Question about non-blocking Windows pipes

Corinna Vinschen
Tue Apr 6 12:57:38 GMT 2021

Hi Ken,

On Apr  1 10:39, Ken Brown via Cygwin-developers wrote:
> Hi Corinna,
> There are several places in where you make a
> distinction between the blocking and nonblocking cases with code like this:
>   cygwait (evt ?: get_handle (),...)

I only see this in fhandler_socket_unix::listen_pipe, actually.

> Here evt is an event handle in the blocking case and is NULL in the
> nonblocking case.  See, for example, fhandler_socket_unix::listen_pipe.
> What's the reasoning behind this?  Why not just always create an event or
> always use the handle?

In the nonblocking case, the status code returned from NtFsControlFile
is either a useful status code like STATUS_SUCCESS or an error code,
or it is STATUS_PENDING.  STATUS_PENDING only means the call is still
not finished.  To get a useful result, you still need a useful status
code.  You get that by waiting for the handle.  Note that waiting for
the handle doesn't mean to wait for the connecting client.  Rather, it's
signalled as soon as the async NtFsControlFile call finished.  If the
status code is STATUS_PIPE_LISTENING then, you know that no client tries
to connect, so you can return EAGAIN.

The completion event object OTOH, is only signalled if a client actually
connected, so that's blocking mode.

Two problems with using an event object in nonblocking mode:

- The event object is referenced in the call.  If NtFsControlFile returns
  STATUS_PENDING and you leave the function, you have to use a globally
  available event object, because this address is used as event object
  until completion of the NtFsControlFile call (and a client connected).

- You also have a pending NtFsControlFile until a client connects.
  This is contrary to what you want in a non-blocking call:  You only
  want to know *if* a client connects, not wait for it either way.

Does that help?  I'm not claiming there isn't another way to handle this
scenario, that's just what I came up with.


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