CTIME(3)

NAME
asctime, asctime_r, ctime, ctime_r, ctime_rz, difftime, gmtime, gmtime_r, localtime, localtime_r, localtime_rz, mktime, mktime_z, tzalloc, tzgetname, tzfree, - 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);
char *
ctime_rz(const timezone_t tz, 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 *
localtime_rz(const timezone_t tz, 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);
time_t
mktime_z(const timezone_t tz, struct tm *tm);
timezone_t
tzalloc(const char *zone);
const char *
tzgetname(const timezone_t tz, int isdst);
void
tzfree(const timezone_t tz);

DESCRIPTION
ctime() converts a pointed to by clock, representing the time in seconds since 00:00:00 UTC, 1970-01-01, and returns a pointer to a string of the form
    Thu Nov 24 18:22:48 1986\n\0
Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form
    Thu Nov 24 18:22:48 81986\n\0
with five spaces before the year. These unusual formats are designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years.

ctime_r() is similar to ctime(), except it places the result of the conversion on the buf argument which should be 26 or more bytes long, instead of using a global static buffer.

ctime_rz() is similar to ctime_r(), but it also takes a const timezone_t argument, returned by a previous call to tzalloc().

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).

localtime_rz() is similar to localtime_r(), but it also takes a const timezone_t argument, returned by a previous call to tzalloc().

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

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; in this case it does not use a consistent rule and may give a different answer when later presented with the same argument.) 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 (time_t)-1.

mktime_z() is similar to mktime() but it also takes a const timezone_t argument, returned by a previous call to tzalloc().

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

tzalloc() takes as an argument a timezone name and returns a timezone_t object suitable to be used in ctime_rz(), localtime_rz(), and mktime_z(). Instead of setting the environment variable TZ, and globally changing the behavior of the calling program, one can use multiple timezones at the same time by using separate timezone_t objects allocated by tzalloc() and calling the “z” variants of the functions.

tzgetname() returns the name for the given tz. If isdst is 0, the call is equivalent to tzname[0]. If isdst is set to 1 the call is equivalent to tzname[1].

tzfree() frees the tz argument previously returned by tzalloc.

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. The tm_zone field will become invalid and point to freed storage if the corresponding struct tm was returned by localtime_rz() and the const timezone_t tz argument has been freed by tzfree().

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.

RETURN VALUES
On success the asctime() and ctime() functions return a pointer to a static character buffer, and the asctime_r(), ctime_r(), and ctime_rz() function return a pointer to the user-supplied buffer. On failure they all return NULL and no errors are defined for them. On success the gmtime(), and localtime() functions return a pointer to a statically allocated struct tm whereas the gmtime_r(), localtime_r(), and localtime_rz(), functions return a pointer to the user-supplied struct tm. On failure they all return NULL and the global variable errno is set to indicate the error. The mktime() and mktime_z() function returns the specified time since the Epoch as a time_t type value. If the time cannot be represented, then mktime() and mktime_z() return (time_t)-1 setting the global variable errno to indicate the error. The tzalloc() function returns a pointer to a timezone_t object or NULL on failure, setting errno to indicate the error. tzgetzone() function returns string containing the name of the timezone given in tz.

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.

ERRORS
The gmtime_r(), localtime_r(), localtime_rz(), gmtime(), localtime(), and mktime(), and mktime_z() will fail when:

[EINVAL]
    The result cannot be represented.

[EOVERFLOW]
    The result cannot be represented.

All functions that return values except their “z” variants, can also return the same errors as open(2) and malloc(3).

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)).

asctime() and ctime() behave strangely for years before 1000 or after 9999. The 1989 and 1999 editions of the C Standard say that years from -99 through 999 are converted without extra spaces, but this conflicts with longstanding tradition and with this implementation. Traditional implementations of these two functions are restricted to years in the range 1900 through 2099. To avoid this portability mess, new programs should use strftime() instead.

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