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POWhen is PyEval_InitThreads meant to be called?
 

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  1. POWhen is PyEval_InitThreads meant to be called?
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    <p>I'm a bit confused about when I'm supposed to call <code>PyEval_InitThreads</code>. In general, I understand that <code>PyEval_InitThreads</code> must be called whenever a non-Python thread (i.e. a thread that is spawned within an extension module) is used. </p> <p>However, I'm confused if <code>PyEval_InitThreads</code> is for C programs which embed the Python interpreter, or Python programs which import C-extension modules, or both.</p> <p>So, if I write a C extension module that will internally launch a thread, do I need to call <code>PyEval_InitThreads</code> when initializing the module?</p> <p>Also, <code>PyEval_InitThreads</code> <a href="http://docs.python.org/dev/c-api/init.html#PyEval_InitThreads"><strong>implicitly acquires the Global Interpreter Lock</strong></a>. So after calling <code>PyEval_InitThreads</code>, presumably the GIL must be <em>released</em> or deadlock will ensue. So how do you release the lock? After reading the documentation, <a href="http://docs.python.org/dev/c-api/init.html#PyEval_ReleaseLock"><strong><code>PyEval_ReleaseLock()</code></strong></a> appears to be the way to release the GIL. However, in practice, if I use the following code in a C extension module:</p> <pre><code> PyEval_InitThreads(); PyEval_ReleaseLock(); </code></pre> <p>...then at runtime Python aborts with:</p> <pre><code>Fatal Python error: drop_gil: GIL is not locked </code></pre> <p>So how do you release the GIL after acquiring it with <code>PyEval_InitThreads</code>?</p>
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    1. COTry without the `PyEval_ReleaseLock()`. The GIL is acquired for a good reason; releasing it before calling other C API functions from Python is going to crash.
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