On Jul 19, 4:30 am, Nick Craig-Wood <[EMAIL PROTECTED]> wrote:
> Murali <[EMAIL PROTECTED]> wrote:
> > After some investigation, I found out that this problem had nothing to
> > do with my GUI app not getting refreshed and I was able to reproduce
> > this problem with normal python scripts. Here is one such script
>
> > #File test.py
> > from subprocess import Popen
> > from subprocess import PIPE
> > import sys
> > if __name__ == '__main__':
> > prog = sys.argv[1]
> > proc = Popen(prog, shell = True, stdout = PIPE, bufsize = 0)
> > out = proc.stdout
> > while proc.poll() is None:
> > print out.readline()
>
> > Run this program as follows
> > $ test.py "ping -c 10www.google.com"
>
> > Now, you would see the responses as if you just launched ping on the
> > command line. Now, lets look at another program. Here is a simple C++
> > program that prints numbers 1 to 10 at the passage of every second
> > (sort of a stopwatch)
>
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <sys/time.h>
> > main ( )
> > {
> > timeval t1, t2;
> > gettimeofday(&t1, NULL);
> > int prev = -1;
> > int cur = 0;
> > while (true)
> > {
> > gettimeofday(&t2,NULL);
> > if(t2.tv_sec - t1.tv_sec > 10)
> > break;
> > else
> > {
> > cur = t2.tv_sec-t1.tv_sec;
> > if (cur != prev)
> > {
> > printf("%d\r\n",cur);
> > prev = cur;
> > }
> > }
> > }
> > }
>
> > if you would build this program and call it lets say timer ($ g++ -o
> > timer timer.cpp) and run it with our python script like this
>
> > $python test.py "./timer"
>
> > you would see that every time you run the program your results vary
> > and on top of this the stdout of the timer program gets displayed all
> > at once presumably when the timer program has completed execution.
>
> > Why this discrepancy between the ping and timer programs? Is my
> > test.py script correct? Is there a better or a preferred method for
> > doing interprocess communication in Python.
>
> Buffering is your problem.
>
> If you add a fflush(stdout); after the printf(...); you'll find the
> c++ program works as you expect.
>
> It is just a fact of life of the C stdio system. If it is connected
> to a terminal then it will turn off buffering. If it is connected
> anything else (eg a pipe via subprocess) then it will buffer stuff as
> you've seen.
>
> So you can
>
Thanks Nick. fflush fixed it. Thanks for your pointers on pexpect and
pty module too.
Murali.
> a) modify the c++ prog to add fflush() in or use setvbuf()
> b) use the pexpect module -http://pexpect.sourceforge.net/
> c) use the pty module (unix only)
>
> The pexpect module will connect to the subprogram with pseudo-ttys,
> fooling the program, and the C library, into thinking that it is
> speaking to a terminal and turn off buffering. Pexpect doesn't work
> on windows.
>
> The fact that ping works is because it uses fflush() - you can see
> this if you "ltrace" it.
>
> --
> Nick Craig-Wood <[EMAIL PROTECTED]> --http://www.craig-wood.com/nick
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