sending data continuously to a subprocess

Question

I am testing the communication with a subprocess. I have to start a server, sending periodically data. The final target is a process that acquires meteo data and a plotting server. The sampling rate is of the order or minutes .... I wrote the these two fragments of code to understand the basics of ipc in python, but I am not even able to make them working. The syncronism is not an issue.

main process

import sys, time
from subprocess import Popen, PIPE

print 'starting'
proc = Popen (['python.exe',
    'C:\Documents and Settings\Administrator\Desktop\sub.py'], 
                stdin = PIPE, stdout = PIPE)
print 'launched'

w = 0
while True:
    w += 1 
    time.sleep (2)
    print 'writing', w
    proc.stdin.write (repr(w))
    proc.stdin.flush()
    print proc.stdout.read()

subprocess:

import sys, time

print 'reading'
v = 0
while True:
    v = sys.stdin.read()
    sys.stdout.write('ACK')
    sys.stdout.flush ()
    time.sleep (4)

The main process is blocking, apparently the sub is not reading-sending the ACK. where am I wrong ??? thanks

Answer 1

The call to sys.stdin.read() blocks, as it is trying to read the entire stream, thus it can't return until the stream is closed.

Try using sys.stdin.readline() and add a newline when writing using sys.stdout.write() (in both processes), e.g. sys.stdout.write('ACK\n'). This should make sure the reading commands will block only until a single line is read.