A X11/Xlib program that reads the KeyPress and KeyRelease events from the window and prints them to the standard output. Used to debug the keyboard within X.

xreadkeys.c

#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>
 
int main()
{
    Display *display;
    Window window;
    XEvent event;
    int s;
 
    /* open connection with the server */
    display = XOpenDisplay(NULL);
    if (display == NULL)
    {
        fprintf(stderr, "Cannot open display\n");
        exit(1);
    }
 
    s = DefaultScreen(display);
 
    /* create window */
    window = XCreateSimpleWindow(display, RootWindow(display, s), 10, 10, 200, 200, 1,
                           BlackPixel(display, s), WhitePixel(display, s));
 
    /* select kind of events we are interested in */
    XSelectInput(display, window, KeyPressMask | KeyReleaseMask );
 
    /* map (show) the window */
    XMapWindow(display, window);
 
    /* event loop */
    while (1)
    {
        XNextEvent(display, &event);
 
        /* keyboard events */
        if (event.type == KeyPress)
        {
            printf( "KeyPress: %x\n", event.xkey.keycode );

            /* exit on ESC key press */
            if ( event.xkey.keycode == 0x09 )
                break;
        }
        else if (event.type == KeyRelease)
        {
            printf( "KeyRelease: %x\n", event.xkey.keycode );
        }
    }

    /* close connection to server */
    XCloseDisplay(display);
 
    return 0;
}

I'm trying to detect the moment a key is pressed and the moment a key is released. Using your program I'm getting a "KeyRelease:" message even when I'm holding down a key. Can you help me detect only when a Key is pressed and only when it's released?

I'm afraid you have to filter out the additional KeyPress events (and KeyRelease events if the autorepeat function is enabled).

Hey Javier,

I have a little C text mode application running under Gnome Terminal.

After receiving a keystring through the terminal, it can proceed with its own function associated with it. Let's say it receives "^H".

The string "^H" is produced pressing ctrl-h (two keys) or ctrl-backspace, also two keys. There is no way to change that in the terminal.

I would like to use your little xreadkeys.c to read the keycodes and tell which keys were pressed.

However, how can I change your program and make it use the window already open without creating a new one?

Thanks and regards

frank

@javiercantero How would I intercept a keypress on the root window? (The one that X draws everything on?)