ASCTIME(3)                 POSIX Programmer's Manual                ASCTIME(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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(3)
