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"udptar.gz" is an example of UDP socket.

UDP echo client and server code.

The server creates a UDP socket and waits for incoming UDP datagrams
(after printing out the port assigned by the operating system).  Each
time a new datagram is received, the server sends the datagram
unchanged back to the sender.

The server also prints out the IP address and port number of the client.

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The client reads from standard input until end of file (^D) and sends
the whole thing as a single datagram to the server specified by the
command line parameters.  The parameters are the hostname and the port
number. The client calls gethostbyname to determine the IP address of
the server. For example, to tell the client that the server is running
on the machine foo.cse.cuhk.edu.hk and on port 2030 the client should be
started like this:

client foo.cse.cuhk.edu.hk 2030

Once the client sends a datagram it waits until it receives a datagram
and then writes the data received to stdout. Since the server is the only
process that knows the port number of the client, we assume it is the
only process that could be sending anything...