CTIME(3) BSD Library Functions Manual CTIME(3)
asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r,
localtime, localtime_r, mktime -- convert date and time to ASCII
Standard C Library (libc, -lc)
extern char *tzname;
ctime(const time_t *clock);
ctime_r(const time_t *clock, char *buf);
difftime(time_t time1, time_t time0);
asctime(const struct tm *tm);
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);
mktime(struct tm *tm);
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() differ-
ing 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 adjust-
ments (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-char-
acter string, as shown in the above example, and returns a pointer to the
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 exam-
ple, 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 func-
tions 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.
/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
getenv(3), strftime(3), time(3), tzset(3), tzfile(5)
The ctime(), difftime(), asctime(), localtime(), gmtime() and mktime()
functions conform to ANSI X3.159-1989 (``ANSI C89'') The ctime_r(),
asctime_r(), localtime_r() and gmtime_r() functions conform to IEEE Std
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.
BSD March 31, 2001 BSD