How to pass a null-terminated array of unions to a C library function in ctypes?

Table of Contants

Introduction

When interfacing Python with C libraries, you may need to pass unions to C functions. If the C function expects a null-terminated array of unions, you can achieve this using ctypes in Python. This guide will walk you through the process of defining a union, creating a null-terminated array of unions, and passing it to a C function.

Defining Unions and Creating a Null-Terminated Array

1. Import the ctypes Module

Start by importing the ctypes module, which provides the necessary tools to define and manipulate C-style unions and arrays in Python.

2. Define a Union

In ctypes, you can define a union by subclassing ctypes.Union. Here's an example of a simple union that can hold either an integer or a float, which matches the corresponding C union.

3. Create a Null-Terminated Array of Unions

Next, you need to create an array of unions. To pass a null-terminated array of unions to a C function, append a None value to the array to serve as the null terminator.

4. Pass the Array to the C Function

Let’s assume the C function has the following signature, which accepts a null-terminated array of unions:

To call this function from Python, you need to load the shared library, set the argument types for the function, and pass the null-terminated array:

Practical Example: Passing a Null-Terminated Array of Unions

Here's a complete example that demonstrates defining a union, creating a null-terminated array, and passing it to a C library function using ctypes.

Explanation:

1. Union Definition: MyUnion is a ctypes.Union that can store either an integer (i) or a float (f).
2. Pointer Type: MyUnionPtr is a pointer to the union.
3. Array of Unions: An array of unions is created using (MyUnion * 3) to hold three elements.
4. Null-Terminator: A null terminator (None) is appended to the array to make it null-terminated.
5. Function Call: The array is passed to the C function using ctypes after defining the argument types.

Conclusion

Passing a null-terminated array of unions to a C library function using ctypes involves creating a union, defining a null-terminated array, and calling the C function. By setting up the correct argument types and appending a None value as the null terminator, you can seamlessly pass the array of unions to C functions. This technique is useful for integrating Python with C libraries that expect union-based data structures.