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[example] how to get popen2() working? [XP, MSVC 2.9.15-octave]

From: andreas.naessl
Subject: [example] how to get popen2() working? [XP, MSVC 2.9.15-octave]
Date: Fri, 30 Nov 2007 16:25:24 +0100

hi all,

i try to establish a simple bidirectional pipe-connection between octave and 
a little "server"-programm written in python, for direct data-transfer without
producing temp-file-trash; i thought of using octave's popen2(). the server 
should be running continuously in the background, waiting for requests.

but i cannot get it working really.

[my System: win XP + Michaels MSVC-octave 2.9.15] 

first: if i communitcate to my "server" directly at octave-prompt, 
i get something back
(although i don't understand the need of the fclear(b) command 
when i want a second readline back from my server-app.) 
please see octave-commandline-output below:

  octave.exe:35> [a,b,i]=popen2('python','')
  a =  8
  b =  7
  i =  20204
  octave.exe:36> fputs(a,"test_command_at promt\n")
  ans = 0  
  octave.exe:37> fgets(b)
  ans = got command  test_command_at promt    ### ok
  octave.exe:38> fputs(a,"0\n")
  ans = 0
  octave.exe:39> fgets(b)
  ans = -1                        ### why this ? ###
  octave.exe:40> fclear(b)
  octave.exe:41> fgets(b)
  ans = got command  0           ### ok

  octave.exe:42> fclose(a)
  ans = 0
  octave.exe:43> fclose(b)
  ans = 0

second: if i put the same commands into a m-file (octave_cli.m),
i don't get back anything to read. why this?

code of "octave_cli.m":
  # [a,b,i]=popen2('python',['-u','']) # unbuffered python doesn't 
  # fgets(b)   gives on prompt here: ans = -1, so ...
  fgets(b)    # ...after fclear: ans = got command  0, but only on prompt 

gives the output:
  octave.exe:34> octave_cli
  a =  9
  b =  8
  i =  20360
  ans = 0
  ans = -1    # here i should get back my sent command-string
  ans = 0
  ans = -1    # here also 
  ans = 0
  ans = 0

third - just to be complete and going a bit octave-off-topic:
the code the pythonic testing-server ""
is added below. does anybody see a problem there?  
with a python-client-connection i managed it working.

  import sys
  # little test-server-app., just echo-ing incoming commands
  # command "0" terminates server
  def main():
    for line in iter(sys.stdin.readline,""):
      ln = line
      print "got command ", ln.rstrip('\n')   # answer-echo on stdout; remove 
      # later insert data-transfer here
      if ln == "0\n":
        print ln, "exit.."
      sys.stdout.flush()     # important 
  if __name__ == "__main__":

sorry for the long msg... the prob might be just a stupid mistake by me, 
as i'm not really a programming-expert...
anyway, i would strongly appreciate any hints by the community

thanks, best regards

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