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From: grw@cam-orl.UUCP (Geoff Wiginton)
Newsgroups: comp.windows.x
Subject: Re: Design problem with XSetIOErrorHandler
Message-ID: <3230.9101241014@quince.cam-orl.co.uk>
Date: 24 Jan 91 10:14:28 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 84


I have encountered the same problem when connecting to several displays. The 
solution I adopted was a bit of a hack, but does allow you to return from the
IOErrorHandler. This involves the use of setjmp and longjmp and means you have 
to know where the problem is likely to occur within your code. The best
 explanation is an example.......



#include <stdio.h> 
#include <setjmp.h>

static Display      *display;
static int           IO_error_type;
static jmp_buf       Popup_error_env;
static jmp_buf       Destroy_error_env, Popdown_error_env;

       int           NewIOErrorHandler();




main()
{
     .........
     XSetIOErrorHandler( NewIOErrorHandler );
     .........
}


ProcedureA(........)
{
	IO_error_type = EXIT;

	if ( setjmp( ProcA_error_env ) == 0)
	  {
	   ..........
	   Code using X display that may not exist anymore.
	   .........
	   }
	else
	   {
	   ...........
	   This code will be executed if the attempt to use the display fails.
	   ...........
	   }
}



int NewIOErrorHandler( display )
   Display     *display;
{
  switch (IO_error_type)
    {
        case EXIT:    {  longjmp( ProcA_error_env, 1); break; }

        case POPUP:   {  longjmp( Popup_error_env, 1);   break; }

	case POPDOWN: {  longjmp( Popdown_error_env, 1); break; }

	default:      {  printf("Fatal X IO error. Sorry.\n"); Die(0,0,NULL,NULL); }
	}
}



 Essentially, longjmp returns to the setjmp that set up the environment
(in this case ProcA_error_env) with a non-zero value. All other times
the setjmp will return a 0 value. This allows you to check if an error
has occurred in a part of the code and to handle it accordingly. The
crucial part is that the error handler is never returned from and so
the application does not exit.



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-------------------------------------------------------------------------------

Geoff Wiginton, Olivetti Research Lab             grw@uk.co.cam-orl
Old Adennbrookes Site, Cambridge                  (0223) 343398