localtime(3)
- NetBSD Manual Pages
CTIME(3) NetBSD Programmer's Manual CTIME(3)
NAME
asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r,
localtime, localtime_r, mktime - convert date and time to ASCII
LIBRARY
Standard C Library (libc, -lc)
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() 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 be-
low. 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
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 en-
coding as that of the values returned by the time(3) function. The orig-
inal values of the tm_wday and tm_yday components of the structure are
ignored, and the original values of the other components are not re-
stricted to their normal ranges. (A positive or zero value for tm_isdst
causes mktime() to presume initially that summer time (for example, Day-
light 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 ar-
rangements 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 ANSI X3.159-1989 (``ANSI C'') The ctime_r(),
asctime_r(), localtime_r() and gmtime_r() functions conform to IEEE Std
1003.1c-1995 (``POSIX'').
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.
NetBSD 1.6 March 31, 2001 3
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