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 .