#include <sys/types.h>
#include <sys/socket.h>

s = socket(af, type, protocol)
int s, af, type, protocol;

The af parameter specifies an address format with which addresses specified in later operations using the socket should be interpreted. These formats are defined in the include file <sys/socket.h> . The currently understood formats are

AF_UNIX (UNIX path names),
AF_INET (ARPA Internet addresses),
AF_PUP (Xerox PUP-I Internet addresses), and
AF_IMPLINK (IMP \*(lqhost at IMP\*(rq addresses).

The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:

SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
SOCK_RDM

A SOCK_STREAM type provides sequenced, reliable, two-way connection based byte streams with an out-of-band data transmission mechanism. A SOCK_DGRAM socket supports datagrams (connectionless, unreliable messages of a fixed (typically small) maximum length). SOCK_RAW sockets provide access to internal network interfaces. The types SOCK_RAW, which is available only to the super-user, and SOCK_SEQPACKET and SOCK_RDM, which are planned, but not yet implemented, are not described here.

The protocol specifies a particular protocol to be used with the socket. Normally only a single protocol exists to support a particular socket type using a given address format. However, it is possible that many protocols may exist in which case a particular protocol must be specified in this manner. The protocol number to use is particular to the \*(lqcommunication domain\*(rq in which communication is to take place; see services (3N) and protocols (3N).

Sockets of type SOCK_STREAM are full-duplex byte streams, similar to pipes. A stream socket must be in a connected state before any data may be sent or received on it. A connection to another socket is created with a connect (2) call. Once connected, data may be transferred using read (2) and write (2) calls or some variant of the send (2) and recv (2) calls. When a session has been completed a close (2) may be performed. Out-of-band data may also be transmitted as described in send (2) and received as described in recv (2).

The communications protocols used to implement a SOCK_STREAM insure that data is not lost or duplicated. If a piece of data for which the peer protocol has buffer space cannot be successfully transmitted within a reasonable length of time, then the connection is considered broken and calls will indicate an error with -1 returns and with ETIMEDOUT as the specific code in the global variable errno. The protocols optionally keep sockets \*(lqwarm\*(rq by forcing transmissions roughly every minute in the absence of other activity. An error is then indicated if no response can be elicited on an otherwise idle connection for a extended period (e.g. 5 minutes). A SIGPIPE signal is raised if a process sends on a broken stream; this causes naive processes, which do not handle the signal, to exit.

SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in send (2) calls. It is also possible to receive datagrams at such a socket with recv (2).

An fcntl (2) call can be used to specify a process group to receive a SIGURG signal when the out-of-band data arrives.

The operation of sockets is controlled by socket level options . These options are defined in the file < sys/socket.h > and explained below. Setsockopt and getsockopt (2) are used to set and get options, respectively. Options other than SO_LINGER take an int parameter, which should non-zero if the option is to be enabled, or zero if it is to be disabled; SO_LINGER uses a struct linger parameter, defined in < sys/socket.h >, which specifies the desired state of the option and the linger interval (see below).

SO_DEBUG turn on recording of debugging information
SO_REUSEADDR allow local address reuse
SO_KEEPALIVE keep connections alive
SO_DONTROUTE do not route outgoing messages
SO_LINGER linger on close if data present
SO_BROADCAST permit transmission of broadcast messages

SO_DEBUG enables debugging in the underlying protocol modules. SO_REUSEADDR indicates the rules used in validating addresses supplied in a bind (2) call should allow reuse of local addresses. SO_KEEPALIVE enables the periodic transmission of messages on a connected socket. Should the connected party fail to respond to these messages, the connection is considered broken and processes using the socket are notified via a SIGPIPE signal. SO_DONTROUTE indicates that outgoing messages should bypass the standard routing facilities. Instead, messages are directed to the appropriate network interface according to the network portion of the destination address. SO_LINGER controls the action taken when unsent messags are queued on socket and a close (2) is performed. If the socket promises reliable delivery of data and SO_LINGER is set, the system will block the process on the close attempt until it is able to transmit the data or until it decides it is unable to deliver the information (a timeout period, termed the linger interval, is specified in the setsockopt call when SO_LINGER is requested). If SO_LINGER is disabled and a close is issued, the system will process the close in a manner that allows the process to continue as quickly as possible.

20 [EAFNOSUPPORT] The specified address family is not supported in this version of the system.

20 [ESOCKTNOSUPPORT] The specified socket type is not supported in this address family.

20 [EPROTONOSUPPORT] The specified protocol is not supported.

20 [EMFILE] The per-process descriptor table is full.

20 [ENOBUFS] No buffer space is available. The socket cannot be created.

``A 4.2BSD Interprocess Communication Primer''.