libc/stdtime/ctime.3

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.\"     From: @(#)ctime.3	8.1 (Berkeley) 6/4/93
.\"
.Dd March 26, 2024
.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 ,
.Nm timegm
.Nd transform binary date and time values
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Vt extern char *tzname[2] ;
.Ft char *
.Fn asctime "const struct tm *tm"
.Ft char *
.Fn asctime_r "const struct tm *tm" "char *buf"
.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 struct tm *
.Fn gmtime "const time_t *clock"
.Ft struct tm *
.Fn gmtime_r "const time_t *clock" "struct tm *result"
.Ft struct tm *
.Fn localtime "const time_t *clock"
.Ft struct tm *
.Fn localtime_r "const time_t *clock" "struct tm *result"
.Ft time_t
.Fn mktime "struct tm *tm"
.Ft time_t
.Fn timegm "struct tm *tm"
.Sh DESCRIPTION
The
.Fn ctime ,
.Fn gmtime ,
and
.Fn localtime
functions all take as argument a pointer to a time value representing
the time in seconds since the Epoch (00:00:00 UTC on January 1, 1970;
see
.Xr time 3 ) .
.Pp
The
.Fn localtime
function converts the time value pointed to by
.Fa clock ,
and returns a pointer to a
.Vt struct tm
(described below) which contains
the broken-out time information for the value after adjusting for the current
time zone (see
.Xr tzset 3 ) .
When the specified time translates to a year that will not fit in an
.Vt int ,
.Fn localtime
returns
.Dv NULL .
The
.Fn localtime
function uses
.Xr tzset 3
to initialize time conversion information if
.Xr tzset 3
has not already been called by the process.
.Pp
After filling in the
.Vt struct tm ,
.Fn localtime
sets the
.Va tm_isdst Ns 'th
element of
.Va tzname
to a pointer to an ASCII string that is the time zone abbreviation to be
used with
.Fn localtime Ns 's
return value.
.Pp
The
.Fn gmtime
function similarly converts the time value, but without any time zone
adjustment, and returns a pointer to a
.Vt struct tm .
.Pp
The
.Fn ctime
function
adjusts the time value for the current time zone in the same manner as
.Fn localtime ,
and returns a pointer to a 26-character string of the form:
.Bd -literal -offset indent
Thu Nov 24 18:22:48 1986\en\e0
.Ed
.Pp
All the fields have constant width.
.Pp
The
.Fn asctime
function converts the broken down time in the
.Vt struct tm
pointed to by
.Fa tm
to the form shown in the example above.
.Pp
The
.Fn ctime_r
and
.Fn asctime_r
functions
provide the same functionality as
.Fn ctime
and
.Fn asctime
except the caller must provide the output buffer
.Fa buf ,
which must be at least 26 characters long, to store the result in.
The
.Fn localtime_r
and
.Fn gmtime_r
functions provide the same functionality as
.Fn localtime
and
.Fn gmtime
respectively, except the caller must provide the output buffer
.Fa result .
.Pp
The functions
.Fn mktime
and
.Fn timegm
convert the broken-down time in the
.Vt struct tm
pointed to by
.Fa tm
into a time value with the same encoding as that of the values
returned by the
.Xr time 3
function (that is, seconds from the Epoch, UTC).
The
.Fn mktime
function interprets the input structure according to the current
timezone setting (see
.Xr tzset 3 )
while the
.Fn timegm
function interprets the input structure as representing Universal
Coordinated Time
.Pq UTC .
.Pp
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, and will be
normalized if needed.
For example,
October 40 is changed into November 9,
a
.Fa tm_hour
of \-1 means 1 hour before midnight,
.Fa tm_mday
of 0 means the day preceding the current month, and
.Fa tm_mon
of \-2 means 2 months before January of
.Fa tm_year .
(A positive or zero value for
.Fa tm_isdst
causes
.Fn mktime
to presume initially that summer time (for example, Daylight Saving Time)
is or is not in effect for the specified time, respectively.
A negative value for
.Fa tm_isdst
causes the
.Fn mktime
function to attempt to guess whether summer time is in effect for the
specified time.
The
.Fa tm_isdst
and
.Fa tm_gmtoff
members are forced to zero by
.Fn timegm . )
.Pp
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.
The
.Fn mktime
function
returns the specified calendar time; if the calendar time cannot be
represented, it returns \-1 and sets
.Xr errno 3
to an appropriate value.
.Pp
Note that \-1 is a valid result (representing one second before
midnight UTC on the evening of 31 December 1969), so this cannot be
relied upon to indicate success or failure; instead,
.Fa tm_wday
and / or
.Fa tm_yday
should be set to an out-of-bounds value (e.g. \-1) prior to calling
.Fn mktime
or
.Fn timegm
and checked after the call.
.Pp
The
.Fn difftime
function returns the difference in seconds between two time values,
.Fa time1
\-
.Fa time0 .
.Pp
External declarations as well as the definition of
.Vt struct tm
are in the
.In time.h
header.
The
.Vt tm
structure includes at least the following fields:
.Bd -literal -offset indent
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 */
.Ed
.Pp
The
.Fa tm_isdst
field is non-zero if summer time is in effect.
.Pp
The
.Fa tm_gmtoff
field is the offset in seconds of the time represented from UTC,
with positive values indicating a time zone ahead of UTC (east of the
Prime Meridian).
.Sh SEE ALSO
.Xr date 1 ,
.Xr clock_gettime 2 ,
.Xr gettimeofday 2 ,
.Xr getenv 3 ,
.Xr time 3 ,
.Xr tzset 3 ,
.Xr tzfile 5
.Sh STANDARDS
The
.Fn asctime ,
.Fn ctime ,
.Fn difftime ,
.Fn gmtime ,
.Fn localtime ,
and
.Fn mktime
functions conform to
.St -isoC ,
and conform to
.St -p1003.1-96
provided the selected local timezone does not contain a leap-second table
(see
.Xr zic 8 ) .
.Pp
The
.Fn asctime_r ,
.Fn ctime_r ,
.Fn gmtime_r ,
and
.Fn localtime_r
functions are expected to conform to
.St -p1003.1-96
(again provided the selected local timezone does not contain a leap-second
table).
.Pp
The
.Fn timegm
function is not specified by any standard; its function cannot be
completely emulated using the standard functions described above.
.Sh HISTORY
This manual page is derived from
the time package contributed to Berkeley by
.An Arthur Olson
and which appeared in
.Bx 4.3 .
.Pp
The functions
.Fn asctime ,
.Fn gmtime ,
and
.Fn localtime
first appeared in
.At v5 ,
.Fn difftime
and
.Fn mktime
in
.Bx 4.3 Reno ,
and
.Fn timegm
and
.Fn timelocal
in SunOS 4.0.
.Pp
The
.Fn asctime_r ,
.Fn ctime_r ,
.Fn gmtime_r
and
.Fn localtime_r
functions have been available since
.Fx 8.0 .
.Sh BUGS
Except for
.Fn difftime ,
.Fn mktime ,
and the
.Fn \&_r
variants of the other functions,
these functions leave their result in an internal static object and return
a pointer to that object.
Subsequent calls to these
function will modify the same object.
.Pp
The C Standard provides no mechanism for a program to modify its current
local timezone setting, and the POSIX-standard
method is not reentrant.
(However, thread-safe implementations are provided
in the POSIX threaded environment.)
.Pp
The
.Va tm_zone
field of a returned
.Vt tm
structure points to a static array of characters,
which will also be overwritten by any subsequent calls (as well as by
subsequent calls to
.Xr tzset 3 ) .
.Pp
Use of the external variable
.Fa tzname
is discouraged; the
.Fa tm_zone
entry in the tm structure is preferred.