title: How does NSMutableData work?
body: <p>I have one problem to work with <code>NSMutableData</code>. </p>
  
  <p>I defined one <code>NSMutableData *receivedData</code>, and tried to copy several <code>NSData*</code> data to the <code>receivedData</code>. I just called <code>[receivedData appendData:data]</code>, but appears the data is not copied:</p>
  
  <pre><code>....
  NSLog(@"get data! Received %d bytes of data",[data length]);
    // output is not zero, say 1231.
  
  [receivedData appendData:data];
  NSLog(@"after append! length is %d bytes of data",[receivedData length]);
    // showing zero
  </code></pre>
  
  <p>Thanks.</p>