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Methods for Python Calling C/C++ Programs

Recently, while debugging, I encountered a situation where Python was running very slowly. Therefore, I researched methods to embed C++ programs in Python and recorded them for future reference.

Generally, calling C/C++ programs in Python can be divided into 3 steps:

• 1. Write the C/C++ implementation program. – 2. Compile the C/C++ program into a dynamic library. – 3. Call the compiled library in Python. There are some differences to note when Python calls C/C++ programs.

1. Python Calling C Functions

Calling C language programs from Python is relatively simple. Compile the C program and then use the ctypes module in Python to call it.

C Language Source File: called_c.c

// Compilation command gcc -o libpycall.so -shared -fPIC called_c.c
#include<stdio.h>
int foo(int a, int b){<!-- -->
printf("a:%d, b:%d.", &a, &b);
return 0;
}

Input the following in the command line or terminal:

gcc -o libpycall.so -shared -fPIC called_c.c

This generates the libpycall.so dynamic library file, which can then be used to call the foo function in Python. Python File: py_call_c.py

import ctypes
dll = ctypes.cdll.LoadLibrary
lib = dll('./libpycall.so') // Path of the library file just generated
lib.foo(1, 3)

Running py_call_c.py outputs:

a:1, b:3

2. Python Calling C++ Classes

Since C++ supports function overloading, when compiled with g++ in C++ mode, the compiler adds additional information to the function names, making it impossible for the ctypes module to find the functions generated by g++. Therefore, to allow g++ to compile in C language mode so that it can find the generated function names, the extern keyword must be used to wrap the code.

C++ Source File: cpp_called.cpp

// Python calling c++ (class) dynamic link library
#include <iostream>
using namespace std;
 
class TestLib
{<!-- -->
    public:
        void display();
        void display(int a);
};
void TestLib::display() {<!-- -->
    cout<<"First display"<<endl;
}
 
void TestLib::display(int a) {<!-- -->
    cout<<"Second display:"<<a<<endl;
}
extern "C" {<!-- -->
    TestLib obj;
    void display() {<!-- -->
        obj.display();
      }
    void display_int(int a) {<!-- -->
        obj.display(a);
      }
}

Input the compilation command in the command line or terminal:

g++ -o libpycallcpp.so -shared -fPIC cpp_called.cpp

Explanation of compilation parameters: -fPIC: Generates position-independent code suitable for dynamic linking; -L path: Indicates to search for library files in the path directory, such as -L. means in the current directory; -I path: Indicates to search for header files in the path directory; -o file: Specifies the output file as file; -shared: Generates a shared library file;

Generate libpycallcpp.so and call it in Python. Python File: py_call_c.py

import ctypes
dll = ctypes.cdll.LoadLibrary
lib = dll('./libpycallcpp.so') // Path of the library file just generated
lib.display()
lib.display_int(0)

Output:

First display
Second display:0

OK, basic functionality achieved, advanced calls will be filled in later

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