c++ - 无法从/dev/input/eventx 读取原始键盘事件

Question

我正在尝试从/dev/input/event2 读取原始键盘缓冲区

ls -l /dev/input/by-id/
lrwxrwxrwx 1 root root 9 Mar 14 21:08 usb-0461_HP_USB_Multimedia_Keyboard-event-if01 -> ../event3
lrwxrwxrwx 1 root root 9 Mar 14 21:08 usb-0461_HP_USB_Multimedia_Keyboard-event-kbd -> ../event2
lrwxrwxrwx 1 root root 9 Mar 14 16:07 usb-Logitech_USB_Receiver-if01-event-mouse -> ../event4
lrwxrwxrwx 1 root root 9 Mar 14 16:07 usb-Logitech_USB_Receiver-if01-mouse -> ../mouse0

我也试过这个:sudo cat/dev/input/event2
当我按下任何键或释放任何键时，我可以看到打印了一些二进制代码。
所以我知道/dev/input/event2 假设是我键盘的原始缓冲区。

这是我的代码

int fd1 = 0;
if( (fd1 = open("/dev/input/event2", O_RDONLY)) > 0 ) 
{
    struct input_event event;
    unsigned int scan_code = 0;
    bool exit_sign = false;
    ssize_t evt_size = sizeof(event);

    printf("Try read %ld bytes\n", evt_size);
    int flags = fcntl(fd1, F_GETFL, 0);
    fcntl(fd1, F_SETFL, flags | O_NONBLOCK);

    while (!exit_sign)
    {
        usleep(1000);
        ssize_t n = read(fd, &event, evt_size);
        if (n == (ssize_t)-1)
        {
            if (errno == EAGAIN || errno == EWOULDBLOCK )
                continue;
            else
            {
                printf("Unknown error:%d\n", errno);
                break;
            }
        }
        else if (n != evt_size)
        {
            printf("Unexpected event:%ld\n", n);
            return -1; // Keyboard events are always of type EV_KEY
        }

        printf("type:%x, code:%x, value:%x\n", event.type, event.code, event.value);
        if(event.type != EV_KEY)
        {
            printf("Unexpected event type:%d\n", event.type);
            return -1; // Keyboard events are always of type EV_KEY
        }

        if(event.value == EV_RELEASED)
        {
            scan_code = event.code;
            printf("read back scan_code is: %d \n", scan_code);
            if (scan_code == KEY_X)
                exit_sign = true;
        }
    }
    close(fd1);
}

当我运行上面的代码时，它真的让我发疯了。
当我按任意键时，似乎读取函数不会读取任何原始缓冲区，而是读取解释键。例如，如果我快速按键盘上的“b”，我希望读取 2 input_event。 1 键向下和 1 键向上；具有正确的类型和正确的键值。但相反，它不会返回(因为缓冲区中只有一个“b”，对于 struct input_event 来说还不够)。如果我在按下足够多的键之前按下回车，似乎 read 会返回并告诉我只读取了几个字节。这些字节是解释键，而不是原始信息 (input_event)。
我做错什么了？如何从 key 缓冲区中读取原始信息？

Answer 1

几天前，我不得不抓取并解释一个 USB RFID 阅读器的输出。该设备模仿键盘，因此任务与您的有些相似。
我使用 EVTEST (https://github.com/freedesktop-unofficial-mirror/evtest) 来了解流程并让我的生活更轻松。
以下是输入字符串“Hello world”的工具输出:

Event: time 1602980447.706476, -------------- SYN_REPORT ------------
Event: time 1602980450.314493, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e5
Event: time 1602980450.314493, type 1 (EV_KEY), code 54 (KEY_RIGHTSHIFT), value 1
Event: time 1602980450.314493, -------------- SYN_REPORT ------------
Event: time 1602980450.386495, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000b
Event: time 1602980450.386495, type 1 (EV_KEY), code 35 (KEY_H), value 1
Event: time 1602980450.386495, -------------- SYN_REPORT ------------
Event: time 1602980450.434491, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e5
Event: time 1602980450.434491, type 1 (EV_KEY), code 54 (KEY_RIGHTSHIFT), value 0
Event: time 1602980450.434491, -------------- SYN_REPORT ------------
Event: time 1602980450.474410, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000b
Event: time 1602980450.474410, type 1 (EV_KEY), code 35 (KEY_H), value 0
Event: time 1602980450.474410, -------------- SYN_REPORT ------------
Event: time 1602980451.898496, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70008
Event: time 1602980451.898496, type 1 (EV_KEY), code 18 (KEY_E), value 1
Event: time 1602980451.898496, -------------- SYN_REPORT ------------
Event: time 1602980451.978502, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70008
Event: time 1602980451.978502, type 1 (EV_KEY), code 18 (KEY_E), value 0
Event: time 1602980451.978502, -------------- SYN_REPORT ------------
Event: time 1602980452.602491, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000f
Event: time 1602980452.602491, type 1 (EV_KEY), code 38 (KEY_L), value 1
Event: time 1602980452.602491, -------------- SYN_REPORT ------------
Event: time 1602980452.682505, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000f
Event: time 1602980452.682505, type 1 (EV_KEY), code 38 (KEY_L), value 0
Event: time 1602980452.682505, -------------- SYN_REPORT ------------
Event: time 1602980452.738487, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000f
Event: time 1602980452.738487, type 1 (EV_KEY), code 38 (KEY_L), value 1
Event: time 1602980452.738487, -------------- SYN_REPORT ------------
Event: time 1602980452.810502, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70012
Event: time 1602980452.810502, type 1 (EV_KEY), code 24 (KEY_O), value 1
Event: time 1602980452.810502, -------------- SYN_REPORT ------------
Event: time 1602980452.850493, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000f
Event: time 1602980452.850493, type 1 (EV_KEY), code 38 (KEY_L), value 0
Event: time 1602980452.850493, -------------- SYN_REPORT ------------
Event: time 1602980452.898498, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70012
Event: time 1602980452.898498, type 1 (EV_KEY), code 24 (KEY_O), value 0
Event: time 1602980452.898498, -------------- SYN_REPORT ------------
Event: time 1602980453.514499, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002c
Event: time 1602980453.514499, type 1 (EV_KEY), code 57 (KEY_SPACE), value 1
Event: time 1602980453.514499, -------------- SYN_REPORT ------------
Event: time 1602980453.594490, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002c
Event: time 1602980453.594490, type 1 (EV_KEY), code 57 (KEY_SPACE), value 0
Event: time 1602980453.594490, -------------- SYN_REPORT ------------
Event: time 1602980453.762489, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7001a
Event: time 1602980453.762489, type 1 (EV_KEY), code 17 (KEY_W), value 1
Event: time 1602980453.762489, -------------- SYN_REPORT ------------
Event: time 1602980453.842490, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7001a
Event: time 1602980453.842490, type 1 (EV_KEY), code 17 (KEY_W), value 0
Event: time 1602980453.842490, -------------- SYN_REPORT ------------
Event: time 1602980453.898499, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70012
Event: time 1602980453.898499, type 1 (EV_KEY), code 24 (KEY_O), value 1
Event: time 1602980453.898499, -------------- SYN_REPORT ------------
Event: time 1602980453.970490, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70012
Event: time 1602980453.970490, type 1 (EV_KEY), code 24 (KEY_O), value 0
Event: time 1602980453.970490, -------------- SYN_REPORT ------------
Event: time 1602980454.034420, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70015
Event: time 1602980454.034420, type 1 (EV_KEY), code 19 (KEY_R), value 1
Event: time 1602980454.034420, -------------- SYN_REPORT ------------
Event: time 1602980454.114500, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70015
Event: time 1602980454.114500, type 1 (EV_KEY), code 19 (KEY_R), value 0
Event: time 1602980454.114500, -------------- SYN_REPORT ------------
Event: time 1602980454.170496, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000f
Event: time 1602980454.170496, type 1 (EV_KEY), code 38 (KEY_L), value 1
Event: time 1602980454.170496, -------------- SYN_REPORT ------------
Event: time 1602980454.226502, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7000f
Event: time 1602980454.226502, type 1 (EV_KEY), code 38 (KEY_L), value 0
Event: time 1602980454.226502, -------------- SYN_REPORT ------------
Event: time 1602980454.258497, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70007
Event: time 1602980454.258497, type 1 (EV_KEY), code 32 (KEY_D), value 1
Event: time 1602980454.258497, -------------- SYN_REPORT ------------
Event: time 1602980454.330501, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70007
Event: time 1602980454.330501, type 1 (EV_KEY), code 32 (KEY_D), value 0

上述输出的第一件事是，您不仅要处理 EV_KEY 事件。
我将以下代码用于我的目的:

extern int read_card_number(int file_handle, char* buffer, int max_size) {
    if (max_size <= 0) return -1;

    struct input_event event;
    int event_size = sizeof(struct input_event);

    char *internal_buffer = malloc(sizeof(char) * max_size);
    int symbol_counter = 0;

    while (1) {
        int bytes_red = read(file_handle, &event, event_size);
        if (bytes_red < event_size) {
            fprintf(stderr, "failed to read %d bytes, got only %d bytes\n", event_size, bytes_red);
            return -2;
        }

        if (event.type != EV_KEY) continue;
        if (event.value != 1) continue;
        char symbol = get_printable_symbol(event.code);
        printf("symbol: %c\n", symbol);
        if (symbol == -1) continue;
        if (symbol_counter >= max_size - 1) {
            fprintf(stderr, "internal_buffer string is larger then expected");
            return -3;
        }
        if (symbol == '\n' || symbol == '\0') {
            internal_buffer[symbol_counter] = '\0';
            memccpy(buffer, internal_buffer, 0, symbol_counter);
            return 0;
        }

        internal_buffer[symbol_counter] = symbol;
        symbol_counter += 1;
    }
}

如果从列表中添加 fcntl 调用(带有 O_NONBLOCK 的 F_SETFL)，代码会损坏，因此主要问题在这里。我建议从简单地删除开始

int flags = fcntl(fd1, F_GETFL, 0);
fcntl(fd1, F_SETFL, flags | O_NONBLOCK);

另见 https://github.com/freedesktop-unofficial-mirror/evtest/blob/master/evtest.c作为一个很好的例子。