Question

the prototype of the function ctime is

char *ctime(const time_t *timep);

As we can see, it return a string. but, where the sting be contained?

and why we shouldn’t free the string’s memory

This is sample code will get a lots of error message

char *p;
p = ctime(...);
...
free(p);

*** glibc detected *** ./a.out: free(): invalid pointer: 0x00007f0b365b4e60 ***

Answer 1

It returns a pointer to a static buffer, and must not be free()d. From man ctime:

The four functions asctime(), ctime(), gmtime() and localtime() return a pointer to static data and hence are not thread-safe.

The C99 standard, section 7.23.3.2 The ctime function states that calling ctime(timer) function is equivalent to asctime(localtime(timer)), and the asctime() implementation (as illustrated in the same document) is equivalent to:

char *asctime(const struct tm *timeptr)
{
    static const char wday_name[7][3] = {
        "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
    };

    static const 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;
}

The argument passed to free() must be a pointer returned by a call to malloc(), calloc() or realloc() only, otherwise the behaviour is undefined.