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DESCRIPTION

The domain parameter specifies a communications domain within which communication will take place; this selects the protocol family which should be used. These families are defined in the include file ⟨sys/socket.h⟩. The currently understood formats are

AF_UNIX		(UNIX internal protocols),
AF_INET		(ARPA Internet protocols),
AF_ISO		(ISO protocols),
AF_NS		(Xerox Network Systems protocols), and
AF_IMPLINK	(IMP “host at IMP” link layer).

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. An out-of-band data transmission mechanism may be supported. A SOCK_DGRAM socket supports datagrams (connectionless, unreliable messages of a fixed (typically small) maximum length). A SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer may be required to read an entire packet with each read system call. This facility is protocol specific, and presently implemented only for PF_NS. SOCK_RAW sockets provide access to internal network protocols and interfaces. The types SOCK_RAW, which is available only to the super-user, and SOCK_RDM, which is 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 within a given protocol family. 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 “communication domain” in which communication is to take place; see protocols(5).

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 “warm” 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_SEQPACKET sockets employ the same system calls as SOCK_STREAM sockets. The only difference is that read(2) calls will return only the amount of data requested, and any remaining in the arriving packet will be discarded.

SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in send(2) calls. Datagrams are generally received with recvfrom(2), which returns the next datagram with its return address.

An fcntl(2) call can be used to specify a process group to receive a SIGURG signal when the out-of-band data arrives. It may also enable non-blocking I/O and asynchronous notification of I/O events via SIGIO.

The operation of sockets is controlled by socket level options. These options are defined in the file ⟨sys/socket.h⟩. Setsockopt(2) and getsockopt(2) are used to set and get options, respectively.