Sockets

A socket is an endpoint for communication between two programs — across the network or on the same machine. The BSD socket API exposes it as a file descriptor, so once a connection is set up you read/write (or send/recv) it much like a file. This week turns the networking concepts into working code.

Domains and types

A socket is created with socket(domain, type, protocol). Two choices define its behaviour:

protocol is almost always 0 (the default for the type).

Byte order and addresses

Networks use big-endian ("network byte order"), which may differ from your host — so ports and addresses must be converted:

• htons / htonl — host → network (16/32-bit); ntohs / ntohl — the reverse
• inet_pton / inet_ntop — convert an address string ↔ its binary form

Addresses are passed in family-specific structs (struct sockaddr_in for IPv4), always cast to the generic struct sockaddr * when calling the API.

TCP: the call sequences

TCP is connection-oriented, so server and client follow a fixed choreography:

server:  socket → bind → listen → accept ⇄ recv / send → close
client:  socket →                connect ⇄ send / recv → close

• Server: socket() → bind() a local address/port → listen() (mark passive, set a backlog) → accept() (blocks; returns a new socket per client while the listener keeps listening) → recv/send → close.
• Client: socket() → connect() (performs the three-way handshake) → send/recv → close.

UDP: connectionless

With SOCK_DGRAM there is no listen/accept/connect. Each message carries its own destination: the server binds and loops on recvfrom (which also yields the sender's address) and replies with sendto. Datagrams preserve message boundaries but may be lost, duplicated, or reordered.

Handling many clients

A plain accept loop is iterative — one client at a time. To serve many at once you can:

• fork a process per connection, or spawn a thread per connection, or
• use I/O multiplexing (select/poll/epoll) to handle many sockets in one thread — the most scalable approach, and the subject of I/O multiplexing.

Practice / lab

• Write a TCP echo server (socket/bind/listen/accept/recv/send) and test it with nc; then write a matching client.
• Reimplement the echo service over UDP with sendto/recvfrom; note the structural differences.
• Make the TCP server handle multiple clients — first with fork, then with threads.
• Use getaddrinfo() to resolve a hostname to addresses (IPv4 and IPv6).

Homework

• NET-1: Echo Server — a TCP echo server and client using the BSD socket API.

References & resources

Required

• Beej's Guide to Network Programming — the definitive socket tutorial
• Socket programming in C (video)
• Socket programming (GeeksforGeeks)
• The Linux Programming Interface (Kerrisk) — Ch. 56–60: sockets

Optional / enrichment

• socket(2) · bind(2) · accept(2) · connect(2) · getaddrinfo(3)
• The C10K problem — scaling socket servers
• Stevens, Unix Network Programming, Vol. 1

Self-check

1. What does a socket represent, and why is it exposed as a file descriptor?
2. What is the difference between AF_INET and AF_UNIX?
3. How do SOCK_STREAM and SOCK_DGRAM differ?
4. Why are htons/htonl necessary?
5. Why does accept() return a new socket, and what is the full TCP server call sequence?