libc/time/ctime.3

.\" $NetBSD: ctime.3,v 1.46 2011/11/02 23:06:08 christos Exp $
.\"
.\" XXX: License missing?
.\"
.Dd November 2, 2011
.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 asctime "const struct tm *tm"
.Ft char *
.Fn asctime_r "const struct tm restrict tm" "char * restrict buf"
.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 struct tm *
.Fn gmtime "const time_t *clock"
.Ft struct tm *
.Fn gmtime_r "const time_t * restrict clock" "struct tm * restrict result"
.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 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 void
.Fn tzfree "const timezone_t tz"
.Ft const char *
.Fn tzgetname "const timezone_t tz" "int isdst"
.Sh DESCRIPTION
The
.Nm
family of functions provide various standard library routines
to operate with time and conversions related to time.
.Sh FUNCTIONS
.Bl -tag -width abcd
.It Fn asctime "tm"
The
.Fn asctime
function converts a time value contained in the
.Fa tm
structure to a string with the following general format:
.Bd -literal -offset indent
.D1 Thu Nov 24 18:22:48 1986\en\e0
.Ed
.Pp
The
.Fa tm
structure is described in
.Xr tm 3 .
.It Fn asctime_r "tm" "buf"
The
.Fn asctime_r
has the same behavior as
.Fn asctime ,
but the result is stored to
.Fa buf ,
which should have a size of at least 26 bytes.
.It Fn ctime "clock"
The
.Fn ctime
function 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 with the format described above.
Years requiring fewer than four characters are padded with leading zeroes.
For years longer than four characters, the string is of the form
.Bd -literal -offset indent
.D1 "Thu Nov 24 18:22:48     81986\en\e0"
.Ed
.Pp
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.
.It Fn ctime_r "clock" "buf"
The
.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.
.It Fn ctime_rz "tz" "clock" "buf"
The
.Fn ctime_rz
function is similar to
.Fn ctime_r ,
but it also takes a
.Ft "const timezone_t"
argument, as returned by a previous call to
.Fn tzalloc .
.It Fn difftime "time1" "time2"
The
.Fn difftime
function returns the difference between two calendar times,
.Fa ( time1 No - Fa time0 ) ,
expressed in seconds.
.It Fn gmtime "clock"
The
.Fn gmtime
function converts to Coordinated Universal Time
.Pq Tn UTC
and returns a pointer to the
.Va tm
structure described in
.Xr tm 3 .
.It Fn gmtime_r "clock" "result"
The
.Fn gmtime_r
provides the same functionality as
.Fn gmtime ,
differing in that the caller must supply a buffer area
.Fa result
to which the result is stored.
.It Fn localtime "clock"
Also
.Fn localtime
is comparable to
.Fn gmtime .
However,
.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, the function sets the
.Fa tm_isdst Ns 'th
element of
.Fa tzname
to a pointer to an
ASCII string that is the time zone abbreviation to be used with
.Fn localtime Ns 's
return value.
.It Fn localtime_r "clock" "result"
As
.Fn gmtime_r ,
the
.Fn localtime_r
takes an additional buffer
.Fa result
as a parameter and stores the result to it.
Note however that
.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 .
.It Fn localtime_rz "tz" "clock" "result"
The
.Fn localtime_rz
function is similar to
.Fn localtime_r ,
but it also takes a
.Ft "const timezone_t"
argument, returned by a previous call to
.Fn tzalloc .
.It Fn mktime "tm"
The
.Fn mktime
function converts the broken-down time,
expressed as local time in the
.Xr tm 3
structure, into a calendar time value with
the same encoding as that of the values returned by the
.Xr time 3
function.
The following remarks should be taken into account.
.Bl -bullet
.It
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.
.It
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.
.It
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.
.El
.Pp
The function returns the specified calendar time;
if the calendar time cannot be represented, it returns
.Va "(time_t)-1" .
This can happen either because the resulting conversion would not fit
in a
.Vt time_t
variable, or because the time specified happens to be in the daylight
savings gap and
.Fa tm_isdst
was set to
.Dv \-1 .
Other
.Fn mktime
implementations do not return an error in the second case and return
the appropriate time offset after the daylight savings gap.
There is code to mimick this behavior, but it is not enabled by default.
.It Fn mktime_z "tz" "tm"
The
.Fn mktime_z
function is similar to
.Fn mktime
but it also takes a
.Ft "const timezone_t"
argument, returned by a previous call to
.Fn tzalloc .
.It Fn tzalloc "zone"
The
.Fn tzalloc
function takes as an argument a timezone name and returns a
.Ft timezone_t
object suitable to be used in the
.Fn ctime_rz ,
.Fn localtime_rz ,
and
.Fn mktime_z
functions.
.Pp
Note that 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.
.It Fn tzfree "tz"
The
.Fn tzfree
function deallocates
.Fa tz ,
which was previously allocated by
.Fn tzalloc .
.It Fn "tzgetname"
Finally,
.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] .
.El
.Sh RETURN VALUES
.Bl -bullet
.It
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.
.It
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.
.It
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.
.It
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.
.It
.Fn tzgetzone
function returns string containing the name of the timezone given in
.Fa tz .
.El
.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 described functions may fail with
.Bl -tag -width Er
.It Bq Er EINVAL
The result cannot be represented because a parameter is incorrect, or
the conversion failed because no such time exists (for example a time
in the DST gap).
.It Bq Er EOVERFLOW
The result cannot be represented because the time requested is out of bounds
and the time calculation resulted in overflow.
.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 tm 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 .
Rest of the functions conform to
.St -p1003.1-2008 .
.Sh CAVEATS
The functions that do not take an explicit
.Ft timezone_t
argument return values point to static data; the data is overwritten by
each call.
For the above functions 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 ) .
The functions that do take an explicit
.Ft timezone_t
argument and set the fields of a supplied
.Va "struct tm"
should not call
.Fn tzfree
since the
.Fa tm_zone
field of the
.Va "struct tm"
points to data allocated by
.Fn tzalloc .
.Pp
The
.Fn asctime
and
.Fn ctime
functions 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.