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    <p>Your understanding is correct: invoking <code>PyEval_InitThreads</code> does, among other things, acquire the GIL. In a correctly written Python/C application, this is not an issue because the GIL will be unlocked in time, either automatically or manually.</p> <p>If the main thread goes on to run Python code, there is nothing special to do, because Python interpreter will automatically relinquish the GIL after a number of instructions have been executed (allowing another thread to acquire it, which will relinquish it again, and so on). Additionally, whenever Python is about to invoke a blocking system call, e.g. to read from the network or write to a file, it will release the GIL around the call.</p> <p>The original version of this answer pretty much ended here. But there is one more thing to take into account: the <em>embedding</em> scenario.</p> <p>When embedding Python, the main thread often initializes Python and goes on to execute other, non-Python-related tasks. In that scenario there is nothing that will <em>automatically</em> release the GIL, so this must be done by the thread itself. That is in no way specific to the call that calls <code>PyEval_InitThreads</code>, it is expected of <a href="https://docs.python.org/2/c-api/init.html#releasing-the-gil-from-extension-code" rel="nofollow noreferrer">all Python/C code</a> invoked with the GIL acquired.</p> <p>For example, the <code>main()</code> might contain code like this:</p> <pre><code>Py_Initialize(); PyEval_InitThreads(); Py_BEGIN_ALLOW_THREADS ... call the non-Python part of the application here ... Py_END_ALLOW_THREADS Py_Finalize(); </code></pre> <p>If your code creates threads manually, they need to acquire the GIL before doing <em>anything</em> Python-related, even as simple as <code>Py_INCREF</code>. To do so, use <a href="https://docs.python.org/2/c-api/init.html#non-python-created-threads" rel="nofollow noreferrer">the following</a>:</p> <pre><code>// Acquire the GIL PyGILState_STATE gstate; gstate = PyGILState_Ensure(); ... call Python code here ... // Release the GIL. No Python API allowed beyond this point. PyGILState_Release(gstate); </code></pre>
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    1. CO`PyGILState_Ensure()/PyGILState_Release()` should be used to invoke Python from a C thread instead of `PyEval_{Save,Restore}Thread()` See [Thread State and the Global Interpreter Lock](http://docs.python.org/3.4/c-api/init.html#thread-state-and-the-global-interpreter-lock)
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    2. CO@J.F.Sebastian Agreed, I've now edited the answer to advertise the correct API.
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    3. COBut, as I mentioned in the question, if the *main thread* is holding the GIL (which it will be after it calls `PyEval_InitThreads()`, then when a worker thread calls `PyGILState_Ensure`, it's going to deadlock.
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