CODEWITHSUNDEEP
winapi
Noitidart
Noitidart

Reputation: 37268

Why do WM_APPCOMMAND LPARAM have to be multiplied by 65536

I am trying to control the master volume. I am able to succesfully do that with this:

HWND mainhwnd = CreateWindow(szWindowClass, _T("window-noit-ext-profilist"), 0, 0, 0, 0, 0, HWND_MESSAGE, NULL, wcex.hInstance, NULL);
if (!mainhwnd) {
    MessageBox(NULL, _T("Profilist: Call to CreateWindow failed!"), _T("window-noit-ext-profilist"), NULL);
    return 1;
}


SendMessage(mainhwnd, WM_APPCOMMAND, (WPARAM)mainhwnd, (LPARAM)(APPCOMMAND_VOLUME_MUTE * 65536)); // mute

SendMessage(mainhwnd, WM_APPCOMMAND, (WPARAM)mainhwnd, (LPARAM)(APPCOMMAND_VOLUME_DOWN * 65536)); // vol down

SendMessage(mainhwnd, WM_APPCOMMAND, (WPARAM)mainhwnd, (LPARAM)(APPCOMMAND_VOLUME_UP * 65536)); // vol up

Why do I have to multiply by 65,536? The docs do not state this. IF I don't multiply, then it doesn't work.

Upvotes: 3

Views: 873

Answers (1)

Soonts
Soonts

Reputation: 21956

For WM_APPCOMMAND, the lParam parameter packs three values in a single integer.

The lower 16bit word, dwKeys, indicates whether various virtual keys are down.

The higher 16bit word packs two fields: the highest 4 bits, uDevice, specifies the input device that is generating the input event. The lower 12 bits, cmd, contains the application command.

Multiplying by 65536 is same as bit shifting by 16 bits to the left (because 65536 = 0x10000 in hexadecimal). So, when you send the message with APPCOMMAND_VOLUME_UP * 65536, you are specifying the cmd is APPCOMMAND_VOLUME_UP, and the uDevice and dwKeys are both zero.

Upvotes: 8

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