Example4: udpecho_server

Description

This example implements a simple UDP echo server.  It listens on Port 7 
for connection requests.  After a client connects, it will echo back 
each message that it receives. This code was intended as a simple 
example of a UDP server program. In order to keep the code simple, it 
will handle only one connection at a time. It will only return the first 
255 bytes of any requests that are longer than 255 bytes. 

You can connect to the example echo server with another embTCP system 
running the UDP echo client of Example1, or you may use any other UDP 
echo client. 

The initialization routine for all of the embTCP example programs is 
called example_init. For this reason, you can only link with one 
example at a time on an embTCP system.  When example_init is called, 
it will create and initialize a task and begin running.

Code Walkthrough

In udpecho_server.c, after a few definitions and prototypes, the next 
section defines the UDP task info structure and calls TK_CREATE to 
create the UDP echo server task.  

The function example_init() creates udp echo server task.
Task body tk_ts_echo waits until the network is ready and 
then calls udp_secho_init.  udp_secho_init() calls the main loop of the 
echo server. udp_secho_init() only returns if there is an error setting 
up the listening socket. If it does return, the task will be deleted.  

The udp_secho_init() routine does the following:
* Sets up a socket address structure for listening on port 7 for a 
  connection request from any IP4 address. 
* Calls t_socket()and t_bind() 
* Uses t_setsocketopt to put the socket in non-blocking mode.  This is 
  important because the echo server uses two sockets: one for listening 
  for connection requests and another that is created to listen for 
  incoming messages from an accepted connection. If the listening socket 
  were to block waiting for another connection request, the echo server 
  would not be able to service the requests for the connected socket.
* Allocates a read buffer and calls udp_echo_poll();

The udp_echo_poll() routine does the following:
* Goes into a loop.  The "for(;;)" expression means it will only exit 
  when "return" is called. 
* Call t_recvfrom to read any messages received on es_sock. A return of 0 
  means the peer closed the socket. A zero return causes us to return
  and close the socket.
* If len > 0, we call tesvrecho() to echo back the received data.
  The t_recvfrom call fills out the socket address structure "him" with 
  the address of the caller.

The udp_secho_close() routine simply prints a closing message and 
returns.