Address families

Most socket functions require a socket descriptor and a generic socket address structure called a sockaddr. The exact format of the sockaddr structure depends on the address family. For IPv4 sockets, the sockaddr structure is sockaddr_in. For IPv6, the sockaddr structure sockaddr_in6 is used.

The following socket functions have a sockaddr structure as one of their parameters:

bind()
connect()
sendmsg()
sendto()
accept()
recvfrom()
recvmsg()
getpeername()
getsockname()

The sockaddr structure that is used in these functions must be the proper structure for the socket family.

For IPv4 (AF_INET), the sockaddr (sockaddr_in) contains the information shown in Table 1.

Table 1. sockaddr format for AF_INET
Description	Length	Contents
sockaddr length	1 byte	Not used, should be set to 0
family	1 byte	AF_INET
port	2 bytes	TCP or UDP port number
IP address	4 bytes	IPv4 IP address
reserved bytes	8 bytes	Not used

For IPv6 (AF_INET6), the sockaddr (sockaddr_in6) contains additional information. Also, note that the IP address for IPv6 is 16 bytes long instead of 4 bytes long as in IPv4.

Table 2. sockaddr format for AF_INET6
Description	Length	Contents
sockaddr length	1 byte	Not used, should be set to 0
family	1 byte	AF_INET6
port	2 bytes	TCP or UDP port number (same as v4)
flowinfo	4 bytes	Flow information
IP address	16 bytes	IPv6 IP address
scope ID	4 bytes	Used to determine IP address scope