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From: bjaspan@athena.mit.edu (Barr3y Jaspan)
Newsgroups: comp.windows.x
Subject: Re: XtAddInput problem
Keywords: XtAddInput XtAppAddInput select
Message-ID: <15783@bloom-beacon.MIT.EDU>
Date: 10 Nov 89 00:40:57 GMT
References: <8911081827.AA12003@turnpike.sun.com> <294@nap1.cds.wpafb.af.mil>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: bjaspan@athena.mit.edu (Barr3y Jaspan)
Organization: Massachusetts Institute of Technology
Lines: 79

I've seen random discussion about the use of XtAddInput() and about
how it supposedly does not work.  It *does* work, and I will attempt
to clear up the confusion.

Anyone who is familiar with UNIX realizes that the intrinsics use
select() to detect X and other events, and that XtAddInput() merely
adds filedescriptors to the parameters passed to select.  The man page
for select() says that it returns when there a file descriptor is
"ready for reading, ready for writing, ..."

The confusion, I think, is in "ready for writing."  That means that
the fd is in a state such that data CAN be written to it, not that
data actually has been written to it.  Specifically, stdout is ALWAYS
"ready for writing" (unless it has been messed with) and so *IF YOU DO
AN XtAddInput ON FD 1, IT WILL TO CALL THE FUNCTION CONSTANTLY* and
that is the correct behavior.

For input events, XtAddInput also does the right thing: it calls the
function whenever there is data waiting to be read.  For example, if
you do XtAddInput() on fd 0 with XtInputReadMask, the callback will be
called when the user has typed a line (or a character in cbreak mode).
The function IS NOT called constantly.

As proof of my argument, I present the following source code and
execution output.

--- snip snip ---

#include <stdio.h>

#include <X11/Intrinsic.h>
#include <X11/StringDefs.h>
#include <X11/Label.h>

void input();

main(argc, argv)
   int argc;
   char **argv;
{
     Widget	top;

     top = XtInitialize("Hello", "World", NULL, 0, &argc, argv);
     (void) XtCreateManagedWidget("label", labelWidgetClass, top,
				  NULL, 0);
     
     XtAddInput(0, XtInputReadMask, input, NULL);
     XtRealizeWidget(top);
     XtMainLoop();
}

void input(client_data, source, input_id)
   caddr_t	client_data;
   int		*source;
   XtInputId	*input_id;
{
     char	buf[BUFSIZ];
     
     printf("Input received on fd %d.\n", *source);
     scanf("%s", buf);
     printf("\"%s\"\n", buf);
}

--- snip snip ---

~% cc -I/mit/x11/include -L/mit/x11/vaxlib input-example.c -lXaw -lXmu -lXt -lX11
~% a.out
testing
Input received on fd 0.
"testing"
Hi there!!
Input received on fd 0.
"Hi"
^C~%

Barr3y Jaspan, MIT-Project Athena
bjaspan@athena.mit.edu
Barry Jaspan, MIT-Project Athena
bjaspan@athena.mit.edu