From: Tom Felker <tcfelker@...>
Date: 2003-05-28T07:29:02+09:00
Subject: Re: system calls

On Tue, 2003-05-27 at 11:56, J.Hawkesworth wrote:

> This is almost what you want, except that the exit code is wrong and I
> don't
> know how to fix it:
> 
> require 'open3'
> module Kernel
>   private
>   def really_cool_system(*args)
>     inp, out, err = Open3.popen3(*args)
>     ret1 = ret2 = nil
>     t1 = Thread.new do
>       inp.close
>     end
>     t2 = Thread.new do
>       ret1 = out.read
>       out.close
>     end
>     t3 = Thread.new do
>       ret2 = err.read
>       err.close
>     end
>     t1.join
>     t2.join
>     t3.join
>     return ret1, ret2, $?
>   end
> end
> 
> irb(main):024:0> really_cool_system("ls","/dev/null","/dev/foo")
> => ["/dev/null\n", "ls: /dev/foo: No such file or directory\n", 65280]
> 
> Regards,
> 
> Brian.

Yes, it's broken: true and false (which return 0 and 1 respectively)
give the same value.

The problem is in Open3.popen3.  It forks twice, so the command it runs
isn't Ruby's child.  (I don't know why you'd want this.)  That means $?
is the return from the first fork, or something like that.  There could
be races also.

To be used in a program with it's own children, you'd want to waitpid on
the system call you spawned to get it's return value.  I think
Open3.popen3 should return [in, out, err, pid] so you could do this, but
it doesn't, so you can't.  Without waiting on the pid of your child, you
can't reliably get the return value.

-- 
Tom Felker

They made a crude but listenable crystal radio out of a light bulb, a
crayon, a square of toilet paper, and a rock. They say they'll have
Linux running on it by next week.