Re: C style , one argument, recursive, string reverse

All previous advices were good and appreciated..
Sorry about my stubborness I am really intrested to know why my code
does not work
I remarked it better.

I know it is probably not the best way to do it but can you see a
problem in there?
not for the sake of solving the problem , just for education...
Thanks.

/*
Algorithm pseudo:

   reverse(str)
    if(str="")
        return ""
    else
       return append(last_char(str),
                           reverse(all_but_last_char(str))
*/

char * C_recReverse(char * str)
{
    // Sub is storing the substring
    // res is the returned string
    // tmp is for storing the previos result so it can be deleted
    char* sub, *res, *tmp;

    if (strlen(str) == 0)
        return "";
    else
    {
        /* the sub and res are allocated per recursion stack*/
        sub = (char*)malloc(sizeof(char)*strlen(str)-1);
        res = (char*)malloc(sizeof(char)*strlen(str));

        /* prepering the substring, stored on the heap*/
        strcpy(sub,str);
        sub[strlen(str)-1] = '\0';

        /* the return value is preperaed , filled with last char*/
        strcpy(res,"");
        strncpy(res,&str[strlen(str)-1],1);

        /* calling the reverse function of the "sub" stored
                                    on local variable tmp*/
        tmp = C_recReverse(sub);

        /* adding the result of substring reverse to the result string (res)
*/
        strcat(res,tmp);

        /*"closing" the string (???) */
        res[strlen(str)] = '\0';

        /* Freeing the substring, I dont really need it anymore*/
        free(sub);

        /* Freeing the "previous" recursion closure,
                                  Can I do it? I think this is the
problem */
        free(tmp);
        return res;
    }
}

Thanks.