How do I declare a bool function in a window.h thread?

Question

For my assignment I need to write a multithreaded program that outputs all the prime numbers less than or equal to the number entered by the user, in a separate thread.

I am new to the threads and do not fully understand how to correctly implement them. But I created a boolean function which determines whether an integer is a prime number or not and I named it bool isPrime. However, the bool isPrime gives me the following error "declaration is incompatible with "LPVOID isPrime". I am not sure how to fix it, I've also noticed that my compiler (visual c++) gives a red underline for "cout" and "cin" but if I declare using namespace std, than the compiler underlines the "std" and gives this error "name must be a namespace name", I don't know what it supposed to mean. How could I fix these two problems?

#include <Windows.h>

DWORD Prime;


DWORD WINAPI primeThreadFun(LPVOID isPrime)
{
    bool isPrime(int n)
    {    
        float sqrtn = sqrt(n);
        if (n < 2) return false;
        if (n < 4) return true;
        if (n % 2 == 0) return false;
        for (int d = 3; d <= sqrtn; d += 2)
        if (n % d == 0) return false;
        return true;
    }

}

int main(int argc, char *argv [])
{
    DWORD ThreadId;

    HANDLE ThreadHandle;

    int n;
    cout << "Enter a number: " << endl;
    cin >> n;
    for (int i = 0; i <= n; i++)
    if (isPrime(i)) cout << i << " ";
    cout endl;
}

ThreadHandle = CreateThread(NULL, 0, primeThreadFun, (LPVOID) isPrime, NULL, ThreadID []);

if (ThreadHandle != NULL)
{
    WaitForSingleObject(ThreadHandle INFINITE);

    CloseHandle(ThreadHandle);
}

}

Answer 1

First, CreateThread requires a specific signature (reference):

DWORD WINAPI ThreadProc(LPVOID lpParam);

So your thread declaration will need the same signature.

Second, what you are attempting to do is not a good way to separate a thread. Think of a thread as a separate process. It will do some task and then end.

You need to do something like (NOTE: the following is not debugged and should only be used as a starting point - this is done intentionally as I'm not here to do your homework for you):

struct MyData
{
    std::vector<unsigned int> myVec;
    unsigned int myCap;
};

DWORD WINAPI MyThreadProc(LPVOID lpParam)
{
    MyData* pData = (MyData*)lpParam;
    for (int i = 4; i <= myCap; ++i)
    {
        int root = (int)std::sqrt(i);
        bool isPrime = true;
        for (int j = 2; j <= root; ++j)
        {
            if (i % j == 0)
            {
                isPrime = false;
                break;
            }
        }

        if (isPrime)
        {
            pData->myVec.push_back(i);
        }
    }

}

int main()
{
    MyData data;
    std::cin >> data.myCap;
    HANDLE hThread = CreateThread(NULL, 0, MyThreadProc, &data, NULL, NULL);
    std:: cout << "Finding Primes" << std::endl;
    while (WaitForSingleObject(hThread, 1000) != WAIT_OBJECT_0)
    {
        std::cout << ".";
    }
    std::cout << std::endl;
    std::copy(data.begin(), data.end(), std::ostream_iterator<unsigned int>(std::cout, " "));
    return 0;
}

Answer 2

C/C++ doesn't support nested functions -- isPrime is nested in primeThreadFun. Move it out, and call it from primeThreadFun.

also, you have a global variable named "isPrime" - you can't have that and a global function "isPrime()"

For cout you need to include "iostream"