The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


asctime, asctime_r - convert date and time to a string


[OB] [Option Start] #include <time.h>

char *asctime(const struct tm *
[Option End]
[OB CX] [Option Start] char *asctime_r(const struct tm *restrict tm, char *restrict buf); [Option End]


For asctime(): [CX] [Option Start]  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. [Option End]

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.

[CX] [Option Start] 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. [Option End]


Upon successful completion, asctime() shall return a pointer to the string. [CX] [Option Start]  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. [Option End]


No errors are defined.

The following sections are informative.




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, ctime, difftime, gmtime, localtime, mktime, strftime, strptime, time, utime

XBD <time.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Normative text previously in the APPLICATION USAGE section is moved to the DESCRIPTION.

The asctime_r() function is included for alignment with the POSIX Threads Extension.

A note indicating that the asctime() function need not be reentrant is added to the DESCRIPTION.

Issue 6

The asctime_r() function is marked as part of the Thread-Safe Functions option.

Extensions beyond the ISO C standard are marked.

The APPLICATION USAGE section is updated to include a note on the thread-safe function and its avoidance of possibly using a static data area.

The DESCRIPTION of asctime_r() is updated to describe the format of the string returned.

The restrict keyword is added to the asctime_r() prototype for alignment with the ISO/IEC 9899:1999 standard

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/17 is applied, adding the CX extension in the RETURN VALUE section requiring that if the asctime() function is unsuccessful it returns NULL.

Issue 7

Austin Group Interpretation 1003.1-2001 #053 is applied, marking these functions obsolescent.

Austin Group Interpretation 1003.1-2001 #156 is applied.

The asctime_r() function is moved from the Thread-Safe Functions option to the Base.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0033 [86,429] is applied.

End of informative text.


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