Socket Programming Interface

The socket() Function

The function signature is:

#include <sys/types.h>
#include <sys/socket.h>
int socket(int domain, int type, int protocol);

The socket() function behaves similarly to open(), creating a communication endpoint for network interaction. On success, it returns a file descriptor, commonly referred to as a socket descriptor. This descriptor functions like a regular file descriptor.

• domain: Specifies the communication domain, determining the protocol family used for communication. For TCP/IP applications, AF_INET is typically chosen.
• type: Defines the socket type.
• protocol: Usually set to 0, indicating the default protocol for the specified domain.

Example usage:

int sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock < 0) {
    perror("socket creation failed");
    exit(EXIT_FAILURE);
}
...
close(sock);

The bind() Function

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

The bind() function associates a IP address or port with a socket. It binds the socket identified by sockfd to the address specified by addr. Returns zero on success, or -1 on failure, setting errno accordingly.

The generic sockaddr structure is defined as:

struct sockaddr {
    sa_family_t sa_family;
    char sa_data[14];
};

The sa_data array holds addressing information, but it's not user-friendly due to its opaque nature. In practice, developers use sockaddr_in, which is more convenient and compatible with sockaddr.

struct sockaddr_in {
    sa_family_t sin_family;
    in_port_t sin_port;
    struct in_addr sin_addr;
    unsigned char sin_zero[8];
};

Usage example:

struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY);
addr.sin_port = htons(5555);
bind(sock, (struct sockaddr *)&addr, sizeof(addr));

The listen() Function

int listen(int sockfd, int backlog);

The accept() Function

After calling listen(), the server enters a listening state, waiting for client connection requests. The accept() function retrieves these requests and establishes connections. Its prototype is:

int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

A server typically follows these steps:

1. Create a socket using socket().
2. Bind the socket to an address with bind().
3. Start lisetning with listen() for incoming connections.
4. Accept connections using accept().

The connect() Function

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

This function is used by clients to establish a connection to a remote server. For TCP connections, it initiates a handshake process. For UDP, it merely records the server's address without sending data.

Sending and Receiving Data

The recv() Function

ssize_t recv(int sockfd, void *buf, size_t len, int flags);

The send() Function

ssize_t send(int sockfd, const void *buf, size_t len, int flags);

Closing the Socket

Use close() to release socket resources.