On Feb 3, 2009, at 12:51 PM, Victor Lin wrote:
It seems that my program can't call to Python's function from thread
directly, because there is Global Interpreter Lock. The python's GIL
is so complex, I have no idea how it works. I'm sorry, what I can do
is to ask. My question is. What should I do before and after I call a
Python function from threads?
Hi Victor,
I asked a similar question yesterday and have gotten no response yet
-- I hope you'll have better luck. I'm writing a C extension that
wants to implement a callback in a new thread. Here's what I think
I've learned from reading the threads API doc. Please note that this
is a classic case of the blind leading the blind! I'm sure some (most?
all?) of the ideas below are wrong, but I'm hoping that thinking
through some of this "out loud" will help both of us. Or maybe some
more knowledgeable person will take pity on/be appalled by my
ignorance and come to our rescue. =)
Python's infamous GIL doesn't exist when a program is single-threaded.
Before a new thread is created, the main thread must call
PyEval_InitThreads() to create the GIL. However, "It is not safe to
call this function when it is unknown which thread (if any) currently
has the global interpreter lock." Therefore my extension must do this:
i_have_the_gil = 0;
if (!PyEval_ThreadsInitialized()) {
PyEval_InitThreads();
/* "...when this function initializes the lock, it also acquires
it." */
i_have_the_gil = 1;
}
That ensures that the GIL is created.
My extension will be calling from a newly-created C thread. The
*Python* thread doesn't exist yet; so next I have to create it.
Therefore --
if (!i_have_the_gil)
PyEval_AcquireLock();
// Might not actually be the main thread but conceptually
// it is OK to assume so here.
main_thread = PyThreadState_Get();
callback_thread = PyThreadState_New(main_thread->interp);
PyThreadState_Swap(callback_thread);
gstate = PyGILState_Ensure();
call_callback_function();
// Now unwind the above
PyGILState_Release(gstate);
PyThreadState_Swap(main_thread);
PyThreadState_Clear(callback_thread);
PyThreadState_Delete(callback_thread);
PyEval_ReleaseLock();
I haven't yet tested this! But hopefully it is the right idea and just
needs a little fine tuning.
I found this discussion useful:
http://mail.python.org/pipermail/python-list/2006-November/413088.html
It includes the quote, "The current thread state API doc, as you read
it from top to bottom now, is in fact totally confusing for anyone who
didn't develop Python himself"
I like! =)
Cheers
Philip
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