title: using 'extern' for integer array results in segmentation fault
body: <p>I have the following code:</p>
  
  <p>File1.c</p>
  
  <pre><code>int arr[10];
  </code></pre>
  
  <p>File2.c</p>
  
  <pre><code>extern int *arr;
  int main()
  {
      arr[0] = 1;
      return 0;
  }
  </code></pre>
  
  <p>Please explain me why accessing the arr in File2.c will cause segmentation fault?<br>
  compiler:gcc
  OS:Linux.</p>