Questions about select for sockets

[Corinna Vinschen]

On Apr  6 11:44, Ken Brown wrote:
> On 4/6/2021 10:33 AM, Corinna Vinschen wrote:
> > On Apr  6 16:20, Corinna Vinschen wrote:
> > > On Apr  3 14:16, Ken Brown via Cygwin-developers wrote:
> > > > 4. Why do we set
> > > > 
> > > >    except_ready = saw_shutdown_write () || saw_shutdown_read () ?
> > > > 
> > > > I can't find any documentation that says that a shutdown should be
> > > > considered an exceptional condition.  On the other hand, POSIX does say,
> > > > "Other circumstances under which a socket may be considered to have an
> > > > exceptional condition pending are protocol-specific and
> > > > implementation-defined."  So maybe there's some Cygwin-specific reason for
> > > > doing this?
> > > 
> > > Nope, this is old (and wrong) cruft.  Neither Steven's book nor the
> > > Linux man pages, nor testing on Linux imply that ready for exception
> > > is used to indicate anything other than out-of-band data.  This should
> > > be fixed.
> > 
> > We may also have to change the saw_shutdown_read/saw_shutdown_write
> > handling.  I checked this on Linux and what happens is:
> > 
> >    After shutdown (fd, SHUT_RD), the socket is ready for reading and writing
> 
> This seems surprising to me.  Is it really the shutdown that caused it to be
> ready for writing in your test, or was it ready for writing anyway (e.g.,
> because the relevant buffer was empty)?

I guess so, too.  How to make sure the socket isn't ready for writing
without going to great lengths?


Corinna