path: root/man3/ctime.3

'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 19:49:27 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Fri Apr 26 12:38:55 MET DST 1996 by Martin Schulze (joey@linux.de)
.\" Modified 2001-11-13, aeb
.\" Modified 2001-12-13, joey, aeb
.\" Modified 2004-11-16, mtk
.\"
.TH ctime 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
localtime_r \- transform date and time to broken-down time or ASCII
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <time.h>
.PP
.BI "char *asctime(const struct tm *" tm );
.BI "char *asctime_r(const struct tm *restrict " tm ,
.BI "                    char " buf "[restrict 26]);"
.PP
.BI "char *ctime(const time_t *" timep );
.BI "char *ctime_r(const time_t *restrict " timep ,
.BI "                    char " buf "[restrict 26]);"
.PP
.BI "struct tm *gmtime(const time_t *" timep );
.BI "struct tm *gmtime_r(const time_t *restrict " timep ,
.BI "                    struct tm *restrict " result );
.PP
.BI "struct tm *localtime(const time_t *" timep );
.BI "struct tm *localtime_r(const time_t *restrict " timep ,
.BI "                    struct tm *restrict " result );
.PP
.BI "time_t mktime(struct tm *" tm );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR asctime_r (),
.BR ctime_r (),
.BR gmtime_r (),
.BR localtime_r ():
.nf
    _POSIX_C_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
The
.BR ctime (),
.BR gmtime (),
and
.BR localtime ()
functions all take
an argument of data type \fItime_t\fP, which represents calendar time.
When interpreted as an absolute time value, it represents the number of
seconds elapsed since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
.PP
The
.BR asctime ()
and
.BR mktime ()
functions both take an argument
representing broken-down time, which is a representation
separated into year, month, day, and so on.
.PP
Broken-down time is stored in the structure
.IR tm ,
described in
.BR tm (3type).
.PP
The call
.BI ctime( t )
is equivalent to
.BI asctime(localtime( t )) \fR.
It converts the calendar time \fIt\fP into a
null-terminated string of the form
.PP
.in +4n
.EX
"Wed Jun 30 21:49:08 1993\en"
.EE
.in
.PP
The abbreviations for the days of the week are "Sun", "Mon", "Tue", "Wed",
"Thu", "Fri", and "Sat".
The abbreviations for the months are "Jan",
"Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", and
"Dec".
The return value points to a statically allocated string which
might be overwritten by subsequent calls to any of the date and time
functions.
The function also sets the external
variables \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP (see
.BR tzset (3))
with information about the current timezone.
The reentrant version
.BR ctime_r ()
does the same, but stores the
string in a user-supplied buffer
which should have room for at least 26 bytes.
It need not
set \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP.
.PP
The
.BR gmtime ()
function converts the calendar time \fItimep\fP to
broken-down time representation, expressed in Coordinated Universal Time
(UTC).
It may return NULL when the year does not fit into an integer.
The return value points to a statically allocated struct which might be
overwritten by subsequent calls to any of the date and time functions.
The
.BR gmtime_r ()
function does the same, but stores the data in a
user-supplied struct.
.PP
The
.BR localtime ()
function converts the calendar time \fItimep\fP to
broken-down time representation,
expressed relative to the user's specified timezone.
The function acts as if it called
.BR tzset (3)
and sets the external variables \fItzname\fP with
information about the current timezone, \fItimezone\fP with the difference
between Coordinated Universal Time (UTC) and local standard time in
seconds, and \fIdaylight\fP to a nonzero value if daylight savings
time rules apply during some part of the year.
The return value points to a statically allocated struct which might be
overwritten by subsequent calls to any of the date and time functions.
The
.BR localtime_r ()
function does the same, but stores the data in a
user-supplied struct.
It need not set \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP.
.PP
The
.BR asctime ()
function converts the broken-down time value
\fItm\fP into a null-terminated string with the same format as
.BR ctime ().
The return value points to a statically allocated string which might be
overwritten by subsequent calls to any of the date and time functions.
The
.BR asctime_r ()
function does the same, but stores the string in
a user-supplied buffer which should have room for at least 26 bytes.
.PP
The
.BR mktime ()
function converts a broken-down time structure, expressed
as local time, to calendar time representation.
The function ignores
the values supplied by the caller in the
.I tm_wday
and
.I tm_yday
fields.
The value specified in the
.I tm_isdst
field informs
.BR mktime ()
whether or not daylight saving time (DST)
is in effect for the time supplied in the
.I tm
structure:
a positive value means DST is in effect;
zero means that DST is not in effect;
and a negative value means that
.BR mktime ()
should (use timezone information and system databases to)
attempt to determine whether DST is in effect at the specified time.
.PP
The
.BR mktime ()
function modifies the fields of the
.I tm
structure as follows:
.I tm_wday
and
.I tm_yday
are set to values determined from the contents of the other fields;
if structure members are outside their valid interval, they will be
normalized (so that, for example, 40 October is changed into 9 November);
.I tm_isdst
is set (regardless of its initial value)
to a positive value or to 0, respectively,
to indicate whether DST is or is not in effect at the specified time.
Calling
.BR mktime ()
also sets the external variable \fItzname\fP with
information about the current timezone.
.PP
If the specified broken-down
time cannot be represented as calendar time (seconds since the Epoch),
.BR mktime ()
returns
.I (time_t)\ \-1
and does not alter the
members of the broken-down time structure.
.SH RETURN VALUE
On success,
.BR gmtime ()
and
.BR localtime ()
return a pointer to a
.IR "struct\ tm" .
.PP
On success,
.BR gmtime_r ()
and
.BR localtime_r ()
return the address of the structure pointed to by
.IR result .
.PP
On success,
.BR asctime ()
and
.BR ctime ()
return a pointer to a string.
.PP
On success,
.BR asctime_r ()
and
.BR ctime_r ()
return a pointer to the string pointed to by
.IR buf .
.PP
On success,
.BR mktime ()
returns the calendar time (seconds since the Epoch),
expressed as a value of type
.IR time_t .
.PP
On error,
.BR mktime ()
returns the value
.IR "(time_t)\ \-1" .
The remaining functions return NULL on error.
On error,
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EOVERFLOW
The result cannot be represented.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR asctime ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:asctime locale
T}
T{
.na
.nh
.BR asctime_r ()
T}	Thread safety	T{
.na
.nh
MT-Safe locale
T}
T{
.na
.nh
.BR ctime ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:tmbuf
race:asctime env locale
T}
T{
.na
.nh
.BR ctime_r (),
.BR gmtime_r (),
.BR localtime_r (),
.BR mktime ()
T}	Thread safety	T{
.na
.nh
MT-Safe env locale
T}
T{
.na
.nh
.BR gmtime (),
.BR localtime ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:tmbuf env locale
T}
.TE
.sp 1
.SH VERSIONS
POSIX doesn't specify the parameters of
.BR ctime_r ()
to be
.IR restrict ;
that is specific to glibc.
.PP
In many implementations, including glibc, a 0 in
.I tm_mday
is interpreted as meaning the last day of the preceding month.
.PP
According to POSIX.1-2001,
.BR localtime ()
is required to behave as though
.BR tzset (3)
was called, while
.BR localtime_r ()
does not have this requirement.
.\" See http://thread.gmane.org/gmane.comp.time.tz/2034/
For portable code,
.BR tzset (3)
should be called before
.BR localtime_r ().
.SH STANDARDS
.TP
.BR asctime ()
.TQ
.BR ctime ()
.TQ
.BR gmtime ()
.TQ
.BR localtime ()
.TQ
.BR mktime ()
C11, POSIX.1-2008.
.TP
.BR asctime_r ()
.TQ
.BR ctime_r ()
.TQ
.BR gmtime_r ()
.TQ
.BR localtime_r ()
POSIX.1-2008.
.SH HISTORY
.TP
.BR gmtime ()
.TQ
.BR localtime ()
.TQ
.BR mktime ()
C89, POSIX.1-2001.
.TP
.BR asctime ()
.TQ
.BR ctime ()
C89, POSIX.1-2001.
Marked obsolete in POSIX.1-2008 (recommending
.BR strftime (3)).
.TP
.BR gmtime_r ()
.TQ
.BR localtime_r ()
POSIX.1-2001.
.TP
.BR asctime_r ()
.TQ
.BR ctime_r ()
POSIX.1-2001.
Marked obsolete in POSIX.1-2008 (recommending
.BR strftime (3)).
.SH NOTES
The four functions
.BR asctime (),
.BR ctime (),
.BR gmtime (),
and
.BR localtime ()
return a pointer to static data and hence are not thread-safe.
The thread-safe versions,
.BR asctime_r (),
.BR ctime_r (),
.BR gmtime_r (),
and
.BR localtime_r (),
are specified by SUSv2.
.PP
POSIX.1-2001 says:
"The
.BR asctime (),
.BR ctime (),
.BR gmtime (),
and
.BR localtime ()
functions shall return values in one of two static objects:
a broken-down time structure and an array of type
.IR char .
Execution of any of the functions may overwrite the information returned
in either of these objects by any of the other functions."
This can occur in the glibc implementation.
.SH SEE ALSO
.BR date (1),
.BR gettimeofday (2),
.BR time (2),
.BR utime (2),
.BR clock (3),
.BR difftime (3),
.BR strftime (3),
.BR strptime (3),
.BR timegm (3),
.BR tzset (3),
.BR time (7)