how to quit the blocking of xlib’s XNextEvent

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No. This is why most UI frameworks (Gtk, KDE, etc) use custom main loops to be able to listen for more event sources.

Internally, XNextEvent uses a socket, so it calls select() to know when input is available. Call ConnectionNumber(display) to get the file descriptor that you need to pass select()

That allows you to listen for several file descriptors.

Sample code from http://www.linuxquestions.org/questions/showthread.php?p=2431345#post2431345

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

Display *dis;
Window win;
int x11_fd;
fd_set in_fds;

struct timeval tv;
XEvent ev;

int main() {
    dis = XOpenDisplay(NULL);
    win = XCreateSimpleWindow(dis, RootWindow(dis, 0), 1, 1, 256, 256, \
        0, BlackPixel (dis, 0), BlackPixel(dis, 0));

    // You don't need all of these. Make the mask as you normally would.
    XSelectInput(dis, win, 
        ExposureMask | KeyPressMask | KeyReleaseMask | PointerMotionMask |
        ButtonPressMask | ButtonReleaseMask  | StructureNotifyMask 
        );

    XMapWindow(dis, win);
    XFlush(dis);

    // This returns the FD of the X11 display (or something like that)
    x11_fd = ConnectionNumber(dis);

    // Main loop
    while(1) {
        // Create a File Description Set containing x11_fd
        FD_ZERO(&in_fds);
        FD_SET(x11_fd, &in_fds);

        // Set our timer.  One second sounds good.
        tv.tv_usec = 0;
        tv.tv_sec = 1;

        // Wait for X Event or a Timer
        int num_ready_fds = select(x11_fd + 1, &in_fds, NULL, NULL, &tv);
        if (num_ready_fds > 0)
            printf("Event Received!\n");
        else if (num_ready_fds == 0)
            // Handle timer here
            printf("Timer Fired!\n");
        else
            printf("An error occured!\n");

        // Handle XEvents and flush the input 
        while(XPending(dis))
            XNextEvent(dis, &ev);
    }
    return(0);
}

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