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Proper handling of nonblocking sockets?

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Joined: 17 Jul 2007
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PostPosted: Wed Jul 18, 2007 7:30 am    Post subject: Proper handling of nonblocking sockets? Reply with quote

My experience with sockets so far only covers the most basic socket operations in C and Java (which I learned at school). Now I tried to write a basic echo-server and client in D and got it working. Anything typed into the console will get sent to the server, returned and displayed again.
The next thing I want to do is make the client use a nonblocking socket. Right now it's rather pointless because the console still blocks, but I wanted a proof-of-concept to base further development on.

This is the (client) code I got so far:

import std.stdio;
import std.socket;

int main ( ) {
   // Setup
   Socket sock = new TcpSocket;
   scope (exit) sock.close();
   sock.blocking = false;
   sock.connect (new InternetAddress ("", 4711));

   while (sock.isAlive()) {
      // Get input from console and transmit it
      byte[] input = cast(byte[])readln();
      int len = sock.send (input);

      // Try to handle errors
      if ((len == sock.ERROR) && !sock.isAlive()) {
         writefln ("Error on send: %d", sock.ERROR); return 1;

      if (len > 0) writefln("Sent [%d]: %s", len, cast(char[])(input[0..len]));

      // Receive the answer if there is one.
      len = sock.receive (input);

      // Receive error handling
      if ((len == sock.ERROR) && !sock.isAlive()) {
         writefln ("Error on receive: %d", sock.ERROR); return 1;

      if (len > 0) writefln("Received [%d]: %s", len, cast(char[])(input[0..len]));

   return 0;

My problem is this: At first everything works alright, I can send and receive text messages. (I know that returned messages print after the next sent message which looks strange on the console, but I don't care - for the purposes of this example) Also, if the server closes the connection, the client will also leave the loop and exit. But if I kill the server with Ctrl+C or the task manager, there is no sign of a lost connection!
When using blocking sockets, I still get -1 from receive() to indicate a transmission error. But with nonblocking sockets, that's just the value I get when the server hasn't sent anything yet. Also, isAlive() still indicates a connected socket when in fact, the server has closed.
I know that for the server, I could use a SocketSet like in the listener.d sample because that class seems to not have any problems telling the difference between an idle connection and a broken one. But it seems weird to use this method for my single client socket, so I haven't even tried Embarassed

What is the right way to use nonblocking sockets?
How can I distinguish between idle and broken connections?
Do I have to use threads? (another strange chapter in D, where's my Mutex?)
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