How can I get the keyboard state in Linux?

Question

I want to check if the user pressed down the Shift key when the program starts. (That means, press down the Shift key before the program is started) It's a simple console program, nothing related to X.

This maybe similar to the Win32 GetKeyboardState() function.

I want to know whether I can do this and how, but not any pros and cons with accessing the terminal directly.

Answer 1

I think there would be a way to do this. The thing is that you would have to read directly from the keyboard device. You would not be getting input from the terminal. I have the same problem. I have a program that runs (in the background) and I want to know if the user is holding down the shift key.

I believe this is possible and a place to start might be /dev/input/by-path/*-kbd.

This file does give input every time a key is pressed or reptadly if it is held down so it might be worth a look. (Try cat /dev/input/by-path/*-kbd)

If you do figure this out I would love to hear how you did it.

EDIT: I have found the solution

I have figured out how do do this. My program is as follows:

#include <stdlib.h>
#include <stdio.h>

#include <linux/input.h>

void usage ( int argc, char *argv[] )
{
    printf("Usage:\n\t%s key\n\nvalid keys are:\n\tlshift\t- Left Shift key\n" , argv[0]);

    exit(EXIT_FAILURE);
}

int main ( int argc, char *argv[], char *env[] )
{
    if ( argc != 2 )    usage(argc, argv);

    int key;

    if ( strcmp(argv[1], "lshift") == 0 )       key = KEY_LEFTSHIFT;
    else if ( strcmp(argv[1], "rshift") == 0 )  key = KEY_RIGHTSHIFT;
    else if ( strcmp(argv[1], "lalt") == 0 )    key = KEY_LEFTALT;
    else if ( strcmp(argv[1], "ralt") == 0 )    key = KEY_RIGHTALT;
    else if ( strcmp(argv[1], "lctrl") == 0 )   key = KEY_LEFTCTRL;
    else if ( strcmp(argv[1], "rctrl") == 0 )   key = KEY_RIGHTCTRL;


    FILE *kbd = fopen("/dev/input/by-path/platform-i8042-serio-0-event-kbd", "r");

    char key_map[KEY_MAX/8 + 1];    //  Create a byte array the size of the number of keys

    memset(key_map, 0, sizeof(key_map));    //  Initate the array to zero's
    ioctl(fileno(kbd), EVIOCGKEY(sizeof(key_map)), key_map);    //  Fill the keymap with the current keyboard state

    int keyb = key_map[key/8];  //  The key we want (and the seven others arround it)
    int mask = 1 << (key % 8);  //  Put a one in the same column as out key state will be in;

    return !(keyb & mask);  //  Returns true if pressed otherwise false

}

The info message is lacking (I'm too lazy). But essentially the first argument is compared to a list of keys and the appropriate key identifier is used. It returns true if the key is pressed and false if not.

Please Note

You will need to change the name of they keyboard device. I do not know of a way to find the default keyboard device. (if you know I would love to hear ;) )

This works beautifully: I use it to start the autostart of Xorg if I hold down the shift key.

Answer 2

AFAIK this cannot be done without Xlib (aka. X) with no root level permissions. Using XQueryKeymap() will do what you want. however you pointed out that X cannot be used. Regardless, opening display connection will also be required.

#include <X11/Xlib.h>
#include <X11/keysym.h>
#include <stdbool.h>
#include <stdio.h>

int main()
{
    Display* dpy = XOpenDisplay(NULL);
    char keys_return[32];
    XQueryKeymap( dpy, keys_return );
    KeyCode kc2 = XKeysymToKeycode( dpy, XK_Shift_L );
    bool bShiftPressed = !!( keys_return[ kc2>>3 ] & ( 1<<(kc2&7) ) );
    printf("Shift is %spressed\n", bShiftPressed ? "" : "not ");
    XCloseDisplay(dpy);
}

Answer 3

I have found a very simple way through gtk/gdk.

int main ( int argc, char *argv[], char *env[] )
{
    gtk_init(&argc, &argv);

    GdkModifierType button_state;
    gdk_window_get_pointer(NULL, NULL, NULL, &button_state);
    if(button_state & GDK_CONTROL_MASK) {
        printf("ctrl key is pressed");
    }
}

Answer 4

You can't.

The Shift key isn't considered as a character key, so even if you access the terminal directly, you won't be able to detect this key.

Maybe you shouldn't have to. Imagine for example that you are using a US keyboard where numbers are accessible on the top row without modifiers, and also checking for the Shift key. People with other keyboard layout may have to use Shift modifiers to access the numbers. If your program react to this Shift press, then your program is basically unusable. The same thing applies for other modifier keys : you may detect some of them only after a normal character key is pressed. Or worse, they may need to use the Shift key to use 'enter' to run your program.

Also, what Shift key do you want to monitor? the one on the local machine, or the one where the user is? remember that SSH exists and is commonly used to access a pseudoterminal remotely.

If you are root and want to monitor the Shift key on the local machine, you can read the evdev devices for events about the Shift key. But this is only possible because of automatic key repeating, so you won't detect a Shift key that is pressed right before running your program, but only a few second before.

Of course you can't do that on the remote machine, that would be a security flaw.

And anyway, why would you want to do that? Wouldn't an X application be the right thing to do in your case?