ASCTIME(3P) POSIX Programmer's Manual ASCTIME(3P)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.1‐2008 defers to the ISO C standard.
The asctime() function shall convert the broken-down time in the struc‐
ture 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 struc‐
ture 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 imple‐
mentations. 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 pro‐
vides the strftime() function which can be used to avoid these prob‐
lems.
FUTURE DIRECTIONS
These functions may be removed in a future version.
SEE ALSOclock(), ctime(), difftime(), gmtime(), localtime(), mktime(), strf‐
time(), strptime(), time(), utime()
The Base Definitions volume of POSIX.1‐2008, <time.h>
COPYRIGHT
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 Electri‐
cal 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 http://www.unix.org/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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 ASCTIME(3P)
