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From: moss@brl.mil (Gary S. Moss (VLD/VMB) <moss>)
Newsgroups: comp.windows.x
Subject: Re: How do I get XtAppAddInput to work correctly
Message-ID: <14801@smoke.brl.mil>
Date: 9 Jan 91 21:30:13 GMT
References: <1990Dec31.170612.7040@lta.com> <5184@trantor.harris-atd.com>
Sender: news@smoke.brl.mil
Reply-To: moss@brl.mil
Organization: Ballistic Research Laboratory
Lines: 64

In article <5184@trantor.harris-atd.com>, wdavis@x102c.harris-atd.com (davis william 26373) writes:
|> 
|> I will remove the input for X if there is a way to know when the
|> file has something more added to the end of what has already been
|> read.  Any ideas on this?

The proper semantics of available data for input are clear when
reading from a pipe or socket, but when reading from a file, it becomes
something of a religious argument.  I was fooled by the documentation
also, so perhaps it should be tweaked.  Anyway, I have combined the
XtAppAddInput with code which checks for available input in the file.
It is fairly simple so I have included it below; the application
monitors a log file (like running a "tail -f" into an AsciiText widget.
This doesn't solve the problem of your registered function being called
in a tight loop so that your work proc doesn't get scheduled, but you
could perhaps not use XtAppAddInput at all.

Hope this helps,
-Gary

...

	/* Register error log monitoring function. */
	(void) XtAppAddInput( apcon, fileno(errorfp), XtInputReadMask,
				displayLogMsg, (XtPointer) errorfp );
	/* Loop to handle events. */
	XtAppMainLoop( apcon );

...

/*
	void displayLogMsg( XtPointer client_data, int *source, XtInputId *id )

	Display the next line of the log file in the "errors" text widget.
 */
/*ARGSUSED*/
STATIC void
displayLogMsg( client_data, source, id )
XtPointer client_data;
int *source;
XtInputId *id;
	{	register int ct;
		long addr; /* save file offset of beginning of line */
		FILE *fp = (FILE *) client_data;
		char buffer[MAXLOGLNLEN];
	if( fp == NULL )
		return; /* monitoring of log errors turned off */
	addr = ftell( fp );
	if( fgets( buffer, MAXLOGLNLEN, fp ) != NULL )
		{	register int len = strlen( buffer );
		/* if( len <= 1 )
			return; /* ignore blank lines */
		if( buffer[len-1] != '\n' )
			{ /* A missing newline means that a write to the
				log is in progress, so we are done. */
			(void) fseek( fp, addr, 0 ); /* beginning of line */
			return;
			}
		sendToTextWidget( XtNameToWidget( toplevel, "main.errors" ),
				buffer );
		}
	else
		clearerr( fp ); /* for 4.2 BSD or SUNs, need to clear EOF */
	}