gmtime, _gmtime32, _gmtime64

아티클
12/18/2017

The latest version of this topic can be found at gmtime, _gmtime32, _gmtime64.

Converts a time value to a structure. More secure versions of these functions are available; see gmtime_s, _gmtime32_s, _gmtime64_s.

Syntax

struct tm *gmtime(   
   const time_t *timer   
);  
struct tm *_gmtime32(   
   const __time32_t *timer   
);  
struct tm *_gmtime64(   
   const __time64_t *timer   
);

Parameters

timer
Pointer to the stored time. The time is represented as seconds elapsed since midnight (00:00:00), January 1, 1970, coordinated universal time (UTC).

Return Value

A pointer to a structure of type tm. The fields of the returned structure hold the evaluated value of the timer argument in UTC rather than in local time. Each of the structure fields is of type int, as follows:

tm_sec
Seconds after minute (0 – 59).

tm_min
Minutes after hour (0 – 59).

tm_hour
Hours since 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
Always 0 for gmtime.

Both the 32-bit and 64-bit versions of gmtime, mktime, mkgmtime, and localtime all use one common tm structure per thread for the conversion. Each call to one of these functions destroys the result of any previous call. If timer represents a date before midnight, January 1, 1970, gmtime returns NULL. There is no error return.

_gmtime64, which uses the __time64_t structure, enables dates to be expressed up through 23:59:59, December 31, 3000, UTC, whereas _gmtime32 only represent dates through 23:59:59 January 18, 2038, UTC. Midnight, January 1, 1970, is the lower bound of the date range for both functions.

gmtime is an inline function that evaluates to _gmtime64, and time_t is equivalent to __time64_t unless _USE_32BIT_TIME_T is defined. If you must force the compiler to interpret time_t as the old 32-bit time_t, you can define _USE_32BIT_TIME_T, but doing so causes gmtime to be in-lined to _gmtime32 and time_t to be defined as __time32_t. We recommend that you do not do this, because it is not allowed on 64-bit platforms and in any case your application may fail after January 18, 2038.

These functions validate their parameters. If timer is a null pointer, or if the timer value is negative, these functions invoke an invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, the functions return NULL and set errno to EINVAL.

Remarks

The _gmtime32 function breaks down the timer value and stores it in a statically allocated structure of type tm, defined in TIME.H. The value of timer is typically obtained from a call to the time function.

Note

In most cases, the target environment tries to determine whether daylight savings time is in effect. The C run-time library assumes that the United States rules for implementing the calculation of Daylight Saving Time (DST) are used.

Requirements

Routine	Required header
`gmtime`	<time.h>
`_gmtime32`	<time.h>
`_gmtime64`	<time.h>

For additional compatibility information, see Compatibility.

Example

  
      // crt_gmtime.c  
// compile with: /W3  
// This program uses _gmtime64 to convert a long-  
// integer representation of coordinated universal time  
// to a structure named newtime, then uses asctime to  
// convert this structure to an output string.  
  
#include <time.h>  
#include <stdio.h>  
  
int main( void )  
{  
   struct tm *newtime;  
   __int64 ltime;  
   char buff[80];  
  
   _time64( &ltime );  
  
   // Obtain coordinated universal time:  
   newtime = _gmtime64( &ltime ); // C4996  
   // Note: _gmtime64 is deprecated; consider using _gmtime64_s  
   asctime_s( buff, sizeof(buff), newtime );  
   printf( "Coordinated universal time is %s\n", buff );  
}

Coordinated universal time is Tue Feb 12 23:11:31 2002

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