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Unix Version 7 - man page for ctime (v7 section 3)

CTIME(3)			     Library Functions Manual				 CTIME(3)

NAME
       ctime, localtime, gmtime, asctime, timezone -  convert date and time to ASCII

SYNOPSIS
       char *ctime(clock)
       long *clock;

       #include <time.h>

       struct tm *localtime(clock)
       long *clock;

       struct tm *gmtime(clock)
       long *clock;

       char *asctime(tm)
       struct tm *tm;

       char *timezone(zone, dst)

DESCRIPTION
       Ctime  converts	a  time  pointed  to  by clock such as returned by time(2) into ASCII and
       returns a pointer to a 26-character string in the following form.   All	the  fields  have
       constant width.

	   Sun Sep 16 01:03:52 1973\n\0

       Localtime  and  gmtime  return  pointers  to  structures  containing the broken-down time.
       Localtime corrects for the time zone and possible daylight savings time;  gmtime  converts
       directly  to  GMT,  which  is  the time UNIX uses.  Asctime converts a broken-down time to
       ASCII and returns a pointer to a 26-character string.

       The structure declaration from the include file is:

	      /* Copyright (C) 1991-2018 Free Software Foundation, Inc.
		 This file is part of the GNU C Library.

		 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
		 <http://www.gnu.org/licenses/>.  */

	      /*
	       *   ISO C99 Standard: 7.23 Date and time    <time.h>
	       */

	      #ifndef	_TIME_H
	      #define _TIME_H	  1

	      #include <features.h>

	      #define __need_size_t
	      #define __need_NULL
	      #include <stddef.h>

	      /* This defines CLOCKS_PER_SEC, which is the number of processor clock
		 ticks per second, and possibly a number of other constants.   */
	      #include <bits/time.h>

	      /* Many of the typedefs and structs whose official home is this header
		 may also need to be defined by other headers.	*/
	      #include <bits/types/clock_t.h>
	      #include <bits/types/time_t.h>
	      #include <bits/types/struct_tm.h>

	      #if defined __USE_POSIX199309 || defined __USE_ISOC11
	      # include <bits/types/struct_timespec.h>
	      #endif

	      #ifdef __USE_POSIX199309
	      # include <bits/types/clockid_t.h>
	      # include <bits/types/timer_t.h>
	      # include <bits/types/struct_itimerspec.h>
	      struct sigevent;
	      #endif

	      #ifdef __USE_XOPEN2K
	      # ifndef __pid_t_defined
	      typedef __pid_t pid_t;
	      #  define __pid_t_defined
	      # endif
	      #endif

	      #ifdef __USE_XOPEN2K8
	      # include <bits/types/locale_t.h>
	      #endif

	      #ifdef __USE_ISOC11
	      /* Time base values for timespec_get.  */
	      # define TIME_UTC 1
	      #endif

	      __BEGIN_DECLS

	      /* Time used by the program so far (user time + system time).
		 The result / CLOCKS_PER_SEC is program time in seconds.  */
	      extern clock_t clock (void) __THROW;

	      /* Return the current time and put it in *TIMER if TIMER is not NULL.  */
	      extern time_t time (time_t *__timer) __THROW;

	      /* Return the difference between TIME1 and TIME0.  */
	      extern double difftime (time_t __time1, time_t __time0)
		   __THROW __attribute__ ((__const__));

	      /* Return the `time_t' representation of TP and normalize TP.  */
	      extern time_t mktime (struct tm *__tp) __THROW;

	      /* Format TP into S according to FORMAT.
		 Write no more than MAXSIZE characters and return the number
		 of characters written, or 0 if it would exceed MAXSIZE.  */
	      extern size_t strftime (char *__restrict __s, size_t __maxsize,
			     const char *__restrict __format,
			     const struct tm *__restrict __tp) __THROW;

	      #ifdef __USE_XOPEN
	      /* Parse S according to FORMAT and store binary time information in TP.
		 The return value is a pointer to the first unparsed character in S.  */
	      extern char *strptime (const char *__restrict __s,
			       const char *__restrict __fmt, struct tm *__tp)
		   __THROW;
	      #endif

	      #ifdef __USE_XOPEN2K8
	      /* Similar to the two functions above but take the information from
		 the provided locale and not the global locale.  */

	      extern size_t strftime_l (char *__restrict __s, size_t __maxsize,
			       const char *__restrict __format,
			       const struct tm *__restrict __tp,
			       locale_t __loc) __THROW;
	      #endif

	      #ifdef __USE_GNU
	      extern char *strptime_l (const char *__restrict __s,
			      const char *__restrict __fmt, struct tm *__tp,
			      locale_t __loc) __THROW;
	      #endif

	      /* Return the `struct tm' representation of *TIMER
		 in Universal Coordinated Time (aka Greenwich Mean Time).  */
	      extern struct tm *gmtime (const time_t *__timer) __THROW;

	      /* Return the `struct tm' representation
		 of *TIMER in the local timezone.  */
	      extern struct tm *localtime (const time_t *__timer) __THROW;

	      #ifdef __USE_POSIX
	      /* Return the `struct tm' representation of *TIMER in UTC,
		 using *TP to store the result.  */
	      extern struct tm *gmtime_r (const time_t *__restrict __timer,
				 struct tm *__restrict __tp) __THROW;

	      /* Return the `struct tm' representation of *TIMER in local time,
		 using *TP to store the result.  */
	      extern struct tm *localtime_r (const time_t *__restrict __timer,
				    struct tm *__restrict __tp) __THROW;
	      #endif	/* POSIX */

	      /* Return a string of the form "Day Mon dd hh:mm:ss yyyy0
		 that is the representation of TP in this format.  */
	      extern char *asctime (const struct tm *__tp) __THROW;

	      /* Equivalent to `asctime (localtime (timer))'.  */
	      extern char *ctime (const time_t *__timer) __THROW;

	      #ifdef __USE_POSIX
	      /* Reentrant versions of the above functions.  */

	      /* Return in BUF a string of the form "Day Mon dd hh:mm:ss yyyy0
		 that is the representation of TP in this format.  */
	      extern char *asctime_r (const struct tm *__restrict __tp,
			     char *__restrict __buf) __THROW;

	      /* Equivalent to `asctime_r (localtime_r (timer, *TMP*), buf)'.  */
	      extern char *ctime_r (const time_t *__restrict __timer,
			      char *__restrict __buf) __THROW;
	      #endif	/* POSIX */

	      /* Defined in localtime.c.  */
	      extern char *__tzname[2];     /* Current timezone names.	*/
	      extern int __daylight;	    /* If daylight-saving time is ever in use.	*/
	      extern long int __timezone;   /* Seconds west of UTC.  */

	      #ifdef	__USE_POSIX
	      /* Same as above.  */
	      extern char *tzname[2];

	      /* Set time conversion information from the TZ environment variable.
		 If TZ is not defined, a locale-dependent default is used.  */
	      extern void tzset (void) __THROW;
	      #endif

	      #if defined __USE_MISC || defined __USE_XOPEN
	      extern int daylight;
	      extern long int timezone;
	      #endif

	      #ifdef __USE_MISC
	      /* Set the system time to *WHEN.
		 This call is restricted to the superuser.  */
	      extern int stime (const time_t *__when) __THROW;
	      #endif

	      /* Nonzero if YEAR is a leap year (every 4 years,
		 except every 100th isn't, and every 400th is).  */
	      #define __isleap(year)	 ((year) % 4 == 0 && ((year) % 100 != 0 || (year) % 400 == 0))

	      #ifdef __USE_MISC
	      /* Miscellaneous functions many Unices inherited from the public domain
		 localtime package.  These are included only for compatibility.  */

	      /* Like `mktime', but for TP represents Universal Time, not local time.  */
	      extern time_t timegm (struct tm *__tp) __THROW;

	      /* Another name for `mktime'.  */
	      extern time_t timelocal (struct tm *__tp) __THROW;

	      /* Return the number of days in YEAR.  */
	      extern int dysize (int __year) __THROW  __attribute__ ((__const__));
	      #endif

	      #ifdef __USE_POSIX199309
	      /* Pause execution for a number of nanoseconds.

		 This function is a cancellation point and therefore not marked with
		 __THROW.  */
	      extern int nanosleep (const struct timespec *__requested_time,
			      struct timespec *__remaining);

	      /* Get resolution of clock CLOCK_ID.  */
	      extern int clock_getres (clockid_t __clock_id, struct timespec *__res) __THROW;

	      /* Get current value of clock CLOCK_ID and store it in TP.  */
	      extern int clock_gettime (clockid_t __clock_id, struct timespec *__tp) __THROW;

	      /* Set clock CLOCK_ID to value TP.  */
	      extern int clock_settime (clockid_t __clock_id, const struct timespec *__tp)
		   __THROW;

	      # ifdef __USE_XOPEN2K
	      /* High-resolution sleep with the specified clock.

		 This function is a cancellation point and therefore not marked with
		 __THROW.  */
	      extern int clock_nanosleep (clockid_t __clock_id, int __flags,
				 const struct timespec *__req,
				 struct timespec *__rem);

	      /* Return clock ID for CPU-time clock.  */
	      extern int clock_getcpuclockid (pid_t __pid, clockid_t *__clock_id) __THROW;
	      # endif

	      /* Create new per-process timer using CLOCK_ID.  */
	      extern int timer_create (clockid_t __clock_id,
			      struct sigevent *__restrict __evp,
			      timer_t *__restrict __timerid) __THROW;

	      /* Delete timer TIMERID.	*/
	      extern int timer_delete (timer_t __timerid) __THROW;

	      /* Set timer TIMERID to VALUE, returning old value in OVALUE.  */
	      extern int timer_settime (timer_t __timerid, int __flags,
			       const struct itimerspec *__restrict __value,
			       struct itimerspec *__restrict __ovalue) __THROW;

	      /* Get current value of timer TIMERID and store it in VALUE.  */
	      extern int timer_gettime (timer_t __timerid, struct itimerspec *__value)
		   __THROW;

	      /* Get expiration overrun for timer TIMERID.  */
	      extern int timer_getoverrun (timer_t __timerid) __THROW;
	      #endif

	      #ifdef __USE_ISOC11
	      /* Set TS to calendar time based in time base BASE.  */
	      extern int timespec_get (struct timespec *__ts, int __base)
		   __THROW __nonnull ((1));
	      #endif

	      #ifdef __USE_XOPEN_EXTENDED
	      /* Set to one of the following values to indicate an error.
		   1  the DATEMSK environment variable is null or undefined,
		   2  the template file cannot be opened for reading,
		   3  failed to get file status information,
		   4  the template file is not a regular file,
		   5  an error is encountered while reading the template file,
		   6  memory allication failed (not enough memory available),
		   7  there is no line in the template that matches the input,
		   8  invalid input specification Example: February 31 or a time is
		   specified that can not be represented in a time_t (representing
		   the time in seconds since 00:00:00 UTC, January 1, 1970) */
	      extern int getdate_err;

	      /* Parse the given string as a date specification and return a value
		 representing the value.  The templates from the file identified by
		 the environment variable DATEMSK are used.  In case of an error
		 `getdate_err' is set.

		 This function is a possible cancellation point and therefore not
		 marked with __THROW.  */
	      extern struct tm *getdate (const char *__string);
	      #endif

	      #ifdef __USE_GNU
	      /* Since `getdate' is not reentrant because of the use of `getdate_err'
		 and the static buffer to return the result in, we provide a thread-safe
		 variant.  The functionality is the same.  The result is returned in
		 the buffer pointed to by RESBUFP and in case of an error the return
		 value is != 0 with the same values as given above for `getdate_err'.

		 This function is not part of POSIX and therefore no official
		 cancellation point.  But due to similarity with an POSIX interface
		 or due to the implementation it is a cancellation point and
		 therefore not marked with __THROW.  */
	      extern int getdate_r (const char *__restrict __string,
			      struct tm *__restrict __resbufp);
	      #endif

	      __END_DECLS

	      #endif /* time.h.  */

       These quantities give the time on a 24-hour clock, day of  month  (1-31),  month  of  year
       (0-11),	day  of  week  (Sunday = 0), year - 1900, day of year (0-365), and a flag that is
       nonzero if daylight saving time is in effect.

       When local time is called for, the program consults the system to determine the time  zone
       and  whether the standard U.S.A. daylight saving time adjustment is appropriate.  The pro-
       gram knows about the peculiarities of this conversion in 1974 and 1975;	if  necessary,	a
       table for these years can be extended.

       Timezone  returns  the  name of the time zone associated with its first argument, which is
       measured in minutes westward from Greenwich.  If the second argument is	0,  the  standard
       name is used, otherwise the Daylight Saving version.  If the required name does not appear
       in a table built into  the  routine,  the  difference  from  GMT  is  produced;	e.g.   in
       Afghanistan timezone(-(60*4+30), 0) is appropriate because it is 4:30 ahead of GMT and the
       string GMT+4:30 is produced.

SEE ALSO
       time(2)

BUGS
       The return values point to static data whose content is overwritten by each call.

											 CTIME(3)


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