title: PyEval_InitThreads in Python 3: How/when to call it? (the saga continues ad nauseam)
body: <p>Basically there seems to be <strong><em>massive</em></strong> confusion/ambiguity over when exactly <a href="http://docs.python.org/3.2/c-api/init.html#PyEval_InitThreads" rel="noreferrer"><strong><code>PyEval_InitThreads()</code></strong></a> is supposed to be called, and what accompanying API calls are needed.  The <a href="http://docs.python.org/3.2/c-api/init.html" rel="noreferrer">official Python documentation</a> is unfortunately very ambiguous.  There are already <a href="https://stackoverflow.com/questions/10625584/embedding-python-in-multithreaded-c-application/10702119">many questions on stackoverflow</a> regarding this topic, and indeed, I've personally already <a href="https://stackoverflow.com/questions/14916815/when-is-pyeval-initthreads-meant-to-be-called">asked a question almost identical</a> to this one, so I won't be particularly surprised if this is closed as a duplicate; but consider that there seems to be no definitive answer to this question.  (Sadly, I don't have Guido Van Rossum on speed-dial.)</p>
  
  <p>Firstly, let's define the scope of the question here: <strong>what do I want to do?</strong>  Well... I want to write a Python extension module in C that will:</p>
  
  <ol>
  <li>Spawn worker threads using the <code>pthread</code> API in C </li>
  <li>Invoke Python callbacks from within these C threads</li>
  </ol>
  
  <p>Okay, so let's start with the Python docs themselves.  The <a href="http://docs.python.org/3.2/c-api/init.html#PyEval_InitThreads" rel="noreferrer"><strong>Python 3.2 docs</strong></a> say:</p>
  
  <blockquote>
    <p><strong>void PyEval_InitThreads()</strong></p>
    
    <p>Initialize and acquire the global interpreter lock. It should be
    called in the main thread before creating a second thread or engaging
    in any other thread operations such as PyEval_ReleaseThread(tstate).
    It is not needed before calling PyEval_SaveThread() or
    PyEval_RestoreThread().</p>
  </blockquote>
  
  <p>So my understanding here is that:</p>
  
  <ol>
  <li>Any C extension module which spawns threads must call
  <code>PyEval_InitThreads()</code> from the main thread before any other threads
  are spawned </li>
  <li>Calling <code>PyEval_InitThreads</code> locks the GIL</li>
  </ol>
  
  <p>So common sense would tell us that any C extension module which creates threads must call <code>PyEval_InitThreads()</code>, and then release the Global Interpreter Lock.  Okay, seems straightforward enough.  So <em>prima facie</em>, all that's required would be the following code:</p>
  
  <pre><code>PyEval_InitThreads(); /* initialize threading and acquire GIL */
  PyEval_ReleaseLock(); /* Release GIL */
  </code></pre>
  
  <p>Seems easy enough... but unfortunately, the Python 3.2 docs <strong>also</strong> say that <a href="http://docs.python.org/3.2/c-api/init.html#PyEval_ReleaseLock" rel="noreferrer"><strong><code>PyEval_ReleaseLock</code></strong> has been <strong>deprecated</strong></a>.  Instead, we're supposed to use <a href="http://docs.python.org/3.2/c-api/init.html#PyEval_SaveThread" rel="noreferrer"><strong><code>PyEval_SaveThread</code></strong></a> in order to release the GIL:</p>
  
  <blockquote>
    <p><strong>PyThreadState* PyEval_SaveThread()</strong></p>
    
    <p>Release the global interpreter lock (if it has been created and thread
    support is enabled) and reset the thread state to NULL, returning the
    previous thread state (which is not NULL). If the lock has been
    created, the current thread must have acquired it.</p>
  </blockquote>
  
  <p>Er... okay, so I guess a C extension module needs to say:</p>
  
  <pre><code>PyEval_InitThreads();
  PyThreadState* st = PyEval_SaveThread();
  </code></pre>
  
  <p><br>
  Indeed, this is exactly what <a href="https://stackoverflow.com/questions/10625584/embedding-python-in-multithreaded-c-application/10702119#10702119"><strong>this stackoverflow answer</strong></a> says.  Except when I actually <em>try</em> this in practice, the Python interpreter immediately seg-faults when I import the extension module.  &nbsp;&nbsp;&nbsp;Nice.
  <br>
  <br></p>
  
  <hr>
  
  <p>Okay, so now I'm giving up on the official Python documentation and turning to Google.  So, <strong><a href="http://dandesousa.com/2011/12/11/boost-python-calling-back-into-python-from-native-c-threads/" rel="noreferrer">this random blog</a></strong> claims all you need to do from an extension module is to call <code>PyEval_InitThreads()</code>.  Of course, the documentation claims that <code>PyEval_InitThreads()</code> acquires the GIL, and indeed, a <a href="https://github.com/certik/python-3.2/blob/master/Python/ceval.c#L304" rel="noreferrer"><strong><em>quick inspection of the source code for <code>PyEval_InitThreads()</code> in <code>ceval.c</code></em></strong></a> reveals that it does indeed call the internal function <code>take_gil(PyThreadState_GET());</code>  </p>
  
  <p>So <code>PyEval_InitThreads()</code> <em>definitely</em> acquires the GIL.  I would think then that you would absolutely need to somehow release the GIL after calling <code>PyEval_InitThreads()</code>. &nbsp;&nbsp;<strong><em>But how?</em></strong>  <code>PyEval_ReleaseLock()</code> is deprecated, and <code>PyEval_SaveThread()</code> just inexplicably seg-faults.</p>
  
  <p>Okay... so maybe for some reason which is currently beyond my understanding, a C extension module <em>doesn't</em> need to release the GIL.  I tried that... and, as expected, as soon as another thread attempts to acquire the GIL (using <a href="http://docs.python.org/3.2/c-api/init.html#PyGILState_Ensure" rel="noreferrer"><strong>PyGILState_Ensure</strong></a>), the program hangs from a deadlock.  So yeah... you <em>really do</em> need to release the GIL after calling <code>PyEval_InitThreads()</code>.</p>
  
  <p>So again, the question is: <strong><em>how do you release the GIL after calling <a href="http://docs.python.org/3.2/c-api/init.html#PyEval_InitThreads" rel="noreferrer"><code>PyEval_InitThreads()</code></a>?</em></strong></p>
  
  <p>And more generally: <strong>what exactly does a C-extension module have to do to be able to safely invoke Python code from worker C-threads?</strong></p>