Re: forkpty with streams

[Andries Brouwer <aebr@win.tue.nl>]

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