diff options
author | Bruno Haible <bruno@clisp.org> | 2003-09-01 17:47:35 +0000 |
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committer | Bruno Haible <bruno@clisp.org> | 2003-09-01 17:47:35 +0000 |
commit | 963409a8758faf2b24658c0a23cf0424211263d3 (patch) | |
tree | 6185f4b0c98f23332b8a5866a14fd21f46a2de6b /lib/timegm.c | |
parent | c5622b3775a13b69c1b5c404ea1e47b65e68708c (diff) | |
download | gnulib-963409a8758faf2b24658c0a23cf0424211263d3.tar.gz |
New module 'timegm'.
Diffstat (limited to 'lib/timegm.c')
-rw-r--r-- | lib/timegm.c | 96 |
1 files changed, 96 insertions, 0 deletions
diff --git a/lib/timegm.c b/lib/timegm.c new file mode 100644 index 0000000000..a7b953da5b --- /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)); +} |