Understanding Socket Programming

• Socket programming helps applications communicate over a network
• Achieved by establishing a connection between two endpoints
• Endpoints are represented by sockets using protocols like TCP or UDP
• Sockets are fundamental for building network applications

Key Concepts

• Sockets are endpoints for network communication, identified by an IP address and port number
• The client-server model is a common architecture where a server listens for connections from clients
• Protocols are rules that govern how data is transmitted
• TCP offers reliable, ordered data
• UDP provides faster, unordered data
• Socket programming typically involves functions such as:
• socket()
• bind()
• listen()
• accept()
• connect()
• send()
• recv()
• close()

Types of Sockets

• Stream Sockets (TCP) are reliable and connection-oriented
• Datagram Sockets (UDP) are fast and connectionless
• Think of a socket as a phone, where it connects one to another person or device to send or receive data

Client-Server Model

• A network communication model where the client initiates the request and the server provides services or data
• Clients are passive and request service with temporary connections
• Servers provide service are active, always listening, and always ready

TCP Sockets

• The client side establishes a TCP socket by:
• Creating a socket using the socket() function
• Connecting the socket to the server using the connect() function
• Sending and receiving data using the read() and write() functions
• Closing the connection using the close() function
• The server side establishes a TCP socket by:
• Creating a socket with the socket() function
• Binding the socket to an address using the bind() function
• Listening for connections with the listen() function
• Accepting a connection using the accept() function call
• This call typically blocks until a client connects with the server
• Sending and receiving data using the send() and receive() functions
• Closing the connection using the close() function

UDP Sockets

• The client side establishes a UDP socket by:
• Creating a socket using the socket() function
• Sending and receiving data using the recvfrom() and sendto() functions
• The server side establishes a UDP socket by:
• Creating a socket with the socket() function
• Binding the socket to an address using the bind() function
• Sending and receiving data using the recvfrom() and sendto() functions

Socket Functions

• socket(): Creates a new socket as a communication endpoint
• int sockfd = socket(int domain, int type, int protocol);
• domain: Address family (e.g., AF_INET for IPv4)
• type: Socket type (SOCK_STREAM for TCP, SOCK_DGRAM for UDP)
• protocol: Usually 0 for default
• Used by both the server and client
• Returns a file descriptor (socket ID), or -1 on error
• bind(): Assigns an IP address and port to the socket
• bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
• It is required for the server to define where to listen (IP + Port)
• Not usually needed for clients unless binding to a specific port/IP
• It is used by the Server
• listen(): Marks the socket as passive, ready to accept incoming connections
• listen(int sockfd, int backlog);
• backlog: Number of pending connections the queue will hold
• It is used by the Server
• accept(): Accepts a new incoming connection on a listening socket
• int clientfd = accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
• Blocks until a connection is made
• Returns a new socket descriptor for the client
• It is used by the Server
• connect(): Initiates a connection to a server socket
• connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
• It is used by the client to connect to the server's IP and port
• Returns 0 on success, -1 on error
• Its is used by the Client
• send(): Sends data over a connected socket
• send(int sockfd, const void *buffer, size_t length, int flags);
• Sends bytes of data
• Flags is often set to 0
• It is used by both the Server and Client
• recv(): Receives data from a connected socket
• recv(int sockfd, void *buffer, size_t length, int flags);
• Waits for data from peer
• Returns the number of bytes received
• It is used by both the Server and Client
• close(): Closes the socket and releases resources
• close(int sockfd);
• It is used by both the Server & Client

Server Example Flow Summary

• int sockfd = socket(AF_INET, SOCK_STREAM, 0);
• bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
• listen(sockfd, 5);
• int clientfd = accept(sockfd, (struct sockaddr*)&client_addr, &addrlen);
• recv(clientfd, buffer, size, 0);
• send(clientfd, response, size, 0);
• close(clientfd);
• close(sockfd);

Client Example Flow Summary

• int sockfd = socket(AF_INET, SOCK_STREAM, 0);
• connect(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
• send(sockfd, message, size, 0);
• recv(sockfd, buffer, size, 0);
• close(sockfd);