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 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 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);
}