title: Going backwards by one in file stream
body: <p>Say I have a file pointer like this:</p>
  
  <pre><code>file_ptr = fopen(“test.txt”, “r+”);
  </code></pre>
  
  <p>and I want to change each <code>char</code>'s ASCII value as I move through the stream (I am doing this as a cipher, so if there is a better or more efficient way to do this, please let me know).</p>
  
  <p>I am trying to use a <code>while(!feof(file_ptr)) {}</code> loop with <code>fgetc()</code> and <code>fputc</code>, but the <code>file_ptr</code> will be pointing to the next char, so i want to know if there is something i can do point it backwards one spot.</p>
  
  <p>Essentially, I want to know the file stream equivalent for:</p>
  
  <pre><code>char* blah="blah blah";
  char* index=blah;
  index++;/*how to navigate a file stream in this way*/
  </code></pre>