CTIME(3)

NAME
asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r, localtime, localtime_r, mktime - convert date and time to ASCII

LIBRARY
libc

SYNOPSIS
#include <time.h>
extern char *tzname[2];
char *
ctime(const time_t *clock);
char *
ctime_r(const time_t *clock, char *buf);
double
difftime(time_t time1, time_t time0);
char *
asctime(const struct tm *tm);
char *
asctime_r(const struct tm restrict tm, char * restrict buf);
struct tm *
localtime(const time_t *clock);
struct tm *
localtime_r(const time_t * restrict clock, struct tm * restrict result);
struct tm *
gmtime(const time_t *clock);
struct tm *
gmtime_r(const time_t * restrict clock, struct tm * restrict result);
time_t
mktime(struct tm *tm);

DESCRIPTION
ctime() converts a long integer, pointed to by clock, representing the time in seconds since 00:00:00 UTC, 1970-01-01, and returns a pointer to a 26-character string of the form
    Thu Nov 24 18:22:48 1986\n\0
All the fields have constant width.

The ctime_r() function provides the same functionality as ctime() differing in that the caller must supply a buffer area buf with a size of at least 26 bytes, in which the result is stored.

localtime() and gmtime() return pointers to tm structures, described below. localtime() corrects for the time zone and any time zone adjustments (such as Daylight Saving Time in the U.S.A.). After filling in the tm structure, localtime() sets the tm_isdst'th element of tzname to a pointer to an ASCII string that's the time zone abbreviation to be used with localtime()'s return value.

gmtime() converts to Coordinated Universal Time.

The gmtime_r() and localtime_r() functions provide the same functionality as gmtime() and localtime() differing in that the caller must supply a buffer area result in which the result is stored; also, localtime_r() does not imply initialization of the local time conversion information; the application may need to do so by calling tzset(3).

asctime() converts a time value contained in a tm structure to a 26-character string, as shown in the above example, and returns a pointer to the string.

The asctime_r() function provides the same functionality as asctime() differing in that the caller must supply a buffer area buf with a size of at least 26 bytes, in which the result is stored.

mktime() converts the broken-down time, expressed as local time, in the structure pointed to by tm into a calendar time value with the same encoding as that of the values returned by the time(3) function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to their normal ranges. (A positive or zero value for tm_isdst causes mktime() to presume initially that summer time (for example, Daylight Saving Time in the U.S.A.) respectively, is or is not in effect for the specified time. A negative value for tm_isdst causes the mktime() function to attempt to divine whether summer time is in effect for the specified time.) On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to their normal ranges; the final value of tm_mday is not set until tm_mon and tm_year are determined. mktime() returns the specified calendar time; if the calendar time cannot be represented, it returns -1.

difftime() returns the difference between two calendar times, (time1 - time0), expressed in seconds.

The structure (of type) struct tm includes the following fields:

    int tm_sec;	/* seconds after the minute [0,61] */
    int tm_min;	/* minutes after the hour [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;	/* day of week [0,6] (Sunday = 0) */
    int tm_yday;	/* day of year [0,365] (Jan 1 = 0) */
    int tm_isdst;	/* daylight savings flag */
    long tm_gmtoff;	/* offset from UTC in seconds */
    char *tm_zone;	/* abbreviation of timezone name */

The tm_zone and tm_gmtoff fields exist, and are filled in, only if arrangements to do so were made when the library containing these functions was created. There is no guarantee that these fields will continue to exist in this form in future releases of this code.

tm_isdst is non-zero if summer time is in effect.

tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values indicating east of the Prime Meridian.

FILES
/etc/localtime
    local time zone file
/usr/share/zoneinfo
    time zone information directory
/usr/share/zoneinfo/posixrules
    used with POSIX-style TZ's
/usr/share/zoneinfo/GMT
    for UTC leap seconds

If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/share/zoneinfo/posixrules.

SEE ALSO
getenv(3), strftime(3), time(3), tzset(3), tzfile(5)

STANDARDS
The ctime(), difftime(), asctime(), localtime(), gmtime() and mktime() functions conform to ansiC The ctime_r(), asctime_r(), localtime_r() and gmtime_r() functions conform to p1003.1c-95.

NOTES
The return values point to static data; the data is overwritten by each call. The tm_zone field of a returned struct tm points to a static array of characters, which will also be overwritten at the next call (and by calls to tzset(3)).

Avoid using out-of-range values with mktime() when setting up lunch with promptness sticklers in Riyadh.