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
IEEE Std 1003.1-2001 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
The ctime() function need not be reentrant. A function that is not
required to be reentrant is not required to 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 thread-safe version ctime_r() is not required
to set tzname.
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.
Values for the broken-down time structure can be obtained by calling
gmtime() or localtime(). The ctime() function is included
for compatibility with older implementations, and does not support localized
date and time formats. Applications should use the strftime() function
to achieve maximum portability.
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.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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