asctime, asctime_r, ctime, ctime_r, difftime, gmtime,
gmtime_r,
localtime, localtime_r, mktime, timegm, timelocal - convert
date and time
to ASCII
#include <sys/types.h>
#include <time.h>
extern char *tzname[2];
void
tzset(void);
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 *tm, char *buf);
struct tm *
localtime(const time_t *clock);
struct tm *
localtime_r(const time_t *clock, struct tm *result);
struct tm *
gmtime(const time_t *clock);
struct tm *
gmtime_r(const time_t *clock, struct tm *result);
time_t
mktime(struct tm *tm);
time_t
timegm(struct tm *tm);
time_t
timelocal(struct tm *tm);
The ctime() function converts a time_t, 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
All the fields have constant width.
The ctime_r() function converts the calendar time pointed to
by clock to
local time in exactly the same way as ctime() and puts the
string into
the array pointed to by buf (which contains at least 26
bytes) and returns
buf. Unlike ctime(), the thread-safe version
ctime_r() is not required
to set tzname.
The localtime() and gmtime() functions 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 the return value of localtime().
gmtime() converts to Coordinated Universal Time.
The localtime_r() and gmtime_r() functions convert the calendar time
pointed to by clock into a broken-down time in exactly the
same way as
their non-reentrant counterparts, localtime() and gmtime(),
but instead
store the result directly into the structure pointed to by
result. Unlike
localtime(), the reentrant version is not required to
set tzname.
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. asctime_r() uses the buffer pointed to by buf
(which should contain
at least 26 bytes) and then returns buf.
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() 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.
timelocal() is a deprecated interface that is equivalent to
calling
mktime() with a negative value for tm_isdst.
timegm() is a deprecated interface that converts the brokendown time, as
returned by gmtime(), into a calendar time value with the
same encoding
as that of the values returned by the time() function.
difftime() returns the difference between two calendar
times, (time1 -
time0), expressed in seconds.
Declarations of all the functions and externals, and the tm
structure,
are in the <time.h> header file. The structure (of type)
struct tm includes
the following fields:
int tm_sec; /* seconds (0 - 60) */
int tm_min; /* minutes (0 - 59) */
int tm_hour; /* hours (0 - 23) */
int tm_mday; /* day of month (1 - 31) */
int tm_mon; /* month of year (0 - 11) */
int tm_year; /* year - 1900 */
int tm_wday; /* day of week (Sunday = 0) */
int tm_yday; /* day of year (0 - 365) */
int tm_isdst; /* is summer time in effect?
*/
char *tm_zone; /* abbreviation of timezone
name */
long tm_gmtoff; /* offset from UTC in seconds
*/
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.
/usr/share/zoneinfo time zone information directory
/etc/localtime local time zone file
/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.
getenv(3), strftime(3), time(3), tzset(3), tzfile(5), zic(8)
The return values of the non re-entrant functions 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()).
The default system time zone may be set by running ``zic -l
timezone'' as
the superuser.
Avoid using out-of-range values with mktime() when setting
up lunch with
promptness sticklers in Riyadh.
OpenBSD 3.6 February 16, 1999
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