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

[Marc-André Lureau]

Hi

On Tue, Oct 18, 2016 at 2:32 PM Dr. David Alan Gilbert <address@hidden>
wrote:

> * 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.
>

glibc can have custom streams iirc:
https://www.gnu.org/software/libc/manual/html_node/Custom-Streams.html#Custom-Streams

I haven't looked in details if that would solve it.


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