Le 12/05/2017 à 15:54, Willy Tarreau a écrit : > Hi Benoît, > > On Thu, May 04, 2017 at 08:50:33AM +0200, Benoît GARNIER wrote: > (...) >> If you do the following operation : time_t => localtime() => struct tm >> => timegm() => time_t, your result will be shift by the timezone time >> offset (but without any DST applied). >> >> Technically, if you live in Great Britain, the operation will succeed >> during winter (but will offset the result by 1 hour during summer, since >> DST is applied here). > So in short you're saying that we should always use mktime() instead ? > > Willy
No, not at all !!! To sum up, these are the basic functions to work with time : - time() return a time_t which is timezone agnostic since it's just a precise moment in time (it represents the same moment for everybody) - localtime() takes this time_t and build a representation of this time in the current time zone (struct tm) - mktime() take a struct tm representing a specific time in the current timezone et return a time_t gmtime() and timegm() are the same as localtime() and mktime() but will ignore the timezone and DST: they only work with UTC time. So you can use timegm() on a struct tm only if you know that struct tm represents a GMT time (for example if it was build with gmtime()). Similarly, using mktime() is only valid if this struct tm represents the time in the current time zone (i.e. if it was build with localtime() with the same timezone). For example if you parse a log file with GMT time in it you'll use timegm() to build a time_t representing the precise time of the log. If you parse a file with local time in it, you'll use mktime() but you'll also have to know what was the timezone used to build it. 1) Time zone agnostic: time() 2) Current time zone: localtime() and mktime() 3) UTC time: gmtime() and timegm() As a rule of thumb, you cannot mix functions in categories 2 and 3. Benoît
