New version of timegm

[Hrvoje Niksic <hniksic@arsdigita.com>]

I noticed that apt uses Wget's mktime_from_utc (renamed to timegm)
where GNU libc's timegm is not available.  You might be interested in
the latest version of that function, which corrects some DST-related
problems and is better documented.

(Needless to say, feel free to trim the parts of the comment that
don't apply, rename it back to timegm, etc.)

/* Converts struct tm to time_t, assuming the data in tm is UTC rather
   than local timezone.

   mktime is similar but assumes struct tm, also known as the
   "broken-down" form of time, is in local time zone.  mktime_from_utc
   uses mktime to make the conversion understanding that an offset
   will be introduced by the local time assumption.

   mktime_from_utc then measures the introduced offset by applying
   gmtime to the initial result and applying mktime to the resulting
   "broken-down" form.  The difference between the two mktime results
   is the measured offset which is then subtracted from the initial
   mktime result to yield a calendar time which is the value returned.

   tm_isdst in struct tm is set to 0 to force mktime to introduce a
   consistent offset (the non DST offset) since tm and tm+o might be
   on opposite sides of a DST change.

   Some implementations of mktime return -1 for the nonexistent
   localtime hour at the beginning of DST.  In this event, use
   mktime(tm - 1hr) + 3600.

   Schematically
     mktime(tm)   --> t+o
     gmtime(t+o)  --> tm+o
     mktime(tm+o) --> t+2o
     t+o - (t+2o - t+o) = t

   Note that glibc contains a function of the same purpose named
   `timegm' (reverse of gmtime).  But obviously, it is not universally
   available, and unfortunately it is not straightforwardly
   extractable for use here.  Perhaps configure should detect timegm
   and use it where available.

   Contributed by Roger Beeman <beeman@cisco.com>, with the help of
   Mark Baushke <mdb@cisco.com> and the rest of the Gurus at CISCO.
   Further improved by Roger with assistance from Edward J. Sabol
   based on input by Jamie Zawinski.  */

static time_t
mktime_from_utc (struct tm *t)
{
  time_t tl, tb;
  struct tm *tg;

  tl = mktime (t);
  if (tl == -1)
    {
      t->tm_hour--;
      tl = mktime (t);
      if (tl == -1)
	return -1; /* can't deal with output from strptime */
      tl += 3600;
    }
  tg = gmtime (&tl);
  tg->tm_isdst = 0;
  tb = mktime (tg);
  if (tb == -1)
    {
      tg->tm_hour--;
      tb = mktime (tg);
      if (tb == -1)
	return -1; /* can't deal with output from gmtime */
      tb += 3600;
    }
  return (tl - (tb - tl));
}


-- 
To UNSUBSCRIBE, email to deity-request@lists.debian.org
with a subject of "unsubscribe". Trouble? Contact listmaster@lists.debian.org