libc/stdtime/ctime.3

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.\"     From: @(#)ctime.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd March 6, 2023
.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 ctime "const time_t *clock"
.Ft double
.Fn difftime "time_t time1" "time_t time0"
.Ft char *
.Fn asctime "const struct tm *tm"
.Ft struct tm *
.Fn localtime "const time_t *clock"
.Ft struct tm *
.Fn gmtime "const time_t *clock"
.Ft time_t
.Fn mktime "struct tm *tm"
.Ft time_t
.Fn timegm "struct tm *tm"
.Ft char *
.Fn ctime_r "const time_t *clock" "char *buf"
.Ft struct tm *
.Fn localtime_r "const time_t *clock" "struct tm *result"
.Ft struct tm *
.Fn gmtime_r "const time_t *clock" "struct tm *result"
.Ft char *
.Fn asctime_r "const struct tm *tm" "char *buf"
.Sh DESCRIPTION
The functions
.Fn ctime ,
.Fn gmtime
and
.Fn localtime
all take as an argument a time value representing the time in seconds since
the Epoch (00:00:00
.Tn UTC ,
January 1, 1970; see
.Xr time 3 ) .
.Pp
The function
.Fn localtime
converts the time value pointed at by
.Fa clock ,
and returns a pointer to a
.Dq Fa struct tm
(described below) which contains
the broken-out time information for the value after adjusting for the current
time zone (and any other factors such as Daylight Saving Time).
When the specified time translates to a year that will not fit in an
.Dv int ,
.Fn localtime
returns NULL.
Time zone adjustments are performed as specified by the
.Ev TZ
environment variable (see
.Xr tzset 3 ) .
The function
.Fn localtime
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 tm structure,
.Fn localtime
sets the
.Fa tm_isdst Ns 'th
element of
.Fa tzname
to a pointer to an
.Tn ASCII
string that is the time zone abbreviation to be
used with
.Fn localtime Ns 's
return value.
.Pp
The function
.Fn gmtime
similarly converts the time value, but without any time zone adjustment,
and returns a pointer to a tm structure (described below).
.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 ctime_r
function
provides the same functionality as
.Fn ctime
except the caller must provide the output buffer
.Fa buf
to store the result, which must be at least 26 characters long.
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
.Fn asctime
function
converts the broken down time in the structure
.Fa tm
pointed at by
.Fa *tm
to the form
shown in the example above.
.Pp
The
.Fn asctime_r
function
provides the same functionality as
.Fn asctime
except the caller provide the output buffer
.Fa buf
to store the result, which must be at least 26 characters long.
.Pp
The functions
.Fn mktime
and
.Fn timegm
convert the broken-down time in the structure
pointed to by 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,
.Tn UTC ) .
The
.Fn mktime
function
interprets the input structure according to the current timezone setting
(see
.Xr tzset 3 ) .
The
.Fn timegm
function
interprets the input structure as representing Universal Coordinated Time
.Pq Tn 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 divine 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;
.Pp
The
.Fn difftime
function
returns the difference between two calendar times,
.Pf ( Fa time1
-
.Fa time0 ) ,
expressed in seconds.
.Pp
External declarations as well as the tm structure definition are in the
.In time.h
include file.
The 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
field
.Fa tm_isdst
is non-zero if summer time is in effect.
.Pp
The field
.Fa tm_gmtoff
is the offset (in seconds) of the time represented from
.Tn UTC ,
with positive
values indicating 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 functions
.Fn asctime_r ,
.Fn ctime_r ,
.Fn gmtime_r ,
and
.Fn localtime_r
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
.Tn POSIX Ns No \&-standard
method is not reentrant.
(However, thread-safe implementations are provided
in the
.Tn 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.