Using a C++ DLL from Python by involving ctypes package

DllTest.cpp

#ifdef _MSC_VER
#define _SCL_SECURE_NO_WARNINGS
#define _CRT_SECURE_NO_WARNINGS
#endif

#include <string>
#include <sstream>
#include <string.h>

#include "DllTest.h"

__declspec( dllexport )
int
doubler( int i )
{
    return 2 * i;
}

__declspec( dllexport )
int
triple( int i )
{
    return 3 * i;
}

__declspec( dllexport )
void
triple_str( int i, char* buffer, int* len )
{
    std::stringstream ss;
    ss << "Triple of " << i << " equal to " << 3 * i;
    strncpy( buffer, ss.str().c_str(), *len );
    *len = ss.str().size();
}

DllTest.py

import os
import sys
import ctypes

if (sys.platform == "win32"):
    libname = "Release/DllTest.dll"
else:
    libname = "libDllTest.so"

lib = ctypes.cdll.LoadLibrary(libname)

print(lib.doubler(3))
print(lib.triple(6))

len = ctypes.c_int(200)
s   = ctypes.create_string_buffer(len.value)
lib.triple_str(7, s, ctypes.byref(len))

print(repr(s.value))
print(len.value)

len = ctypes.c_int(200)
lib.triple_str(10, s, ctypes.byref(len))

print(repr(s.value))
print(len.value)

len = ctypes.c_int(200)
lib.triple_str(1000, s, ctypes.byref(len))

print(repr(s.value))
print(len.value)

DllTestApp.cpp

#include <iostream>

#include "DllTest.h"

#define BUFFER_SIZE 200

int main()
{
    std::cout << doubler( 2 )    << std::endl;
    std::cout << triple( 3 )     << std::endl;
    
    char buffer[BUFFER_SIZE];
    int  buffer_len = BUFFER_SIZE;
    triple_str( 4, buffer, &buffer_len );
    std::cout << buffer << " and len = " << buffer_len << std::endl;
}