bpo-43816: add 'extern "C"' to pyctype.h

[aytey]

Issue

With Python 3.9.4, and when compiling with Visual Studio 2019, we have noticed that the variable _Py_ctype_table is not scoped with in an extern "C" block, and where the Python library (python39.lib) has been compiled with a C compiler.

This causes an issue when trying to refer to _Py_ctype_table from a C++ file, as the compiler tries to name-mangle the use of _Py_ctype_table, but the linker cannot then tie the mangled name to non-mangled named from python39.lib.

Example:

#include "Python.h"
int main() { return _Py_ctype_table[0]; }

Compilation:

cl.exe /Fe:test.exe /TP /I include test.cpp /link libs/python39.lib
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29336 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cpp
Microsoft (R) Incremental Linker Version 14.28.29336.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test.exe
libs/python39.lib
test.obj
test.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) unsigned int const * const _Py_ctype_table" (__imp_?_Py_ctype_table@@3QBIB) referenced in function main
test.exe : fatal error LNK1120: 1 unresolved externals

With cl.exe:

cl.exe /Bv  
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29336 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

Compiler Passes:
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\cl.exe:        Version 19.28.29336.0
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\c1.dll:        Version 19.28.29336.0
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\c1xx.dll:      Version 19.28.29336.0
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\c2.dll:        Version 19.28.29336.0
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\c1xx.dll:      Version 19.28.29336.0
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\link.exe:      Version 14.28.29336.0
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\mspdb140.dll:  Version 14.28.29336.0
 Z:\home\avj\visual_studio\MSVC\14.28.29333\bin\HostX64\x64\1033\clui.dll: Version 19.28.29336.0

A naïve check of Python.h (e126547) seems to suggest that:

• There are 82 includes
• 64 of these contain extern "C"
• 8 do not contain extern "C"
• The remaining 10 are either system includes or pyconfig.h

For the 8 that do not contain extern "C", none of these use PyAPI_DATA. This leads me to believe that it is an oversight that pyctype.h does not have extern "C"

Resolution

This PR resolves this issue by adding an extern "C" declaration to the top of pyctype.h (if compiling as C++).

Signed-off-by: Andrew V. Jones andrew.jones@vector.com

https://bugs.python.org/issue43816

[aytey]

Any idea why test_ssl failed on macOS after the rebase?

[miss-islington]

Thanks @andrewvaughanj for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[miss-islington]

Thanks @andrewvaughanj for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[miss-islington]

Sorry @andrewvaughanj and @vstinner, I had trouble checking out the 3.9 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 54db51c9114ac49030832f5134979ca866ffd21c 3.9

[bedevere-bot]

GH-25386 is a backport of this pull request to the 3.8 branch.

[miss-islington]

Thanks @andrewvaughanj for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-25387 is a backport of this pull request to the 3.9 branch.