This program is the direct counterpart of the basic client example: it is the simplest possible program of its kind. This program simply sets up a listener on the network interface and port passed on the program's command line, accepts incoming connections, and echoes the remote peer's packets back to them.

The skeleton main() function in main.cpp calls our DoWinsock() function, which contains the main loop of the program. The function starts off by setting up the listener. This binds the program to a particular network interface and port and tells Winsock to forward all connection requests to the program.

The program then enters an endless loop to wait for connections. It accepts each incoming connection and then bounces packets back to the client until the client closes the connection. When that happens, the server closes its side of the socket and then goes back to waiting for connections.

Notice that you must pass an address on the command line to this program, due to the way the common main() function works. For this program, you can safely use "0.0.0.0" all the time: when you bind() to the "any" address, Winsock sends incoming connections from any network interface to your program.

Interesting Behavior

This server exhibits some interesting behavior that you may want to experiment with:

• If your computer has more than one network interface (e.g., a modem for connecting to the Internet and a network card for a LAN connection) you can pass a specific network address to the program on the command line. This makes the program bind to only that address, so incoming connection requests from the other network interface are ignored.

• This server will only handle a single connection at a time, but because of Winsock's connection backlog mechanism, multiple clients can have connections pending at one time. The first client that connects is accepted, and the server will echo its packets back until it closes the connection. Meanwhile, one other client can connect, because we set the connection backlog size to 1 with the listen() call. That additional client will then send data which the stack will queue up, but it won't see a reply until the first client disconnects so that the server can accept() this subsequent connection. Prove this to yourself by running two copies of basic-client against this server with the "shutdown delay" set to some largish value. Then try running three copies of basic-client.

• Try running the netstat command line program while basic-server is running to see its listener record. (Note that netstat may only work correctly for this under Windows NT derivatives.)

Building the Program

The only module you will need to compile this program, aside from the common files listed on the main examples page, is basic-server.cpp. The comment at the top of the file gives complete compilation instructions; alternately, you can use the common Makefile.