Geek asks and answers

Ctime () returns a string, why don't we need to free () the memory of this string?

function prototype ctimeis

char *ctime(const time_t *timep);

As we can see, it returns a string. but where is the sting?

and why we should not free the memory line

This sample code will receive many error messages.

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

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

+5
source share
2 answers

It returns a pointer to a buffer staticand should not be free()d. From man ctime :

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

C99, 7.23.3.2 ctime , ctime(timer) asctime(localtime(timer)), asctime() ( ) :

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;
}

, free(), , malloc(), calloc() realloc(), undefined.

+8

malloc'd.

+1

More articles:


All Articles