ctime

CTIME(3P)                   POSIX Programmer's Manual                  CTIME(3P)



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 .



IEEE/The Open Group                   2017                             CTIME(3P)