Re: Waiting for something to happen on a pipe/socket on Windows

[Graham Bloice <graham.bloice () trihedral com>]

I'll be able to look into this when I get back from vacation next week if
no one else gets there first.

On 27 June 2017 at 22:18, Roland Knall <rknall () gmail com> wrote:

> This is one of the best and most comprehensive explanations for Windows
> sockets I read so far.
>
> @Stig - You could take a look at extcap_spawn.c, there you will find code,
> also in extcap.c for implementing waiting on that thing, Windows likes to
> call a "Named Pipe"
>
> cheers,
> Roland
>
> On Tue, Jun 27, 2017 at 10:59 PM, Guy Harris <guy () alum mit edu> wrote:
>
> > (Sending to a broader list, in case anybody has answers.)
> >
> > On Jun 26, 2017, at 10:53 AM, Stig Bjørlykke <stig () bjorlykke org> wrote:
> >
> > > The extcap interface toolbar is not supported on Windows yet,
> > > basically because I don't know how the pipe handling is done when
> > > using select(),
> >
> > Sadly, Windows' handling of "wait for something to happen" is a bit
> > clumsier than UN*X's.
> >
> > UN*Xes have select() and poll(), with the possible addition of some other
> > similar mechanisms such as Linux's epoll() or *BSD/Darwin's kqueues.  You
> > can wait for a device, a socket, or a pipe with them.  (Sadly, in the
> > general case, you can't wait for a process or thread to terminate with
> > select(), although you can do it with kqueues on at least some platforms.
> > I'm not sure whether the inability to wait for a mutex to unlock or a
> > condition variable to be signaled is a significant issue or not.)
> >
> > Windows has select()
> >
> >         https://msdn.microsoft.com/en-us/library/windows/desktop/ms7
> > 40141(v=vs.85).aspx
> >
> > which works on sockets, sockets, sockets, and sockets, and
> > WaitForMultipleObjects():
> >
> >         https://msdn.microsoft.com/en-us/library/windows/desktop/ms6
> > 87025(v=vs.85).aspx
> >
> > which works on a number of things - only one of which is the equivalent
> > of a readable file descriptor, namely "console input".  It doesn't appear
> > that you can use a HANDLE for a pipe (named or unnamed), and, while
> > Microsoft seem to indicate that you can do regular ReadFile()/WriteFile()
> > I/O on a socket:
> >
> >         https://msdn.microsoft.com/en-us/library/windows/desktop/ms7
> > 40522(v=vs.85).aspx
> >
> > they don't indicate what the connection is between a socket descriptor
> > and a socket handle (they have the same size, 32 bits in Win32 and 64 bits
> > in Win64, but are SOCKET values usable as HANDLEs, or is there a "get a
> > handle for this socket descriptor" call?) - and even if you get a HANDLE it
> > probably works no better with WaitForMultipleEvents() than do pipe HANDLEs.
> >
> > For named pipes, you can use "overlapped mode" for asynchronous I/O:
> >
> >         https://msdn.microsoft.com/en-us/library/aa365788(VS.85).aspx
> >
> > and can create an event handle that's signaled when a read or write
> > completes, and hand *that* to WaitForMultipleObjects().  This appears, from
> > some Web searching, to be the closest thing to waiting for a read to be
> > possible, but it would be done differently - instead of
> >
> >         select();
> >         if (pipe is readable) {
> >                 read into buffer;
> >                 process that data if we can;
> >         }
> >
> > it'd be more like
> >
> >         start an asynchronous read into the buffer;
> >         WaitForMultipleObjects();
> >         if (event for that pipe was triggered) {
> >                 process the data that was read if we can;
> >         }
> >
> > Anonymous pipes are just named pipes with created unique names:
> >
> >         https://msdn.microsoft.com/en-us/library/aa365141(v=vs.85).aspx
> >
> > but, as that page says, "Asynchronous (overlapped) read and write
> > operations are not supported by anonymous pipes.".
> >
> > For sockets, it appears you can do an operation that's similar to
> > select(), but, instead of waiting, returns an event handle:
> >
> >         https://msdn.microsoft.com/en-us/library/windows/desktop/ms7
> > 41576(v=vs.85).aspx
> >
> > However, those event handles may, or may not, be like regular Windows
> > event handles; they are documented as working with
> > WSAWaitForMultipleEvents() rather than with WaitForMultipleObjects():
> >
> >         https://msdn.microsoft.com/en-us/library/windows/desktop/ms7
> > 42219(v=vs.85).aspx
> >
> > and they also don't indicate whether you can use regular event objects
> > with WSAWaitForMultipleEvents().
> >
> > Furthermore, GUI applications should probably use
> > MsgWaitForMultipleObjects().  I think that's what's used in the GUI event
> > loops of GTK+ and Qt on Windows.
> >
> > So doing the equivalent of select()/poll() on Windows appears to be a
> > challenge.
> >
> > If anybody has more information than what I found, that'd be helpful.
> > ____________________________________________________________
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>
>
>
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-- 
Graham Bloice
Software Developer
Trihedral UK Limited

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