int socket(int domain, int type, int protocol);
The domain parameter specifies a communications domain within which communication will take place; this selects the protocol family which should be used. The protocol family generally is the same as the address family for the addresses supplied in later operations on the socket. These families are defined in the include file sys/socket.h. There must be an entry in the netconfig(4bnu) file for at least each protocol family and type required. If protocol has been specified, but no exact match for the tuplet family, type, protocol is found, then the first entry containing the specified family and type with zero for protocol will be used. The currently understood formats are:
The socket has the indicated type, which specifies the communication semantics. Currently defined types are:
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, multiple 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. If a protocol is specified by the caller, then it will be packaged into a socket level option request and sent to the underlying protocol layers.
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 call. Once connected, data may be transferred using read and write calls or some variant of the send and recv calls. When a session has been completed, a close may be performed. Out-of-band data may also be transmitted as described on the send(3sock) manual page and received as described on the recv(3sock) manual page.
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 (for instance 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 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 datagrams to be sent to correspondents named in sendto calls. Datagrams are generally received with recvfrom, which returns the next datagram with its return address.
An fcntl 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 with SIGIO signals.
The operation of sockets is controlled by socket level options. These options are defined in the file sys/socket.h. setsockopt and getsockopt are used to set and get options, respectively.