Re: [Qemu-devel] [PATCH 2/3] Always use nonblocking mode for qemu_chr_open_fd.

[Daniel P. Berrange]

On Wed, Jul 23, 2008 at 01:48:14PM +0200, Gerd Hoffmann wrote:
> Daniel P. Berrange wrote:
> 
> > In the case of the PTY backend, the only time I'd expect data to be dropped
> > is if there was no active slave open. If an application has the PTY open
> > and is interacting, then I'd want to get all data.  
> 
> Yep, that would be most useful (and also matches what is done for tcp
> for example).  Now the interesting question is:  How can qemu figure (in
> a portable way) whenever there is some process listening on the slave side?

Well you get SIGHUP when the slave side closes, but the problem is then
detecting the re-connect. If you keep polling on the master PTY you'll just
spin getting SIGHUP all the time. On Linux you can deal with this by using
epoll() in edge-triggered mode instead of poll which is level-triggered.
Unfortunately epoll is Linux specific, so you'd need to have alternate
impls of the guts of the QEMU event loop for every OS :-(

According to epoll(2), on FreeBSD you can use kqueue and on Solaris
you can use /dev/poll. The PTY code isn't supported on Windows so
that wouldn't neccessarily be a show stopper.

Daniel
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