Plot pixels on screen in loop crashes linux

#include <iostream>
#include <stdio.h>
#include <cstdlib>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <unistd.h>


/*
root@ST-LAPTOP:/usr/include/X11# grep "XGetPixel" ./*
./Xutil.h:extern unsigned long XGetPixel(
./Xutil.h:#define XGetPixel(ximage, x, y) \
*/

// Compile: g++ mypointer.c -o my -lX11

// cc prog.c -o prog -L/usr/X11/lib -lX11
// or perhaps this (for a system with release 6 of X11):

//cc prog.c -o prog -L/usr/X11R6/lib -lX11

// On SunOs 4 systems, the X libraries are placed in /usr/openwin/lib:
// cc prog.c -o prog -L/usr/openwin/lib -lX11



#define GetRValue(dwColor) dwColor & 0x000000FF
#define GetGValue(dwColor) (dwColor & 0x0000FF00) >> 8
#define GetBValue(dwColor) dwColor >> 16


int GetPixel(int x, int y)
{
    Display *dpy;
    XImage *image;
    XColor col;

    dpy = XOpenDisplay(NULL);
    image = XGetImage(dpy, DefaultRootWindow(dpy),
                      x, y, 1, 1, AllPlanes, ZPixmap);
    col.pixel = XGetPixel(image, 0, 0);
    XDestroyImage(image);

    // The XQueryColor() function returns the current RGB value for the pixel in the XColor structure and sets the DoRed, DoGreen, and DoBlue flags.
    // XQueryColor() can generate BadColor and BadValue errors.
    // BadColor A value for a Colormap argument does not name a defined Colormap.
    // BadValue Some numeric value falls outside the range of values accepted by the request.
    // Unless a specific range is specified for an argument, the full range defined by the argument's type
    // is accepted. Any argument defined as a set of alternatives can generate this error.
    XQueryColor(dpy, DefaultColormap(dpy, DefaultScreen(dpy)), &col);
    // The colormap is a lookup table located on the server the pixel value is used as an index into the table.

    XCloseDisplay(dpy);

    printf("R: %03d G: %03d B: %03d\n", (int)((double) col.red/257.0), (int)((double) col.green/257.0), (int)((double) col.blue/257.0));
    return (col.red&0xff00)<<8 | (col.green)&0xff00 | (col.blue) >> 8; // = windows return value
}


int main()
{
    // this variable will contain the pointer to the Display structure
    // returned when opening a connection.
    Display* display;
    int screen_num;
    Screen *screen;
    Window root_win;
    XGCValues gc_val;

    // open the connection to the display "simey:0".
    display = XOpenDisplay(0);// display = XOpenDisplay("simey:0");
    if (display == NULL)
    {
        fprintf(stderr, "Cannot connect to X server %s\n", "simey:0");
        exit (EXIT_FAILURE);
    }

    XGrabServer(display);

    screen_num = DefaultScreen(display);
    screen = XScreenOfDisplay(display, screen_num);
    root_win = RootWindow(display, XScreenNumberOfScreen(screen));

    // Create a GC (Graphics Context) for the line
    gc_val.function           = GXclear;
    gc_val.plane_mask         = AllPlanes;
    gc_val.foreground         = WhitePixel(display, screen_num);
    gc_val.background         = BlackPixel(display, screen_num);
    gc_val.line_width         = 4;
    gc_val.line_style         = LineSolid;
    gc_val.cap_style          = CapButt;
    gc_val.join_style         = JoinMiter;
    gc_val.fill_style         = FillOpaqueStippled;
    gc_val.fill_rule          = WindingRule;
    gc_val.graphics_exposures = False;
    gc_val.clip_x_origin      = 0;
    gc_val.clip_y_origin      = 0;
    gc_val.clip_mask          = None;
    gc_val.subwindow_mode     = IncludeInferiors;

    GC  gc_line = XCreateGC(display, root_win, GCFunction | GCPlaneMask |  GCForeground | GCBackground | GCLineWidth | GCLineStyle |
                            GCCapStyle  | GCJoinStyle  |  GCFillStyle  |  GCFillRule  |  GCGraphicsExposures |
                            GCClipXOrigin |  GCClipYOrigin  |  GCClipMask  | GCSubwindowMode, &gc_val);

    while( 1 )
    {
        // XSetForeground(display, gc_line, some_color);
        // XDrawRectangle(display, root_win, gc_line, 50, 50, 400, 400);
        for (int ix = 0; ix < 500;++ix)
        {
            for (int iy = 0; iy < 500;++iy)
            {
                XDrawPoint(display, root_win, gc_line, ix, iy); // SetPixel
            }
        }

        usleep(100);
    }

    XFlush(display);

    XUngrabServer(display);


    XCloseDisplay( display );

    int iMyColor =  GetPixel(20,20) ;
    printf("Sole color: 0x%08X\n", iMyColor);
    printf("R: %03d G: %03d B: %03d\n", GetRValue(iMyColor), GetGValue(iMyColor), GetBValue(iMyColor));

    // printf("Sizof short: %ld\n", sizeof(unsigned short)) ; // 2 Bytes
    return EXIT_SUCCESS ;
}