When do you need to pass arguments to `Thread.new`?

When you pass a variable into a thread like that, then the thread makes a local copy of the variable and uses it, so modifications to it do not affect the variable outside of the thread you passed in

a = "foo"
Thread.new{ a = "new"}
p a # => "new"
Thread.new(a){|d| d = "old"} 
p a # => "new"
p d # => undefined

I think I hit the actual problem. With a code like this:

    sock = Socket.unix_server_socket(SOCK)
    sock.listen 10
    while conn = sock.accept do
        io, address = conn
        STDERR.puts "#{io.fileno}: Accepted connection from '#{address}'"
        Thread.new{ serve io }
    end

it appears to work when accepting few connections. The problem comes when accepting connections quickly one after another. The update to local variable io will be reflected in multiple concurrent threads unless passed as argument to Thread.new