Using thread APC as FIFO queue to avoid using explicit locking/synchronization when enqueueing/dequeueing.

apc_as_fifo_queue.cpp

#include <Windows.h>

typedef struct _param_t {
    int code;
} param_t;

HANDLE hTerm = NULL;    // terminate event
HANDLE hThr = NULL;     // apc thread
HANDLE hReady = NULL;   // apc thread ready event

// This is our FIFO queue thread.
DWORD __stdcall InternalApcDispatcher(LPVOID lpData)
{
    _tprintf(L"[%d] dispatcher started.\n", GetCurrentThreadId());

    SetEvent(hReady);

    while (TRUE) {
        // All we need to do here is to set this thread to alertable wait to be able to service
        // user mode APCs. Clients will queue APCs to this thread for servicing.
        if (WaitForSingleObjectEx(hTerm, INFINITE, TRUE) == WAIT_OBJECT_0) break;
    }

    return 0;
}

// Helper function to add work to our queue.
DWORD QueueWork(PAPCFUNC pFunction, ULONG_PTR pData)
{
    return QueueUserAPC(pFunction, hThr, pData);
}

// Sample work to be queued.
VOID CALLBACK APCProc(_In_ ULONG_PTR dwParam)
{
    int i = (int)dwParam;
    _tprintf(L"threadid: %d, data: %d\n", GetCurrentThreadId(), i);
}

int main()
{
    hTerm = CreateEvent(NULL, TRUE, FALSE, NULL);
    hReady = CreateEvent(NULL, TRUE, FALSE, NULL);

    // create the FIFO queue thread
    hThr = CreateThread(NULL, 0, InternalApcDispatcher, NULL, 0, NULL);

    // wait for it to be ready
    WaitForSingleObject(hReady, INFINITE);

    // queue 10 work items with index as our data
    for (int i = 0; i < 10; i++) {
        QueueWork(APCProc, (ULONG_PTR)i);
    }

    // wait a bit to allow APC processing
    Sleep(3000);

    // close everything
    SetEvent(hTerm);
    CloseHandle(hTerm);
    CloseHandle(hReady);
    WaitForSingleObject(hThr, INFINITE);

    return 0;
}