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

Table of Contants

Introduction

When interfacing Python with a C library, it’s common to pass arrays of structures. If the C function expects a null-terminated array of structures, you need to carefully define the structure in ctypes, create an array, and add a null terminator. This guide explains how to pass a null-terminated array of structures to a C library function using Python's ctypes.

Defining Structures and Creating a Null-Terminated Array

1. Import the ctypes Module

First, import the ctypes module to access the necessary types for defining C-style structures and arrays.

2. Define a Structure

Create a structure using ctypes.Structure to match the C structure that will be passed to the C function. Here’s an example of a simple C structure with an integer and a float field.

3. Create a Null-Terminated Array of Structures

Create an array of structures using the defined MyStruct class. After creating the array, append a null terminator (None) to the end.

4. Pass the Array to the C Function

Assume you have a C function like this, which accepts a null-terminated array of structures:

To pass the array to this function in Python using ctypes, you can set up the argument types and call the function:

Practical Example: Passing a Null-Terminated Array of Structures

Here’s a complete example of how to define a structure, create a null-terminated array, and pass it to a C library function:

Explanation:

1. Structure Definition: MyStruct mimics the C structure, with two fields (i and f).
2. Pointer Type: MyStructPtr is defined as a pointer to MyStruct.
3. Array of Structures: An array of three structures is created using (MyStruct * 3).
4. Null-Terminator: A null terminator (None) is appended to create a null-terminated array.
5. C Function Call: The array is passed to the process_structs function, which processes the array in C.

Conclusion

Passing a null-terminated array of structures to a C function using ctypes involves creating the structure in Python, setting up an array of structures, and appending a null terminator (None). You can then pass this array to the C function after setting the correct argument types. This method is essential when working with C functions that expect null-terminated arrays for memory-safe operations.