Calling C from Python

ctypes is Python library for calling C code from Python. Following is a very simple example of how to build a shared object (*.so file) from C code which can be called from Python using ctypes.

Simple example §

1. Write the C library.

liblife.c §

int life(void);

int life()
{
    return 42;
}

2. Compile.

gcc -shared -o liblife.so -fPIC liblife.c

3. Use Python ctypes to load library and execute.

life.py §

import ctypes
import os

here = os.path.dirname(os.path.abspath(__file__))
libc = ctypes.cdll.LoadLibrary(os.path.join(here, "liblife.so"))
print(libc.life())

Running life.py will echo 42 to stdout.

Step 2 in detail §

To expand upon step 2, it is actually the following two steps combined:

1. Compile the code with the Position Independent Code Flag, generating testlib.o:

gcc -c -fPIC liblife.c -o liblife.o

2. Generated a shared object file from the object file:

gcc liblife.o -shared -o liblife.so

Further reading §

• Use -soname option (and example).
• More compilation details.
• Ensure shared object is 32-bit or 64-bit as appropriate.
• Passing more complex data types such as arrays.

Alternatives §

• cffi is another Python library, which prefers working at the API level, in contrast to ctypes which works at the ABI level.
• SWIG is written in C/C++, and supports interfacing between C/C++ and a variety of high-level languages, including Python. It requires writing a SWIG interface file.
• Cython.
• pybind11 (for C++11).