[squeak-dev] reading and writing from STDIN and STDOUT

Jason Rogers jacaetevha at gmail.com
Fri May 15 05:31:57 UTC 2009

On Tue, May 12, 2009 at 8:59 PM, David T. Lewis <lewis at mail.msen.com> wrote:
> On Tue, May 12, 2009 at 04:59:27PM -0400, Jason Rogers wrote:
>> I am competing in a friendly Code Smackdown at work.  Smalltalk vs.
>> All on-comers (Haskell, Erlang, LISP, Java, C and Python).  The
>> challenge is to write a client for a Chinese Poker game.
>> Everything is going well except communicating with the Python-based
>> server.  I am having trouble reading and writing from STDIN and
>> STDOUT.  I've installed OSProcess which should give me access to these
>> streams, but I don't see how to read one line at a time from STDIN and
>> then get a handle on STDOUT to write a line.  Anyone have pointers or
>> sample code?
>> Also, the image needs to be launched by Python with code like...
>>    subprocess.Popen(["/Application/Squeak/Contents/MacOS/Squeak VM
>> Opt", "/Users/jrogers/Squeak/images/chinese_poker.image", "Squeak
>> Client"], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
>> This will obviously need to change for a server running on another OS.
>>  The question I have here is how to start a process reading from STDIN
>> as soon as the VM is fired up.  Should I just create an object that is
>> waiting on STDIN, reading a line at a time, before saving the image?
>> That way it would just keep trying to read when the image was fired up
>> again.
> Hi Jason,
> I'm assuming that you are using OS X or other unix based OS (note
> that OSProcess does not do pipes correctly on Windows, so you won't
> be able to do that).
> Please install CommandShell in addition to OSProcess, as that will give
> you better ways to control external processes connected by pipes. I'm
> not clear from your question as to whether Python or Squeak is the
> "client" in this scenario, but it sounds like you will want to have
> Squeak making requests to a Python server, but you want Python to
> set up the pipes and invoke Squeak as a subprocess. I don't have
> an example of this scenario exactly, but the following may help.
> For an example of Squeak as the client, but where Squeak (not Python)
> does all the work of setting up the pipes connections to the server,
> see PipeableOSProcess class>>tkExample. This shows Squeak running
> a tk/tcl interpreter and putting up a trivial UI with buttons connected
> to Squeak.
> For an example of Squeak as the server, communicating through its
> STDIN and STDOUT streams (which might be pipes set up by Python in
> your scenario), see class ExternalCommandShell. After evaluating
> "ExternalCommandShell start" your Squeak image will read command
> lines from STDIN and respond on STDOUT. The ExternalCommandShell
> will evaluate input commands and Smalltalk expressions, returning
> its output to the STDOUT stream. This does not sound like what you
> want for your project, but it should give you an idea of how these
> things can be done.
> In general, the STDIN steam is "OSProcess thisOSProcess stdIn" and
> the STDOUT and STDERR streams are "OSProcess thisOSProcess stdOut"
> and "OSProcess thisOSProcess stdErr". Do not save these in variables,
> but evaluate them at runtime so your Squeak image can find the right
> input/output streams even after an image restart. That means you can
> start a Process in Squeak that reads from stdIn and writes to stdOut.
> Leave the process running when you save the image, and it will carry
> on where it left off whenever your Python program restarts the Squeak
> image.
> Note that the stdIn stream for Squeak is (and must be) non-blocking in
> order to prevent locking up the Squeak VM on a blocked read. That means
> that your process will need to read available data from stdIn, possibly
> looping until you get a complete string of whatever you are looking for.
> For example, if ten characters are currently available in your STDIN
> input stream, then "OSProcess thisOSProcess stdIn next: 10000" will
> answer a string of length ten.
> ExternalCommandShell does this all in an event-driven manner with
> no polling, so you can copy that if you want to get fancy. Otherwise
> a simple polling loop in your process will probably work well enough.
> Put a 100 ms delay in whatever loop is reading from stdIn an you will
> get tolerable performance.
> Dave

Thanks again Dave for the help.  My tack was to "copy"
ExternalCommandShell's pattern.  StdIn is working fine, but when I
write to StdOut nothing seems to actually "get out".  Any ideas?

I'm running on Mac, I don't have the AioPlugin installed (I've
commented out the inform: that tells about the plugin not being there
and therefore polling will be used).  I'm not verbose enough at
building plugins.

Any ideas why?  I can post my Monticello file(s) somewhere if that would help.

Jason Rogers

"I am crucified with Christ: nevertheless I live;
yet not I, but Christ liveth in me: and the life
which I now live in the flesh I live by the faith of
the Son of God, who loved me, and gave
himself for me."
    Galatians 2:20

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