Issue 22945: Ctypes inconsistent between Linux and OS X (original) (raw)

Created on 2014-11-26 04:46 by Daniel.Standage, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Messages (5)
msg231692 - (view)	Author: Daniel Standage (Daniel.Standage)	Date: 2014-11-26 04:46
The ctypes documentation describes the '._as_parameter_' attribute, which simplifies using Python objects to encapsulate C objects. If a ctypes object with the '._as_parameter_' attribute defined is passed as a parameter to a C function, the value of '._as_parameter_' is supplied to the C function instead of a pointer to the Python project. At least that's how it's advertised to work. And it does indeed work as advertised on Linux, but on Mac OS X the mechanism doesn't appear to work correctly. I have created a small dummy example demonstrating this at https://github.com/standage/ctypes-demo.
msg231693 - (view)	Author: Ned Deily (ned.deily) *	Date: 2014-11-26 06:16
FWIW, your test does seem to run if the C test program is instead built in 32-bit mode (modifying CFLAGS to include "-arch i386"). For example, with the Apple OS X 10.10 system Python: DYLD_LIBRARY_PATH=.. arch -i386 /usr/bin/python2.7 test.py Gizmo 1: 15 Gizmo 2: 20 Empty data
msg231697 - (view)	Author: Eryk Sun (eryksun) *	Date: 2014-11-26 08:13
c_void_p's getfunc returns the address as an integer. In turn, an integer argument gets converted to a C int. This mean you need to a create a new c_void_p from the address for _as_parameter_, e.g. self._as_parameter_ = c_void_p(self.g) Or set restype to a subclass of c_void_p. ctypes won't call the getfunc in this case, e.g. class my_void_p(c_void_p): pass mylib.my_gizmo_create.restype = my_void_p ConvParam https://hg.python.org/cpython/file/ee879c0ffa11/Modules/_ctypes/callproc.c#l613 GetResult https://hg.python.org/cpython/file/ee879c0ffa11/Modules/_ctypes/callproc.c#l903
msg232716 - (view)	Author: Ned Deily (ned.deily) *	Date: 2014-12-16 06:30
Thanks, eryksun. Any objections to closing this issue?
msg232744 - (view)	Author: Daniel Standage (Daniel.Standage)	Date: 2014-12-16 14:35
Sorry for my radio silence. Your example was very helpful, and restores the behavior I was looking for. Thanks!