How to return a structure from a function in ctypes?

Table of Contants

Introduction

The ctypes library in Python allows you to interface with C libraries, including returning structures from C functions. This can be useful when you need to encapsulate related data in a single entity and return it to your Python code. This guide will show you how to define a structure in Python, call a C function, and retrieve the structure from it.

Returning a Structure from a Function in ctypes

1. Defining a Structure

First, define the structure in Python using ctypes.Structure. You need to specify the fields and their data types.

2. Declaring the C Function

Next, declare the C function prototype in Python, including the return type.

3. Calling the Function

Finally, you can call the function and retrieve the structure.

Example: Returning a Structure

C Code (mylib.c)

Let’s consider the following C function that returns a structure defined in a file named mylib.c:

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

Python Code

Here’s how to return a structure from the C function using ctypes:

Explanation

1. Structure Definition: The Data class is defined using ctypes.Structure, with _fields_ specifying its fields and types.
2. Load Library: The C library is loaded using ctypes.CDLL().
3. Function Prototypes: The argument types and return type for the create_data function are set using argtypes and restype.
4. Function Call: The function is called with two arguments (an integer and a float), and the returned structure is assigned to result.
5. Accessing Fields: The fields of the returned structure can be accessed directly through the result variable.

Practical Example

Complete Example

Here is a complete example, including both the C code and Python code, to demonstrate how to return a structure.

C Code (mylib.c)

Compile the C code into a shared library:

Python Code

Output

When you run the Python code, the output will display the values of the fields in the returned structure:

Conclusion

Returning structures from C functions using the ctypes library in Python allows you to manage related data efficiently. By following the steps outlined above, you can successfully retrieve structures from C code and utilize them in your Python applications. This integration enhances the capabilities of your Python programs, enabling you to leverage existing C libraries effectively.