Communications Programming Concepts

Socket Header Files

Socket header files contain data definitions, structures, constants, macros, and options used by socket subroutines. An application program must include the appropriate header file to make use of structures or other information a particular socket subroutine requires. Commonly used socket header files are:

/usr/include/netinet/in.h	Defines Internet constants and structures.
/usr/include/arpa/nameser.h	Contains Internet name server information.
/usr/include/netdb.h	Contains data definitions for socket subroutines.
/usr/include/resolv.h	Contains resolver global definitions and variables.
/usr/include/sys/socket.h	Contains data definitions and socket structures.
/usr/include/sys/socketvar.h	Defines the kernel structure per socket and contains buffer queues.
/usr/include/sys/types.h	Contains data type definitions.
/usr/include/sys/un.h	Defines structures for the UNIX interprocess communication domain.
/usr/include/sys/ndd_var.h	Defines structures for the operating system Network Device Driver (NDD) domain.
/usr/include/sys/atmsock.h	Contains constants and structures for the Asynchronous Transfer Mode (ATM) protocol in the operating system NDD domain.

In addition to commonly used socket header files, Internet address translation subroutines require the inclusion of the inet.h file. The inet.h file is located in the /usr/include/arpa directory.

Socket Address Data Structures

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 for an illustration of this data structure.

Figure 9-3. sockaddr 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.

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:

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_ns	Defines the Xerox Network Services (XNS) sockets to be used for reliable, full-duplex, connection-oriented services to an application. The /usr/include/netns/ns.h file contains the sockaddr_ns 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.