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Re: [Qemu-devel] chardev's and fd's in monitors

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] chardev's and fd's in monitors
Date: Tue, 18 Oct 2016 12:32:02 +0100
User-agent: Mutt/1.7.1 (2016-10-04)

* Daniel P. Berrange (address@hidden) wrote:
> On Wed, Oct 12, 2016 at 08:15:02PM +0100, Dr. David Alan Gilbert wrote:
> > Hi,
> >   I had a look at a couple of readline like libraries;
> > editline and linenoise.  A difficulty with using them is that
> > they both want fd's or FILE*'s; editline takes either but
> > from a brief look I think it's expecting to extract the fd.
> > That makes them tricky to integrate into qemu, where
> > the chardev's hide a whole bunch of non-fd things; in particular
> > tls, mux, ringbuffers etc.
> > 
> > If we could get away with just a FILE* then we could use fopencookie,
> > but that's GNU only.
> > 
> > Is there any sane way of shepherding all chardev's into having an
> > fd?
> The entire chardev abstraction model exists precisely because we cannot
> make all chardevs look like a single fd. Even those which are fd based
> may have separate FDs for input and output.

Note that editline takes separate in/out streams, but it does want those streams
to be FILE*'s.

> IMHO the only viable approach would be to enhance linenoise/editline to
> not assume use of fd* or FILE * abstractions.

I think if it came to that then we'd probably end up sticking with what we
had for a very long time; I'd assume it would take a long time before
any mods we made to the libraries would come around to be generally useful.

> BTW, what is the actual thread issue you are facing ? Chardevs at least
> ought to be usable from a separate thread, as long as each distinct
> chardev object instance was only used from one thread at a time ?

Marc-André pointed that out; I hadn't realised they were thread safe.
But what are the rules? You say 'only used from one thread at a time' -
what happens if we have a mux and the different streams to the mux come
from different threads?

My actual thoughts for threads came from a few sides:
  a) Maybe I could have a shim thread that fed the editline fd from a chardev
  b) I'd eventually like multiple monitor threads.


> Regards,
> Daniel
> -- 
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Dr. David Alan Gilbert / address@hidden / Manchester, UK

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