http://docs.python.org/dev/3.0/library/ctypes.html#callback-functions

ctypes.xFUNCTYPE are another opportunity to advertise decorators.  
Please consider inserting yet another qsort example written as a 
decorator, perhaps as follows.  Or---it could be that I'm slow and the 
average pythonista will figure this out on first read.


@CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
def py_cmp_func(*args):
    (a,b,) = (t[0] for t in args)
    print("py_cmp_func", a, b)
    return a-b

qsort(ia,len(ia),sizeof(c_int),py_cmp_func)

As far as I know, the above code will fail randomly in release mode if 
you have multiple threads running Python, because when ctypes calls out 
to CFUNCTYPE functions, it releases the GIL.

For decorating a python function to give to qsort, maybe you can use 
PYFUNCTYPE?

[David Lambert]
> > @CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
> > def py_cmp_func(*args):
> >     (a,b,) = (t[0] for t in args)
> >     print("py_cmp_func", a, b)
> >     return a-b
> > 
> > qsort(ia,len(ia),sizeof(c_int),py_cmp_func)

[Kevin Watters]
> > As far as I know, the above code will fail randomly in release mode if 
> > you have multiple threads running Python, because when ctypes calls out 
> > to CFUNCTYPE functions, it releases the GIL.
> > 
> > For decorating a python function to give to qsort, maybe you can use 
> > PYFUNCTYPE?

Kevin is wrong - the code is perfect!  Sure does calling a CFUNCTYPE instance
release the GIL, but ctypes aquires the GIL again before executing the python
code in the wrapped py_cmp_func above.

I guess that using PYFUNCTYPE instead would possibly crash because the call to
'qsort' releases the GIL, and the PYFUNCTYPE instance assumes the GIL but does not
acquire it.

But I have no time to work on the docs, sorry.

I suspect we'll be working on this PR a little, but a least it's a start to get this moving.

New changeset 379e9d639a52766f79c7a206c5096c8333d1896f by Berker Peksag (Andrés Delfino) in branch 'master':
bpo-4260: Document that ctypes.xFUNCTYPE are decorators (GH-7924)
https://github.com/python/cpython/commit/379e9d639a52766f79c7a206c5096c8333d1896f

New changeset 08c1da71030e0d6019b3493e0ffaf060a6aa5d8e by Miss Islington (bot) in branch '3.7':
bpo-4260: Document that ctypes.xFUNCTYPE are decorators (GH-7924)
https://github.com/python/cpython/commit/08c1da71030e0d6019b3493e0ffaf060a6aa5d8e

New changeset 083a836937b734fcc81b2482d2c52825caff0115 by Miss Islington (bot) in branch '3.6':
bpo-4260: Document that ctypes.xFUNCTYPE are decorators (GH-7924)
https://github.com/python/cpython/commit/083a836937b734fcc81b2482d2c52825caff0115

I think we can now close this one. Thanks for the report, David, and thanks for the PR Andreas.

(And thanks for creating and maintaining this awesome module for years, Thomas :))