The ctime() function converts the time pointed to by clock, representing the time in seconds since the Epoch (00:00:00 UTC, January 1, 1970), to local time in the form of a 26-character string, as shown below. Time zone and daylight savings corrections are made before string generation. The fields are in constant width:

Fri Sep 13 00:00:00 1986\n\0

The ctime() function is equivalent to:

asctime(localtime(clock))

The ctime(), asctime(), gmtime(), and localtime() functions return values in one of two static objects: a broken-down time structure and an array of char. Execution of any of the functions can overwrite the information returned in either of these objects by any of the other functions.

The ctime_r() function has the same functionality as ctime() except that the caller must supply a buffer buf with length buflen to store the result; buf must be at least 26 bytes. The POSIX ctime_r() function does not take a buflen parameter.

The localtime() and gmtime() functions return pointers to tm structures (see below). The localtime() function corrects for the main time zone and possible alternate ("daylight savings") time zone; the gmtime() function converts directly to Coordinated Universal Time (UTC), which is what the UNIX system uses internally.

The localtime_r() and gmtime_r() functions have the same functionality as localtime() and gmtime() respectively, except that the caller must supply a buffer res to store the result.

The asctime() function converts a tm structure to a 26-character string, as shown in the previous example, and returns a pointer to the string.

The asctime_r() function has the same functionality as asctime() except that the caller must supply a buffer buf with length buflen for the result to be stored. The buf argument must be at least 26 bytes. The POSIX asctime_r() function does not take a buflen parameter. The asctime_r() function returns a pointer to buf upon success. In case of failure, NULL is returned and errno is set.

Declarations of all the functions and externals, and the tm structure, are in the <time.h> header. The members of the tm structure are:

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 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. (specifically, the years 1974, 1975, and 1987). They start handling the new daylight savings time starting with the first Sunday in April, 1987.

The tzset() function uses the contents of the environment variable TZ to override the value of the different external variables. It is called by asctime() and can also be called by the user. See environ(5) for a description of the TZ environment variable.

Starting and ending times are relative to the current local time zone. If the alternate time zone start and end dates and the time are not provided, the days for the United States that year will be used and the time will be 2 AM. If the start and end dates are provided but the time is not provided, the time will be 2 AM. The effects of tzset() change the values of the external variables timezone, altzone, daylight, and tzname.

Note that in most installations, TZ is set to the correct value by default when the user logs on, using the local /etc/default/init file (see TIMEZONE(4)).

The ctime_r() and asctime_r() functions will fail if:

ERANGE

The length of the buffer supplied by the caller is not large enough to store the result.

These functions do not support localized date and time formats. The strftime(3C) function can be used when localization is required.

The localtime(), localtime_r(), gmtime(), gmtime_r(), ctime(), and ctime_r() functions assume Gregorian dates. Times before the adoption of the Gregorian calendar will not match historial records.

The zoneinfo timezone data files do not transition past Tue Jan 19 03:14:07 2038 UTC. Therefore for 64-bit applications using zoneinfo timezones, calculations beyond this date might not use the correct offset from standard time, and could return incorrect values. This affects the 64-bit version of localtime(), localtime_r(), ctime(), and ctime_r().

See attributes(5) for descriptions of the following attributes:

time(2), Intro(3), getenv(3C), mktime(3C), printf(3C), putenv(3C), setlocale(3C), strftime(3C), TIMEZONE(4), attributes(5), environ(5)

When compiling multithreaded programs, see Intro(3), Notes On Multithreaded Applications.

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. The system administrator must change the Julian start and end days annually.

The asctime(), ctime(), gmtime(), and localtime() functions are unsafe in multithread applications. The asctime_r() and gmtime_r() functions are MT-Safe. The ctime_r(), localtime_r(), and tzset() functions are MT-Safe in multithread applications, as long as no user-defined function directly modifies one of the following variables: timezone, altzone, daylight, and tzname. These four variables are not MT-Safe to access. They are modified by the tzset() function in an MT-Safe manner. The mktime(), localtime_r(), and ctime_r() functions call tzset().

Solaris 2.4 and earlier releases provided definitions of the ctime_r(), localtime_r(), gmtime_r(), and asctime_r() functions as specified in POSIX.1c Draft 6. The final POSIX.1c standard changed the interface for ctime_r() and asctime_r(). Support for the Draft 6 interface is provided for compatibility only and might not be supported in future releases. New applications and libraries should use the POSIX standard interface.

For POSIX.1c-compliant applications, the _POSIX_PTHREAD_SEMANTICS and _REENTRANT flags are automatically turned on by defining the _POSIX_C_SOURCE flag with a value >= 199506L.