Help a newbie use Python's OS module:

[Kevin O'Gorman]

Donn Cave wrote:
> 
> Quoth Kevin O'Gorman <kogorman at pacbell.net>:
> 
> | I can use os.fork() to create a child process, but I cannot
> | get messages from the child to the parent through the pipe
> | I've created.
> |
> | If anyone's got a little sample program that shows how this
> | is done, I'd sure appreciate it.
> |
> | At the moment, my (non-working) version complains that the
> | parent's attemts to read the pipe are failing.  It looks
> | like this:
> |
> |
> | #! /usr/bin/python
> | # $Id: fork.py,v 1.2 2000/10/06 04:17:32 kevin Exp kevin $
> |
> | import string
> | import sys
> | import re
> | import os
> |
> | print "Running $Id: fork.py,v 1.2 2000/10/06 04:17:32 kevin Exp kevin $"
> |
> | osi=sys.stdin
> | oso=sys.stdout
> | ose=sys.stderr
> |
> | rw=os.pipe()
> | r=rw[0]
> | w=rw[1]
> | print "Reading on",r,", and writing on",w
> | child=os.fork()
> | if child==0:
> |         sys.stderr.write("In child\n")
> |         os.close(1)
> |         os.dup(w)
> |         sys.stdout=os.fdopen(1,"w")
> |         os.close(r)
> |         os.close(w)
> |
> |         sys.stderr.write("Writing\n")
> |         print "foo"
> |         print "bar"
> |         print "foobuar"
> |         sys.stdout.close()
> |         sys.exit(0)
> |
> | os.close(0)
> | os.dup(r)
> | sys.stdin=os.fdopen(r)           ## <-----
> | os.close(r)
> | os.close(w)
> |
> | print "Stdin is ",sys.stdin
> | print "about to read"
> | lin=sys.stdin.readline()
> | if not lin:
> |         print "Read failed",lin
> |         sys.exit(1)
> | while lin:
> |         print lin
> |
> | lin=sys.stdin.readline()
> 
> I think you will recognize the nature of the problem right away.
> Try sys.stdin = os.fdopen(0), in the parent.
> 
> I find dup2() easier to use than dup().  I rarely use file objects
> in this kind of thing, because of their buffering.  In this example
> I don't see any problems with either of those, though.
> 
>         Donn Cave, donn at u.washington.edu


Right on, of course.  Thanks for the help.  Everything will be
so much easier now that I have a working example.

I must admit, though, that I didn't quite understand the comment
about no using file objects.  Doesn't it get cumbersome to
direct output to the raw descriptors?  What do you use?
I didn't see anything like fprintf, or a way to set sys.stdout
to a descriptor (a bare int doesn't work).

++ kevin


-- 
Kevin O'Gorman  (805) 650-6274  mailto:kogorman at pacbell.net
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