How to declare a function pointer in ctypes?

Table of Contants

Introduction

In Python, the ctypes library allows you to interface with C libraries, including the ability to declare and use function pointers. A function pointer is a variable that stores the address of a function, enabling you to call the function indirectly. This guide explains how to declare a function pointer in ctypes and provide a practical example.

Declaring a Function Pointer in ctypes

1. Import the ctypes Module

Start by importing the ctypes module to access its functionalities.

2. Define the Function Prototype

To declare a function pointer, you need to define the prototype of the function it will point to. This includes the argument types and the return type. You can use ctypes.CFUNCTYPE for this purpose.

For example, if you have a C function that takes two integers and returns an integer, the prototype would be defined as follows:

3. Declare the Function Pointer

After defining the function prototype, you can declare a function pointer using the type you just defined. Here’s how to do it:

4. Assign a C Function to the Function Pointer

You can then assign a C function to this function pointer. For instance, if you have a C function defined as follows:

You can load the shared library and assign the function to your pointer:

5. Call the Function Through the Function Pointer

Once the function pointer is assigned, you can call the function using the pointer like this:

Practical Example: Using a Function Pointer in ctypes

Here's a complete example that demonstrates how to declare a function pointer in ctypes and use it to call a C function.

C Code (save as example.c):

Compile the C Code:

Compile the C code to a shared library (Linux):bash

Python Code:

Explanation:

1. Function Prototype: The FunctionType defines a function pointer type that takes two int arguments and returns an int.
2. Shared Library Loading: The libexample.so shared library is loaded using ctypes.CDLL.
3. Function Pointer Assignment: The add function from the C library is assigned to func_ptr.
4. Function Call: The function is called via the pointer, and the result is printed.

Conclusion

Declaring and using function pointers in ctypes is straightforward. By defining a function prototype using ctypes.CFUNCTYPE, you can create function pointers that allow you to call C functions from Python. This technique is essential for integrating Python with C libraries and enables more complex interactions, such as callbacks and event handling.