Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!wiley!wendar
From: wendar@wiley.UUCP (Wendar Fu)
Newsgroups: comp.windows.x
Subject: XtAddInput question
Message-ID: <9702@wiley.UUCP>
Date: 2 May 90 22:13:08 GMT
Organization: TRW Inc., Redondo Beach, CA
Lines: 108

I'm a new x user, and I am having problem using XtAddInput().

I'm trying to write a program that forks a child process, and the
parent process captures all the data that goes to standard output
and standard error from the child process, and process them.
The problem I have is it seems to enter an infinite loop, in which
the handler routine is called repeatedly.  The simplified program follows:

------------------------
char	*btn_text;

main(argc, argv)
unsigned int argc;
char **argv;
{
	int	n, i;
	int	cpid, fd[2];
	Widget	toplevel, button;
	Arg	args[10];

	toplevel = XtInitialize("main", "XMdemos", NULL, NULL, &argc, argv);
	btn_text = XmStringCreateLtoR("Push to Quit", XmSTRING_DEFAULT_CHARSET);
	n = 0;
	XtSetArg(args[n], XmNlabelType, XmSTRING); n++;
	XtSetArg(args[n], XmNlabelString, btn_text); n++;
	button = XtCreateManagedWidget("button", xmPushButtonWidgetClass,
		toplevel, args, n);
	XtAddCallback(button, XmNactivateCallback, activateCB, NULL);

	fd[0] = fd[1] = -1;	/* initialize file descriptors */
	if(pipe(&fd[0]) != 0)
	{
		printf("main: pipe call failure\n");
		exit(-1);
	}

	/* set all file descriptors execept stdin, stdout, stderr, and the */
	/* that will be used by the new process for pipes, to be closed */
	/* when the new process is forked */
	for(i=3; i<getdtablesize(); i++)
	{
		if ((i==fd[0]) || (i==fd[1]))
			fcntl(i, F_SETFD, 0);	/* keep open through fork */
		else
			fcntl(i, F_SETFD, 1);	/* close during fork */
	}

	/* fork the child process */
	switch(cpid = fork())
	{
	case 0:		/* child process */
		/* Close fd[0] and assign stdout(1) and stderr(2) to fd[1]. */
		/* Once this is done, fd[1] can be closed */
		close(fd[0]);
		dup2(fd[1], 1);		/* stdout */
		dup2(fd[1], 2);		/* stderr */
		close(fd[1]);
		execl("child", 0);
		exit(1);
	case -1:	/* fork failed */
		printf("main: forking child process failure\n");
		/* close all opened descriptors created by pipe call */
		for(i=0; i<2; i++)
		{
			if (fd[i] >= 0)
				close(fd[i]);
		}
		exit(-1);
	default:	/* parent process */
		close(fd[1]);
		XtAddInput(fd[0], XtInputReadMask, handler, 0);
		break;
	}

	XtRealizeWidget(toplevel);
	XtMainLoop();
	close(fd[0]);
}

void activateCB(w, client_data, call_data)
Widget	w;
caddr_t	client_data;
caddr_t	call_data;
{
	XtFree(btn_text);
	exit(-1);
}


void handler()
{
	/* read message from file descriptor and manages them */
}

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

The child process does nothing except for print a line to its standard
error every couple of seconds.  The handler routine is stilled called
repeated when:
	(1)	execl() is not called,
	(2)	fd[0] and fd[1] are not closed.

Please respond if you have see any error, or if you have a different
approach to handle this.

Thanks in advance!

--- Wendar ---