In mouse hooking, the wheel delta is always 0 - c++

Question

#include <iostream>
#include <windows.h>
LRESULT CALLBACK mouseProc(int nCode, WPARAM wParam, LPARAM lParam){
if(wParam == WM_MOUSEWHEEL){
std::cout << "wheel: " << GET_WHEEL_DELTA_WPARAM(wParam) << std::endl;
}else{
MOUSEHOOKSTRUCT* mouselparam = (MOUSEHOOKSTRUCT*)lParam;
std::cout << "etc: " << wParam << " - " << mouselparam->pt.x << "x" << mouselparam->pt.y << std::endl;
}
return CallNextHookEx(NULL, nCode, wParam, lParam);
}
int main(int argc, char *argv[]) {
HHOOK hhkLowLevelMouse = SetWindowsHookEx(WH_MOUSE_LL, mouseProc, 0, 0);
MSG msg;
while (!GetMessage(&msg, NULL, NULL, NULL)) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
UnhookWindowsHookEx(hhkLowLevelMouse);
return 0;
}
This is the full code.
"etc:" works perfectly as I intended.
"wheel:" always outputs 0.
am i missing something?
Using HIWORD instead of GET_WHEEL_DELTA_WPARAM gives the same result.

Answer 1

GET_WHEEL_DELTA_WPARAM() only works with the wParam of a real WM_MOUSEWHEEL window message, not the wParam of a WH_MOUSE_LL hook.
In the hook, the wParam is just the message identifier by itself, nothing more. ALL of the mouse details are stored in a MSLLHOOKSTRUCT struct pointed by the lParam. Which you attempted to look at for all mouse messages other than WM_MOUSEWHEEL, but you are looking at the wrong struct (MOUSEHOOKSTRUCT is used by WH_MOUSE, not WH_MOUSE_LL).
Per the LowLevelMouseProc callback function documentation:
wParam [in]
Type: WPARAM
The identifier of the mouse message. This parameter can be one of the following messages: WM_LBUTTONDOWN, WM_LBUTTONUP, WM_MOUSEMOVE, WM_MOUSEWHEEL, WM_MOUSEHWHEEL, WM_RBUTTONDOWN, or WM_RBUTTONUP.
lParam [in]
Type: LPARAM
A pointer to an MSLLHOOKSTRUCT structure.
And the MSLLHOOKSTRUCT structure documentation:
mouseData
Type: DWORD
If the message is WM_MOUSEWHEEL, the high-order word of this member is the wheel delta. The low-order word is reserved. A positive value indicates that the wheel was rotated forward, away from the user; a negative value indicates that the wheel was rotated backward, toward the user. One wheel click is defined as WHEEL_DELTA, which is 120.
Try this instead:
LRESULT CALLBACK mouseProc(int nCode, WPARAM wParam, LPARAM lParam){
if (nCode == HC_ACTION) {
MSLLHOOKSTRUCT* mouselparam = (MSLLHOOKSTRUCT*)lParam;
if (wParam == WM_MOUSEWHEEL) {
short delta = HIWORD(mouselparam->mouseData);
// alternatively, GET_WHEEL_DELTA_WPARAM() would also work here:
// short delta = GET_WHEEL_DELTA_WPARAM(mouselparam->mouseData);
std::cout << "wheel: " << delta << std::endl;
}else{
std::cout << "etc: " << wParam << " - " << mouselparam->pt.x << "x" << mouselparam->pt.y << std::endl;
}
}
return CallNextHookEx(NULL, nCode, wParam, lParam);
}