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 .