title: C on Xcode: bad access
body: <p>I am trying to finish an assignment for my <code>C</code> programming class on <code>Xcode</code>. It's simple. It reads a text file, which contains a list of student IDs and their test scores. And then it calculates the overall score based on these data, and writes them to another text file. However, I keep getting this <code>Bad Access</code> error message that says:</p>
  
  <pre><code>Thread 1: EXC_BAD_ACCESS(code=1, address=0x7fff00000de4)
  </code></pre>
  
  <p>I don't know what it means. What should I do?</p>