NAME

asctime — convert date and time to a string

SYNOPSIS

[OB] #include <time.h>

char *asctime(const struct tm *timeptr);

DESCRIPTION

^[CX] 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.1-2024 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;
}

If any of the members of the broken-down time contain values that are outside their normal ranges (see XBD <time.h>), the behavior of the asctime() function is undefined. Likewise, if the calculated year exceeds four digits or is less than the year 1000, the behavior is undefined.

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 that return a pointer to one of these object types may overwrite the information in any object of the same type pointed to by the value returned from any previous call to any of them.

The asctime() function need not be thread-safe; however, asctime() shall avoid data races with all functions other than itself, ctime(), gmtime(), and localtime().

RETURN VALUE

Upon successful completion, asctime() shall return a pointer to the string. ^[CX]  If the function is unsuccessful, it shall return NULL.

ERRORS

No errors are defined.

EXAMPLES

None.

APPLICATION USAGE

This function is included only for compatibility with older implementations. It has undefined behavior if the resulting string would be too long, so the use of this function should be discouraged. On implementations that do not detect output string length overflow, it is possible to overflow the output buffer in such a way as to cause applications to fail, or possible system security violations. Also, this function does 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().

RATIONALE

The standard developers decided to mark the asctime() function obsolescent even though it 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.

FUTURE DIRECTIONS

This function may be removed in a future version, but not until after it has been removed from the ISO C standard.

SEE ALSO

clock, ctime, difftime, futimens, gmtime, localtime, mktime, strftime, strptime, time

XBD <time.h>

CHANGE HISTORY

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.

Issue 8

Austin Group Defect 469 is applied, clarifying the conditions under which the behavior of asctime() is undefined.

Austin Group Defect 1302 is applied, aligning this function with the ISO/IEC 9899:2018 standard.

Austin Group Defect 1330 is applied, changing the FUTURE DIRECTIONS section.

Austin Group Defect 1376 is applied, removing CX shading from some text derived from the ISO C standard and updating it to match the ISO C standard.

Austin Group Defect 1410 is applied, removing the asctime_r() function.