CTIME
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
ctime,
ctime_r
--- convert a time value to a date and time string
SYNOPSIS
#include <time.h>
char *ctime(const time_t *clock);
char *ctime_r(const time_t *clock, char *buf);
DESCRIPTION
For
ctime():
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here and the
ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.
The
ctime()
function shall convert the time pointed to by
clock,
representing time in seconds since the Epoch, to local time in the form
of a string. It shall be equivalent to:
-
asctime(localtime(clock))
The
asctime(),
ctime(),
gmtime(),
and
localtime()
functions shall return values in one of two static objects: a
broken-down time structure and an array of
char.
Execution of any of the functions may overwrite the information
returned in either of these objects by any of the other functions.
The
ctime()
function need not be thread-safe.
The
ctime_r()
function shall convert the calendar time pointed to by
clock
to local time in exactly the same form as
ctime()
and put the string into the array pointed to by
buf
(which shall be at least 26 bytes in size) and return
buf.
Unlike
ctime(),
the
ctime_r()
function is not required to set
tzname.
If
ctime_r()
sets
tzname,
it shall also set
daylight
and
timezone.
If
ctime_r()
does not set
tzname,
it shall not set
daylight
and shall not set
timezone.
RETURN VALUE
The
ctime()
function shall return the pointer returned by
asctime()
with that broken-down time as an argument.
Upon successful completion,
ctime_r()
shall return a pointer to the string pointed to by
buf.
When an error is encountered, a null pointer shall be returned.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
These functions are included only for compatibility with older
implementations. They have undefined behavior if the resulting string
would be too long, so the use of these functions should be discouraged.
On implementations that do not detect output string length overflow, it
is possible to overflow the output buffers in such a way as to cause
applications to fail, or possible system security violations. Also,
these functions do not support localized date and time formats. To
avoid these problems, applications should use
strftime()
to generate strings from broken-down times.
Values for the broken-down time structure can be obtained by calling
gmtime()
or
localtime().
The
ctime_r()
function is thread-safe and shall return values in a user-supplied
buffer instead of possibly using a static data area that may be
overwritten by each call.
Attempts to use
ctime()
or
ctime_r()
for times before the Epoch or for times beyond the year 9999 produce
undefined results. Refer to
asctime().
RATIONALE
The standard developers decided to mark the
ctime()
and
ctime_r()
functions obsolescent even though they are in the ISO C standard due to the
possibility of buffer overflow. The ISO C standard also provides the
strftime()
function which can be used to avoid these problems.
FUTURE DIRECTIONS
These functions may be removed in a future version.
SEE ALSO
asctime(),
clock(),
difftime(),
gmtime(),
localtime(),
mktime(),
strftime(),
strptime(),
time(),
utime()
The Base Definitions volume of POSIX.1-2017,
<time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .