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.1‐2017 defers to the ISO C

       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

       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 problems.


       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.1‐2017, time.h(0p)

COPYRIGHT         top

       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 .

       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               2017                       ASCTIME(3P)

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