How can I get the keyboard state in Linux?

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);
}

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.