Admin
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I want to call some c function from python to be able to improve performance of my code. But I cannot find online whether when I call a C function using the ctypes libraries the GIL is released. As a simple example:
from ctypes import *
fun = cdll.LoadLibrary("libc.so.6")
fun.sleep.argtypes = [c_uint]
fun.sleep(c_uint(5))
Is the GIL released during the call to fun.sleep?
According to [Python.Docs]: ctypes.CFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False) (emphasis is mine):
The returned function prototype creates functions that use the standard C calling convention. The function will release the GIL during the call.
Also (in order to link *FUNCTYPE and *DLL), [Python.Docs]: class ctypes.PyDLL(name, mode=DEFAULT_MODE, handle=None) (emphasis still mine):
Instances of this class behave like CDLL instances, except that the Python GIL is not released during the function call
Therefore, the answer to your question is: YES.
As a side note, if fun.sleep is called many times (like in a loop or something), there's time "lost" in marshalling data between Python and C for every call, and in that case you might want to consider writing the whole loop in C.
It would be nice if this were started directly in the CLL documentation, but the following article provides a convincing test : http://caswenson.com/2009_06_13_bypassing_the_python_gil_with_ctypes.html
Edit: I spoke too soon: the CFUNCTYPE documentation is pretty explicit about releasing the GIL: https://docs.python.org/3/library/ctypes.html#ctypes.CFUNCTYPE
