[libvirt] [PATCHv2 1/4] util: new function virTimeLocalOffsetFromUTC

[Marcelo Tosatti]

On Thu, May 22, 2014 at 02:07:27PM +0300, Laine Stump wrote:
> Since there isn't a single libc API to get this value, this patch
> supplies one which gets the value by grabbing current UTC, then
> converting that into a struct tm with localtime_r(), then back to a
> time_t using mktime; it again does the same operation, but using
> gmtime_r() instead (for UTC). It then subtracts utc time from the
> localtime, and finally adjusts if dst is set in the localtime timeinfo
> (because for some reason mktime doesn't take that into account).
> 
> This function should be POSIX-compliant, and is threadsafe, but not
> async signal safe. If it was ever necessary to know this value in a
> child process, we could cache it with a one-time init function when
> libvirtd starts, then just supply the cached value, but that
> complexity isn't needed for current usage.
> ---
> 
> Change from V1: add test cases with TZ set to different values (if
> someone knows how to force DST on/off, I would gladly add some test
> cases for this as well).


man tzset:

       The second format is used when there is daylight saving time:

              std offset dst [offset],start[/time],end[/time]