From: "ara.t.howard" <ara.t.howard@...>
Date: 2008-07-18T03:43:00+09:00
Subject: Re: Reading stdout & stderr from a pipe with popen3


On Jul 17, 2008, at 12:22 PM, Matt Mencel wrote:

> Well drat....
>
> pid, stdin, stdout, stderr = Open4::popen4 "zmprov"
> stdin.puts "sm shares"
> stdin.puts "gaf"
> stdin.close
> puts stderr.read.strip if !stderr.read.strip.empty?
> puts stdout.read.strip if stderr.read.strip.empty?
> stdin.puts "sm shares2" # line 52
> stdin.puts "gaf"
> stdin.close
> puts stderr.read.strip if !stderr.read.strip.empty?
> puts stdout.read.strip if stderr.read.strip.empty?
> exit
>
>
> Trying to do that doesn't work, it says the stream is closed and I  
> can't send anything new to it.
> ./pipe_test.rb:52:in `write': closed stream (IOError)
>
> So I don't want to stdin.close until I'm done processing a bunch of  
> accounts, but I need to check stdout/stderr after I check each  
> account without the stdin.close.  Is that possible?
>
> Matt

it's entirely up to the client program - this has nothing to do with  
open4 or any other program.

you've got all sorts if issues happening here

- you simply need to flush stdin, not close it, if you plan to  
continue to use it
- you are reading to the end of stdout and stderr, this blocks until a  
program exits, which will not happen until you close stdin, which is  
why you are needing to do that to see any output
- zmprov may or may not buffer stdout and stderr, i have no idea how  
this program is written an so cannot say

basically, if we assume zmprov is written in a line oriented fashion  
you *might* be able to do something like

   stdin.puts input

   output = stdout.gets output

   if output =~ /error/
    ....

but i see you are checking stderr too, so you have a problem: you have  
to read from both stdout and stderr and one, or both, might be empty.   
you program is going to block until you check both.  so you have options

- use threads and a queue to check both, develop your own protocol  
using the code from open4 as an example of how to read from two io  
objects simultaneously
- use select to read from either stdout or stderr, whichever becomes  
available
- re-write zmprov so send all output on stderr and check only that

in short your question is impossible to answer without understanding  
what, *exactly* zmprov does and how it produces output on stdout and  
stderr.  it's also impossible to solve without a solid understanding  
of how io and, specifically io buffering works.

hope that helps.

a @ http://codeforpeople.com/
--
we can deny everything, except that we have the possibility of being  
better. simply reflect on that.
h.h. the 14th dalai lama