Question

I need to create two threads that strictly alternates. Here is sample code what I use:

#include <Windows.h>
#include <iostream>
using std::cout;
using std::endl;

HANDLE g_hMutex1;
HANDLE g_hMutex2;

DWORD WINAPI ThreadFunc1(LPVOID lpParam);
DWORD WINAPI ThreadFunc2(LPVOID lpParam);

int main(void)
{
    int nCalcNumber = 10;
    DWORD dwThreadId;
    HANDLE pThreadHandles[2];

    g_hMutex1 = CreateMutex(NULL, FALSE, NULL);
    g_hMutex1 = CreateMutex(NULL, FALSE, NULL);

    pThreadHandles[0] = CreateThread(
        NULL,
        0,
        ThreadFunc1,
        static_cast<void*>(&nCalcNumber),
        0,
        &dwThreadId);

    pThreadHandles[1] = CreateThread(
        NULL,
        0,
        ThreadFunc2,
        static_cast<void*>(&nCalcNumber),
        0,
        &dwThreadId);

    WaitForMultipleObjects(2, pThreadHandles, TRUE, INFINITE);

    CloseHandle(pThreadHandles[0]);
    CloseHandle(pThreadHandles[1]);
    CloseHandle(g_hMutex1);
    CloseHandle(g_hMutex2);

    return 0;
}

DWORD WINAPI ThreadFunc1(LPVOID lpParam)
{
    int* nCalcNumber = static_cast<int*>(lpParam);

    for (int i = 0; i < *nCalcNumber; i++)
    {
        WaitForSingleObject(g_hMutex1, INFINITE);

        cout << "Func 1" << endl;

        ReleaseMutex(g_hMutex1);
    }

    return 0;
}

DWORD WINAPI ThreadFunc2(LPVOID lpParam)
{
    int* nCalcNumber = static_cast<int*>(lpParam);

    for (int i = 0; i < *nCalcNumber; i++)
    {
        WaitForSingleObject(g_hMutex1, INFINITE);

        cout << "Func 2" << endl;

        ReleaseMutex(g_hMutex1);
    }

    return 0;
}

And a result, which I expect to receive:

 Func 1
 Func 2
 Func 1
 Func 2
 Func 1
 Func 2
 ...and so one

What should be added to get the desired result. Can I use for that the second mutex?

Answer 1

If you can use semaphore: You can use semaphore instead of mutex, it’s easy to use same as mutex.

This code works fine:

#include <windows.h>
#include <iostream>
using std::cout;
using std::endl;

PHANDLE sem1;
PHANDLE sem2;

DWORD WINAPI ThreadFunc1(LPVOID lpParam);
DWORD WINAPI ThreadFunc2(LPVOID lpParam);

int main(void)
{
    int nCalcNumber = 10;
    DWORD dwThreadId;
    HANDLE pThreadHandles[2];

    sem1 = (PHANDLE) CreateSemaphore(NULL, 1, 1, NULL);
    sem2 = (PHANDLE) CreateSemaphore(NULL, 0, 1, NULL);


    pThreadHandles[0] = CreateThread(
                                     NULL,
                                     0,
                                     ThreadFunc1,
                                     static_cast<void*> (&nCalcNumber),
                                     0,
                                     &dwThreadId);

    pThreadHandles[1] = CreateThread(
                                     NULL,
                                     0,
                                     ThreadFunc2,
                                     static_cast<void*> (&nCalcNumber),
                                     0,
                                     &dwThreadId);

    WaitForMultipleObjects(2, pThreadHandles, TRUE, INFINITE);

    CloseHandle(pThreadHandles[0]);
    CloseHandle(pThreadHandles[1]);
    CloseHandle(sem1);
    CloseHandle(sem2);

    return 0;
}

DWORD WINAPI ThreadFunc1(LPVOID lpParam)
{
    int* nCalcNumber = static_cast<int*> (lpParam);

    for (int i = 0; i < *nCalcNumber; i++)
    {
        WaitForSingleObject(sem1, INFINITE);

        cout << "Func 1" << endl;

        ReleaseSemaphore(sem2, 1 ,NULL);
    }

    return 0;
}

DWORD WINAPI ThreadFunc2(LPVOID lpParam)
{
    int* nCalcNumber = static_cast<int*> (lpParam);

    for (int i = 0; i < *nCalcNumber; i++)
    {

        WaitForSingleObject(sem2, INFINITE);

        cout << "Func 2" << endl;

        ReleaseSemaphore(sem1, 1 ,NULL);
    }

    return 0;
}