Windows keyboard messages and Alt keys - c++

Question

I'm trying to understand how does the Message Loop and generally keyboard messages works in the Windows systems. I've written a simple program featuring the Message Loop:
// include the basic windows header file
#include <windows.h>
#include <windowsx.h>
#include <stdio.h>
// the WindowProc function prototype
LRESULT CALLBACK WindowProc(HWND hWnd,
UINT message,
WPARAM wParam,
LPARAM lParam);
char MsgBuff[300];
template <typename T>
char* BinVal(T SrcVal) {
const short BitsCount = sizeof(T) * CHAR_BIT;
const short SeparatorCount = sizeof(T) * 2 - 1;
static char BinValStr[BitsCount + SeparatorCount] = { 0 };
printf("BitsCount: %d\n", BitsCount);
printf("SeparatorCount: %d\n", SeparatorCount);
printf("BinValStr size: %d\n", BitsCount + SeparatorCount);
int i = 0;
int j = 0;
for (i = BitsCount + SeparatorCount - 1; i >= 0; i--) {
if ((j + 1) % 5 == 0) {
BinValStr[i] = ' ';
}
else {
if ((SrcVal & 1) == 1)
BinValStr[i] = '1';
else
BinValStr[i] = '0';
SrcVal >>= 1;
}
j++;
//printf(": %d, %d\n", i, SrcVal&1);
}
return BinValStr;
}
// the entry point for any Windows program
int WINAPI WinMain(
_In_ HINSTANCE hInstance,
_In_opt_ HINSTANCE hPrevInstance,
_In_ LPSTR lpCmdLine,
_In_ int nCmdShow) {
// the handle for the window, filled by a function
HWND hWnd;
// this struct holds information for the window class
WNDCLASSEX wc;
// clear out the window class for use
ZeroMemory(&wc, sizeof(WNDCLASSEX));
// fill in the struct with the needed information
wc.cbSize = sizeof(WNDCLASSEX);
wc.style = CS_HREDRAW | CS_VREDRAW;
wc.lpfnWndProc = WindowProc;
wc.hInstance = hInstance;
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)COLOR_WINDOW;
wc.lpszClassName = L"WindowClass1";
// register the window class
RegisterClassEx(&wc);
// create the window and use the result as the handle
hWnd = CreateWindowEx(NULL,
L"WindowClass1", // name of the window class
L"Our First Windowed Program", // title of the window
WS_OVERLAPPEDWINDOW, // window style
300, // x-position of the window
300, // y-position of the window
500, // width of the window
400, // height of the window
NULL, // we have no parent window, NULL
NULL, // we aren't using menus, NULL
hInstance, // application handle
NULL); // used with multiple windows, NULL
// display the window on the screen
ShowWindow(hWnd, nCmdShow);
// enter the main loop:
// this struct holds Windows event messages
MSG msg;
// Enter the infinite message loop
while (TRUE) {
// Check to see if any messages are waiting in the queue
while (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
{
// translate keystroke messages into the right format
TranslateMessage(&msg);
// send the message to the WindowProc function
DispatchMessage(&msg);
}
// If the message is WM_QUIT, exit the while loop
if (msg.message == WM_QUIT)
break;
}
// return this part of the WM_QUIT message to Windows
return msg.wParam;
}
int LoopCnt = 0;
int BinMask = 0b00000000'00000000'00000000'11111111;
// this is the main message handler for the program
LRESULT CALLBACK WindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {
switch (message) {
case WM_DESTROY: {
PostQuitMessage(0);
} break;
case WM_KEYDOWN: {
sprintf_s(MsgBuff, 300, "WM_KEYDOWN(#%i) wParam (virtual-key code): %d (%Xh), scan code: %X, lParam(bin): %s\n", LoopCnt, wParam, wParam, lParam & BinMask, BinVal(lParam));
OutputDebugStringA(MsgBuff);
} break;
case WM_CHAR: {
sprintf_s(MsgBuff, 300, "WM_CHAR(#%i) wParam (virtual-key code): %d (%Xh), scan code: %X, lParam(bin): %s\n", LoopCnt, wParam, wParam, lParam & BinMask, BinVal(lParam));
OutputDebugStringA(MsgBuff);
} break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
LoopCnt++;
}
There i examine the WM_KEYDOWN message lParam param for the scan code send.
Now i have following questions:
Why pressing the left Alt key doesn't generate a WM_KEYDOWN message while pressing right Alt does? (the debugger doesn't go into the WM_KEYDOWN: case statement when pressing the left Alt)
Why pressing the right Alt key generates TWO WM_KEYDOWN messages simultaneously - having 1D(hex) & 38 (hex) scan codes respectively?