title: why the sendto function needs the third parameter, in socket programming
body: <p>I was new to socket program. 
  When learning the <code>sendto</code> function, as the prototype:</p>
  
  <pre><code>ssize_t sendto(int socket, const void *message, size_t length,
                  int flags, const struct sockaddr *dest_addr,
                  socklen_t dest_len);
  </code></pre>
  
  <p>I get to know that the "message" has contain the target IP, and the <code>dest_addr</code> argument also specifies the target IP address.
  Is there other usage of the <code>dest_addr</code> argument?</p>