sockaddrin

sockaddr_in, commonly written as sockaddr_in, is the IPv4 socket address structure used in the BSD sockets API to describe an endpoint for IPv4 communication. It is defined in the header netinet/in.h and is used with socket operations such as bind, connect, sendto, and recvfrom. The generic interface often requires casting to a pointer to struct sockaddr for these functions, which enables IPv4 addresses to be used interchangeably with other address families.

The structure contains several fields:

- sin_family, which must be set to AF_INET to indicate IPv4.

- sin_port, the port number in network byte order; it is typically populated using the htons function.

- sin_addr, a struct in_addr whose s_addr holds the IPv4 address in network byte order; conversion functions such as inet_pton or inet_addr are commonly used to set this field.

- sin_zero, padding to make the structure the same size as the generic sockaddr; it is usually initialized to zero.

Typical usage involves zeroing the structure, setting the family and port, and assigning the address, then passing a pointer to it cast to struct sockaddr for socket calls. For example, a server would fill sin_family with AF_INET, sin_port with htons(port), and sin_addr with the desired address (often INADDR_ANY to listen on all interfaces).

sockaddr_in is specific to IPv4, while the generic sockaddr structure serves as a common header for all address families. For IPv6, a parallel structure, sockaddr_in6, is used. The API remains largely consistent across platforms, including Unix-like systems and Windows, making sockaddr_in a foundational element of IPv4 networking.