Tom Wong Tom Wong - 1 year ago 120
C Question

how to quit the blocking of xlib's XNextEvent

Under windows, the GUI thread usually call GetMessage to waiting for message,
when another thread use PoseMessage put a message into the queue, then the
GUI thread will return GetMessage (quit blocking).

Does anyone can tell me, when I use XNextEvent under XWindows to waiting for
event, how can I "wakeup" the GUI thread in another thread. Is there some
API like PoseMessage I can use ?.

Answer Source

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. So can you: 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

#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);

    // 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_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");
            printf("An error occured!\n");

        // Handle XEvents and flush the input 
            XNextEvent(dis, &ev);
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