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.
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.1‐2008 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:
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() does not set tzname, it shall not set daylight and shall
not set timezone.
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.
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
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
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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
IEEE/The Open Group 2013 CTIME(3P)