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Date:	Thu, 12 Mar 2020 16:08:33 +0100
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: Setting termios VMIN > 0 and VTIME > 0 on non blocking file
Message-ID:	<20200312150833.GQ4042@calimero.vinschen.de>
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On Mar 12 15:44, Corinna Vinschen wrote:
> On Mar 12 15:20, =C3=85ke Rehnman via Cygwin wrote:
> > I think the problem is if the number of bytes requested are more than w=
hat
>=20
> To clarify: number of bytes =3D=3D VMIN?
>=20
> > is in the buffer it is going to overlap the read function (because of V=
TIME)
> > and immediately after that CancelIO is called. Contrary to what is ment=
ioned
> > in the source code I think CancelIO is actually discarding data...
>=20
> So far we didn't have that experience.  CancelIO is usually safe
> in this regard.
>=20
> > =C2=A0 /* Use CancelIo rather than PurgeComm (PURGE_RXABORT) since
> > =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0 PurgeComm apparently discar=
ds in-flight bytes while CancelIo
> > =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0 only stops the overlapped I=
O routine. */
> >=20
> >=20
> > My suggestion is the following patch:
> >=20
> > diff --git a/winsup/cygwin/fhandler_serial.cc
> > b/winsup/cygwin/fhandler_serial.cc
> > index 69e5768f6..afa8871bf 100644
> > --- a/winsup/cygwin/fhandler_serial.cc
> > +++ b/winsup/cygwin/fhandler_serial.cc
> > @@ -898,7 +898,11 @@ fhandler_serial::tcsetattr (int action, const stru=
ct
> > termios *t)
> > =C2=A0=C2=A0 {
> > =C2=A0=C2=A0=C2=A0=C2=A0 memset (&to, 0, sizeof (to));
> >=20
> > -=C2=A0=C2=A0=C2=A0 if ((vmin_ > 0) && (vtime_ =3D=3D 0))
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (is_nonblocking())
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 {
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0 to.ReadIntervalTimeout =3D MAXDWORD;
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
> > +=C2=A0=C2=A0=C2=A0 else if ((vmin_ > 0) && (vtime_ =3D=3D 0))
>=20
> What if you switch to !O_NONBLOCK after calling tcsetattr?  The
> setting of ReadIntervalTimeout would be lost then.
>=20
> Either we have to repeat calling SetCommTimeouts every time
> we switch mode, or we have to do the above setting temporary
> every time we call ReadFile in non blocking mode.

What about this:

diff --git a/winsup/cygwin/fhandler_serial.cc b/winsup/cygwin/fhandler_seri=
al.cc
--- a/winsup/cygwin/fhandler_serial.cc
+++ b/winsup/cygwin/fhandler_serial.cc
@@ -68,6 +68,16 @@ fhandler_serial::raw_read (void *ptr, size_t& ulen)
 	goto err;
       else if (ev)
 	termios_printf ("error detected %x", ev);
+      else if (is_nonblocking ())
+	{
+	  if (!st.cbInQue)
+	    {
+	      tot =3D -1;
+	      set_errno (EAGAIN);
+	      goto out;
+	    }
+	  inq =3D st.cbInQue;
+	}
       else if (st.cbInQue && !vtime_)
 	inq =3D st.cbInQue;
       else if (!is_nonblocking () && !overlapped_armed)


Corinna

--=20
Corinna Vinschen
Cygwin Maintainer

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