title: Find header file that defines a C function
body: <p>Shouldn't be hard, right? Right?</p>
  
  <p>I am currently trawling the OpenAFS codebase to find the header definition of pioctl. I've thrown everything I've got at it: checked ctags, grepped the source code for pioctl, etc. The closest I've got to a lead is the fact that there's a file pioctl_nt.h that contains the definition, except it's not actually what I want because none of the userspace code directly includes it, and it's Windows specific.</p>
  
  <p>Now, I'm not expecting you to go and download the OpenAFS codebase and find the header file for me. I am curious, though: what are your techniques for finding the header file you need when everything else fails? What are the worst case scenarios that could cause a grep for pioctl in the codebase to not actually come up with anything that looks like a function definition?</p>
  
  <p>I should also note that I have access to two independent userspace programs that have done it properly, so in theory I could do an O(n) search for the function. But none of the header files pop out to me, and n is large...</p>
  
  <p><strong>Edit:</strong> The immediate issue has been resolved: pioctl() is defined implicitly, as shown by this:</p>
  
  <pre><code>AFS.xs:2796: error: implicit declaration of function ‘pioctl’
  </code></pre>