.\"	$NetBSD: ctime.3,v 1.24 2001/09/16 01:35:53 wiz Exp $
.Dd March 31, 2001
.Dt CTIME 3
.Os
.Sh NAME
.Nm asctime ,
.Nm asctime_r ,
.Nm ctime ,
.Nm ctime_r ,
.Nm difftime ,
.Nm gmtime ,
.Nm gmtime_r ,
.Nm localtime ,
.Nm localtime_r ,
.Nm mktime
.Nd convert date and time to ASCII
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <time.h>
.Dv extern char *tzname[2];
.Ft char *
.Fn ctime "const time_t *clock"
.Ft char *
.Fn ctime_r "const time_t *clock"  "char *buf"
.Ft double
.Fn difftime "time_t time1" "time_t time0"
.Ft char *
.Fn asctime "const struct tm *tm"
.Ft char *
.Fn asctime_r "const struct tm restrict tm" "char * restrict buf"
.Ft struct tm *
.Fn localtime "const time_t *clock"
.Ft struct tm *
.Fn localtime_r "const time_t * restrict clock" "struct tm * restrict result"
.Ft struct tm *
.Fn gmtime "const time_t *clock"
.Ft struct tm *
.Fn gmtime_r "const time_t * restrict clock" "struct tm * restrict result"
.Ft time_t
.Fn mktime "struct tm *tm"
.Sh DESCRIPTION
.Fn ctime
converts a long integer, pointed to by
.Fa 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
.Dl Thu Nov 24 18:22:48 1986\en\e0
All the fields have constant width.
.Pp
The
.Fn ctime_r
function provides the same functionality as
.Fn ctime
differing in that the caller must supply a buffer area
.Fa buf
with a size of at least 26 bytes, in which the result is stored.
.Pp
.Fn localtime
and
.Fn gmtime
return pointers to
.Va tm
structures, described below.
.Fn 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
.Va tm
structure,
.Fn localtime
sets the
.Fa tm_isdst Ns 'th
element of
.Fa tzname
to a pointer to an
ASCII string that's the time zone abbreviation to be used with
.Fn localtime Ns 's
return value.
.Pp
.Fn gmtime
converts to Coordinated Universal Time.
.Pp
The
.Fn gmtime_r
and
.Fn localtime_r
functions provide the same functionality as
.Fn gmtime
and
.Fn localtime
differing in that the caller must supply a buffer area
.Fa result
in which the result is stored; also,
.Fn localtime_r
does not imply initialization of the local time conversion information;
the application may need to do so by calling
.Xr tzset 3 .
.Pp
.Fn asctime
converts a time value contained in a
.Va tm
structure to a 26-character string, as shown in the above example,
and returns a pointer to the string.
.Pp
The
.Fn asctime_r
function provides the same functionality as
.Fn asctime
differing in that the caller must supply a buffer area
.Fa buf
with a size of at least 26 bytes, in which the result is stored.
.Pp
.Fn mktime
converts the broken-down time, expressed as local time,
in the structure pointed to by
.Fa tm
into a calendar time value with the same encoding as that of the values
returned by the
.Xr time 3
function.
The original values of the
.Fa tm_wday
and
.Fa 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
.Fa tm_isdst
causes
.Fn 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
.Fa tm_isdst
causes the
.Fn mktime
function to attempt to divine whether summer time is in effect
for the specified time.)
On successful completion, the values of the
.Fa tm_wday
and
.Fa 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
.Fa tm_mday
is not set until
.Fa tm_mon
and
.Fa tm_year
are determined.
.Fn mktime
returns the specified calendar time; if the calendar time cannot be
represented, it returns -1.
.Pp
.Fn difftime
returns the difference between two calendar times,
.Fa ( time1 No - Fa time0 ) ,
expressed in seconds.
.Pp
The structure (of type)
.Va "struct tm"
includes the following fields:
.Bd -literal -offset indent
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 */
.Ed
.Pp
The
.Fa tm_zone
and
.Fa 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.
.Pp
.Fa tm_isdst
is non-zero if summer time is in effect.
.Pp
.Fa tm_gmtoff
is the offset (in seconds) of the time represented
from UTC, with positive values indicating east
of the Prime Meridian.
.Sh FILES
.Bl -tag -width /usr/share/zoneinfo/posixrules -compact
.It Pa /etc/localtime
local time zone file
.It Pa /usr/share/zoneinfo
time zone information directory
.It Pa /usr/share/zoneinfo/posixrules
used with POSIX-style TZ's
.It Pa /usr/share/zoneinfo/GMT
for UTC leap seconds
.El
.Pp
If
.Pa /usr/share/zoneinfo/GMT
is absent, UTC leap seconds are loaded from
.Pa /usr/share/zoneinfo/posixrules .
.Sh SEE ALSO
.Xr getenv 3 ,
.Xr strftime 3 ,
.Xr time 3 ,
.Xr tzset 3 ,
.Xr tzfile 5
.Sh STANDARDS
The
.Fn ctime ,
.Fn difftime ,
.Fn asctime ,
.Fn localtime ,
.Fn gmtime
and
.Fn mktime
functions conform to
.St -ansiC
The
.Fn ctime_r ,
.Fn asctime_r ,
.Fn localtime_r
and
.Fn gmtime_r
functions conform to
.St -p1003.1c-95 .
.Sh NOTES
The return values point to static data; the data is overwritten by
each call.  The
.Fa tm_zone
field of a returned
.Va "struct tm"
points to a static array of characters, which will also be overwritten
at the next call (and by calls to
.Xr tzset 3 ) .
.Pp
Avoid using out-of-range values with
.Fn mktime
when setting up lunch with promptness sticklers in Riyadh.
.\" @(#)newctime.3     7.14