How to pass a union to a function in ctypes?

Table of Contants

Introduction

In Python, the ctypes library facilitates interfacing with C libraries, allowing you to pass complex data types such as unions to C functions. Unions are similar to structures but can hold only one of their members at a time, making them useful for saving memory when multiple data types are needed but not simultaneously.

Passing a Union to a Function in ctypes

1. Defining a Union

To pass a union to a C function, you must first define the union in Python using ctypes.Union. This involves specifying the fields and their corresponding data types.

2. Declaring the C Function

Next, you need to declare the C function prototype in Python, indicating the argument types and return types.

3. Creating an Instance and Passing It

You can then create an instance of the union and pass it to the C function.

Example: Passing a Union

C Code (mylib.c)

Let’s consider the following C function defined in a file named mylib.c:

Compile this C code into a shared library (e.g., mylib.so on Linux or mylib.dll on Windows).

Python Code

Here’s how to pass a union to the C function using ctypes:

Explanation

1. Union Definition: The Data class is defined using ctypes.Union, with _fields_ specifying its fields and types.
2. Load Library: The C library is loaded using ctypes.CDLL().
3. Function Prototypes: The argument and return types for the print_data function are set with argtypes and restype.
4. Union Instance: An instance of Data is created. Since a union can hold only one value at a time, only one of its fields should be set before passing it to the function.
5. Function Calls: The print_data function is called three times, passing the union instance after setting different fields.

Practical Example

Complete Example

Here is a complete example, including both the C code and Python code, to demonstrate how to pass a union.

C Code (mylib.c)

Compile the C code into a shared library:

Python Code

Output

The output will display the values of the union's fields as they are passed to the C function, but note that only the last set value is valid when printed, since a union can only hold one value at a time.

Conclusion

Passing unions to C functions using the ctypes library in Python allows for efficient handling of different data types in a memory-efficient way. By following the steps outlined above, you can successfully integrate C code with Python, utilizing the flexibility of unions.