Using APC as FIFO queue in Windows

2017-04-20

April 20, 2017

1 minute

In this way, we can implement a FIFO queue without using explicit locking/synchronization for enqueueing/dequeueing.

For self reference:

	#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;
	}