Why Can\'t I Load My DLL Module in Python 3.8 on Windows?

Troubleshooting Loading DLL Modules in Python on Windows

Recent attempts to import a compiled libuvc DLL on Windows have yielded the error "Could not find module 'libuvc.dll'". Despite correctly finding the file's path using ctypes.util.find_library, Python fails to load it. The issue lies in the winmode parameter in the ctypes.cdll.LoadLibrary function.

Understanding the Winmode Parameter

Prior to Python 3.8, winmode did not exist and mode was set directly to ctypes.DEFAULT_MODE, which corresponded to a value of zero. In Python 3.8, winmode was introduced.

When winmode is set to None, the search mode defaults to nt._LOAD_LIBRARY_SEARCH_DEFAULT_DIRS. This search mode does not take into account changes to environment variables or paths.

However, setting winmode to 0 specifies that the full path should be used, and the library is loaded successfully. This behavior is documented in the Microsoft documentation (https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-loadlibraryexa).

Resolving the Issue

To resolve the loading issue, simply specify winmode=0 when calling ctypes.cdll.LoadLibrary, as in the following code snippet:

import ctypes

name = ctypes.util.find_library('libuvc')
lib = ctypes.cdll.LoadLibrary(name, winmode=0)

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