Equivalent of "Invalidate Rect" / "WM_PAINT" in X11

Question

I'm Porting some code from Windows to XLib. In the windows code, I can force a redraw by calling InvalidateRect and then handling the corresponding WM_PAINT message. However, I am having trouble finding out how to do this in X11/XLib. I see there is an Expose message but not sure if that is the same thing.

If it matters, I need to do this to force the window to render at a certain frame rate for an OpenGL based program.

Answer 1

You need to handle Expose events. This tutorial explains with an example how to handle Expose events :

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xos.h>
#include <X11/Xatom.h>
#include <X11/keysym.h>
/*Linux users will need to add -ldl to the Makefile to compile 
 *this example.
 */
Display *dis;
Window win;
XEvent report;
GC green_gc;
XColor green_col;
Colormap colormap;
/*
Try changing the green[] = below to a different color.
The color can also be from /usr/X11R6/lib/X11/rgb.txt, such as RoyalBlue4.
A # (number sign) is only needed when using hexadecimal colors.
*/
char green[] = "#00FF00";

int main() {
    dis = XOpenDisplay(NULL);
    win = XCreateSimpleWindow(dis, RootWindow(dis, 0), 1, 1, 500, 500, 0, BlackPixel (dis, 0), BlackPixel(dis, 0));
    XMapWindow(dis, win);
    colormap = DefaultColormap(dis, 0);
    green_gc = XCreateGC(dis, win, 0, 0);
    XParseColor(dis, colormap, green, &green_col);
    XAllocColor(dis, colormap, &green_col);
    XSetForeground(dis, green_gc, green_col.pixel);

    XSelectInput(dis, win, ExposureMask | KeyPressMask | ButtonPressMask);

    XDrawRectangle(dis, win, green_gc, 1, 1, 497, 497);
    XDrawRectangle(dis, win, green_gc, 50, 50, 398, 398);
    XFlush(dis);

    while (1)  {
    XNextEvent(dis, &report);
        switch  (report.type) {
        case Expose:   
            fprintf(stdout, "I have been exposed.\n");
                XDrawRectangle(dis, win, green_gc, 1, 1, 497, 497);
                XDrawRectangle(dis, win, green_gc, 50, 50, 398, 398);
                XFlush(dis);
            break;
            case KeyPress:
        /*Close the program if q is pressed.*/
                if (XLookupKeysym(&report.xkey, 0) == XK_q) {
                exit(0);
                }
            break;
        }
    }

return 0;
}

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I may have misunderstood the question. If you want to create Expose events in your application, you can create and set expose event, and send it using XSendEvent.