ctime(3C) UNIX System V(C Programming Language Utilities) ctime(3C)
NAME
ctime, localtime, gmtime, asctime, tzset - convert date and time to
string
SYNOPSIS
#include <time.h>
char *ctime (const timet *clock);
struct tm *localtime (const timet *clock);
struct tm *gmtime (const timet *clock);
char *asctime (const struct tm *tm);
extern timet timezone, altzone;
extern int daylight;
extern char *tzname[2];
void tzset (void);
DESCRIPTION
ctime, localtime, and gmtime accept arguments of type timet, pointed to
by clock, representing the time in seconds since 00:00:00 UTC, January 1,
1970. ctime returns a pointer to a 26-character string as shown below.
Time zone and daylight savings corrections are made before the string is
generated. The fields are constant in width:
Fri Sep 13 00:00:00 1986\n\0
localtime and gmtime return pointers to tm structures, described below.
localtime corrects for the main time zone and possible alternate
(``daylight savings'') time zone; gmtime converts directly to Coordinated
Universal Time (UTC), which is the time the UNIX system uses internally.
asctime converts a tm structure to a 26-character string, as shown in the
above example, and returns a pointer to the string.
Declarations of all the functions and externals, and the tm structure,
are in the time.h header file. The structure declaration is:
struct tm {
int tmsec; /* seconds after the minute - [0, 61] */
/* for leap seconds */
int tmmin; /* minutes after the hour - [0, 59] */
int tmhour; /* hour since midnight - [0, 23] */
int tmmday; /* day of the month - [1, 31] */
int tmmon; /* months since January - [0, 11] */
int tmyear; /* years since 1900 */
int tmwday; /* days since Sunday - [0, 6] */
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int tmyday; /* days since January 1 - [0, 365] */
int tmisdst; /* flag for alternate daylight */
/* savings time */
};
The value of tmisdst is positive if daylight savings time is in effect,
zero if daylight savings time is not in effect, and negative if the
information is not available. (Previously, the value of tmisdst was
defined as non-zero if daylight savings time was in effect.)
The external timet variable altzone contains the difference, in seconds,
between Coordinated Universal Time and the alternate time zone. The
external variable timezone contains the difference, in seconds, between
UTC and local standard time. The external variable daylight indicates
whether time should reflect daylight savings time. Both timezone and
altzone default to 0 (UTC). The external variable daylight is non-zero
if an alternate time zone exists. The time zone names are contained in
the external variable tzname, which by default is set to:
char *tzname[2] = { "GMT", " " };
These functions know about the peculiarities of this conversion for
various time periods for the U.S. (specifically, the years 1974, 1975,
and 1987). They will handle the new daylight savings time starting with
the first Sunday in April, 1987.
tzset uses the contents of the environment variable TZ to override the
value of the different external variables. The function tzset is called
by asctime and may also be called by the user. See environ(5) for a
description of the TZ environment variable.
tzset scans the contents of the environment variable and assigns the
different fields to the respective variable. For example, the most
complete setting for New Jersey in 1986 could be
EST5EDT4,116/2:00:00,298/2:00:00
or simply
EST5EDT
An example of a southern hemisphere setting such as the Cook Islands
could be
KDT9:30KST10:00,63/5:00,302/20:00
In the longer version of the New Jersey example of TZ, tzname[0] is EST,
timezone will be set to 5*60*60, tzname[1] is EDT, altzone will be set to
4*60*60, the starting date of the alternate time zone is the 117th day at
2 AM, the ending date of the alternate time zone is the 299th day at 2 AM
(using zero-based Julian days), and daylight will be set positive.
Starting and ending times are relative to the alternate time zone. If
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ctime(3C) UNIX System V(C Programming Language Utilities) ctime(3C)
the alternate time zone start and end dates and the time are not
provided, the days for the United States that year will be used and the
time will be 2 AM. If the start and end dates are provided but the time
is not provided, the time will be 2 AM. The effects of tzset are thus to
change the values of the external variables timezone, altzone, daylight,
and tzname. ctime, localtime, mktime, and strftime will also update
these external variables as if they had called tzset at the time
specified by the timet or struct tm value that they are converting.
Note that in most installations, TZ is set to the correct value by
default when the user logs on, via the local /etc/profile file [see
profile(4) and timezone(4)].
FILES
/usr/lib/locale/language/LCTIME - file containing locale specific date
and time information
SEE ALSO
time(2), getenv(3C), mktime(3C), putenv(3C), printf(3S), setlocale(3C),
strftime(3C), cftime(4), profile(4), timezone(4), environ(5).
NOTES
The return values for ctime, localtime, and gmtime point to static data
whose content is overwritten by each call.
Setting the time during the interval of change from timezone to altzone
or vice versa can produce unpredictable results. The system
administrator must change the Julian start and end days annually.
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