[Newbies] Re: Read a filestream (named pipe) with a timeout
dsl101
dl3 at pale-eds.co.uk
Fri Jan 10 16:26:17 UTC 2014
Thanks - I'm persevering with the busy wait for now, but may look at async.
I also thought it looked much trickier, but your example code is great so
thanks for that.
I did originally try using sockets, but it triggers all kinds of security
warnings on a PC when a socket server is set up. I didn't want to worry end
users, or required them to put exceptions in their firewalls, etc., when
the communication is entirely local. I have also tried using plain files
(which works), but watching for changes on the filesystem is ugly and prone
to missing things.
Ideally, I'd like to use messages (e.g. via WndProc on Win32). In fact,
I've been able to *send* windows messages from Squeak to a vb.net app by
calling RegisterWindowMessage / SendMessage in user32.dll, but I couldn't
find any way of receiving messages using this system, and I need
bi-directional. I presume the VM would need to capture and pass those on to
the Squeak image, which it doesn't currently do. If there is some kind of
'user' event channel I could use, please do suggest something...
Hence pipes was the least worst option so far.
Thanks again,
Dave
On Fri, Jan 10, 2014 at 3:37 PM, Levente Uzonyi-2 [via Smalltalk] <
ml-node+s1294792n4735726h10 at n4.nabble.com> wrote:
> On Fri, 10 Jan 2014, dsl101 wrote:
>
> > Hi Ron,
> > Thanks for that - but I can only see #dataAvailable for Sockets, not for
> FileStream (named pipes). I think the same kind of thing is available for
> pipes (you can do `pipe size` to see how much data is there),
> > but it still doesn't wait. I'm trying to avoid a busy loop waiting for
> the data - like this:
>
> FileStreams don't have semaphores, so you can only use busy waiting with
> them. However there's AsyncFile, which can do what you want. But its
> interface is a bit cumbersome, and it's hardly tested/used at all. Here's
> how it could work:
>
> | syncSemaphore file message |
> syncSemaphore := Semaphore new.
> file := AsyncFile new.
> file open: 'your_pipe' forWrite: true.
> message := 'Here''s Johnny!!!!'.
> file writeBuffer: message atFilePosition: 0 onCompletionDo: [
> syncSemaphore signal ].
> (syncSemaphore waitTimeoutMSecs: 3000) ifTrue: [
> "handle timeout" ].
> file readByteCount: 32 fromFilePosition: 0 onCompletionDo: [ :response |
> message := response.
> syncSemaphore signal ].
> (syncSemaphore waitTimeoutMSecs: 3000) ifTrue: [
> "handle timeout" ].
> Transcript show: 'Received: ', message; cr.
> file close.
>
> Using syncSemaphore is a must, because the callbacks are evaluated from
> another process.
>
>
> Levente
>
> P.S.: If you want to communicate with another program from Squeak, then
> you should use Sockets if possible, since those are versatile and well
> tested.
>
> >
> > start := DateAndTime millisecondClockValue.
> > (pipe size < 32) & (DateAndTime millisecondClockValue - start < 3000)
> ifTrue: [
> > (Delay forMilliseconds 50) wait.
> > ]
> > pipe size = 32 ifTrue: [
> > "Get data"
> > ] ifFalse: [
> > "Deal with timeout"
> > ]
> >
> > The shorter the 'wait', the more responsive the code is to data arriving
> on the pipe, but the more CPU it will use as it spins round the loop. The
> longer the 'wait', the more lag it has for data coming back.
> > That's what I'm trying to avoid by blocking on the read, but with a way
> to escape after some timeout.
> >
> > I'm guessing the call to 'pipe next:' is a primitive, and blocks there,
> which is why valueWithin:onTimeout: doesn't return after the timeout, but
> does eventually return the correct answer. So, I'm guessing
> > I'll have to do something like this:
> > * Set up a semaphore
> > * Fork the blocking read process, which will signal the semaphore if
> it ever returns its 32 bytes
> > * In the main thread, wait for up to 3 seconds for the semaphore to be
> signalled
> > * If the semaphore times out, kill the forked process
> > Obviously there's a potential race at the end there, but the worst case
> is we throw away data which was returned at the last moment. Is there
> anything else you can see wrong with this approach?
> >
> > Thanks,
> >
> > Dave
> >
> >
> > On Thu, Jan 9, 2014 at 9:16 PM, Ron Teitelbaum [via Smalltalk] <[hidden
> email]> wrote:
> > Hi Dave,
> >
> > See #dataAvailable ??and #recieveAvailableData.
> >
> > It's never good to call for data if you don't know you have any.
> ??Better to
> > setup a wait for data until call instead. ??
> >
> > All the best,
> >
> > Ron Teitelbaum
> > Head Of Engineering
> > 3d Immersive Collaboration Consulting
> > [hidden email]
> > Follow Me On Twitter: @RonTeitelbaum
> > www.3dicc.com
> > https://www.google.com/+3dicc
> >
> >
> > > -----Original Message-----
> > > From: [hidden email] [mailto:[hidden email]
> > > [hidden email]] On Behalf Of dsl101
> > > Sent: Thursday, January 09, 2014 10:16 AM
> > > To: [hidden email]
> > > Subject: [Newbies] Read a filestream (named pipe) with a timeout
> > >
> > > I'm using Squeak 4.2 and working on the smalltalk end of a named
> pipe
> > > connection, which sends a message to the named pipe server with:
> > >
> > > msg := 'Here''s Johnny!!!!'.
> > > pipe nextPutAll: msg; flush.
> > >
> > > It should then receive an acknowledgement, which will be a
> 32-byte md5
> > hash of
> > > the received message (which the smalltalk app can then verify). It's
> > possible the
> > > named pipe server may have gone away or otherwise been unable to deal
> with
> > > the request, and so I'd like to set a timeout on reading the
> > acknowledgement.
> > > I've tried using this:
> > >
> > > ack := [ pipe next: 32 ] valueWithin: (Duration seconds: 3)
> > onTimeout: [
> > > 'timeout'. ].
> > >
> > > and then made the pipe server pause artificially to test the code. But
> the
> > > smalltalk thread blocks on the read and doesn't carry on (even after
> the
> > > timeout), although if I then get the pipe server to send the correct
> > response
> > > (after a 5 second delay, for example), the value of 'ack' is
> 'timeout'.
> > Obviously
> > > the timeout did what it's supposed to do, but couldn't 'unblock' the
> > blocking
> > > read on the pipe.
> > >
> > > Is there a way to accomplish this even with a blocking FileStream
> read?
> > I'd rather
> > > avoid a busy wait on there being 32 characters available if at all
> > possible.
> > >
> > > Thanks,
> > >
> > > Dave
> > >
> > >
> > >
> > > --
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> > > pipe-with-a-timeout-tp4735456.html
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