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authorBruno Haible <bruno@clisp.org>2003-09-01 17:47:35 +0000
committerBruno Haible <bruno@clisp.org>2003-09-01 17:47:35 +0000
commit963409a8758faf2b24658c0a23cf0424211263d3 (patch)
tree6185f4b0c98f23332b8a5866a14fd21f46a2de6b /lib/timegm.c
parentc5622b3775a13b69c1b5c404ea1e47b65e68708c (diff)
downloadgnulib-963409a8758faf2b24658c0a23cf0424211263d3.tar.gz
New module 'timegm'.
Diffstat (limited to 'lib/timegm.c')
-rw-r--r--lib/timegm.c96
1 files changed, 96 insertions, 0 deletions
diff --git a/lib/timegm.c b/lib/timegm.c
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+++ b/lib/timegm.c
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+/* 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));
+}