Re: forkpty with streams

[Matti Aarnio <matti.aarnio@zmailer.org>]

On Fri, Jul 25, 2003 at 10:59:04AM +0000, Andrew Barton wrote:
...
> > > 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);
> > > 	}
...
> 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?

PTY file-handle is full-duplex bi-directional thing, and sometimes
it may need reading, or you get unwanted deadlocks.

> 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?

dup() helps you to have two fd:s,  fdopen() for both, one with "w",
other "r".   Things should not need that dup() actually.
Also fcntl() the fd's to be non-blocking.

Actually I am always nervous with stdio streams in places
where I want to use non-blocking file handles, and carefull
read()ing and write()ng along with select()s to handle
non-stagnation of this type of communications.

> I appreciate the help.

/Matti Aarnio