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From: Tomas Ukkonen <tomas.ukkonen@protonmail.ch>
Newsgroups: comp.lang.python
Subject: PyGILState_Ensure() deadlocks, why?
Date: Sun, 07 Jul 2024 19:24:04 +0000
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Hello
Is this python c api bug? The following C++ code (g++) deadlocks on Ubuntu=
 Linux.

/*=C2=A0* This code deadlocks on python3-dev 3.12.3 (ubuntu 24.04 lts)
=C2=A0*
=C2=A0* g++ python_thread_test.cpp `python3-config --cflags --libs --embed=
`
=C2=A0* ./a.out
=C2=A0*
=C2=A0* uname:
=C2=A0* Linux softice 6.8.0-36-generic SMP PREEMPT_DYNAMIC x86_64 GNU/Linu=
x
=C2=A0*/

#include <Python.h>
#include <thread>
#include <vector>
#include <iostream>

void perform_long_operation() {
=C2=A0 =C2=A0 // Simulate a long-running task
=C2=A0 =C2=A0 std::this_thread::sleep_for(std::chrono::seconds(5));
}

void thread_function() {
=C2=A0 =C2=A0 // Ensure this thread has the GIL
=C2=A0 =C2=A0 PyGILState_STATE gstate =3D PyGILState_Ensure();

=C2=A0 =C2=A0 // Execute some Python code
=C2=A0 =C2=A0 PyRun_SimpleString("print('Hello from std::thread!')");

=C2=A0 =C2=A0 // Release the GIL for long operation
=C2=A0 =C2=A0 Py_BEGIN_ALLOW_THREADS
=C2=A0 =C2=A0 perform_long_operation();
=C2=A0 =C2=A0 Py_END_ALLOW_THREADS

=C2=A0 =C2=A0 // Re-acquire the GIL and execute more Python code
=C2=A0 =C2=A0 gstate =3D PyGILState_Ensure();
=C2=A0 =C2=A0 PyRun_SimpleString("print('Thread operation completed!')");

=C2=A0 =C2=A0 // Release the GIL
=C2=A0 =C2=A0 PyGILState_Release(gstate);
}

int main() {
=C2=A0 =C2=A0 // Initialize the Python Interpreter
=C2=A0 =C2=A0 Py_Initialize();

=C2=A0 =C2=A0 // Create a vector of threads
=C2=A0 =C2=A0 std::vector<std::thread> threads;

=C2=A0 =C2=A0 // Launch threads
=C2=A0 =C2=A0 for (int i =3D 0; i < 5; ++i) {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 threads.push_back(std::thread(thread_function)=
);
=C2=A0 =C2=A0 }

=C2=A0 =C2=A0 // Join threads
=C2=A0 =C2=A0 for (auto& t : threads) {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 t.join();
=C2=A0 =C2=A0 }

=C2=A0 =C2=A0 // Finalize the Python Interpreter
=C2=A0 =C2=A0 Py_Finalize();

=C2=A0 =C2=A0 return 0;
}



Tomas Ukkonen
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