Socket Address Data Structures

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The socket data structure defines the socket. During a socket subroutine, the system dynamically creates the socket data structure.

The socket address is specified by a data structure that is defined in a header file. See the sockaddr Structure figure (Figure 1) for an illustration of this data structure.

This diagram shows the sockaddr structure containing the following from the left: len, family, and socket address_data. The second line of the diagram gives the size of the sections in the first line as follows: len and family together equal 2 bytes, socket address_data is a variable size. — Figure 1. sockaddr Structure

The /usr/include/sys/socket.h file contains the sockaddr structure. The contents of the sa_data structure depend on the protocol in use.

The types of socket-address data structures are as follows:

Item	Description
struct sockaddr_in	Defines sockets used for machine-to-machine communication across a network and interprocess communication (IPC). The /usr/include/netinet/in.h file contains the sockaddr_in structure.
struct sockaddr_un	Defines UNIX domain sockets used for local IPC only. These sockets require complete path name specification and do not traverse networks. The /usr/include/sys/un.h file contains the sockaddr_un structure.
struct sockaddr_ndd	Defines the operating system NDD sockets used for machine-to-machine communication across a physical network. The /usr/include/sys/ndd_var.h file contains the sockaddr_ndd structure. Depending upon socket types and protocol, other header files may need to be included.