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

       asctime, asctime_r — convert date and time to a string

SYNOPSIS

       #include <time.h>

       char *asctime(const struct tm *timeptr);
       char *asctime_r(const struct tm *restrict tm, char *restrict buf);

DESCRIPTION

       For asctime(): 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  asctime()  function shall convert the broken-down time in the structure pointed to by timeptr into a
       string in the form:

           Sun Sep 16 01:03:52 1973\n\0

       using the equivalent of the following algorithm:

           char *asctime(const struct tm *timeptr)
           {
               static char wday_name[7][3] = {
                   "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
               };
               static char mon_name[12][3] = {
                   "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                   "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
               };
               static char result[26];

               sprintf(result, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",
                   wday_name[timeptr->tm_wday],
                   mon_name[timeptr->tm_mon],
                   timeptr->tm_mday, timeptr->tm_hour,
                   timeptr->tm_min, timeptr->tm_sec,
                   1900 + timeptr->tm_year);
               return result;
           }

       However, the behavior is undefined if timeptr->tm_wday or  timeptr->tm_mon  are  not  within  the  normal
       ranges  as  defined in <time.h>, or if timeptr->tm_year exceeds {INT_MAX}−1990, or if the above algorithm
       would attempt to generate more than 26 bytes of output (including the terminating null).

       The tm structure is defined in the <time.h> header.

       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 type char.  Execution of any of the functions may
       overwrite the information returned in either of these objects by any of the other functions.

       The asctime() function need not be thread-safe.

       The asctime_r() function shall convert the broken-down time in the structure pointed  to  by  tm  into  a
       string  (of  the same form as that returned by asctime(), and with the same undefined behavior when input
       or output is out of range) that is placed in the user-supplied buffer pointed  to  by  buf  (which  shall
       contain at least 26 bytes) and then return buf.

RETURN VALUE

       Upon  successful  completion,  asctime()  shall  return  a  pointer  to  the  string.  If the function is
       unsuccessful, it shall return NULL.

       Upon successful completion, asctime_r() shall return a pointer to a character string containing the  date
       and  time.  This  string  is  pointed  to by the argument buf.  If the function is unsuccessful, it shall
       return NULL.

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  asctime_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.

RATIONALE

       The standard developers decided to mark the asctime() and asctime_r() functions obsolescent  even  though
       asctime()  is  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

       clock(), ctime(), difftime(), gmtime(), localtime(), mktime(), strftime(), strptime(), time(), utime()

       The Base Definitions volume of POSIX.1‐2008, <time.h>

COPYRIGHT

       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .