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NEWCTIME(3) Library Functions Manual NEWCTIME(3)
NAME
asctime, ctime, difftime, gmtime, localtime, mktime - convert date and
time to ASCII
SYNOPSIS
extern char *tzname[2];
void tzset()
#include <sys/types.h>
char *ctime(clock)
const time_t *clock;
double difftime(time1, time0)
time_t time1;
time_t time0;
#include <time.h>
char *asctime(tm)
const struct tm *tm;
struct tm *localtime(clock)
const time_t *clock;
struct tm *gmtime(clock)
const time_t *clock;
time_t mktime(tm)
struct tm *tm;
cc ... -ltz
DESCRIPTION
Ctime converts a long integer, pointed to by clock, 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.
The *clock time stamp represents the time in seconds since 1970-01-01
00:00:00 Coordinated Universal Time (UTC). The POSIX standard says
that time stamps must be nonnegative and must ignore leap seconds.
Many implementations extend POSIX by allowing negative time stamps, and
can therefore represent time stamps that predate the introduction of
UTC and are some other flavor of Universal Time (UT). Some
implementations support leap seconds, in contradiction to POSIX.
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 United States). 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.
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 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 -1.
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 UT 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 UT,
with positive values indicating east of the Prime Meridian. The
field's name is derived from Greenwich Mean Time, a precursor of UT.
FILES
/usr/local/etc/zoneinfo time zone information directory
/usr/local/etc/zoneinfo/localtime local time zone file
/usr/local/etc/zoneinfo/posixrules used with POSIX-style TZ's
/usr/local/etc/zoneinfo/GMT for UTC leap seconds
If /usr/local/etc/zoneinfo/GMT is absent, UTC leap seconds are loaded
from /usr/local/etc/zoneinfo/posixrules.
SEE ALSO
getenv(3), newstrftime(3), newtzset(3), time(2), tzfile(5)
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).
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.
NEWCTIME(3)
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