Marshalling the following C structure with ctypes in Windows (compiled with VS2015) is not working correctly. As you can see, in the simple demo program I attached, the integer I pass as a parameter gets somehow corrupted. I use cdecl calling convention. The corruption happens only if the returned structure contains 3 booleans, if I add a fourth bool to the structure than I get expected results. In linux (compiling with Gcc) the same code (with 3 booleans) work as expected. typedef uint64_t CustomHandle; typedef struct {	bool first;	bool second;	bool third; } Flags; CustomHandle GetHandle(); Flags GetFlags(CustomHandle handle); As already mention I attached all the files to reproduce the issue with the related cmake file

I think the problem is that FFI layer assumes that MSVC compiler will try to pass any structure less than 8 bytes in registers whereis it is not always true: To be returned by value in RAX, user-defined types must have a length of 1, 2, 4, 8, 16, 32, or 64 bits (from https://msdn.microsoft.com/en-us/library/7572ztz4.aspx). I have a fix, now adding tests.

 New changeset 7843caeb909bd907e903606414e238db4082315a by Victor Stinner (Vladimir Matveev) in branch 'master': bpo-34603, ctypes/libffi_msvc: Fix returning structs from functions (GH-9258) https://github.com/python/cpython/commit/7843caeb909bd907e903606414e238db4082315a 

 New changeset e3f6aa7fe48b91f4ff619b2a51d473249d620bcb by Miss Islington (bot) in branch '3.7': bpo-34603, ctypes/libffi_msvc: Fix returning structs from functions (GH-9258) https://github.com/python/cpython/commit/e3f6aa7fe48b91f4ff619b2a51d473249d620bcb 

 New changeset e53632019816749ffd5be0afab2a99d744dbbe35 by Miss Islington (bot) in branch '3.6': bpo-34603, ctypes/libffi_msvc: Fix returning structs from functions (GH-9258) https://github.com/python/cpython/commit/e53632019816749ffd5be0afab2a99d744dbbe35 

 New changeset b63a16febbd1c943c9dbc5c651326b410aa50698 by Victor Stinner (Vladimir Matveev) in branch '2.7': [2.7] bpo-34603, ctypes/libffi_msvc: Fix returning structs from functions (GH-9258) (GH-9425) https://github.com/python/cpython/commit/b63a16febbd1c943c9dbc5c651326b410aa50698 

Thanks MatteoL for the bug report, it has been fixed in 2.7, 3.6, 3.7 and master. Thanks Vladimir Matveev for the fix!

assertEquals() is deprecated, use assertEqual() instead.

 New changeset 4642d5f59828e774585e9895b538b24d71b9df8e by Serhiy Storchaka in branch 'master': Use assertEqual() instead of assertEquals(). (GH-9721) https://github.com/python/cpython/commit/4642d5f59828e774585e9895b538b24d71b9df8e 

 New changeset 6bffe50f5fff8e8a40ae32c3e9c408622a15caf6 by Serhiy Storchaka in branch '3.7': Use assertEqual() instead of assertEquals(). (GH-9721) (GH-9725) https://github.com/python/cpython/commit/6bffe50f5fff8e8a40ae32c3e9c408622a15caf6 

 New changeset d02490a9a9c238ed7ded1120877fdfdce16364a3 by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6': Use assertEqual() instead of assertEquals(). (GH-9721) (GH-9725) (GH-9727) https://github.com/python/cpython/commit/d02490a9a9c238ed7ded1120877fdfdce16364a3