Re: forkpty with streams

[Andrew Barton <andrevv@users.sourceforge.net>]

On Fri, 2003-07-25 at 15:27, Andries Brouwer wrote:
> On Thu, Jul 24, 2003 at 11:28:36PM +0000, Andrew Barton wrote:
> 
> > I've got the 2.4 kernel, and I'm trying to use the forkpty() system call
> 
> forkpty is not a system call
> 
> > with the standard I/O stream functions. The calls to forkpty() and
> > fdopen() and fprintf() all return successfully, but 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 mentioned that things would improve if I let the parent read from
fd. Will this work using streams? I have tried opening fd in "r+" mode,
but in that case I end up reading my own data. Do I need to lay an
fflush() somewhere inbetween? What is it exactly that causes the data to
be sent to the parent?

I appreciate the help.