From: Mark Zvilius <zvilius@...>
Date: 2007-09-03T09:29:44+09:00
Subject: Blocking UDP recv() interaction with select()

I want to use Ruby for a UDP server application under Windows XP. I
intend to use two threads: one for a simple UI, and a separate thread
for the UDP server (with blocking calls).

As a proof of concept I wrote the following test code, whose intent is
to wait for an incoming message, with the user able to press CTRL-C to
terminate the “server.”

---------------------
# server thread
def bar
  sock = UDPSocket.new()
  sock.bind( "", PORT )
  begin
    msg, adr = sock.recvfrom(256)
  rescue Interrupt
    puts "interrupt"
  else
    print adr.inspect, " -> ", msg.inspect, "\n"
  end
  sock.close
end

# main (ui) thread
t = Thread.new( &method(:bar) )
puts "press ctrl-c to quit"
begin
  select( nil, nil, [STDIN], 1.0 ) while t.alive?
rescue Interrupt
  t.raise( Interrupt )
end
puts "wait for thread"
t.join()
puts "thread reaped"
---------------------

This approach works great for handling CTRL-C: the application exits
without hesitation.

However there are 2 problems:

1. When I send a message to the server, I get the following error on the
select() call:
C:/bin/ruby/z.rb:54:in `select': An operation was attempted on something
that is not a socket. (Errno::ENOTSOCK)

2. While waiting with one thread at the recvfrom() and the other in the
one-second select() loop, this code uses 100% of the available CPU time.

Are these considered bugs, features, or perhaps bugs that are not
fixable? [I am fully aware that Winsock select() only works for sockets,
so the underlying implementation must be nastier for Windows than for
Unix.]

Is there a more appropriate way to provide a “UI thread” alongside a
blocking UDP server thread?

I am using Ruby 1.8.5 on Windows XP.

Thanks,
Mark Zvilius
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