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.1‐2017 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.1‐2017, <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                                       ASCTIME(3POSIX)