asctime(3p) — Linux manual page


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

PROLOG         top

       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         top

       asctime, asctime_r — convert date and time to a string

SYNOPSIS         top

       #include <time.h>

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

DESCRIPTION         top

       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",
                   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         top

       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         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top



       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         top

       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


       These functions may be removed in a future version.

SEE ALSO         top

       clock(3p), ctime(3p), difftime(3p), gmtime(3p), localtime(3p),
       mktime(3p), strftime(3p), strptime(3p), time(3p), utime(3p)

       The Base Definitions volume of POSIX.1‐2008, time.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group                 2013                         ASCTIME(3P)

Pages that refer to this page: time.h(0p)clock(3p)ctime(3p)ctime_r(3p)difftime(3p)gmtime(3p)gmtime_r(3p)localtime(3p)localtime_r(3p)mktime(3p)strftime(3p)strftime_l(3p)time(3p)