New module 'timegm'.

[gnulib.git] / lib / timegm.c

diff --git a/lib/timegm.c b/lib/timegm.c
new file mode 100644 (file)
index 0000000..a7b953d
--- /dev/null
+++ b/lib/timegm.c
@@ -0,0 +1,96 @@
+/* Convert calendar time to simple time, inverse of mktime.
+   Copyright (C) 1995, 1996, 1997, 1998, 2000, 2001, 2003
+   Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
+
+#if HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+/* Specification.  Get mktime and gmtime declarations.  */
+#include "timegm.h"
+
+/* 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.  */
+
+time_t
+timegm (struct tm *tm)
+{
+  time_t tl, tb;
+  struct tm *tg;
+
+  tl = mktime (tm);
+  if (tl == (time_t) -1)
+    {
+      tm->tm_hour--;
+      tl = mktime (tm);
+      if (tl == (time_t) -1)
+       return (time_t) -1;
+      tl += 3600;
+    }
+#if HAVE_GMTIME_R && HAVE_DECL_GMTIME_R
+  tg = gmtime_r (&tl, tg);
+#else
+  tg = gmtime (&tl);
+#endif
+  tg->tm_isdst = 0;
+  tb = mktime (tg);
+  if (tb == (time_t) -1)
+    {
+      tg->tm_hour--;
+      tb = mktime (tg);
+      if (tb == (time_t) -1)
+       return (time_t) -1;
+      tb += 3600;
+    }
+  return (tl - (tb - tl));
+}