Why are Low Level Mouse Hooks Causing Lag - c++

Question

I'm trying to make an application that just detects if the left mouse button is held down. In trying to do this, as well as learn mouse hooks, I copy-pasted a hook from an example source (https://cboard.cprogramming.com/windows-programming/119909-setwindowshookex-lowlevelmouseproc.html) just to see what it would do. The problem is that it lags my computer. Why is this, and how can I fix it?
#define _CRT_SECURE_NO_WARNINGS
#include <windows.h>
#include <iostream>
#include <cstdio>
using namespace std;
HHOOK g_Hook;
HANDLE g_evExit;
LRESULT CALLBACK LowLevelMouseProc (int code, WPARAM wParam, LPARAM lParam)
{
if (code == HC_ACTION)
{
const char *msg;
char msg_buff[128];
switch (wParam)
{
case WM_LBUTTONDOWN: msg = "WM_LBUTTONDOWN"; break;
case WM_LBUTTONUP: msg = "WM_LBUTTONUP"; break;
case WM_RBUTTONDOWN: msg = "WM_RBUTTONDOWN"; break;
case WM_RBUTTONUP: msg = "WM_RBUTTONUP"; break;
default:
sprintf(msg_buff, "Unknown msg: %u", wParam);
msg = msg_buff;
break;
}//switch
const MSLLHOOKSTRUCT *p =
reinterpret_cast<const MSLLHOOKSTRUCT*>(lParam);
cout << msg << " - [" << p->pt.x << ',' << p->pt.y << ']' << endl;
static bool left_down = false;
static bool right_down = false;
switch (wParam)
{
case WM_LBUTTONDOWN: left_down = true; break;
case WM_LBUTTONUP: left_down = false; break;
case WM_RBUTTONDOWN: right_down = true; break;
case WM_RBUTTONUP: right_down = false; break;
}//switch
if (left_down && right_down)
SetEvent(g_evExit);
}//if
return CallNextHookEx(g_Hook, code, wParam, lParam);
}//LowLevelMouseProc
int main()
{
g_evExit = CreateEvent(0, TRUE, FALSE, 0);
if (!g_evExit)
{
cerr << "CreateEvent failed, le = " << GetLastError() << endl;
return 1;
}//if
g_Hook = SetWindowsHookEx(WH_MOUSE_LL, &LowLevelMouseProc,
GetModuleHandle(0), 0);
if (!g_Hook)
{
cerr << "SetWindowsHookEx() failed, le = " << GetLastError() << endl;
return 1;
}//if
cout << "Press both left and right mouse buttons to exit..." << endl;
MSG msg;
DWORD status;
while (1)
{
while (PeekMessage(&msg, 0, 0, 0, PM_REMOVE))
DispatchMessage(&msg);
status = MsgWaitForMultipleObjects(1, &g_evExit, FALSE,
INFINITE, QS_ALLINPUT);
if (status == (WAIT_OBJECT_0 + 1))
{
// there are messages to process, eat em up
continue;
}//if
else
{
// assume g_evExit is signaled
break;
}//else
}//while
cout << "Exiting..." << endl;
UnhookWindowsHookEx(g_Hook);
CloseHandle(g_evExit);
return 0;
}//main

Answer 1

You need a message pump. The code you posted uses PeekMessage in combination with MsgWaitForMultipleObjects, but it's probably a better idea to use GetMessage along with TranslateMessage and DispatchMessage, as showcased in How to manually run message pump in C++:
MSG msg;
while (GetMessage(&msg, NULL, 0, 0) > 1) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
With this loop, I was able to set up a WH_MOUSE_LL hook with SetWindowsHookExA without having the mouse input lag, and correctly receiving all events.
Make sure the hook is installed on the same thread that will pump the messages.