How to define a callback function in ctypes?

Table of Contents

• Introduction
• What is a Callback Function in ctypes?
  • Key Concept:
• Example of Defining a Callback Function
  • Step-by-Step Example
    • 1. Define the Callback Function
    • 2. Define the CFUNCTYPE
    • 3. Convert the Python Function into a C Callback
    • 4. Load the C Library and Define the C Function Prototype
    • 5. Call the C Function with the Callback
  • Full Example Code
• More Complex Callback Examples
  • Callback with Multiple Arguments
  • Callback with a Return Value
• Conclusion

Introduction

In Python, the ctypes library allows calling C functions directly from shared libraries. When working with C libraries, you may encounter situations where a C function expects a callback function. In this guide, we'll explain how to define a callback function in Python using ctypes and pass it to a C function.

What is a Callback Function in ctypes?

A callback function is a function that is passed as an argument to another function. The receiving function can then call this callback during its execution. When using ctypes, Python allows you to pass a Python function as a callback to a C function, provided you properly define the argument and return types.

Key Concept:

• CFUNCTYPE: Used to define the data types of the callback function's arguments and return type in Python when passing it to a C function.

Example of Defining a Callback Function

Consider the following example of a C function that accepts a callback function:

To interact with this function from Python, you need to define a callback in Python and pass it to the C function using ctypes.

Step-by-Step Example

1. Define the Callback Function

First, define the Python function that you want to use as the callback. In this case, the callback takes a single int argument.

2. Define the CFUNCTYPE

Next, use ctypes.CFUNCTYPE to define the prototype of the callback function. This tells Python the types of arguments and the return type the callback should have.

3. Convert the Python Function into a C Callback

Now, convert your Python function into a C-compatible callback by creating an instance of CALLBACK_TYPE.

4. Load the C Library and Define the C Function Prototype

Load the C library and define the C function that accepts the callback.

5. Call the C Function with the Callback

Finally, call the C function, passing the callback and the integer value.

Full Example Code

More Complex Callback Examples

Callback with Multiple Arguments

If your callback function takes multiple arguments, simply adjust the argument list in CFUNCTYPE. For example, a callback that takes two integers:

You would adjust the C function prototype to match:

Callback with a Return Value

For a callback that returns a value, specify the return type in CFUNCTYPE. For example, a callback that returns an integer:

Conclusion

Defining a callback function in Python with ctypes involves specifying the function prototype using CFUNCTYPE and passing a Python function as the callback to a C function. By following this process, you can enable C functions to call Python code, making it easy to integrate complex workflows between Python and C.