ctime(3p) — Linux manual page


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

PROLOG         top

       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         top

       ctime, ctime_r — convert a time value to a date and time string

SYNOPSIS         top

       #include <time.h>

       char *ctime(const time_t *clock);
       char *ctime_r(const time_t *clock, char *buf);

DESCRIPTION         top

       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

       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() 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         top

       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         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top



       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(3p).

RATIONALE         top

       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.


       These functions may be removed in a future version.

SEE ALSO         top

       asctime(3p), clock(3p), difftime(3p), gmtime(3p), localtime(3p),
       mktime(3p), strftime(3p), strptime(3p), time(3p), utime(3p)

       The Base Definitions volume of POSIX.1‐2017, time.h(0p)

COPYRIGHT         top

       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)

Pages that refer to this page: time.h(0p)asctime(3p)clock(3p)clock_getres(3p)difftime(3p)getdate(3p)gettimeofday(3p)gmtime(3p)localtime(3p)mktime(3p)strftime(3p)time(3p)tzset(3p)