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From: mouse@LARRY.MCRCIM.MCGILL.EDU
Newsgroups: comp.windows.x
Subject: Re:  How Do You Monitor Events For More Than 1 Display?
Message-ID: <9012041241.AA01920@Larry.McRCIM.McGill.EDU>
Date: 4 Dec 90 12:41:23 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 109

> I am writing this program which is supposed to wait for KeyPress
> events from more than 1 display, however, after thinking about it for
> a long long time, I don't really know how to do it.  Please enlighten
> me.

It's system-dependent.  I don't know enough about toolkits to say for
sure, but I would hope that most/all toolkits would provide some way of
doing this.

On BSD UNIX (or other Unices which have been BSDified enough to have
select()), the canonical way of doing this is to gobble up all events
that are available on the connections, then select() waiting for
something to arrive.

> To be more concrete, here's what I will do if I only have 1 display:

> 	for (;;) {
> 		XNextEvent(display, &event);
> 	}

Well, you also need to do something with the event; just reading it
isn't going to be very useful.

I have a program that speaks to multiple displays.  Here is its event
loop, with irrelevant code stripped out.  (The real function is about
twice as big as what you see here; it does input from another file
descriptor, and timeouts, in this loop as well.)

(DISP_INFO is a typedef referring to my structure containing all the
interesting information about each connection.  disps is a global
pointer to a linked list of these structures.)

	#define EACH_DISP(var) var=disps;var;var=var->link

	run()
	{
	 XEvent e;
	 DISP_INFO *d;
	 int selrv;
	 int wantflush;
	 int wantupd;
	 fd_set fds;
	 int selcnt;
	 int mustcontinue;
	 struct timeval *tvp;
	
	 for (EACH_DISP(d)) d->xfd = XConnectionNumber(d->disp);
	 wantflush = 1;
	 wantupd = 0;
	 while (1)
	  { mustcontinue = 0;
	    for (EACH_DISP(d))
	     { if (XQLength(d->disp) > 0)
		{ do
		   { XNextEvent(d->disp,&e);
		     handle_event(&e);
		   } while (XQLength(d->disp) > 0);
		  wantupd = 1;
		  mustcontinue = 1;
		}
	     }
	    if (mustcontinue) continue;
	    FD_ZERO(&fds);
	    for (EACH_DISP(d)) FD_SET(d->xfd,&fds);
	    if (wantflush || wantupd)
	     { static struct timeval ztv;
	       ztv.tv_sec = 0;
	       ztv.tv_usec = 0;
	       tvp = &ztv;
	     }
	    else
	     { tvp = 0;
	     }
	    selrv = select(FD_SETSIZE,&fds,(fd_set *)0,(fd_set *)0,tvp);
	    if (selrv < 0)
	     { if (errno == EINTR) continue;
	       perror("select");
	       utexit(1);
	     }
	    if (selrv == 0)
	     { if (wantupd)
		{ update_display();
		  wantflush = 1;
		  wantupd = 0;
		}
	       if (wantflush)
		{ for (EACH_DISP(d)) XFlush(d->disp);
		  if (debugging) fflush(stdout);
		  wantflush = 0;
		}
	       continue;
	     }
	    for (EACH_DISP(d))
	     { if (FD_ISSET(d->xfd,&fds))
		{ int bpend;
		  ioctl(d->xfd,FIONREAD,&bpend);
		  if (bpend == 0) /* socket EOF */
		   { utexit(0);
		   }
		  XEventsQueued(d->disp,QueuedAfterReading);
		}
	     }
	  }
	}

					der Mouse

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