# Question About TCPServer & TCPSocket classes

**URL:** <https://www.ruby-forum.com/t/question-about-tcpserver-tcpsocket-classes/147863>\
**Category:** Ruby\
**Created:** [September 6, 2008, 7:50am UTC](https://www.ruby-forum.com/t/question-about-tcpserver-tcpsocket-classes/147863 "2008-09-06T07:50:40Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Steve\_Lewis](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/5f9b8f/32.png) [@Steve\_Lewis](https://www.ruby-forum.com/u/Steve_Lewis)\
**Post date:** [September 6, 2008, 7:50am UTC](https://www.ruby-forum.com/t/question-about-tcpserver-tcpsocket-classes/147863/1 "2008-09-06T07:50:40Z")

</div>

Hello Rubyists,

I’ve been messing around with socket programming in Ruby lately, and  
I’ve  
hit a snag with some of my experiments. My first, (successful),  
experiment was to create a basic client/server script wherein the client  
sends a string to the server, and the server responds by simply writing  
the string back to the client…that code is as follows:

Server:  
#####################################  
require ‘socket’

class PingServer \< TCPServer

def start\_server  
loop do  
Thread.start(self.accept) do |s|  
p “Connection accepted from server #{s.inspect}”  
request = s.readline.gsub(/\n$/, ‘’)  
p “Request was #{request}”  
s.write “Your request was as follows: #{request}”  
s.write Time.now if request == ‘time’  
s.close  
p “Connection #{s} closed”  
end  
end  
end

end

PingServer.new(‘localhost’, 3000).start\_server

###############################################

The client code is thus:  
########################################  
require ‘socket’

def send\_message(message)  
TCPSocket.open(‘localhost’, 3000) do |client|  
client.write(message)  
stuff = client.read(100)  
client.close  
p stuff  
get\_message  
end  
end

def get\_message  
message = gets  
send\_message(message) unless message.gsub(/\n/, ‘’) == ‘quit’  
exit  
end

get\_message

##################################

As I said, the above code works as expected, I start the server, then  
run  
the client program…I can send strings to the server all day long, and  
the server writes them right back to me.

The problem occurs when I try to send the following instead of a string:

Marshal.dump(%w(foo bar baz))

Sending the resulting string over the wire causes the server to  
hang…it  
doesn’t return the string back to the client…I’m at a loss to figure  
out why it seems to have trouble handling the string…

Ideas would be appreciated!!

Thanks

Steve

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<div class="post-metadata">

**Author:** ![Steve\_Lewis](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/5f9b8f/32.png) [@Steve\_Lewis](https://www.ruby-forum.com/u/Steve_Lewis)\
**Post date:** [September 6, 2008, 8:55am UTC](https://www.ruby-forum.com/t/question-about-tcpserver-tcpsocket-classes/147863/2 "2008-09-06T08:55:36Z")

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Steve Lewis wrote:

> require ‘socket’
> 
> class PingServer \< TCPServer
> 
> def start\_server  
> loop do  
> Thread.start(self.accept) do |s|  
> p “Connection accepted from server #{s.inspect}”  
> request = s.readline.gsub(/\n$/, ‘’)  
> ^^^^^^^^

This method reads until newline, and binary data (such as Marshal.dump  
output) is not generally delimited that way. Use IO#read or Socket#recv  
instead.

If you want to read a binary message, you can do one of several things:

1. pick a delimiter char and escape that wherever it occurs within the  
binary data (yuck)

2. send a length field before sending the data, so the receiver knows  
how much to wait for

3. send one message per socket connection

4. encode the binary data in some non-binary form: e.g. base64, using  
pack(“m”), and delimit using blank lines.

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<div class="post-metadata">

**Author:** ![Steve\_Lewis](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/5f9b8f/32.png) [@Steve\_Lewis](https://www.ruby-forum.com/u/Steve_Lewis)\
**Post date:** [September 6, 2008, 11:05am UTC](https://www.ruby-forum.com/t/question-about-tcpserver-tcpsocket-classes/147863/3 "2008-09-06T11:05:27Z")

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On 06.09.2008 07:45, Steve Lewis wrote:

> require ‘socket’  
> s.write Time.now if request == ‘time’  
> ###############################################  
> TCPSocket.open(‘localhost’, 3000) do |client|  
> send\_message(message) unless message.gsub(/\n/, ‘’) == ‘quit’  
> the server writes them right back to me.
> 
> The problem occurs when I try to send the following instead of a string:
> 
> Marshal.dump(%w(foo bar baz))
> 
> Sending the resulting string over the wire causes the server to hang…it  
> doesn’t return the string back to the client…I’m at a loss to figure  
> out why it seems to have trouble handling the string…
> 
> Ideas would be appreciated!!

Steve, you are mixing #read, #write on one side and #readline on the  
other side. Since Marshal.dump creates binary data your #readline on  
the server won’t properly work. You should either use a binary  
protocoll _or_ a textual (line based) protocol. Mixing both causes  
problems.

Just an additional note: if you use Marshal, you can directly read to  
and write from the socket without the intermediate String. This is  
likely more efficient and also you avoid the issue of not knowing how  
long the Marshal sequence is.

And yet another note: just in case you need a solution with  
communicating processes where all of them are Ruby applications you can  
use DRuby. But I guess at the moment you want to learn about Ruby  
socket programming so this is probably just for keeping in the back of  
your head.

Kind regards

robert

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<div class="post-metadata">

**Author:** ![Steve\_Lewis](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/5f9b8f/32.png) [@Steve\_Lewis](https://www.ruby-forum.com/u/Steve_Lewis)\
**Post date:** [September 6, 2008, 7:45pm UTC](https://www.ruby-forum.com/t/question-about-tcpserver-tcpsocket-classes/147863/4 "2008-09-06T19:45:25Z")

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Robert K. wrote:

> Just an additional note: if you use Marshal, you can directly read to  
> and write from the socket without the intermediate String. This is  
> likely more efficient and also you avoid the issue of not knowing how  
> long the Marshal sequence is.

Good point. I forgot that the output of Marshal records how long the  
data is, and therefore #load can read the correct number of bytes  
directly from an io.

> And yet another note: just in case you need a solution with  
> communicating processes where all of them are Ruby applications you can  
> use DRuby. But I guess at the moment you want to learn about Ruby  
> socket programming so this is probably just for keeping in the back of  
> your head.

Another possibility, if you are communicating to C code or other  
languages:

[http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/177704](http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/177704)

It’s a lib for both C and ruby that uses length fields to make tcp  
sockets “message oriented” rather than stream oriented.

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<div class="post-metadata">

**Author:** ![Steve\_Lewis](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/5f9b8f/32.png) [@Steve\_Lewis](https://www.ruby-forum.com/u/Steve_Lewis)\
**Post date:** [September 7, 2008, 3:05am UTC](https://www.ruby-forum.com/t/question-about-tcpserver-tcpsocket-classes/147863/5 "2008-09-07T03:05:32Z")

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On Sat, 06 Sep 2008 11:04:43 +0200, Robert K. wrote:

> > ##################################### require ‘socket’  
> > Time.now if request == ‘time’ s.close
> > 
> > ```
> > client.write(message)
> > 
> > ```
> > 
> > end  
> > The problem occurs when I try to send the following instead of a  
> > Steve, you are mixing #read, #write on one side and #readline on the  
> > And yet another note: just in case you need a solution with  
> > communicating processes where all of them are Ruby applications you can  
> > use DRuby. But I guess at the moment you want to learn about Ruby  
> > socket programming so this is probably just for keeping in the back of  
> > your head.
> 
> Kind regards
> 
> robert

Robert,

Thanks for the help! You’re correct, my curiosity here is mostly  
academic, but DRuby looks like the practical solution to use when I  
decide I’d like to try something more substantial.