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
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