Xlib: Closing window always causes fatal IO error?

Question

I'm not sure why this happens, but any window I create using Xlib in C++ gives outputs an error to the terminal when I try to close is using the X button. I can close it programmatically with no errors, it's just the X button that does it.

The error is the following:

XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
after 483 requests (483 known processed) with 0 events remaining.

The number of requests is different every time, but there's always 0 events remaining. Why does this happen? The cause doesn't seem to be my code, since it does this no matter what and sends no close events to the queue. I've tried intercepting the Atom WM_WINDOW_DELETE, and it doesn't run over the expected code when I close the window.

Edit: Added event loop code.

while(XPending(display)) {
    XNextEvent(display, &event);

    pthread_mutex_unlock(&mutex);

    if(event.type == Expose) {
        XWindowAttributes getWindowAttributes;

        pthread_mutex_lock(&mutex);

        XGetWindowAttributes(display, window, &getWindowAttributes);

        if(state.currentState == STATE_NORMAL) {
            state.normX = getWindowAttributes.x;
            state.normY = getWindowAttributes.y;
            state.normWidth = getWindowAttributes.width;
            state.normHeight = getWindowAttributes.height;
        }

        pthread_mutex_unlock(&mutex);

        glViewport(0, 0, getWindowAttributes.width, getWindowAttributes.height);
    } else if(event.type == KeyPress) {
        return false;
    } else if(event.type == ClientMessage) {
        std::cout<<"X Button pressed"<<std::endl; //Never run when X-ing window
        if(event.xclient.message_type == XInternAtom(display, "WM_DELETE_WINDOW", True)) {
            return false;
        }
    } else if(event.type == ButtonPress) {
        if(state.currentState != STATE_FULLSCREEN) {
            fullscreen();
        } else {
            normalize();
        }
    } else if(!handleEvent(event)){
        return false;
    }

    pthread_mutex_lock(&mutex);
}

Answer 1

In addition to WM_WINDOW_DELETE you need to listen for and handle the ClientMessage event.

Modifying the example from Rosetta Code to illustrate:

#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
    Display *d;
    Window w;
    XEvent e;
    const char *msg = "Hello, World!";
    int s;

    d = XOpenDisplay(NULL);
    if (d == NULL) {
        fprintf(stderr, "Cannot open display\n");
        exit(1);
    }

    s = DefaultScreen(d);
    w = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 100, 100, 1, BlackPixel(d, s), WhitePixel(d, s));
    XSelectInput(d, w, ExposureMask | KeyPressMask);
    XMapWindow(d, w);

    // I support the WM_DELETE_WINDOW protocol
    Atom WM_DELETE_WINDOW = XInternAtom(d, "WM_DELETE_WINDOW", False); 
    XSetWMProtocols(d, w, &WM_DELETE_WINDOW, 1);

    while (1) {
        XNextEvent(d, &e);
        if (e.type == Expose) {
            XFillRectangle(d, w, DefaultGC(d, s), 20, 20, 10, 10);
            XDrawString(d, w, DefaultGC(d, s), 10, 50, msg, strlen(msg));
        }
        else if (e.type == KeyPress)
            break;
        else if (e.type == ClientMessage)
            // TODO Should check here for other client message types - 
            // however as the only protocol registered above is WM_DELETE_WINDOW
            // it is safe for this small example.
            break;
    }

    XCloseDisplay(d);
    return 0;
}