C: Trouble with Strings

I am new to C89, and don't really understand how strings work. I am developing on Windows 7.

Here is what I am trying to do, in Java:

String hostname = url.substring(7, url.indexOf('/'));

Here is my clumsy attempt to do this in C89:

// well formed url ensured
void get(char *url) {
    int hostnameLength;
    char *firstSlash;
    char *hostname;

    firstSlash = strchr(url + 7, '/');
    hostnameLength = strlen(url) - strlen(firstSlash) - 7;
    hostname = malloc(sizeof(*hostname) * (hostnameLength + 1));
    strncpy(hostname, url + 7, hostnameLength);
    hostname[hostnameLength] = 0; // null terminate
}

Update to reflect answers

For a hostnameLength of 14, hostname is malloc()'开发者_如何学编程d 31 characters. Why does this happen?

// now what? is strncpy():

hostname = malloc(hostnameLength + 1);
strncpy(hostname, url + 7, hostnameLength);
hostname[hostnameLength] = '\0'; // don't forget to null terminate!

After that, you need to do:

hostname = malloc(sizeof(char) * (hostnameLength+1));
strncpy(hostname,  url + 7, hostnameLength);
hostname[hostnameLength] = 0;

See strncpy for details on the copying. It does require your destination pointer to be allocated in advance (hence the malloc), and will only copy so many characters...