Callback from c thread with ctypes

[Victor Lin]

Hi,

I am going to develop a c library binding with ctypes. That c library
will call callback from worker threads it created. Here comes the
problem : Will the GIL be acquired before it goes into Python
function?

I got a little try..

DSPPROC = WINFUNCTYPE(None, DWORD, DWORD, c_void_p, DWORD, c_void_p)

def test(handle, channel, buffer, length, user):
print handle, channel, buffer, length, user

dsp = BASS_ChannelSetDSP(stream, DSPPROC(test), None, 123)

I got "access violation" when I run it... It seems that the ctypes
did't acquire GIL before it call the python callback. As the document
says.

WINFUNCTYPE will release GIL during the call

But it does not mention callback about Python function? How about a
call from another thread? Could somebody help me?

Thanks.
Victor Lin.

 

[Diez B. Roggisch]

Hi,

I am going to develop a c library binding with ctypes. That c library
will call callback from worker threads it created. Here comes the
problem : Will the GIL be acquired before it goes into Python
function?

I got a little try..

DSPPROC = WINFUNCTYPE(None, DWORD, DWORD, c_void_p, DWORD, c_void_p)

def test(handle, channel, buffer, length, user):
print handle, channel, buffer, length, user

dsp = BASS_ChannelSetDSP(stream, DSPPROC(test), None, 123)

I got "access violation" when I run it... It seems that the ctypes
did't acquire GIL before it call the python callback. As the document
says.

WINFUNCTYPE will release GIL during the call

But it does not mention callback about Python function? How about a
call from another thread? Could somebody help me?

The releasing takes only place when entering a c-function. A
python-callback acquires the GIL, see callbacks.c in the python source.

The access violation has nothing to do with that I presume, that's just
a general programming error as it happens with ctypes during development.

Diez

 

[Victor Lin]

Victor Lin schrieb:












The releasing takes only place when entering a c-function. A
python-callback acquires the GIL, see callbacks.c in the python source.

The access violation has nothing to do with that I presume, that's just
a general programming error as it happens with ctypes during development.

Diez

Hi,

Thanks your replying, I try it again, found that my program only crash
when I call it from python's IDLE. If I click it, namely execute it
with python directly, it works fine. Why the program will crash within
IDLE?

 

[Victor Lin]

You can't call Python code from an arbitrary thread. Before you are
allowed to call a Python function from a thread, you must enable
Python's threading subsystem and register the thread. Python need to
store information (like the exception information) in a thread local
variable (TLS).

I can't tell you how to register your thread with Python, though.

Christian

I know I have to call PyEval_InitThreads if my module create threads
that will contact python stuff, for example, call a python callback
function from threads. But however, it is ctypes. I have no idea
should I do that for the imported dll? If it is, how?

Thanks.

 

[Lie Ryan]

Hi,

Thanks your replying, I try it again, found that my program only crash
when I call it from python's IDLE. If I click it, namely execute it
with python directly, it works fine. Why the program will crash within
IDLE?

Usually because IDLE is written in python and Tkinter. IDLE doesn't give
a completely isolated environment for your program to run in, this
causes python program that uses Tkinter often causes various
undescribable oddities.