Ruby TCPSocket: Find out how much data is available

Question

Is there a way to find out how many bytes of data is available on an TCPSocket in Ruby? I.e. how many bytes can be ready without blocking?

Overarm

Answer 1

The standard library io/wait might be useful here. Requiring it gives stream-based I/O (sockets and pipes) some new methods, among which is ready?. According to the documentation, ready? returns non-nil if there are bytes available without blocking. It just so happens that the non-nil value it returns it the number of bytes that are available in MRI.

Here's an example which creates a dumb little socket server, and then connects to it with a client. The server just sends "foo" and then closes the connection. The client waits a little bit to give the server time to send, and then prints how many bytes are available for reading. The interesting stuff for you is in the client:

require 'socket'
require 'io/wait'

# Server

server_socket = TCPServer.new('localhost', 0)
port = server_socket.addr[1]
Thread.new do
  session = server_socket.accept
  sleep 0.5
  session.puts "foo"
  session.close
end

# Client

client_socket = TCPSocket.new('localhost', port)
puts client_socket.ready?    # => nil
sleep 1
puts client_socket.ready?    # => 4

Don't use that server code in anything real. It's deliberately short in order to keep the example simple.

Note: According to the Pickaxe book, io/wait is only available if "FIONREAD feature in ioctl(2)", which it is in Linux. I don't know about Windows & others.

Hertzog

Do you know by any chance if it is available for JRuby? – Overarm

@panzi, I don't know JRuby, so I don't know. This page suggests that the functionality may be available but using different classes than MRI:jruby.org/apidocs/org/jruby/util/io/CRLFStreamWrapper.html (scroll down to ready?). – Hertzog

I tried it under JRuby. The method is there but it does only return 1 instead of the real amount if something can be read. Well, I can implement it without this but it would have been more efficient. – Overarm

@panzi, I wondered if ready? returning a count was a quirk of MRI. Now you've confirmed that it is. I apologize for leading you down a false trail. – Hertzog

I am using the SSLSocket. The function .ready? does not seem to exist there. Is there any other feature I can use on the SSLSocket? – Effector

@MarekKüthe Unfortunately I do not know. – Hertzog

@Jeff Schaller Thank you for your edit, especially to the outdated word I used. It's a small edit but a big improvement. – Hertzog