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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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ASCTIME(3P) POSIX Programmer's Manual ASCTIME(3P)
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.
asctime, asctime_r — convert date and time to a string
#include <time.h>
char *asctime(const struct tm *timeptr);
char *asctime_r(const struct tm *restrict tm, char *restrict buf);
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.
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.
No errors are defined.
The following sections are informative.
None.
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.
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.
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)
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(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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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