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📘 APUE – Chapter 16: Sockets Summary

1. Introduction to Sockets

  • Sockets are an abstraction for network communication.
  • Enable communication between processes over the network.
  • Support multiple protocols: TCP, UDP (IPv4, IPv6).

2. Socket Types

  • SOCK_STREAM – TCP (reliable, connection-oriented).
  • SOCK_DGRAM – UDP (connectionless, unreliable).
  • SOCK_SEQPACKET – sequenced, reliable datagrams (less common).
  • SOCK_RAW – raw access to IP packets (used in advanced networking).

3. Socket System Calls

  1. int socket(int domain, int type, int protocol);
    • Create a new socket descriptor.
  2. int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
    • Assign an address/port to a socket (server side).
  3. int listen(int sockfd, int backlog);
    • Mark socket as passive (ready to accept connections).
  4. int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
    • Accept a new incoming connection (blocking call).
  5. int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
    • Establish connection to a server (client side).
  6. ssize_t read(int sockfd, void *buf, size_t count);
  7. ssize_t write(int sockfd, const void *buf, size_t count);
  8. ssize_t recv(int sockfd, void *buf, size_t len, int flags);
  9. ssize_t send(int sockfd, const void *buf, size_t len, int flags);
  10. int close(int sockfd);
    • Close the socket and free resources.

4. Socket Address Structures

  • IPv4: struct sockaddr_in

    struct sockaddr_in {
    sa_family_t sin_family;  // AF_INET
    in_port_t sin_port;      // port number (network byte order)
    struct in_addr sin_addr; // IP address
};

  • IPv6: struct sockaddr_in6

  • Generic: struct sockaddr for compatibility.

5. Data Conversion Utilities

  • htons(), htonl(): host to network byte order (short/long)
  • ntohs(), ntohl(): network to host byte order
  • inet_pton(): convert string IP to struct in_addr or struct in6_addr
  • inet_ntop(): convert address structure to string

6. TCP Client-Server Model

Server:

  1. socket()
  2. bind()
  3. listen()
  4. accept() → handle client
  5. read()/write() → communicate
  6. close()

Client:

  1. socket()
  2. connect()
  3. read()/write() → communicate
  4. close()

7. UDP Model

  • Connectionless: no listen()/accept().
  • Use sendto() and recvfrom() for communication.
  • No guarantee of delivery; suitable for lightweight or real-time protocols.

8. Concurrency in Servers

  • Process-based: fork() for each client
  • Thread-based: pthread_create() for each client
  • I/O multiplexing: select(), poll(), epoll()

9. Error Handling

  • Check return values of all socket calls.
  • Use perror() or strerror() for diagnostics.
  • Handle EINTR for interruptible syscalls.

10. Key Takeaways

  • Sockets abstract network communication using familiar file descriptor semantics.
  • TCP ensures reliable, ordered delivery; UDP is fast but unreliable.
  • Address conversions (inet_pton, inet_ntop) are essential for portability.
  • Concurrency requires careful design: processes, threads, or event loops.
  • Practice writing small client-server programs to solidify concepts.