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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).

It seems that notwithstanding the issues identified in the original bug report, calls by value of structs containing unions are being used out there in the wild. Examples:
The comtypes library (Windows):
https://github.com/python/cpython/pull/16799#issuecomment-573318591
The ctypesgen library, which is used by other projects across platforms:
https://github.com/davidjamesca/ctypesgen/issues/77
Linux examples:
https://github.com/waveform80/picamera/issues/604
https://github.com/hcpl/xkbgroup/issues/15
https://github.com/enkore/i3pystatus/issues/771
So, it would seem that the simplest course of action is to disable the checks for now. The checks will be commented out and this issue and the related issue bpo-16576 will be reopened.

New changeset c12440c371025bea9c3bfb94945f006c486c2c01 by Vinay Sajip in branch 'master':
bpo-16575: Disabled checks for union types being passed by value. (GH-17960)
https://github.com/python/cpython/commit/c12440c371025bea9c3bfb94945f006c486c2c01

New changeset eb9ba2f66df2195a9c6295e73ab3d545a1445f05 by Vinay Sajip (Miss Islington (bot)) in branch '3.8':
bpo-16575: Disabled checks for union types being passed by value. (GH-17960) (GH-17964)
https://github.com/python/cpython/commit/eb9ba2f66df2195a9c6295e73ab3d545a1445f05

New changeset 9dbf5d3bc2033940cdca35440cf08814544f81e4 by Vinay Sajip in branch '3.7':
[3.7] bpo-16575: Disabled checks for union types being passed by value. (GH-17960) (GH-17970)
https://github.com/python/cpython/commit/9dbf5d3bc2033940cdca35440cf08814544f81e4