title: What does ## mean in C?
body: <p>I was looking at the source code of the berkeley lab checkpoint/restart and found this pre-process macro definition:</p>
  
  <pre><code>  #define io_wrap(_op,_ctx,_file,_buf,_count) \
          cr_##_op((_ctx)-&gt;req-&gt;errbuf,(_file),(_buf),(_count))
  </code></pre>
  
  <p>Any idea what the hell this thing means?</p>
  
  <p>I found this in blcr-0.8.5/vmadump4/vmadump.h</p>
  
  <p>Thank you!</p>