Time zone definitions

[Mark Koennecke]

On Wed, 5 Jan 2000, Ray Osborn wrote:

> A question for the C experts.  When I was trying to get the time zone set
> correctly using the Metrowerks CodeWarrior compiler on the Mac, I found that
> tm_gmtoff is not included in the Mac's tm struct.  However, as you can see
> in the version of NAPI.C that I committed to the server yesterday (search
> for the __MWERKS__ ifdef), I was able to get around the problem by using the
> time difference between the gmtime and localtime functions.
> 
> I'm not absolutely sure about this because I don't have the ANSI C (or ISO
> C) standard in front of me, but this web site seems to confirm this.  If
> that's the case, isn't it safer to use the technique I have defined as the
> default technique, and only use the ifdefs for the pathological non-ANSI
> cases (e.g. DEC C v5)?
> 
> I presume that the tm_gmtoff is a GNU C extension to the ANSI C standard.
> Is that right?
> 
> Any thoughts?
> Ray

  Ray,

  I looked into my copy of the Kernighan & Ritchie which contains a fairly
  complete rendition of the ANSI-C standard. You seem to be right that  
  tm_gmtoff is NOT required by ANSI-C. Whereas gmtime and localtime is. I
  then looked at my time.h on DigitalUnix and found that the tm_gmtoff is
  ifdef'ed with _OSF_SOURCE. This suggests to me that it may be in the
  POSIX-standard. Anyway, I suggest to go with the gmtime-localtime option
  for more portability.

                               Mark