is not set in the user's environment, (g)libc uses the system timezone. So if you have a local struct tm in, say, 'Europe/Paris' and your machine or environment is set to 'America/Denver', the wrong offset will be applied when you convert to GMT. All the time functions call tzset(void) which reads TZ to set char *tzname[2], long timezone (diff, in seconds, from GMT) and int daylight (boolean for DST). Setting these directly has no affect, because tzset() will overwrite them the next time you call localtime, etc. </p> <p>I was faced with the same issue as 'igor' in the original question, while setenv works it seems problematic (re-entran?). I decided to look further to see if I could modify tzset (void) to tzset(char*) to explicitly set the above mentioned variables. Well, of course, that's just a bad idea... but in probing the glibc source and the IANA TZ database source, I came to the conclusion that the setenv approach ain't so bad. </p> <p>First, setenv only modifies the process global 'char **environ' (not the calling shell, so the 'real' TZ is not affected). And, second, glibc actually puts a lock in setenv. The drawback is that setenv/tzset calls are not atomic, so another thread could conceivably write to TZ before the original thread call tzset. But a well-implemented application that uses threads should watch for that anyway.</p> <p>It would be cool if POSIX defined tzset to take a char* for look up in the extensive IANA TZ database (and take NULL to mean, 'use the user or system TZ/), but failing that, setenv seems to be ok. </p>
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