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From: Andries Brouwer <aebr@win.tue.nl>
To: Andrew Barton <andrevv@users.sourceforge.net>
Cc: linux-kernel@vger.kernel.org
Subject: Re: forkpty with streams
Date: Fri, 25 Jul 2003 22:10:00 +0200	[thread overview]
Message-ID: <20030725201000.GB670@win.tue.nl> (raw)
In-Reply-To: <1059130744.13184.11.camel@localhost>

On Fri, Jul 25, 2003 at 10:59:04AM +0000, Andrew Barton wrote:
> On Fri, 2003-07-25 at 15:27, Andries Brouwer wrote:
> > On Thu, Jul 24, 2003 at 11:28:36PM +0000, Andrew Barton wrote:

> > > the data never seems to get to the child process.
> > 
> > > 	pid = forkpty (&fd, 0, 0, 0);
> > > 	if (pid == 0) {
> > > 		execlp ("sh", "sh", (void *)0);
> > > 	} else {
> > > 		F = fdopen (fd, "w");
> > > 		fprintf (F, "exit\n");
> > > 		fflush (F);
> > > 		wait (0);
> > > 	}
> > 
> > Let me see. Your sh gets input from this pseudotty and sends its
> > output there again. But you never read that filedescriptor.
> > No doubt things will improve if you let the parent read from fd.
> > 
> > Andries
> 
> Before I tried using streams, I just used write() to communicate with
> the ptty, but I had the same problem. I found that if I put a read()
> call before and after the write(), it worked. But why? Is this some kind
> of I/O voodoo? How does the reading affect the writing?

You test with bash, which is a complicated program with many
subtleties related to job control and terminal control. Things will be
easier with ash instead of sh.

But to answer your concrete question: bash does
	ioctl(0, TCSETSW, foo);
which is the same as POSIX
	tcsetattr(0, TCSADRAIN, foo);
which does some setting, but first waits for the output buffer to drain.
It will never drain, unless the other side of the pty does some reading.

Andries


      parent reply	other threads:[~2003-07-25 19:54 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-07-24 23:28 Andrew Barton
2003-07-25 13:02 ` Doug McNaught
2003-07-25 15:27 ` Andries Brouwer
2003-07-25 10:59   ` Andrew Barton
2003-07-25 18:46     ` Matti Aarnio
2003-07-25 12:53       ` Andrew Barton
2003-07-25 20:10     ` Andries Brouwer [this message]

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