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 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.
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 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.
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.