libc/time/ctime.3

.\" $NetBSD: ctime.3,v 1.38 2011/01/24 19:37:35 njoly Exp $
.Dd December 14, 2010
.Dt CTIME 3
.Os
.Sh NAME
.Nm asctime ,
.Nm asctime_r ,
.Nm ctime ,
.Nm ctime_r ,
.Nm ctime_rz ,
.Nm difftime ,
.Nm gmtime ,
.Nm gmtime_r ,
.Nm localtime ,
.Nm localtime_r ,
.Nm localtime_rz ,
.Nm mktime ,
.Nm mktime_z ,
.Nm tzalloc ,
.Nm tzgetname ,
.Nm tzfree ,
.Nd convert date and time to ASCII
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Vt extern char *tzname[2];
.Ft char *
.Fn ctime "const time_t *clock"
.Ft char *
.Fn ctime_r "const time_t *clock"  "char *buf"
.Ft char *
.Fn ctime_rz "const timezone_t tz" "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 localtime_rz "const timezone_t tz" "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"
.Ft time_t
.Fn mktime_z "const timezone_t tz" "struct tm *tm"
.Ft timezone_t
.Fn tzalloc "const char *zone"
.Ft const char *
.Fn tzgetname "const timezone_t tz" "int isdst"
.Ft void
.Fn tzfree "const timezone_t tz"
.Sh DESCRIPTION
.Fn ctime
converts a
.Vt time_t ,
pointed to by
.Fa clock ,
representing the time in seconds since
00:00:00 UTC, 1970-01-01,
and returns a pointer to a
string of the form
.D1 Thu Nov 24 18:22:48 1986\en\e0
Years requiring fewer than four characters are padded with leading zeroes.
For years longer than four characters, the string is of the form
.D1 Thu Nov 24 18:22:48     81986\en\e0
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.
.Pp
.Fn ctime_r
is similar to
.Fn ctime ,
except it places the result of the conversion on the
.Fa buf
argument which should be 26 or more bytes long, instead of using a global
static buffer.
.Pp
.Fn ctime_rz
is similar to
.Fn ctime_r ,
but it also takes a
.Ft "const timezone_t"
argument, returned by a previous call to
.Fn tzalloc .
.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 localtime_rz
is similar to
.Fn localtime_r ,
but it also takes a
.Ft "const timezone_t"
argument, returned by a previous call to
.Fn tzalloc .
.Pp
.Fn asctime
converts a time value contained in a
.Dq tm
structure to a string,
as shown in the above example,
and returns a pointer to the string.
.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; 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
.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
.Va "(time_t)-1" .
.Pp
.Fn mktime_z
is similar to
.Fn mktime
but it also takes a
.Ft "const timezone_t"
argument, returned by a previous call to
.Fn tzalloc .
.Pp
.Fn difftime
returns the difference between two calendar times,
.Fa ( time1 No - Fa time0 ) ,
expressed in seconds.
.Pp
.Fn tzalloc
takes as an argument a timezone name and returns a
.Ft timezone_t
object suitable to be used in
.Fn ctime_rz ,
.Fn localtime_rz ,
and
.Fn mktime_z .
Instead of setting the environment variable
.Va TZ ,
and globally changing the behavior of the calling program, one can use
multiple timezones at the same time by using separate
.Ft timezone_t
objects allocated by
.Fn tzalloc
and calling the
.Dq z
variants of the functions.
.Pp
.Fn tzgetname
returns the name for the given
.Fa tz .
If
.Fa isdst
is
.Va 0 ,
the call is equivalent to
.Va tzname[0] .
If
.Fa isdst
is set to
.Va 1
the call is equivalent to
.Va tzname[1] .
.Pp
.Fn tzfree
frees the
.Fa tz
argument previously returned by
.Fa tzalloc .
.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.
The
.Fa tm_zone
field will become invalid and point to freed storage if the corresponding
.Va "struct tm"
was returned by
.Fn localtime_rz
and the
.Ft "const timezone_t"
.Fa tz
argument has been freed by
.Fn tzfree .
.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 RETURN VALUES
On success the
.Fn asctime
and
.Fn ctime
functions return a pointer to a static character buffer, and the
.Fn asctime_r ,
.Fn ctime_r ,
and
.Fn ctime_rz
function return a pointer to the user-supplied buffer.
On failure they all return
.Dv NULL
and no errors are defined for them.
On success the
.Fn gmtime ,
and
.Fn localtime
functions return a pointer to a statically allocated
.Va "struct tm"
whereas the
.Fn gmtime_r ,
.Fn localtime_r ,
and
.Fn localtime_rz ,
functions return a pointer to the user-supplied
.Va "struct tm" .
On failure they all return
.Dv NULL
and the global variable
.Va errno
is set to indicate the error.
The
.Fn mktime
and
.Fn mktime_z
function returns the specified time since the Epoch as a
.Vt time_t
type value.
If the time cannot be represented, then
.Fn mktime
and
.Fn mktime_z
return
.Va "(time_t)-1"
setting the global variable
.Va errno
to indicate the error.
The
.Fn tzalloc
function returns a pointer to a
.Ft timezone_t
object or
.Dv NULL
on failure, setting
.Va errno
to indicate the error.
.Fn tzgetzone
function returns string containing the name of the timezone given in
.Fa tz .
.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 ERRORS
The
.Fn gmtime_r ,
.Fn localtime_r ,
.Fn localtime_rz ,
.Fn gmtime ,
.Fn localtime ,
and
.Fn mktime ,
and
.Fn mktime_z
will fail when:
.Bl -tag -width Er
.It Bq Er EINVAL
The result cannot be represented.
.It Bq Er EOVERFLOW
The result cannot be represented.
.El
.Pp
All functions that return values except their
.Dq z
variants, can also return the same errors as
.Xr open 2
and
.Xr malloc 3 .
.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
.Fn asctime
and
.Fn 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
.Fn strftime
instead.
.Pp
Avoid using out-of-range values with
.Fn mktime
when setting up lunch with promptness sticklers in Riyadh.
.\" @(#)newctime.3	8.3
.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.