localtime_s, _localtime32_s, _localtime64_s

Articolo
07/24/2011

Converts a time value and corrects for the local time zone. These are versions of localtime, _localtime32, _localtime64 with security enhancements as described in Security Features in the CRT.

errno_t localtime_s(
   struct tm* _tm,
   const time_t *time 
);
errno_t _localtime32_s(
   struct tm* _tm,
   const time32_t *time 
);
errno_t _localtime64_s(
   struct tm* _tm,
   const _time64_t *time 
);

Parameters

_tm
Pointer to the time structure to be filled in.
time
Pointer to the stored time.

Return Value

Zero if successful. The return value is an error code if there is a failure. Error codes are defined in Errno.h. For a listing of these errors, see errno.

Error Conditions

_tm	time	Return value	Value in _tm	Invokes invalid parameter handler
NULL	any	EINVAL	Not modified	Yes
Not NULL (points to valid memory)	NULL	EINVAL	All fields set to -1	Yes
Not NULL (points to valid memory)	less than 0 or greater than _MAX__TIME64_T	EINVAL	All fields set to -1	No

In the case of the first two error conditions, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, these functions set errno to EINVAL and return EINVAL.

Remarks

The _localtime32_s function converts a time stored as a time_t value and stores the result in a structure of type tm. The long value timer represents the seconds elapsed since midnight (00:00:00), January 1, 1970, UTC. This value is usually obtained from the time function.

_localtime32_s corrects for the local time zone if the user first sets the global environment variable TZ. When TZ is set, three other environment variables (_timezone, _daylight, and _tzname) are automatically set as well. If the TZ variable is not set, localtime32_s attempts to use the time zone information specified in the Date/Time application in Control Panel. If this information cannot be obtained, PST8PDT, which signifies the Pacific time zone, is used by default. See _tzset for a description of these variables. TZ is a Microsoft extension and not part of the ANSI standard definition of localtime.

Nota

The target environment should try to determine whether daylight saving time is in effect.

_localtime64_s, which uses the __time64_t structure, allows dates to be expressed up through 23:59:59, December 31, 3000, coordinated universal time (UTC), whereas _localtime32_s represents dates through 03:14:07 January 19, 2038, UTC.

localtime_s is an inline function which evaluates to _localtime64_s, and time_t is equivalent to __time64_t. If you need to force the compiler to interpret time_t as the old 32-bit time_t, you can define _USE_32BIT_TIME_T. Doing this will cause localtime_s to evaluate to _localtime32_s. This is not recommended because your application may fail after January 19, 2038, and it is not allowed on 64-bit platforms.

The fields of the structure type tm store the following values, each of which is an int.

tm_sec
Seconds after minute (0 – 59).
tm_min
Minutes after hour (0 – 59).
tm_hour
Hours after midnight (0 – 23).
tm_mday
Day of month (1 – 31).
tm_mon
Month (0 – 11; January = 0).
tm_year
Year (current year minus 1900).
tm_wday
Day of week (0 – 6; Sunday = 0).
tm_yday
Day of year (0 – 365; January 1 = 0).
tm_isdst
Positive value if daylight saving time is in effect; 0 if daylight saving time is not in effect; negative value if status of daylight saving time is unknown. If the TZ environment variable is set, the C run-time library assumes rules appropriate to the United States for implementing the calculation of daylight saving time (DST).

Requirements

Routine	Required header
localtime_s	<time.h>
_localtime32_s	<time.h>
_localtime64_s	<time.h>

For more compatibility information, see Compatibility in the Introduction.

Example

// crt_localtime_s.c
/* This program uses _time64 to get the current time 
 * and then uses _localtime64_s() to convert this time to a structure 
 * representing the local time. The program converts the result 
 * from a 24-hour clock to a 12-hour clock and determines the 
 * proper extension (AM or PM).
 */

#include <stdio.h>
#include <string.h>
#include <time.h>

int main( void )
{
        struct tm newtime;
        char am_pm[] = "AM";
        __time64_t long_time;
        char timebuf[26];
        errno_t err;

        // Get time as 64-bit integer.
        _time64( &long_time ); 
        // Convert to local time.
        err = _localtime64_s( &newtime, &long_time ); 
        if (err)
        {
            printf("Invalid argument to _localtime64_s.");
            exit(1);
        }
        if( newtime.tm_hour > 12 )        // Set up extension. 
                strcpy_s( am_pm, sizeof(am_pm), "PM" );
        if( newtime.tm_hour > 12 )        // Convert from 24-hour 
                newtime.tm_hour -= 12;    // to 12-hour clock. 
        if( newtime.tm_hour == 0 )        // Set hour to 12 if midnight.
                newtime.tm_hour = 12;

        // Convert to an ASCII representation. 
        err = asctime_s(timebuf, 26, &newtime);
        if (err)
        {
           printf("Invalid argument to asctime_s.");
           exit(1);
        }
        printf( "%.19s %s\n", timebuf, am_pm );
}

Sample Output

Fri Apr 25 01:19:27 PM

.NET Framework Equivalent

System::DateTime::ToLocalTime

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