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

Question

Local variables defined outside of a thread seem to be visible from inside so that the following two uses of Thread.new seem to be the same:

a = :foo
Thread.new{puts a} # => :foo
Thread.new(a){|a| puts a} # => :foo

The document gives the example:

arr = []
a, b, c = 1, 2, 3
Thread.new(a,b,c){|d, e, f| arr << d << e << f}.join
arr #=> [1, 2, 3]

but since a, b, c are visible from inside of the created thread, this should also be the same as:

arr = []
a, b, c = 1, 2, 3
Thread.new{d, e, f = a, b, c; arr << d << e << f}.join
arr #=> [1, 2, 3]

Is there any difference? When do you need to pass local variables as arguments to Thread.new?

Answer 1

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

Answer 2

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