Multiple event sources in a main loop without 100% cpu usage

Question

I've written a program in C which has a main loop that looks something like this:

   while (running) {

     int recv = read_line(fd1, buf, sizeof(buf));
     if (recv > -1) {
       processCommand(buf);
     }
 
     while (XPending(display)) {
       XNextEvent(display, &ev); // This is blocking
       // Handle event
     }
   }

You can see that we have 2 event sources, one of which is blocking.

This works fine but it means I have 100% CPU usage. Normally in a program like this, the blocking call would take care of that but because I have 2 things to check, I can't allow the loop to be blocked.

What's the correct way to program this such that:

• The CPU is not used at 100%
• Both event sources are checked every loop.

Answer 1

As mentioned before, 'select' or 'poll' are your friends. To get the file descriptor of your display:

ConnectionNumber(display)

int XConnectionNumber(Display *display);

Both return a connection number for the specified display. On a POSIX-conformant system, this is the file descriptor of the connection.