I use this on a monitoring screen to get my attention: It make the screen flash multiples times (X11)

epilepsy.c

#include <unistd.h>
#include <X11/X.h>
#include <X11/Xlib.h>
#include <strings.h>
#include <memory.h>
#include <stdlib.h>
#include <stdio.h>

/*
 * 2013/10/09. Adrien Pujol.
 *
 * Build me: `gcc -std=gnu99 -Wall epilepsy.c -lX11 -o epilepsy`
 *
 */

const long INTERVAL_MS = 500000;

int main ()
{
    Display* dis = XOpenDisplay(NULL);

    Atom wm_state = XInternAtom(dis, "_NET_WM_STATE", False);
    Atom fullscreen = XInternAtom(dis, "_NET_WM_STATE_FULLSCREEN", False);

    // White Screen Event
    Window whiteWin = XCreateSimpleWindow(dis, RootWindow(dis, 0), 0, 0, 10, 10,
                                     0, WhitePixel (dis, 0), WhitePixel(dis, 0));
    XEvent whiteXev;
    memset(&whiteXev, 0, sizeof(whiteXev));
    whiteXev.type = ClientMessage;
    whiteXev.xclient.window = whiteWin;
    whiteXev.xclient.message_type = wm_state;
    whiteXev.xclient.format = 32;
    whiteXev.xclient.data.l[0] = 1;
    whiteXev.xclient.data.l[1] = fullscreen;
    whiteXev.xclient.data.l[2] = 0;

    Window blackWin = XCreateSimpleWindow(dis, RootWindow(dis, 0), 0, 0, 10, 10,
                                     0, BlackPixel (dis, 0), BlackPixel(dis, 0));
    XEvent blackXev;
    memset(&blackXev, 0, sizeof(blackXev));
    blackXev.type = ClientMessage;
    blackXev.xclient.window = blackWin;
    blackXev.xclient.message_type = wm_state;
    blackXev.xclient.format = 32;
    blackXev.xclient.data.l[0] = 1;
    blackXev.xclient.data.l[1] = fullscreen;
    blackXev.xclient.data.l[2] = 0;

    for (int i = 0; i < 5; i++){

        // White flash

        XMapWindow(dis, whiteWin);
        XSendEvent (dis, DefaultRootWindow(dis), False,
                    SubstructureRedirectMask | SubstructureNotifyMask, &whiteXev);

        usleep(INTERVAL_MS);
        XFlush(dis);

        XMapWindow(dis, blackWin);
        XSendEvent (dis, DefaultRootWindow(dis), False,
                    SubstructureRedirectMask | SubstructureNotifyMask, &blackXev);

        usleep(INTERVAL_MS);
        XFlush(dis);
    }

    return(0);
}