Solving the Python DLL Function Return Value Issue

Discover how to fix the wrong return value from DLL functions in Python using `ctypes` with our step-by-step guide. --- This video is based on the question https://stackoverflow.com/q/62727895/ asked by the user 'Vladimir' ( https://stackoverflow.com/u/11084664/ ) and on the answer https://stackoverflow.com/a/62728881/ provided by the user 'Mark Tolonen' ( https://stackoverflow.com/u/235698/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Python dll function return wrong value Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Solving the Python DLL Function Return Value Issue When working with dynamic link libraries (DLLs) in Python, especially when interacting with functions originally designed for languages like C# or C+ + , one might face unexpected return values. This can be frustrating and confusing, especially if you're expecting a successful return value but end up encountering errors. In this guide, we'll take a closer look at an issue related to invoking a DLL function in Python with ctypes, and how to effectively resolve the problem. The Problem: Unexpected Return Values from DLL A developer encountered an issue when trying to call a DLL function named TnzDDBOpenDatabase using Python's ctypes library. The problem arose because while the function was successfully called in C# with a return code of 0, indicating success, the same function reported incorrect values when accessed through Python. Here's a quick overview of the issue: In C# , the call to TnzDDBOpenDatabase returned: errorCode = 0 dbHandle = 0x140e0690 In Python, however, the results were: result = -2147352576 handle = c_long(-1) Why would these values be different? Let's break down the solution. Solution: Correctly Calling the DLL Function in Python To ensure that TnzDDBOpenDatabase is called correctly in Python, follow these steps: Step 1: Load the DLL First, we need to load the DLL using ctypes.CDLL. Ensure that the path to your DLL is correct: [[See Video to Reveal this Text or Code Snippet]] Step 2: Prepare the Parameters The original parameters used in the C# function provide insight into how we should structure the arguments in Python: The first argument accepts a byte string (which corresponds to a file path). The second argument is expected to be a pointer to a handle. Given this understanding, we will prepare our variables as follows: [[See Video to Reveal this Text or Code Snippet]] Step 3: Set the Function's Return and Argument Types It’s essential to declare the return type and types of the arguments for the function: [[See Video to Reveal this Text or Code Snippet]] Step 4: Create a Handle for Output Since the second parameter is expected to be an out parameter, we will create a suitable variable to hold it: [[See Video to Reveal this Text or Code Snippet]] Step 5: Call the Function Finally, we call the function and pass the parameters correctly. Notice how we use ctypes.byref() to pass the handle by reference: [[See Video to Reveal this Text or Code Snippet]] Conclusion: Verifying the Results After making these adjustments, it's crucial to check the results. You can inspect result and handle: [[See Video to Reveal this Text or Code Snippet]] By understanding the expected types and ensuring proper method calls in Python, you can resolve complex issues related to DLL integrations. In this case, ensuring the correct handling of pointers and argument types allowed the Python code to function similar to its C# counterpart. If you encounter similar issues in the future, always refer back to the function prototype, inspect the expected argument types, and ensure you're using ctypes appropriately. Happy coding!