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newctime.3.txt
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newctime.3.txt
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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,
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 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 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.
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.
Avoid using out-of-range values with mktime when setting up
lunch with promptness sticklers in Riyadh.