windows-nt-4.0/private/crt32/time/localtim.c

/***
*localtim.c - Convert time_t value to time structure
*
*	Copyright (c) 1985-1993, Microsoft Corporation. All rights reserved.
*
*Purpose:
*	Converts time stored as a time_t value to a structure of type
*	struct tm expressed as local time.
*
*Revision History:
*	01-??-84  RLB	Module created
*	05-??-84  DCW	split off from the rest of the ctime routines
*	02-18-87  JCR	made localtime work when gmtime returns null
*	03-31-87  JCR	fixed bug pertaining to uninitialized _isindst(tb)
*	04-10-87  JCR	changed long declaration to time_t and added const
*	11-10-87  SKS	Removed IBMC20 switch
*	11-18-87  SKS	Change tzset() to __tzset()
*	12-11-87  JCR	Added "_LOAD_DS" to declaration
*	11-06-89  KRS	Added (unsigned) to handle years 2040-2099 correctly.
*	03-20-90  GJF	Replaced _LOAD_DS with _CALLTYPE1, added #include
*			<cruntime.h>, removed #include <register.h> and
*			fixed the copyright. Also, cleaned up the formatting
*			a bit.
*	10-04-90  GJF	New-style function declarator. Also, rewrote expr.
*			to avoid using cast as lvalue.
*	01-21-91  GJF	ANSI naming.
*	08-10-92  PBS	Posix support (TZ stuff).
*	03-24-93  GJF	Ported C8-16 version and adapted for exotic Daylight
*			Savings Time conversions which are legal under POSIX.
*	07-15-93  GJF	Replaced _tzset() call with __tzset() call.
*
*******************************************************************************/

#include <cruntime.h>
#include <limits.h>
#include <time.h>
#include <stddef.h>
#include <ctime.h>
#include <internal.h>

/***
*struct tm *localtime(ptime) - convert time_t value to tm structure
*
*Purpose:
*	Convert a value in internal (time_t) format to a tm struct
*	containing the corresponding local time.
*
* NOTES:
*	(1) gmtime must be called before _isindst to ensure that the tb time
*	    structure is initialized.
*	(2) gmtime and localtime use a single statically allocated buffer.
*	    Each call to one of these routines destroys the contents of the
*	    previous call.
*	(3) It is assumed that time_t is a 32-bit long integer representing
*	    the number of seconds since 00:00:00, 01-01-70 (UTC) (i.e., the
*	    Posix/Unix Epoch. Only non-negative values are supported.
*	(4) It is assumed that the maximum adjustment for local time is
*	    less than three days (include Daylight Savings Time adjustment).
*	    This only a concern in Posix where the specification of the TZ
*	    environment restricts the combined offset for time zone and
*	    Daylight Savings Time to 2 * (24:59:59), just under 50 hours.
*
*Entry:
*	time_t *ptime - pointer to a long time value
*
*Exit:
*	If *ptime is non-negative, returns a pointer to the tm structure.
*	Otherwise, returns NULL.
*
*Exceptions:
*	See items (3) and (4) in the NOTES above. If these assumptions are
*	violated, behavior is undefined.
*
*******************************************************************************/

struct tm * _CRTAPI1 localtime (
	const time_t *ptime
	)
{
	REG1 struct tm *ptm;
	long ltime;

	/*
	 * Check for illegal time_t value
	 */
	if ( (long)*ptime < 0L )
		return( NULL );

#ifdef	_POSIX_
	tzset();
#else
	__tzset();
#endif

	if ( (*ptime > 3 * _DAY_SEC) && (*ptime < LONG_MAX - 3 * _DAY_SEC) ) {
		/*
		 * The date does not fall within the first three, or last
		 * three, representable days of the Epoch. Therefore, there
		 * is no possibility of overflowing or underflowing the
		 * time_t representation as we compensate for timezone and
		 * Daylight Savings Time.
		 */

		ltime = (long)*ptime - _timezone;
		ptm = gmtime( &ltime );

		/*
		 * Check and adjust for Daylight Saving Time.
		 */
		if ( _daylight && _isindst( ptm ) ) {
#ifdef	_POSIX_
			ltime -= _dstoffset - _timezone;
#else
			ltime += 3600L;
#endif
			ptm = gmtime( (time_t *)&ltime );
			ptm->tm_isdst = 1;
		}
	}
	else {
		ptm = gmtime( ptime );
		/*
		 * The date falls with the first three, or last three days
		 * of the Epoch. It is possible the time_t representation
		 * would overflow or underflow while compensating for
		 * timezone and Daylight Savings Time. Therefore, make the
		 * timezone and Daylight Savings Time adjustments directly
		 * in the tm structure. The beginning of the Epoch is
		 * 00:00:00, 01-01-70 (UTC) and the last representable second
		 * in theEpoch is 03:14:07, 01-19-2038 (UTC). This will be
		 * used in the calculations below.
		 *
		 * First, adjust for the timezone.
		 */
		ltime = (long)ptm->tm_sec - _timezone;
		ptm->tm_sec = (int)(ltime % 60);
		if ( ptm->tm_sec < 0 ) {
			ptm->tm_sec += 60;
			ltime -= 60;
		}

		ltime = (long)ptm->tm_min + ltime/60;
		ptm->tm_min = (int)(ltime % 60);
		if ( ptm->tm_min < 0 ) {
			ptm->tm_min += 60;
			ltime -= 60;
		}

		ltime = (long)ptm->tm_hour + ltime/60;
		ptm->tm_hour = (int)(ltime % 24);
		if ( ptm->tm_hour < 0 ) {
			ptm->tm_hour += 24;
			ltime -=24;
		}

		ltime /= 24;

		if ( ltime > 0L ) {
			/*
			 * There is no possibility of overflowing the tm_mday
			 * and tm_yday fields since the date can be no later
			 * than January 19.
			 */
			ptm->tm_wday = (ptm->tm_wday + ltime) % 7;
			ptm->tm_mday += ltime;
			ptm->tm_yday += ltime;
		}
		else if ( ltime < 0L ) {
			/*
			 * It is possible to underflow the tm_mday and tm_yday
			 * fields. If this happens, then adjusted date must
			 * lie in December 1969.
			 */
			ptm->tm_wday = (ptm->tm_wday + 7 + ltime) % 7;
			if ( (ptm->tm_mday += ltime) <= 0 ) {
				ptm->tm_mday += 31;
				ptm->tm_yday = 365;
				ptm->tm_mon = 11;
				ptm->tm_year--;
			}
			else {
				ptm->tm_yday += ltime;
			}
		}

#ifdef	_POSIX_
		/*
		 * In Posix, it is possible either the first or last three
		 * days of the Epoch might lie with Daylight Savings Time in
		 * certain time zones.
		 */
		if ( _isindst(ptm) ) {

			ltime = (long)ptm->tm_sec + _dstoffset;
			ptm->tm_sec = (int)(ltime % 60);
			if ( ptm->tm_sec < 0 ) {
				ptm->tm_sec += 60;
				ltime -= 60;
			}

			ltime = (long)ptm->tm_min + ltime/60;
			ptm->tm_min = (int)(ltime % 60);
			if ( ptm->tm_min < 0 ) {
				ptm->tm_min += 60;
				ltime -= 60;
			}

			ltime = (long)ptm->tm_hour + ltime/60;
			ptm->tm_hour = (int)(ltime % 24);
			if ( ptm->tm_hour < 0 ) {
				ptm->tm_hour += 24;
				ltime -=24;
			}

			ltime /= 24;

			if ( ltime > 0L ) {
				/*
				 * There is no possibility of overflowing the
				 * tm_mday and tm_yday fields since the date
				 * can be no later than January 19.
				 */
				ptm->tm_wday = (ptm->tm_wday + ltime) % 7;
				ptm->tm_mday += ltime;
				ptm->tm_yday += ltime;
			}
			else if ( ltime < 0L ) {
				/*
				 * It is possible to underflow the tm_mday
				 * and tm_yday fields. If this happens, then
				 * adjusted date must lie in December 1969.
				 */
				ptm->tm_wday = (ptm->tm_wday + 7 + ltime) % 7;
				if ( (ptm->tm_mday += ltime) <= 0 ) {
					ptm->tm_mday += 31;
					ptm->tm_yday = 365;
					ptm->tm_mon = 12;
					ptm->tm_year--;
				}
				else {
					ptm->tm_yday += ltime;
				}
			}
		}

#endif

	}


	return(ptm);
}