From: Gary Wright <gwtmp01@...>
Date: 2007-03-20T07:07:04+09:00
Subject: Re: Passing File Descriptors?


On Mar 19, 2007, at 12:43 PM, Worky Workerson wrote:
> Is there a ruby way to pass file descriptors to separate processes?
> I'm thinking of interfacing with a C program, which uses the
> SCM_RIGHTS cmsg over a unix-domain socket ... is this idiom
> implemented in Ruby?  How would I marshall an IO object into (and out
> of) the integer file descriptor, or does this "just work"?

After a little investigation, this turned out to be easier than
I thought.  The following code was tested on Mac OS X with ruby 1.8.5.
There are three files below, don't cut and paste the whole thing:

    socketpair.rb      # example with parent/child

    server.rb          # example with unrelated processes
    client.rb

Socketpair.rb and client.rb both take a single argument, the name
of the data file to open.

Start server.rb before running client.rb.

If you try this on something other than Mac OS X ruby 1.8.5, let us
know how it works.

Gary Wright


$ cat socketpair.rb
# Scenario 1:  parent and child related via fork
require 'socket'

read, write = UNIXSocket.pair

puts "parent end: #{read.inspect}, #{read.fileno}"
puts "child  end: #{write.inspect}, #{write.fileno}"

pid = fork {
   file = File.open(ARGV[0])
   puts "child is sending: #{file.inspect}/#{file.fileno} connected  
to #{ARGV[0]}"
   write.send_io(file)
   exit
}

thefile  = read.recv_io
puts "parent received: #{thefile.inspect}/#{thefile.fileno}, contents:"
puts thefile.read

# end of scenario 1

$ cat server.rb
require 'socket'

serv = UNIXServer.new("/tmp/server")
p "server listening for connections: #{serv.inspect}"

client = serv.accept
p "received connection: #{client.inspect}"

clientfile  = client.recv_io
puts "parent received fd: #{clientfile.fileno}, contents:"

puts clientfile.read
# end of server.rb


$ cat client.rb
require 'socket'

server = UNIXSocket.new("/tmp/server")
puts "connected to : #{server.inspect}"

fd = File.open(ARGV[0])
puts "child is sending fd: #{fd.fileno} connected to #{ARGV[0]}"

server.send_io(fd)    # we are *not* sending the data!

exit