How to read this project.

Order of code files

1. showip.c

   • Learn about how getaddrinfo can be used to fetch details about an address (IP or domain).
   • It can be fed with details (hints) to nudge the kind of response you need. (eg: In case you need either IPv4 or IPv6 address info).

2. socket_setup.c

   • Builds on showipc.
   • Uses the res from getaddrinfo to create a socket to the IP/domain supplied to getaddrinfo.

3. bind_setup.c

   • Builds on socket_setup.c
   • Use setsockopt to handle "address already in use errors".
   • Bind the socket to the address received as a response from getaddrinfo.

4. connect_setup.c

   • Builds on bind_setup.c
   • We don't call bind here because we don't need to know about the local port that we are communicating through. The server can get the random port the kernel assigns to this connection.
   • This is what a typical client code will do. Client normally don't need to bind to a specific port because they don't normally need to handle any incoming connections and hence don't need to be listening on some well known port.

5. listen_accept.c

   • Shows how to listen for incoming connections and accept them.
   • listen also takes as an arg the number of connections that can be held in the backlog queue.
   • getaddrinfo -> socket -> bind -> listen -> accept
   • bind is necessary here because we want to fix the port through which we will handle incoming connections.
   • A client connects to a specified port that the server is listening on(the port the server socket is bound to with bind). The server socket will be listening on this port. The incoming client connection will be queued to be accepted. When we call accept to fetch this pending connection. accept returns a brand new socket file descriptor to use for this connection.
   • accept also populates a value-result parameter with the client address info.

6. send_setup.c & recv_setup.c

   • Builds on top of listen_accept.c
   • Shows the use of send and recv between an extremely simple client/server program.

7. DGRAM sendto and recvfrom [No code files for this]

   • sendto and recvfrom work the same way as send and recv but for DGRAM sockets (UDP).
   • The only pieces of additional info are the destination, from addr and their lengths that we specify or receive as a result of value-result params.
   • If we use connect on DGRAM sockets then we can simply use read and send. They still work on UDP but the address is filled by the socket interface automatically.

8. close and shutdown [No code files for this]

   • Simple file descriptor close works on closing sockets. Any attempt to read and write to a closed socket will lead to an error.
   • shutdown allows you to cut off communication in a certain direction or both directions like close.
   • shutdown doesn't close a socket fd. It only changes it usability. Use close to free the socket file descriptor.

9. getpeername [No code files for this]

   • Populates the value-result param addr with the address of who is connected on the other end of the socket.

10. gethostname [No code files for this]

    • Returns the hostname of the system it is called on.
    • Can be used with gethostbyname to fetch the system's IP address.

11. client.c & server.c

    • Simple client-server example code to demonstrate socket communication using the concepts mentioned above.
    • Work on TCP (STREAM).

12. poll_stdin.c

    • A simple example to show how to use the poll syscall, its parameters and how to handle it's result to check for which event occurred in which file descriptor.

13. poll_sv.c

    • A server that uses poll to display non-blocking IO for a simple broadcast chat application.
    • Scales(memory wise, not performance wise) for large number of clients by reallocating the poll array of structs that house the file descriptors.
    • Obvious issue is that when a new client joins the fd_count is updated (increased) forcing the loop to run one more time to check for events.

14. select_stdin.c

• A simple example to show how to use the select syscall, its parameters and how to handle it's result to check for which event occurred.

15. select_sv.c
    • A server that uses select to demonstrate non-blocking IO for a simple broadcast chat application.

Further reading and references

• man pages for each function highlighted.
• Beej's network programming
• The Linux Programming Interface