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

[Eric Blake]

On 05/22/2014 05:07 AM, 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).

Re-reading POSIX:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html

I think you can do something like:

VIR00:30VID

where "VIR" is the normal time, and "VID" is the daylight-savings time 1
hour ahead.  It was the absence of a second name that made POSIX treat
the TZ value as not having daylight savings.

I'll review the actual patch in another mail.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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