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Re: termios VTIME and VMIN prevent SIGALRM from interrupting read
- From: Corinna Vinschen <corinna-cygwin at cygwin dot com>
- To: cygwin at cygwin dot com
- Date: Thu, 25 Oct 2012 10:35:59 +0200
- Subject: Re: termios VTIME and VMIN prevent SIGALRM from interrupting read
- References: <7603230f0f415b46b2bf1bebe58c87c8.squirrel@indra.com>
- Reply-to: cygwin at cygwin dot com
On Oct 23 16:27, J. L. Sloan wrote:
> If I use termios(3) to set VTIME and VMIN to values greater than zero for
> a serial port, I can no longer interrupt a read(2) against the descriptor
> with SIGALRM. Replacing read(2) with nanosleep(2) yields EINTR as
> expected. Not setting VTIME and VMIN, read(2) yields EINTR as desired.
> Test program and cygcheck output attached. Could be this is intended
> behavior. It seems more likely that this is a bug in the driver for my
> USB-to-serial adapter. Cygwin 1.7.17 snapshot 2012-10-16.
VMIN shouldn't have any influence on that behaviour, in theory. If you
use VTIME values > 0, the Cygwin code doesn't use it's own interruptible
wait routine, but rather calls ReadFile directly and relies on OS timeout
handling. So, yes, it's kind of expected right now.
I think it should be possible to rewrite the VTIME behaviour so that
it doesn't rely on the OS timeout but instead adds a timer to the
interruptible wait. Adding the timer is pretty simple, but changing
the setting of the SetCommTimeouts values might be tricky. See
http://cygwin.com/cgi-bin/cvsweb.cgi/src/winsup/cygwin/fhandler_serial.cc?rev=1.89&content-type=text/x-cvsweb-markup&cvsroot=src
especially the VMIN/VTIME comment deep down in fhandler_serial::tcsetattr().
Corinna
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Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
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