evdev设备的访问抓取状态

Question

我需要在程序中检索evdev设备的抓取状态。更具体地说，我需要检索这里看到的grab结构中的evdev指针的状态：https://elixir.bootlin.com/linux/v4.20/source/drivers/input/evdev.c#L42 (如果它是NULL或NULL)。这在用户空间中是完全可能的吗?例如，通过使用/dev/(k)mem读取内核内存，还是需要编写内核模块？我该从哪里开始呢？

Answer 1

要确定设备当前是否被从用户空间抓取，您可以自己尝试抓取它；要么自己使用EVIOCGRAB ioctl，要么使用libevdev中的libevdev_grab：

if (!ioctl(evdevfd, EVIOCGRAB, (void *) 1)) {
    // We grabbed the device, no one else had it; release it
    ioctl(evdevfd, EVIOCGRAB, (void *) 0);
} else if (errno == EBUSY) {
    // The device is busy, someone else has it
} else {
    // Something else went wrong
}

或

struct libevdev *dev;
libevdev_new_from_fd(evdevfd, &dev);
if (!libevdev_grab(dev, LIBEVDEV_GRAB)) {
    // We grabbed the device, no one else had it; release it
    libevdev_grab(dev, LIBEVDEV_UNGRAB);
} else if (errno == EBUSY) {
    // The device is busy, someone else has it
} else {
    // Something else went wrong
}

(大量添加错误处理；evdevfd是您感兴趣的evdev设备的打开文件描述符。)