How to get result of a thread in C++ - c++

Question

I have written a function which It is doing the Fibonacci calculation. I wanted to start and execute it with CreateThread as a thread. Finally, I want to save the result of the thread (the Fibonacci) and show it in the console. What is the problem with my code? It doesn't work properly. It starts the thread, but I don't know how should I store the result of the thread and show it.
#include <Windows.h>
#include <iostream>
DWORD WINAPI Fibonacci(LPVOID arg_repeat) {
DWORD dwValue = *(int*)arg_repeat;
if (dwValue < 3)
return 1;
return Fibonacci((int*)dwValue - 1) + Fibonacci((int*)dwValue - 2);
}
auto main(int argc, const char* argv[]) -> decltype(0) {
DWORD dwFibonacciValue;
std::cout << "Fibonacci Value: ";
std::cin >> dwFibonacciValue;
DWORD dwThreadId;
HANDLE hThreading = CreateThread(NULL, 0, Fibonacci, &dwFibonacciValue, NULL, &dwThreadId);
WaitForSingleObject(hThreading, INFINITE);
std::cout << "Fibonacci Result: " << dwResult << std::endl;
CloseHandle(hThreading);
return 0;
}

Answer 1

Your code is wrong beause (int*)dwValue - 1 is not a valid pointer.
You should separate the thread function from the fibonacci function. There will be much less dubious and wrong casts and code will be much clearer:
#include <Windows.h>
#include <iostream>
// Clean and easy to read fibonacci function without fancy casts
DWORD Fibonacci(DWORD dwValue) {
if (dwValue < 3)
return 1;
return Fibonacci(dwValue - 1) + Fibonacci(dwValue - 2);
}
// Thread function
DWORD WINAPI Thread(LPVOID arg_repeat) {
return Fibonacci(*(DWORD*)arg_repeat); // the only cast int the whole code
}
int main(int argc, const char* argv[]) -> decltype(0) {
// it's 'int main', not 'auto main'
DWORD dwFibonacciValue;
std::cout << "Fibonacci Value: ";
std::cin >> dwFibonacciValue;
DWORD dwThreadId;
HANDLE hThreading = CreateThread(NULL, 0, Thread, &dwFibonacciValue, 0, &dwThreadId);
WaitForSingleObject(hThreading, INFINITE);
// get return value of the thread (your actual question)
DWORD dwResult;
GetExitCodeThread(hThreading, &dwResult);
std::cout << "Fibonacci Result: " << dwResult << std::endl;
CloseHandle(hThreading);
return 0;
}
There is no error check whatsoever in this code for clarity.
Other detail:
CreateThread(NULL, 0, Thread, &dwFibonacciValue, NULL, &dwThreadId);
// ^ you should provide a DWORD
// here and not a pointer
should be
CreateThread(NULL, 0, Thread, &dwFibonacciValue, 0, &dwThreadId);