asctime_r (3p) - Linux Manuals

asctime_r: convert date and time to a string


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 IEEE Std 1003.1-2001 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",
        timeptr->tm_mday, timeptr->tm_hour,
        timeptr->tm_min, timeptr->tm_sec,
        1900 + timeptr->tm_year);
    return result;

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 reentrant. A function that is not required to be reentrant is not required to 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()) 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.

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.




Values for the broken-down time structure can be obtained by calling gmtime() or localtime(). This function is included for compatibility with older implementations, and does not support localized date and time formats. Applications should use strftime() to achieve maximum portability.

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.






Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .


clock(), ctime(), difftime(), gmtime(), localtime(), mktime(), strftime(), strptime(), time(), utime(), the Base Definitions volume of IEEE Std 1003.1-2001, <time.h>