c++ - Global keyboard hook with WH_KEYBOARD_LL and keybd_event (windows)

Question

I am trying to write a simple global keyboard hook program to redirect some keys. For example, when the program is executed, I press 'a' on the keyboard, the program can disable it and simulate a 'b' click. I do not need a graphic ui, just a console is enough (keep it running)

My plan is to use global hook to catch the key input, and then use keybd_event to simulate the keyboard. But I have some problems.

The first problem is that the program can correctly block 'A' but if I hit 'A' on the keyboard once, the printf in the callback function is executed twice, as well as the keybd_event. So if i open a txt file, i click 'A' once, there are two 'B's input. why is that?

The second question is that why the hook using of WH_KEYBOARD_LL can work on other process without a dll? I thought that we had to use a dll to make a global hook until I wrote this example...

#include "stdafx.h"
#include <Windows.h>
#define _WIN32_WINNT 0x050

LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam)
{
    BOOL fEatKeystroke = FALSE;

    if (nCode == HC_ACTION)
    {
        switch (wParam)
        {
        case WM_KEYDOWN:
        case WM_SYSKEYDOWN:
        case WM_KEYUP:
        case WM_SYSKEYUP:
            PKBDLLHOOKSTRUCT p = (PKBDLLHOOKSTRUCT)lParam;
            if (fEatKeystroke = (p->vkCode == 0x41)) {     //redirect a to b
            printf("Hello a
");
            keybd_event('B', 0, 0, 0);
            keybd_event('B', 0, KEYEVENTF_KEYUP, 0);
            break;
            }
            break;
        }
    }
    return(fEatKeystroke ? 1 : CallNextHookEx(NULL, nCode, wParam, lParam));
}

int main()
{
    // Install the low-level keyboard & mouse hooks
    HHOOK hhkLowLevelKybd = SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc, 0, 0);

    // Keep this app running until we're told to stop
    MSG msg;
    while (!GetMessage(&msg, NULL, NULL, NULL)) {    //this while loop keeps the hook
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    UnhookWindowsHookEx(hhkLowLevelKybd);

    return(0);
}

Many thanks!

Answer 1

Your callback function execute twice because of WM_KEYDOWN and WM_KEYUP. When you down a key of your keyboard, windows calls the callback function with WM_KEYDOWN message and when you release the key, windows calls the callback function with WM_KEYUP message. That's why your callback function execute twice.

You should change your switch statement to this:

switch (wParam)
{
    case WM_KEYDOWN:
    case WM_SYSKEYDOWN:
    case WM_KEYUP:
    case WM_SYSKEYUP:
        PKBDLLHOOKSTRUCT p = (PKBDLLHOOKSTRUCT)lParam;
        if (fEatKeystroke = (p->vkCode == 0x41))  //redirect a to b
        {     
            printf("Hello a
");

            if ( (wParam == WM_KEYDOWN) || (wParam == WM_SYSKEYDOWN) ) // Keydown
            {
                keybd_event('B', 0, 0, 0);
            }
            else if ( (wParam == WM_KEYUP) || (wParam == WM_SYSKEYUP) ) // Keyup
            {
                keybd_event('B', 0, KEYEVENTF_KEYUP, 0);
            }
            break;
        }
        break;
}

About your second question, I think you have already got from @Ivan Danilov answer.