Question

I am following the tutorial: http://www.tutorialspoint.com/ruby/ruby_socket_programming.htm and using it to set up the “Simple Client”.

The code is as follows:

    require 'socket'      # Sockets are in standard library

    hostname = 'localhost'
    port = 2000

    s = TCPSocket.open(host, port)

    while line = s.gets   # Read lines from the socket
      puts line.chop      # And print with platform line terminator
    end
    s.close               # Close the socket when done

When I execute a single s.gets statement, I get the first response line from the server. When I do s.gets again, I get the second response line, etc. However, the moment I get to the end of the response and do s.gets then the program goes into a frozen state. (This is if I test the above program from the Rails Console). The same happens when I run the code – the freeze occurs when the while statement does the final s.gets. Any ideas on how to solve this?

For information: The server is a C# server that sends response messages using the following C# code (each line of the response uses the same code and the final line in the response is not treated any differently from the other lines):

    socket.Send(Encoding.ASCII.GetBytes(msg + "\r\n"));

So I am thinking the client freezes because it probably doesn’t interpret the servers “end of response” properly. Any ideas?

Answer 1

It seems like there is a problem with reading the nil value from the socket. Still not sure why this happens, but using a selector as suggested in Recovering from a broken TCP socket in Ruby when in gets() solves the problem very nicely (would have upvoted his answer, but don’t have enough rep).

Solution I followed was to add the selector in a method as follows:

    def next_line_readable?(socket)
      readfds, writefds, exceptfds = select([socket], nil, nil, 0.1)
      p :r => readfds, :w => writefds, :e => exceptfds
      readfds #Will be nil if next line cannot be read
    end

and then in main program to use it like:

    socket = TCPSocket.open(host, port)
    while next_line_readable?(socket)
      puts socket.gets.chop
    end