title: Writing in a file from multiple threads
body: <p>I'm writing a download manager in Objective-C which downloads file from multiple segments at the same times in order to improve the speed. Each segement of the file is downloaded in a thread.</p>
  
  <p>At first, I thought to write each segment in a different file and to put together all the files at the end of the download. But for many reasons, it's not a good solution.</p>
  
  <p>So, I'm searching a way to write in a file at a specific position and which is able to handle multiple thread because in my application, each segment is downloaded inside a thread.
  In Java, I know that <code>FileChannel</code> does the trick perfectly but I have no idea in Objective-C.</p>