title: clang++: fatal error: 'unistd.h' file not found
body: <p>Using OS X 10.10.2, I download <a href="http://llvm.org/releases/download.html#3.6.0">Clang for Mac OS X</a> v. 3.6.0, and try to compile a simple Hello World program.</p>
  
  <p>The commands I use are these:<br>
  (assumes you downloaded <code>clang</code> to <code>.</code>)</p>
  
  <pre><code>cd .
  ./clang+llvm-3.6.0-x86_64-apple-darwin/bin/clang++ main.cpp
  </code></pre>
  
  <p>The result is this:</p>
  
  <pre><code>In file included from main.cpp:1:
  In file included from ./clang+llvm-3.6.0-x86_64-apple-darwin/bin/../include/c++/v1/iostream:37:
  ./clang+llvm-3.6.0-x86_64-apple-darwin/bin/../include/c++/v1/__config:23:10: fatal error: 'unistd.h' file not
    found
  </code></pre>
  
  <p>which makes sense, as there is no file <code>unistd.h</code>, as verifiable by</p>
  
  <pre><code>find . -name unistd.h
  </code></pre>
  
  <p>which yields <code>0</code> results.</p>
  
  <p>I tried downloading the <a href="http://llvm.org/releases/download.html#3.6.0">LibC++ source code</a> v. 3.6.0 but even that contains no <code>unistd.h</code>. Where can I find the official/vanilla version of that file, and the files that are referenced, and meta-referenced, by it?</p>