How to Call a C Function in Python

Calling C functions in Python can greatly enhance the capabilities of your programs by incorporating functionality from C libraries. Python, known for its simplicity and versatility, offers various methods to seamlessly integrate C code, resulting in improved performance and access to low−level system APIs. This article explores the topic of calling C functions in Python, highlighting different approaches that leverage Python's strengths. One popular method is using the ctypes library, which allows direct invocation of C functions from dynamic link libraries. We will also delve into the CFFI library, which provides a high−level interface for calling C functions, and the powerful combination of ctypes with the numpy library to efficiently handle numerical data. Additionally, we will touch upon the Cython language, enabling the creation of C extensions using Python syntax. By mastering these techniques, we can utilize the power of C in your Python code, unlocking more outstanding programming capabilities and enabling the solution of a wide range of tasks.

Using the ctypes Library

The Python ctypes library is a robust resource that empowers us to generate C−compatible data types and directly invoke functions in dynamic link libraries or shared libraries using Python. Below, you'll find a comprehensive walkthrough detailing how to utilize ctypes for calling a C function:

Step 1: Import the ctypes module

import ctypes

Step 2: Load the shared library containing the C function

lib = ctypes.CDLL('./')

This step loads the shared library file into memory, making the functions within it accessible.

Step 3: Declare the function prototype

my_function = lib.my_function
my_function.argtypes = [ctypes.c_int, ctypes.c_int]
my_function.restype = ctypes.c_int

During this step, we establish the function prototype by associating the C function from the library with a Python variable. Furthermore, we define the argument types using argtypes and the return type using restype.

Step 4: Call the C function from Python

result = my_function(5, 10)

Finally, we can call the C function just like any other Python function. Pass the required arguments, and it will execute the corresponding C code. The result returned by the C function can be stored in a variable and printed or used further in your Python program.

Using the CFFI Library

CFFI (C Foreign Function Interface) is another library that enables calling C functions from Python. It provides a higher−level interface compared to ctypes and supports a broader range of C features. Here's an example:

Step 1: Install the cffi library

Use the command pip install cffi to install the CFFI library on your system.

Step 2: Import the cffi module and create a CFFI interface object

In your Python code, import the cffi module to gain access to its functionality. Then, create a CFFI interface object using the following code:

import cffi
ffi = cffi.FFI()

Step 3: Define the C function prototype using the CFFI syntax

When using the CFFI library, it is necessary to define the C function prototype before calling it from Python. To achieve this, we can utilize the ffi.cdef() function, which allows us to specify the function signature using C syntax. By providing the correct function signature, CFFI gains an understanding of the C function's arguments and return type, enabling seamless communication between Python and C. Here is an example code:

    int my_function(int a, int b);

Step 4: Load the shared library and obtain a C function pointer

To load the shared library that contains the desired C function using the CFFI library, we can employ the ffi.dlopen() function. This function is provided by CFFI and enables us to specify the path to the shared library file (such as a .so file). It returns a library handle, granting us access to the C functions within the library. Here's an example illustrating the process:

lib = ffi.dlopen('./')
my_function = lib.my_function

Step 5: Call the C function from Python

After obtaining the C function pointer, calling the C function becomes as straightforward as calling any other Python function. Simply provide the arguments in the format specified by the C function prototype. Here's an example:

result = my_function(5, 10)

By following these steps, we can effectively hold the CFFI library to call C functions from Python, bridging the gap between the two languages and utilizing the power of C within your Python code.

Using the Cython Language

Cython extends Python's syntax, enabling the creation of C extensions in a Python−like manner. It serves as a bridge between Python and C, allowing developers to combine Python's simplicity with C's performance. With Cython, you can leverage Python's high−level features while seamlessly incorporating low−level C functionality into your code.

Step 1: Create a .pyx file (e.g., mymodule.pyx) with the following content

cdef extern from "./myheader.h":
    int my_function(int a, int b)

def call_c_function(int a, int b):
    return my_function(a, b)

Step 2: Create a file with the following content

from setuptools import setup
from Cython.Build import cythonize


Step 3: Compile the Cython module

To compile the Cython module, navigate to the directory containing the file in the terminal and execute the following command:

python build_ext --inplace

This command will generate the necessary C files and compile them into a shared library.

Step 4: Import the module and call the C function

import mymodule

result = mymodule.call_c_function(5, 10)


To summarize, the ability to call C functions in Python provides a robust means of integrating high−performance capabilities and direct system access into Python applications. Through the utilization of libraries like ctypes, CFFI, and Cython, developers can effortlessly connect Python and C, effectively closing the gap between the two languages. Whether you aim to optimize computationally demanding tasks, interact with hardware devices, or leverage existing C libraries, these techniques offer exceptional flexibility and efficiency. By mastering the art of calling C functions in Python, developers gain access to a vast array of possibilities, allowing them to conquer diverse programming challenges while combining the best of both worlds.

Updated on: 20-Jul-2023

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