title: What is #pragma used for?
body: <p>Can anyone help me understand #pragma?</p>
  
  <pre><code>ifndef TARGET_OS_LINUX
  #pragma once
  endif
  </code></pre>
  
  <p>What,when, where, why, an example?</p>
  
  <p>The above is in some code that I am refactoring....</p>