ctime(3C)

ctime, ctime_r, localtime, localtime_r, gmtime, gmtime_r, asctime, asctime_r, tzset, timezone, altzone, daylight, tzname -- convert date and time to string

Synopsis

   #include <time.h>
   
   char *ctime(const time_t *clock);
   

   char *ctime_r(const time_t *clock, char *buffer);
   

   struct tm *localtime(const time_t *clock);
   

   struct tm *localtime_r(const time_t *clock, struct tm *result);
   

   struct tm *gmtime(const time_t *clock);
   

   struct tm *gmtime_r(const time_t *clock, struct tm *result);
   

   char *asctime(const struct tm *tm);
   

   char *asctime_r(const struct tm *tm, char *buffer);
   

   void tzset(void);
   

   extern time_t timezone, altzone;
   

   extern int daylight;
   

   extern char *tzname[2];

Description

The reentrant functions (suffixed by _r) require the user to allocate the necessary space for the return value and pass a pointer to this area using result.

ctime, ctime_r, localtime, localtime_r, gmtime, and gmtime_r accept arguments of type time_t, pointed to by clock, representing the time in seconds since 00:00:00 UTC, January 1, 1970. ctime returns a pointer to a 26-character string as shown below. ctime_r converts the calendar time pointed to by clock to local time in the same format as shown below and places the string into the location pointed to by buffer, which is assumed to hold at least 26 characters. ctime_r returns buffer upon successful completion. Time zone and daylight savings corrections are made before the string is generated. The fields are constant in width:

   Fri Aug 13 00:00:00 1993\n\0

localtime and gmtime return pointers to tm structures, described below. localtime corrects for the main time zone and possible alternate (``daylight savings'') time zone; gmtime converts directly to Coordinated Universal Time (UTC), which is the time the UNIX system uses internally. localtime_r converts the calendar time pointed to by clock into a broken-down time that is stored in the struct tm pointed to by result. It returns result, upon successful completion. gmtime_r converts the calendar time pointed to by clock into a broken-down time expressed as Coordinated Universal Time (UTC). The broken-down time is stored in the struct tm pointed to by result. gmtime_r returns result, upon successful completion.

asctime converts a tm structure to a 26-character string, as shown in the above example, and returns a pointer to the string. asctime_r converts the broken-down time in the structure pointed to by tm into a string that is placed in the location pointed to by buffer, which is assumed to hold at least 26 characters. It returns buffer upon successful completion.

Declarations of all the functions and externals, and the tm structure, are in the time.h header file. The members for this structure include:

   	int	tm_sec;	/* seconds after the minute -- [0, 61] */
   				/* for leap seconds */
   	int	tm_min;	/* minutes after the hour -- [0, 59] */
   	int	tm_hour;	/* hour 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 January 1 -- [0, 365] */
   	int	tm_isdst;	/* flag for alternate daylight */
   				/* savings time */

The value of tm_isdst is positive if daylight savings time is in effect, zero if daylight savings time is not in effect, and negative if the information is not available. (Previously, the value of tm_isdst was defined as non-zero if daylight savings time was in effect.)

The external time_t variable altzone contains the difference, in seconds, between Coordinated Universal Time and the alternate time zone. The external variable timezone contains the difference, in seconds, between UTC and local standard time. The external variable daylight indicates whether time should reflect daylight savings time. Both timezone and altzone default to 0 (UTC). The external variable daylight is non-zero if an alternate time zone exists. The time zone names are contained in the external variable tzname, which by default is set to:

   char *tzname[2] = { "GMT", "   " };

These functions know about the peculiarities of this conversion for various time periods for the U.S.A. (specifically, the years 1974, 1975, and 1987). They will handle the new daylight savings time starting with the first Sunday in April, 1987.

tzset uses the contents of the environment variable TZ to override the value of the different external variables. It also sets the external variable daylight to zero if Daylight Savings Time conversions should never be applied for the time zone in use; otherwise, non-zero. tzset is called by asctime and may also be called by the user. See environ(5) for a description of the TZ environment variable.

Note that in most installations, TZ is set to the correct value by default when the user logs on, via the local /etc/profile file [see profile(4) and timezone(4)].

Files

/usr/lib/locale/language/LC_TIME: file containing locale specific date and time information

References

environ(5), fprintf(3S), getenv(3C), mktime(3C), profile(4), putenv(3C), setlocale(3C), strftime(3C), strftime(4), time(2), timezone(4)

Notices

The return values for ctime, localtime, and gmtime point to static data whose content is overwritten by each call.

Setting the time during the interval of change from timezone to altzone or vice versa can produce unpredictable results.

Use the reentrant functions for multithreaded applications.