Call Windows C-dll (foreign) functions directly via ctypes inside Python: A complete template

With Python ctypes (see 1), Python can directly call (foreign) C functions defined in Windows dll. This makes the speed-up of Python via C truly easy. The details, especially with Windows dll, can be confusing, unfortunately. This package provides the necessary step-by-step guide on how to achieve calling C functions in Python (version 3.13).

Build Windows Dll

To create a dll project in Visual Studio 2022:

1. On VS2022 start-up, choose Create a new project.
2. Choose C++, and choose Dynamic-link Library (DLL).
3. Enter a name, in my case winCDynamic, for your project.
4. Delete dllmain.cpp, framework.h, pch.h, pch.cpp from Project.
5. Move qlcfuncs.cpp to the project folder (i.e., winCDynamic) and add to the project. Note: If you provide a header file qlcfuncs.h, it will NOT work!
6. Set the VS2022 Compiler by opening the Property Pages dialog and Configuration Properties in the left pane:

• C/C++ -> Precompiled Headers: set [Precompiled Headers] to [Not Using Precompiled Headers]

Note: the default calling convention should be [__cdecl] in the VS2022 Compiler. If not, set as follows:

• C/C++ -> Advanced: set [Calling Convention] to [__cdecl (/Gd)]

Now, you can build your Windows dll for Python and copy winCDynamic.dll to your desired folder (in my case libc, see 2).

Python calls C-dll functions

To call dll functions in Python, you must pass the correct argument types (i.e., buffered byte string, c_double, ctypes Structure, or ctypes array) and specify the return type. Read the documentation directly in c4Python.py for what most of you need to know.

If you run c4Python.py (under Command, type: py c4Python.py), you will find three successful calls, such as qlDblRetArgs(). But you will see the following error as well:

• AttributeError: function 'qlArrayArg' not found

The error is caused by missing the qlArrayArg() function that depends on a C++ function defined in a static library. Let’s build the static library then.

Build Windows (Static) Lib

To create a static library project in Visual Studio 2022:

1. On VS2022 start-up, choose Create a new project.
2. Choose C++, and choose Static Library.
3. Enter a name, in my case winCppStatic, for your project.
4. Delete framework.h, pch.h, pch.cpp, winCppStatic.cpp from Project.
5. Move qlcpptools.h & qlcpptools.cpp to the project folder (i.e., winCppStatic) and add them to the project.
6. Set up the VS2022 Compiler: set [Precompiled Headers] to [Not Using Precompiled Headers].

Note: The default calling convention should be [__cdecl] in the VS2022 Compiler. If not, set it as in the Build Windows Dll section.

Build your Windows static library and move qlcpptools.h & winCppStatic.lib to libc (see 2).

Re-build Windows Dll

To rebuild the dll, first set up the VS2022 Compiler options:

• Linker -> Input: add [../libc/winCppStatic.lib] to [Additional Dependencies]
• C/C++ -> Preprocessor: Add [_QL__INCLUDE_STATIC_LIB_] to [Preprocessor Definitions]

Rebuild and run c4Python.py. qlArrayArg() will be successful this time.

ctypes in Python or PyObject in C++

Suppose you have a big and complex C++ application (such as my own convertible bonds pricing Github package). In that case, it makes more sense to wrap functionalities from static libraries inside a function and export the function in a Windows DLL to the Python client. On the other hand, with a few functions to extend Python by C++, you can use PyObject defined in Python.h to define modules and functions in C++ within a Python package directly (see 3 and my C++ files inside Python Github package).

Notes:

[1] ctypes — A foreign function library for Python

[2] To automatically copy files after the build, select Configuration Properties > Build Events > Post-Build Event, and in the Command Line field, enter this command (for the dll project):

xcopy /y /d $(TargetPath) ..\libc

or for the static lib project:

xcopy /y /d $(ProjectDir) qlcpptools.h ..\libc

xcopy /y /d $(TargetPath) ..\libc

[3] Extending Python with C or C++ — Python 3.13.2