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Tutorial 2: A Simple TCP Server

Source

Adapted from Spectranet: Tutorial 2.

The first practical network program is a TCP server that accepts one connection and exchanges simple text. It can be tested from another computer with telnet or nc.

Server flow​

A TCP server normally performs these steps:

  • open a socket
  • bind the socket to a local port
  • put the socket into listening mode
  • accept an incoming connection
  • send and receive data on the accepted socket
  • close the accepted socket when finished

The original listening socket remains available for future connections; the accepted socket is the one used for the actual data transfer.

Opening the socket​

In assembly, a TCP socket is opened by passing SOCK_STREAM and calling the SOCKET routine through HLCALL:

ld c, SOCK_STREAM
ld hl, SOCKET
call HLCALL
jr c, .error
ld (v_sockfd), a

In C, the same operation is:

int sockfd;
sockfd = socket(AF_INET, SOCK_STREAM, 0);

Errors are reported with carry set in assembly, or with a negative return value in C.

Binding and listening​

The server binds the socket to the port it wants to receive connections on, then calls listen. In the tutorial example, port 2000 is used so a PC can connect with:

telnet 172.16.0.38 2000

After accept returns, the program sends a greeting, reads text sent by the remote side, and exits when the input begins with x.

Closing sockets​

Sockets are limited hardware resources. Programs should close accepted sockets and listening sockets when finished. Reset clears socket state, but programs should not rely on reset as normal cleanup.