# Open3 throws exception when preceded by fork

**URL:** <https://www.ruby-forum.com/t/open3-throws-exception-when-preceded-by-fork/143057>\
**Category:** Ruby\
**Created:** [July 14, 2008, 9:10pm UTC](https://www.ruby-forum.com/t/open3-throws-exception-when-preceded-by-fork/143057 "2008-07-14T21:10:48Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![trickyvail](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/df705f/32.png) [@trickyvail](https://www.ruby-forum.com/u/trickyvail)\
**Post date:** [July 14, 2008, 9:10pm UTC](https://www.ruby-forum.com/t/open3-throws-exception-when-preceded-by-fork/143057/1 "2008-07-14T21:10:48Z")

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I am attempting to create a daemon that (when required) spawns off  
child processes to perform “jobs”. It is important that this daemon  
does not block so it can start concurrent jobs, and that it knows when  
jobs have completed. Some of the jobs are written in C++ and send  
their output over standard error (stderr).

Here’s an outline of my daemon program.

#!/usr/bin/ruby  
require ‘open3’  
stop = false  
Signal.trap(‘CLD’) do  
child = Process.wait  
puts “child #{child} closed.”  
stop = true  
end  
child = Kernel.fork()  
if(child == nil)  
stdin, stdout, stderr =  
Open3.popen3(‘program\_that\_sends\_output\_to\_stderr’)  
stderr.each do |line|  
puts line  
end  
Kernel.exit!  
end  
while(! stop)  
sleep 5  
end  
puts “finished”

When tested separately the fork and popen3 calls seem to be just what  
I need to meet my objectives, but when they are run sequentially the  
popen3 call throws an exception:  
Error: “/usr/lib/ruby/1.8/open3.rb:75:in `waitpid’: No child processes  
(Errno::ECHILD)”

Is this a limitation of Open3? Any alternative approaches would be  
gratefully received.  
Thank you.

---

<div class="post-metadata">

**Author:** ![trickyvail](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/df705f/32.png) [@trickyvail](https://www.ruby-forum.com/u/trickyvail)\
**Post date:** [July 14, 2008, 10:57pm UTC](https://www.ruby-forum.com/t/open3-throws-exception-when-preceded-by-fork/143057/2 "2008-07-14T22:57:34Z")

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On Jul 14, 2008, at 1:05 PM, trickyvail wrote:

> s this a limitation of Open3? Any alternative approaches would be  
> gratefully received.

require ‘rubygems’  
require ‘slave’ # gem install slave

class Server  
def run job  
system job  
end  
end

slave = Slave.new{ Sever.new }  
server = slave.object

server.run ‘echo “this is in another process you do not have to ever  
worry about”’

a @ [http://codeforpeople.com/](http://codeforpeople.com/)

---

<div class="post-metadata">

**Author:** ![trickyvail](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/df705f/32.png) [@trickyvail](https://www.ruby-forum.com/u/trickyvail)\
**Post date:** [July 15, 2008, 2:24am UTC](https://www.ruby-forum.com/t/open3-throws-exception-when-preceded-by-fork/143057/3 "2008-07-15T02:24:57Z")

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On Jul 14, 2:52 pm, “ara.t.howard” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> server = slave.object
> 
> server.run ‘echo “this is in another process you do not have to ever  
> worry about”’

Thank you for you reply. Unfortunately I am unable to utilize the  
slave ruby gem (and open4 gem also) due to some rubygems package  
corruption? on my Ubuntu machine. I’ve given up fighting with it and  
come up with a different solution.

daemon:  
#!/usr/bin/ruby  
stop = false  
Signal.trap(‘CLD’) do  
child = Process.wait  
puts “child #{child} closed.”  
stop = true  
end  
exec(‘./job.rb’) if fork.nil?  
while(! stop)  
puts ‘not finished, do some other stuff’  
sleep 5  
end  
puts “finished”

job:  
#!/usr/bin/ruby  
require ‘open3’  
stdin, stdout, stderr = Open3.popen3(‘real\_job\_binary’)  
stderr.each do |line|  
puts line  
end

This way the calls to fork and open3 are isolated in different ruby  
interpreter instances where they can’t interfere with each other.

---

<div class="post-metadata">

**Author:** ![trickyvail](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/df705f/32.png) [@trickyvail](https://www.ruby-forum.com/u/trickyvail)\
**Post date:** [July 15, 2008, 2:33am UTC](https://www.ruby-forum.com/t/open3-throws-exception-when-preceded-by-fork/143057/4 "2008-07-15T02:33:10Z")

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On Jul 14, 2008, at 5:50 PM, trickyvail wrote:

> Thank you for you reply. Unfortunately I am unable to utilize the  
> slave ruby gem (and open4 gem also) due to some rubygems package  
> corruption? on my Ubuntu machine. I’ve given up fighting with it and  
> come up with a different solution.

the tgz files are on rubyforge. just unpack and run

‘sudo ruby install.rb’

cheers.

a @ [http://codeforpeople.com/](http://codeforpeople.com/)