PyEval_InitThreads in Python 3: How/when to call it? (the saga continues ad nauseam)
Your understanding is correct: invoking PyEval_InitThreads 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.
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.
The original version of this answer pretty much ended here. But there is one more thing to take into account: the embedding scenario.
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 automatically release the GIL, so this must be done by the thread itself. That is in no way specific to the call that calls PyEval_InitThreads, it is expected of all Python/C code invoked with the GIL acquired.
For example, the main() might contain code like this:
Py_Initialize();
PyEval_InitThreads();
Py_BEGIN_ALLOW_THREADS
... call the non-Python part of the application here ...
Py_END_ALLOW_THREADS
Py_Finalize();
If your code creates threads manually, they need to acquire the GIL before doing anything Python-related, even as simple as Py_INCREF. To do so, use the following:
// 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);
I have seen symptoms similar to yours: deadlocks if I only call PyEval_InitThreads(), because my main thread never calls anything from Python again, and segfaults if I unconditionally call something like PyEval_SaveThread(). The symptoms depend on the version of Python and on the situation: I am developing a plug-in that embeds Python for a library that can be loaded as part of a Python extension. The code needs therefore to run independent of whether it is loaded by Python as main.
The following worked for be with both python2.7 and python3.4, and with my library running within Python and outside of Python. In my plug-in init routine, which is executed in the main thread, I run:
Py_InitializeEx(0);
if (!PyEval_ThreadsInitialized()) {
PyEval_InitThreads();
PyThreadState* mainPyThread = PyEval_SaveThread();
}
(mainPyThread is actually some static variable, but I don't think that matters as I never need to use it again).
Then I create threads using pthreads, and in each function that needs to access the Python API, I use:
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
// Python C API calls
PyGILState_Release(gstate);
There are two methods of multi threading while executing C/Python API.
1.Execution of different threads with same interpreter - We can execute a Python interpreter and share the same interpreter over the different threads.
The coding will be as follows.
main(){
//initialize Python
Py_Initialize();
PyRun_SimpleString("from time import time,ctime\n"
"print 'In Main, Today is',ctime(time())\n");
//to Initialize and acquire the global interpreter lock
PyEval_InitThreads();
//release the lock
PyThreadState *_save;
_save = PyEval_SaveThread();
// Create threads.
for (int i = 0; i<MAX_THREADS; i++)
{
hThreadArray[i] = CreateThread
//(...
MyThreadFunction, // thread function name
//...)
} // End of main thread creation loop.
// Wait until all threads have terminated.
//...
//Close all thread handles and free memory allocations.
//...
//end python here
//but need to check for GIL here too
PyEval_RestoreThread(_save);
Py_Finalize();
return 0;
}
//the thread function
DWORD WINAPI MyThreadFunction(LPVOID lpParam)
{
//non Pythonic activity
//...
//check for the state of Python GIL
PyGILState_STATE gilState;
gilState = PyGILState_Ensure();
//execute Python here
PyRun_SimpleString("from time import time,ctime\n"
"print 'In Thread Today is',ctime(time())\n");
//release the GIL
PyGILState_Release(gilState);
//other non Pythonic activity
//...
return 0;
}
- Another method is that, we can execute a Python interpreter in the main thread and, to each thread we can give its own sub interpreter. Thus every thread runs with its own separate , independent versions of all imported modules, including the fundamental modules - builtins, __main__ and sys.
The code is as follows
int main()
{
// Initialize the main interpreter
Py_Initialize();
// Initialize and acquire the global interpreter lock
PyEval_InitThreads();
// Release the lock
PyThreadState *_save;
_save = PyEval_SaveThread();
// create threads
for (int i = 0; i<MAX_THREADS; i++)
{
// Create the thread to begin execution on its own.
hThreadArray[i] = CreateThread
//(...
MyThreadFunction, // thread function name
//...); // returns the thread identifier
} // End of main thread creation loop.
// Wait until all threads have terminated.
WaitForMultipleObjects(MAX_THREADS, hThreadArray, TRUE, INFINITE);
// Close all thread handles and free memory allocations.
// ...
//end python here
//but need to check for GIL here too
//re capture the lock
PyEval_RestoreThread(_save);
//end python interpreter
Py_Finalize();
return 0;
}
//the thread functions
DWORD WINAPI MyThreadFunction(LPVOID lpParam)
{
// Non Pythonic activity
// ...
//create a new interpreter
PyEval_AcquireLock(); // acquire lock on the GIL
PyThreadState* pThreadState = Py_NewInterpreter();
assert(pThreadState != NULL); // check for failure
PyEval_ReleaseThread(pThreadState); // release the GIL
// switch in current interpreter
PyEval_AcquireThread(pThreadState);
//execute python code
PyRun_SimpleString("from time import time,ctime\n" "print\n"
"print 'Today is',ctime(time())\n");
// release current interpreter
PyEval_ReleaseThread(pThreadState);
//now to end the interpreter
PyEval_AcquireThread(pThreadState); // lock the GIL
Py_EndInterpreter(pThreadState);
PyEval_ReleaseLock(); // release the GIL
// Other non Pythonic activity
return 0;
}
It is necessary to note that the Global Interpreter Lock still persists and, in spite of giving individual interpreters to each thread, when it comes to python execution, we can still execute only one thread at a time. GIL is UNIQUE to PROCESS, so in spite of providing unique sub interpreter to each thread, we cannot have simultaneous execution of threads
Sources: Executing a Python interpreter in the main thread and, to each thread we can give its own sub interpreter
Multi threading tutorial (msdn)