path: root/time/mktime.c

/* Convert a 'struct tm' to a time_t value.
   Copyright (C) 1993-2025 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Paul Eggert <eggert@twinsun.com>.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

/* The following macros influence what gets defined when this file is compiled:

   Macro/expression            Which gnulib module    This compilation unit
                                                      should define

   _LIBC                       (glibc proper)         mktime

   NEED_MKTIME_WORKING         mktime                 rpl_mktime
   || NEED_MKTIME_WINDOWS

   NEED_MKTIME_INTERNAL        mktime-internal        mktime_internal
 */

#ifndef _LIBC
# include <libc-config.h>
#endif

/* Assume that leap seconds are possible, unless told otherwise.
   If the host has a 'zic' command with a '-L leapsecondfilename' option,
   then it supports leap seconds; otherwise it probably doesn't.  */
#ifndef LEAP_SECONDS_POSSIBLE
# define LEAP_SECONDS_POSSIBLE 1
#endif

#include <time.h>

#include <errno.h>
#include <limits.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

#include <intprops.h>
#include <verify.h>

#ifndef NEED_MKTIME_INTERNAL
# define NEED_MKTIME_INTERNAL 0
#endif
#ifndef NEED_MKTIME_WINDOWS
# define NEED_MKTIME_WINDOWS 0
#endif
#ifndef NEED_MKTIME_WORKING
# define NEED_MKTIME_WORKING 0
#endif

#include "mktime-internal.h"

#if !defined _LIBC && (NEED_MKTIME_WORKING || NEED_MKTIME_WINDOWS)
static void
my_tzset (void)
{
# if NEED_MKTIME_WINDOWS
  /* Rectify the value of the environment variable TZ.
     There are four possible kinds of such values:
       - Traditional US time zone names, e.g. "PST8PDT".  Syntax: see
         <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset>
       - Time zone names based on geography, that contain one or more
         slashes, e.g. "Europe/Moscow".
       - Time zone names based on geography, without slashes, e.g.
         "Singapore".
       - Time zone names that contain explicit DST rules.  Syntax: see
         <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03>
     The Microsoft CRT understands only the first kind.  It produces incorrect
     results if the value of TZ is of the other kinds.
     But in a Cygwin environment, /etc/profile.d/tzset.sh sets TZ to a value
     of the second kind for most geographies, or of the first kind in a few
     other geographies.  If it is of the second kind, neutralize it.  For the
     Microsoft CRT, an absent or empty TZ means the time zone that the user
     has set in the Windows Control Panel.
     If the value of TZ is of the third or fourth kind -- Cygwin programs
     understand these syntaxes as well --, it does not matter whether we
     neutralize it or not, since these values occur only when a Cygwin user
     has set TZ explicitly; this case is 1. rare and 2. under the user's
     responsibility.  */
  const char *tz = getenv ("TZ");
  if (tz != NULL && strchr (tz, '/') != NULL)
    _putenv ("TZ=");
# else
  tzset ();
# endif
}
# undef __tzset
# define __tzset() my_tzset ()
#endif

#if defined _LIBC || NEED_MKTIME_WORKING || NEED_MKTIME_INTERNAL

/* A signed type that can represent an integer number of years
   multiplied by four times the number of seconds in a year.  It is
   needed when converting a tm_year value times the number of seconds
   in a year.  The factor of four comes because these products need
   to be subtracted from each other, and sometimes with an offset
   added to them, and then with another timestamp added, without
   worrying about overflow.

   Much of the code uses long_int to represent __time64_t values, to
   lessen the hassle of dealing with platforms where __time64_t is
   unsigned, and because long_int should suffice to represent all
   __time64_t values that mktime can generate even on platforms where
   __time64_t is wider than the int components of struct tm.  */

#if INT_MAX <= LONG_MAX / 4 / 366 / 24 / 60 / 60
typedef long int long_int;
#else
typedef long long int long_int;
#endif
verify (INT_MAX <= TYPE_MAXIMUM (long_int) / 4 / 366 / 24 / 60 / 60);

/* Shift A right by B bits portably, by dividing A by 2**B and
   truncating towards minus infinity.  B should be in the range 0 <= B
   <= LONG_INT_BITS - 2, where LONG_INT_BITS is the number of useful
   bits in a long_int.  LONG_INT_BITS is at least 32.

   ISO C99 says that A >> B is implementation-defined if A < 0.  Some
   implementations (e.g., UNICOS 9.0 on a Cray Y-MP EL) don't shift
   right in the usual way when A < 0, so SHR falls back on division if
   ordinary A >> B doesn't seem to be the usual signed shift.  */

static long_int
shr (long_int a, int b)
{
  long_int one = 1;
  return (-one >> 1 == -1
	  ? a >> b
	  : (a + (a < 0)) / (one << b) - (a < 0));
}

/* Bounds for the intersection of __time64_t and long_int.  */

static long_int const mktime_min
  = ((TYPE_SIGNED (__time64_t)
      && TYPE_MINIMUM (__time64_t) < TYPE_MINIMUM (long_int))
     ? TYPE_MINIMUM (long_int) : TYPE_MINIMUM (__time64_t));
static long_int const mktime_max
  = (TYPE_MAXIMUM (long_int) < TYPE_MAXIMUM (__time64_t)
     ? TYPE_MAXIMUM (long_int) : TYPE_MAXIMUM (__time64_t));

#define EPOCH_YEAR 1970
#define TM_YEAR_BASE 1900
verify (TM_YEAR_BASE % 100 == 0);

/* Is YEAR + TM_YEAR_BASE a leap year?  */
static bool
leapyear (long_int year)
{
  /* Don't add YEAR to TM_YEAR_BASE, as that might overflow.
     Also, work even if YEAR is negative.  */
  return
    ((year & 3) == 0
     && (year % 100 != 0
	 || ((year / 100) & 3) == (- (TM_YEAR_BASE / 100) & 3)));
}

/* How many days come before each month (0-12).  */