ctypes pretends to support passing arguments to C functions that are unions (not pointers to unions), but that's a lie.  In fact, the underlying libffi does not support it.  The attached example misbehaves on Linux x86-64.

A minimal fix would be to update the documentation.

A more comprehensive fix would be to tweak ctypes to reject unions and bit-fields when running on non-x86 (does this work for ARM and other non-Intel archs?)

An even more comprehensive fix would be to make this work, but that would require forking libffi and should presumably be first implemented upstream.

See also http://bugs.python.org/issue16576.

New changeset 0acd9408b6f1 by Eli Bendersky in branch '3.2':
Add warning in ctypes documentation for #16575 and #16576
http://hg.python.org/cpython/rev/0acd9408b6f1

New changeset bfc159f8e4b4 by Eli Bendersky in branch '3.3':
Add warning in ctypes documentation for #16575 and #16576
http://hg.python.org/cpython/rev/bfc159f8e4b4

New changeset 502624235c7b by Eli Bendersky in branch 'default':
Add warning in ctypes documentation for #16575 and #16576
http://hg.python.org/cpython/rev/502624235c7b

New changeset eece32440a52 by Eli Bendersky in branch '2.7':
Add warning in ctypes documentation for #16575 and #16576
http://hg.python.org/cpython/rev/eece32440a52

I've opened a libffi issue in an attempt to get this fixed upstream:

  https://github.com/atgreen/libffi/issues/33

Link to issue has changed to:

https://github.com/libffi/libffi/issues/33

New changeset 79d4ed102a5069c6cebaed2627cb1645637f0429 by Vinay Sajip in branch 'master':
bpo-16575: Add checks for unions passed by value to functions. (GH-16799)
https://github.com/python/cpython/commit/79d4ed102a5069c6cebaed2627cb1645637f0429

New changeset 952899704800a5aa438e84d50f0b6fc498c72e89 by Vinay Sajip in branch '3.8':
[3.8] bpo-16575: Add checks for unions passed by value to functions. (GH-16799) (GH-17016)
https://github.com/python/cpython/commit/952899704800a5aa438e84d50f0b6fc498c72e89

New changeset 0118d109d54bf75c99a8b0fa9aeae1a478ac4b7e by Vinay Sajip in branch '3.7':
[3.7] bpo-16575: Add checks for unions passed by value to functions. (GH-16799) (GH-17017)
https://github.com/python/cpython/commit/0118d109d54bf75c99a8b0fa9aeae1a478ac4b7e

This change introduced a reference leak on Windows. Example on 3.7:

https://buildbot.python.org/all/#/builders/132/builds/645

test_ctypes leaked [174, 174, 174] references, sum=522
test_ctypes leaked [76, 77, 77] memory blocks, sum=230

Same issue on x86 Gentoo Refleaks 3.7:

test_ctypes leaked [174, 174, 174] references, sum=522
test_ctypes leaked [76, 78, 76] memory blocks, sum=230

https://buildbot.python.org/all/#/builders/114/builds/631

Opened https://github.com/python/cpython/pull/17064 to fix this. Essentially it's a tiny little oversight in the back-porting. In the 3.7 branch, we perform an attribute lookup for `from_param` before the union checking code, so we must remember to DECREF it. This is unlike master where the attribute lookup happens after the union checking code.



λ win32\python_d.exe -m test -R 3:3 test_ctypes
0:00:00 Run tests sequentially
0:00:00 [1/1] test_ctypes
beginning 6 repetitions
123456
......

== Tests result: SUCCESS ==

1 test OK.

Total duration: 14.8 sec
Tests result: SUCCESS

New changeset 484edbf9bf1a9e6bae0fcb10a0c165b89ea79295 by Vinay Sajip (Ammar Askar) in branch '3.7':
bpo-16575: Fix refleak on passing unions in ctypes (GH-17064)
https://github.com/python/cpython/commit/484edbf9bf1a9e6bae0fcb10a0c165b89ea79295

Will close after https://buildbot.python.org/all/#/builders?tags=%2Brefleak&tags=%2B3.7 go back to green.

3.7 refleaks buildbots pass again (ignoring a few warnings).