Robert Klemme
4/28/2009 3:47:00 PM
On 28.04.2009 17:19, Maarten Mortier wrote:
> Hi, I was writing a quick Relay class and ran into a problem.
> Objects of this class act between two ports and log every TCP/IP that is
> sent between those ports - according to a specific protocol (in this
> simple example the first 8 characters contain the length of the data
> that will follow)
>
> These are the important methods of the class. All objects are local to
> the object and not shared between other objects.
>
>
> def start
> @server = TCPServer.new(l_h, l_p)
> @client = TCPSocket.new(r_h, r_p)
>
> loop do
> Thread.start(@server.accept) do |s|
> # handshake(s)
> client_to_server(s)
> server_to_client(s)
> end
> end
> end
>
> def client_to_server(s)
> Thread.new do
> loop do
> len = @client.recv(8)
> msg = @client.recv(len.to_i)
> @logfile.puts("Received " + msg)
> s.write(len)
> s.write(msg)
> end
> end
> end
>
> def server_to_client(s)
> Thread.new do
> loop do
> len = s.recv(8)
> msg = s.recv(len.to_i)
> @logfile.puts("Sent " + msg)
> @client.write(len)
> @client.write(msg)
> end
> end
> end
>
>
> My problem is that the file operations (@logfile.puts) are not performed
> immediately. When I check the file while communication is passing by, it
> is incomplete, sometimes even empty. Only when I kill the connection
> contained in @server.accept is the file properly flushed and is all
> traffic put into it.
> I don't understand why this happens. Why does @logfile.puts not happen
> immediately while @client.write and s.write do?
> I can't see why this would have anything to do with thread safety.
>
> Putting a mutex lock/unlock around the file operation didn't really do
> anything (I didn't really expect that to work either)
>
> Can anybody tell me what is happening here?
You see the effect of IO buffering which indeed does not have anything
to do with multi threading. You basically have two options to remedy that
1. use @client.sync = true for automatic flushing after every write
operation
2. manually invoke #flush
Independent from that you should take measures to make writing to the
logfile thread safe. One option would be to have a reserved writer
thread which pulls log messages from a Queue and writes them to the file.
Another alternative is your approach (synchronization).
Kind regards
robert