The test ctypes.test.test_structures.StructureTestCase.test_pass_by_value fails on 64-bit Cygwin and MinGW using the system libffi with:

======================================================================
FAIL: test_pass_by_value (ctypes.test.test_structures.StructureTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/embray/src/python/cpython/Lib/ctypes/test/test_structures.py", line 416, in test_pass_by_value
    self.assertEqual(s.first, 0xdeadbeef)
AssertionError: 195948557 != 3735928559


It seems that libffi does not handle passing structs by value properly on those platforms as I explained here: https://github.com/libffi/libffi/issues/305

The upstream bug hasn't been confirmed yet by the libffi developers so I could be wrong, but I think this is fairly clearly broken there.

I have a PR forthcoming to work around the issue.

Looks same as #29804.

Thanks for pointing that out.  Indeed it's the same symptom, but slightly different cause.  libffi has different implementations of its calling routines for different architectures/platforms depending on the machine architecture and calling conventions used.  So this case is probably buggy for the arm64 implementation as well, as in #29804.

Off the top of my head I don't know a reliable way to check for this case, but this fix would work around the arm64 issue as well if I could check for that architecture easily at compile time.

A 'defined(__aarch64__)' can be used for the arm64 arch. I will add it to your patch and test it on an arm64 machine to see if the test passes.

I have added `defined(__aarch64__)` check to the if statement and tested the patch on arm64. The test passed.

So could you please also add the check for `defined(__aarch64__)` to the pull request so that it fixes #29804 as well?

Sure, thanks for pointing that out.

Iryna, I updated the pull request with a slightly updated fix.  Could you confirm again, when you get a chance, that it works for arm64?  Thanks.

Thank you, Erik. I have applied your new patch, and ran another build on arm64. The tests passed. Attaching the build log for more information.

Thanks for checking!

Patch LGTM now, perhaps others will concur?

New changeset 9ba3aa4d02a110d1a1ea464a8aff3be7dd9c63c3 by Vinay Sajip (Erik Bray) in branch 'master':
bpo-30353: Fix pass by value for structs on 64-bit Cygwin/MinGW (GH-1559)
https://github.com/python/cpython/commit/9ba3aa4d02a110d1a1ea464a8aff3be7dd9c63c3

This bug affects also the 3.6 branch. Can the fix be backported?

This has a backport PR now for 3.6.  Once that PR is merged I think we can close this as fixed.

New changeset 2f3ba27185a369bcb6b36b13aa3518ffcc970ffa by Ned Deily (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-30353: Fix pass by value for structs on 64-bit Cygwin/MinGW (GH-1559) (GH-5954)
https://github.com/python/cpython/commit/2f3ba27185a369bcb6b36b13aa3518ffcc970ffa