How to call a callback function in ctypes?

Table of Contants

Introduction

In Python, the ctypes library allows you to interact with C libraries, including the ability to call callback functions. A callback function is one that is passed as an argument to another function and is executed at a specific point in that function's execution. This is particularly useful when you need to handle events or data processing in a customizable manner.

Calling a Callback Function in ctypes

1. Define the Callback Function Type

To call a callback function, you first need to define the callback function's prototype using ctypes. This includes specifying its return type and the types of its arguments.

2. Implement the Callback Function

You will then implement the actual callback function in Python.

3. Pass the Callback to a C Function

Finally, you will pass the callback function to a C function that will invoke it during its execution.

Example: Calling a Callback Function

C Code (mylib.c)

Here’s an example of a C function that accepts a callback:

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

Python Code

Below is how you can call the callback function in Python using ctypes:

Explanation

1. Callback Function Type: The CALLBACK_FUNC_TYPE is defined using ctypes.CFUNCTYPE, which specifies that the callback returns None and accepts a single int argument.
2. Implement Callback: The my_callback function in Python prints the value it receives.
3. Wrap Callback: The Python function is wrapped in a ctypes callable object using CALLBACK_FUNC_TYPE.
4. Load Library: The shared C library is loaded using ctypes.CDLL().
5. Invoke Callback: The C function perform_operation is called, passing the callback_instance, which allows the C code to invoke the Python callback.

Output

When you run the Python code, you should see the following output:

Practical Example

Complete Example

Here’s a complete example combining both C and Python code to demonstrate how to call a callback function.

C Code (mylib.c)

Compile the C code into a shared library:

Python Code

Output

When you run the Python code, the output will be:

Conclusion

Calling a callback function in Python using the ctypes library allows for flexible interaction with C libraries. By defining the callback type, implementing the callback in Python, and passing it to a C function, you can easily manage complex operations and event handling. This technique enhances the integration of Python with C, providing a powerful way to extend functionality and control behavior in native libraries.