C – time.h library functions

      All C inbuilt functions which are declared in time.h header file are given below. The source code for time.h header file is also given below for your reference.


List of inbuilt C functions in time.h file:

S.no Functions Description
1 setdate() This function used to modify the system date
2 getdate() This function is used to get the CPU time
3 clock() This function is used to get current system time
4 time() This function is used to get current system time as structure
5 difftime() This function is used to get the difference between two given times
6 strftime() This function is used to modify the actual time format
7 mktime() This function interprets tm structure as calendar time
8 localtime() This function shares the tm structure that contains date and time informations
9 gmtime() This function shares the tm structure that contains date and time informations
10 ctime() This function is used to return string that contains date and time informations
11 asctime() Tm structure contents are interpreted by this function as calendar time. This time is converted into string.

Source code for time.h header file:

      Please find the source code for time.h header file below. This code is taken from DevC++ compiler files for your reference.

 * time.h
 * This file has no copyright assigned and is placed in the Public Domain.
 * This file is a part of the mingw-runtime package.
 * No warranty is given; refer to the file DISCLAIMER within the package.
 * Date and time functions and types.

#ifndef	_TIME_H_
#define	_TIME_H_

/* All the headers include this file. */
#include <_mingw.h>

#define __need_wchar_t
#define __need_size_t
#define __need_NULL
#ifndef RC_INVOKED
#include <stddef.h>
#endif	/* Not RC_INVOKED */

 * Number of clock ticks per second. A clock tick is the unit by which
 * processor time is measured and is returned by 'clock'.
#define	CLOCKS_PER_SEC	((clock_t)1000)

#ifndef RC_INVOKED

 * A type for storing the current time and date. This is the number of
 * seconds since midnight Jan 1, 1970.
 * NOTE: This is also defined in non-ISO sys/types.h.
typedef	long	time_t;

#ifndef __STRICT_ANSI__
/* A 64-bit time_t to get to Y3K */
#ifndef _TIME64_T_DEFINED
typedef __int64 __time64_t;
#define _TIME64_T_DEFINED
 * A type for measuring processor time (in clock ticks).
typedef	long	clock_t;

#ifndef _TM_DEFINED
 * A structure for storing all kinds of useful information about the
 * current (or another) time.
struct tm
	int	tm_sec;		/* Seconds: 0-59 (K&R says 0-61?) */
	int	tm_min;		/* Minutes: 0-59 */
	int	tm_hour;	/* Hours since midnight: 0-23 */
	int	tm_mday;	/* Day of the month: 1-31 */
	int	tm_mon;		/* Months *since* january: 0-11 */
	int	tm_year;	/* Years since 1900 */
	int	tm_wday;	/* Days since Sunday (0-6) */
	int	tm_yday;	/* Days since Jan. 1: 0-365 */
	int	tm_isdst;	/* +1 Daylight Savings Time, 0 No DST,
				 * -1 don't know */
#define _TM_DEFINED

#ifdef	__cplusplus
extern "C" {

_CRTIMP clock_t __cdecl	clock (void);
_CRTIMP time_t __cdecl	time (time_t*);
_CRTIMP double __cdecl	difftime (time_t, time_t);
_CRTIMP time_t __cdecl	mktime (struct tm*);

 * These functions write to and return pointers to static buffers that may
 * be overwritten by other function calls. Yikes!
 * NOTE: localtime, and perhaps the others of the four functions grouped
 * below may return NULL if their argument is not 'acceptable'. Also note
 * that calling asctime with a NULL pointer will produce an Invalid Page
 * Fault and crap out your program. Guess how I know. Hint: stat called on
 * a directory gives 'invalid' times in st_atime etc...
_CRTIMP char* __cdecl		asctime (const struct tm*);
_CRTIMP char* __cdecl		ctime (const time_t*);
_CRTIMP struct tm*  __cdecl	gmtime (const time_t*);
_CRTIMP struct tm*  __cdecl	localtime (const time_t*);

_CRTIMP size_t __cdecl		strftime (char*, size_t, const char*, const struct tm*);

#ifndef __STRICT_ANSI__

extern _CRTIMP void __cdecl	_tzset (void);

#ifndef _NO_OLDNAMES
extern _CRTIMP void __cdecl	tzset (void);

_CRTIMP char* __cdecl	_strdate(char*);
_CRTIMP char* __cdecl	_strtime(char*);

/* These require newer versions of msvcrt.dll (6.10 or higher). */ 
#if __MSVCRT_VERSION__ >= 0x0601
_CRTIMP __time64_t __cdecl  _time64( __time64_t);
_CRTIMP __time64_t __cdecl  _mktime64 (struct tm*);
_CRTIMP char* __cdecl _ctime64 (const __time64_t*);
_CRTIMP struct tm*  __cdecl _gmtime64 (const __time64_t*);
_CRTIMP struct tm*  __cdecl _localtime64 (const __time64_t*);
#endif /* __MSVCRT_VERSION__ >= 0x0601 */

 * _daylight: non zero if daylight savings time is used.
 * _timezone: difference in seconds between GMT and local time.
 * _tzname: standard/daylight savings time zone names (an array with two
 *          elements).
#ifdef __MSVCRT__

/* These are for compatibility with pre-VC 5.0 suppied MSVCRT. */
extern _CRTIMP int* __cdecl	__p__daylight (void);
extern _CRTIMP long* __cdecl	__p__timezone (void);
extern _CRTIMP char** __cdecl	__p__tzname (void);

__MINGW_IMPORT int	_daylight;
__MINGW_IMPORT long	_timezone;
__MINGW_IMPORT char 	*_tzname[2];

#else /* not __MSVCRT (ie. crtdll) */


extern int*	_imp___daylight_dll;
extern long*	_imp___timezone_dll;
extern char**	_imp___tzname;

#define _daylight	(*_imp___daylight_dll)
#define _timezone	(*_imp___timezone_dll)
#define _tzname		(*_imp___tzname)


__MINGW_IMPORT int	_daylight_dll;
__MINGW_IMPORT long	_timezone_dll;
__MINGW_IMPORT char*	_tzname[2];

#define _daylight	_daylight_dll
#define _timezone	_timezone_dll


#endif /* not __MSVCRT__ */

#ifndef _NO_OLDNAMES

#ifdef __MSVCRT__

/* These go in the oldnames import library for MSVCRT. */
__MINGW_IMPORT int	daylight;
__MINGW_IMPORT long	timezone;
__MINGW_IMPORT char 	*tzname[2];

#else /* not __MSVCRT__ */

/* CRTDLL is royally messed up when it comes to these macros.
   TODO: import and alias these via oldnames import library instead 
   of macros.  */

#define daylight        _daylight
/* NOTE: timezone not defined because it would conflict with sys/timeb.h.
   Also, tzname used to a be macro, but now it's in moldname. */
__MINGW_IMPORT char 	*tzname[2];

#endif /* not __MSVCRT__ */

#endif	/* Not _NO_OLDNAMES */
#endif	/* Not __STRICT_ANSI__ */

/* wide function prototypes, also declared in wchar.h */
#ifndef __STRICT_ANSI__
#ifdef __MSVCRT__
_CRTIMP wchar_t* __cdecl	_wasctime(const struct tm*);
_CRTIMP wchar_t* __cdecl	_wctime(const time_t*);
_CRTIMP wchar_t* __cdecl	_wstrdate(wchar_t*);
_CRTIMP wchar_t* __cdecl	_wstrtime(wchar_t*);
#if __MSVCRT_VERSION__ >= 0x0601
_CRTIMP wchar_t* __cdecl	_wctime64 (const __time64_t*);
#endif /*  __MSVCRT__ */
#endif /* __STRICT_ANSI__ */
_CRTIMP size_t __cdecl		wcsftime (wchar_t*, size_t, const wchar_t*, const struct tm*);
#endif /* _WTIME_DEFINED */ 

#ifdef	__cplusplus

#endif	/* Not RC_INVOKED */

#endif	/* Not _TIME_H_ */