# Blocking UDP recv() interaction with select()

**URL:** <https://www.ruby-forum.com/t/blocking-udp-recv-interaction-with-select/113626>\
**Category:** Ruby\
**Created:** [September 3, 2007, 2:29am UTC](https://www.ruby-forum.com/t/blocking-udp-recv-interaction-with-select/113626 "2007-09-03T02:29:40Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![zvilius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9f8e36/32.png) [@zvilius](https://www.ruby-forum.com/u/zvilius)\
**Post date:** [September 3, 2007, 2:29am UTC](https://www.ruby-forum.com/t/blocking-udp-recv-interaction-with-select/113626/1 "2007-09-03T02:29:40Z")

</div>

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 Z.

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<div class="post-metadata">

**Author:** ![zvilius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9f8e36/32.png) [@zvilius](https://www.ruby-forum.com/u/zvilius)\
**Post date:** [September 3, 2007, 9:07am UTC](https://www.ruby-forum.com/t/blocking-udp-recv-interaction-with-select/113626/2 "2007-09-03T09:07:43Z")

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From: “Mark Z.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

> 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.

Your code calling select with STDIN on Windows isn’t going to  
work properly. Here’s what I get in IRB:

> > select( nil, nil, [STDIN], 1.0 )  
> > Errno::ENOTSOCK: An operation was attempted on something that is not a  
> > socket.  
> > from (irb):174:in `select’  
> > from (irb):174

It seems this has changed from the old behavior, which, if I recall  
correctly, didn’t used to raise an exception. I think it used to just  
always immediately return from select indicating the file handle  
was readable (regardless of whether any data was present or  
not.)

But anyway…

I don’t have a particular suggestion, other than to only use sockets  
with select on Windows.

If you need to check whether any input has arrived at the console,  
I’ve used ‘kbhit’ in the past.

( [http://groups.google.com/group/comp.lang.ruby/msg/27eaefd419457a5f](http://groups.google.com/group/comp.lang.ruby/msg/27eaefd419457a5f) )

Hope this helps,

Bill

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<div class="post-metadata">

**Author:** ![zvilius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9f8e36/32.png) [@zvilius](https://www.ruby-forum.com/u/zvilius)\
**Post date:** [September 3, 2007, 4:30pm UTC](https://www.ruby-forum.com/t/blocking-udp-recv-interaction-with-select/113626/3 "2007-09-03T16:30:43Z")

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On 9/2/07, Mark Z. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> 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.”

Look at Ruby/EventMachine, which can solve your problem without threads.  
Sync to the current head revision to get a recent addition which allows  
you  
to respond to keyboard input.