Search code examples
cunixtimefreebsdmktime

mktime and tm_isdst

I saw a lot of different views so thought of asking here.

I read man mktime:

 (A positive or zero value for tm_isdst causes mktime() to presume initially
 that summer time (for example, Daylight Saving Time) is or is not in
 effect for the specified time, respectively.  A negative value for
 tm_isdst causes the mktime() function to attempt to divine whether summer
 time is in effect for the specified time.

My question is, shouldn't tm_isdst be kept as -1 to let the system decide if its dst or not and that way the code becomes dst agnostic?

Am I missing something?

Solution

  • I believe the original reason for that is some timezones do not have daylight savings time. Since mktime is not async-safe nor is it re-entrant allows the implementation to store the current value of daylight savings in the POSIX extern char tzname[2], indexed by daylight [0 or 1]. This means tzname[0]="[std TZ name]" and tzname="[daylight TZ name, e.g. EDT]"

    See your tzset() man page for more information on this. Standards conforming mktime() is required to behave as though it called tzset() anyway. This kind of obviates the use of tm_isdst, IMO.

    Bottom line: your particular implementation and timezone(s) would dictate whether you would use -1, 0, or 1 for tm_isdst. There is no one default correct way for all implementations.