[Newbies] Re: Read a filestream (named pipe) with a timeout
dsl101
dl3 at pale-eds.co.uk
Fri Jan 10 18:02:00 UTC 2014
And here's another one I'm banging my head on. If I open 2 named pipes like
so:
pipeIn := FileStream fileNamed: '\\.\pipe\in'.
pipeOut := FileStream fileNamed: '\\.\pipe\out'.
And test them like this:
1. Write 'Here's Johnny!!!!!' to pipeOut
2. Wait for the md5 ack from the pipe server and verify
3. Wait for a message from the pipe server on pipeIn
4. Computer md5 ack and send it back
all works fine (assuming the pipe server at the other end has a mirror
image of this flow, which it does).
However, if I do this (simulating having several messages waiting in the
'out' pipe):
1. Write 'Here's Johnny!!!!!' to pipeOut (call this message 1)
2. Write 'Another message' to pipeOut (call this message 2)
3. Wait for the md5 ack for message 1 and verify (pipe server only reads
message 1)
4. Wait for a message from the pipe server on pipeIn (pipe server
initiates this once it has sent the ack above)
5. Computer md5 and send it back via pipeIn
I get a 'write failed' at step 5, trying to send the md5 back on pipeIn.
This seems strange, since the only difference I can see is the extra write
to pipeOut at step 2 - nothing to do with pipeIn.
What's even more frustrating is that by step 3, I can see on the pipe
server (via PeekNamedPipe) that there are another 15 bytes available
(message 2), and if I actually read them here before sending the ack back
to the Squeak app, then step 5 doesn't fail. Gah... Can a nextPutAll: on
one StandardFileStream fail because a write on another hasn't been read?
The full code looks like this:
pipeIn := FileStream fileNamed: '\\.\pipe\in'.
pipeOut := FileStream fileNamed: '\\.\pipe\out'.
msg := 'Here''s Johnny!!!!'.
msgHash := ((CMD5Hasher hashMessage: msg) printStringBase: 16 length: 32
padded: true) asLowercase.
pipeOut nextPutAll: msg; flush.
"Queue another message - this will cause the later call to pipeIn
nextPutAll: to fail, unless the pipe server has already read this
message..."
msg := 'Another message'.
pipeOut nextPutAll: msg; flush.
start := DateAndTime millisecondClockValue.
[(pipeOut size < 32) & ((DateAndTime millisecondClockValue) - start <
3000)] whileTrue: [
(Delay forMilliseconds: 200) wait.
].
pipeOut size = 32 ifTrue: [
ack := pipeOut next: 32.
] ifFalse: [
ack := 'timeout'.
].
ack = msgHash ifFalse: [
^ false.
].
"Wait up to 3 seconds for the return connection check from pipe server"
start := DateAndTime millisecondClockValue.
[(pipeIn size = 0) & ((DateAndTime millisecondClockValue) - start < 3000)]
whileTrue: [
(Delay forMilliseconds: 200) wait.
].
msg := pipeIn next: (pipeIn size).
msgHash := ((CMD5Hasher hashMessage: msg) printStringBase: 16 length: 32
padded: true) asLowercase.
"The next line will fail unless the pipe server has read our earlier
message on pipeOut"
pipeIn nextPutAll: msgHash; flush.
^ true.
BTW - the reason I'm using 2 pipes is that both ends (the squeak app and
the vb.net app) could generate events / messages at any time, and so I need
both ends to be 'waiting' for a message from the other. This seemed the
simplest way to achieve this and be able to send an ack without the risk of
the ack getting mangled up with new messages. However, it seems like I
might have been wrong...
On Fri, Jan 10, 2014 at 4:25 PM, David Lomas <dl3 at pale-eds.co.uk> wrote:
> 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
>> > >
>> > >
>> > >
>> > > --
>> > > View this message in context:
>> > http://forum.world.st/Read-a-filestream-named-
>> > > pipe-with-a-timeout-tp4735456.html
>> > > Sent from the Squeak - Beginners mailing list archive at Nabble.com.
>> > > _______________________________________________
>> > > Beginners mailing list
>> > > [hidden email]
>> > > http://lists.squeakfoundation.org/mailman/listinfo/beginners
>> >
>> >
>> > _______________________________________________
>> > Beginners mailing list
>> > [hidden email]
>> > http://lists.squeakfoundation.org/mailman/listinfo/beginners
>> >
>> >
>> >
>> ________________________________________________________________________________________________________________________________________________________________________________________________________________
>>
>> > If you reply to this email, your message will be added to the
>> discussion below:
>> >
>> http://forum.world.st/Read-a-filestream-named-pipe-with-a-timeout-tp4735456p4735547.html
>> > To unsubscribe from Read a filestream (named pipe) with a timeout,
>> click here.
>> > NAML
>> >
>> >
>> >
>> ________________________________________________________________________________________________________________________________________________________________________________________________________________
>>
>> > View this message in context: Re: Read a filestream (named pipe) with a
>> timeout Sent from the Squeak - Beginners mailing list archive at
>> Nabble.com.
>> >
>> _______________________________________________
>> Beginners mailing list
>> [hidden email] <http://user/SendEmail.jtp?type=node&node=4735726&i=0>
>> http://lists.squeakfoundation.org/mailman/listinfo/beginners
>>
>>
>> ------------------------------
>> If you reply to this email, your message will be added to the
>> discussion below:
>>
>> http://forum.world.st/Read-a-filestream-named-pipe-with-a-timeout-tp4735456p4735726.html
>> To unsubscribe from Read a filestream (named pipe) with a timeout, click
>> here<http://forum.world.st/template/NamlServlet.jtp?macro=unsubscribe_by_code&node=4735456&code=ZGwzQHBhbGUtZWRzLmNvLnVrfDQ3MzU0NTZ8MTEyNTU4NzUzMg==>
>> .
>> NAML<http://forum.world.st/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml>
>>
>
>
--
View this message in context: http://forum.world.st/Read-a-filestream-named-pipe-with-a-timeout-tp4735456p4735778.html
Sent from the Squeak - Beginners mailing list archive at Nabble.com.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/beginners/attachments/20140110/783ab5bd/attachment-0001.htm
More information about the Beginners
mailing list