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<HTML
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>Chapter 38. TCP/IP Library Reference</TD
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NAME="NET-COMMON-TCPIP-MANPAGES-SOCKET">socket</H1
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>SOCKET(2) System Calls Manual SOCKET(2)
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NAME
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socket - create an endpoint for communication
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SYNOPSIS
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#include <sys/types.h>
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#include <sys/socket.h>
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int
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socket(int domain, int type, int protocol);
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DESCRIPTION
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socket() creates an endpoint for communication and returns a descriptor.
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The domain parameter specifies a communications domain within which com-
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munication will take place; this selects the protocol family which should
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be used. These families are defined in the include file <sys/socket.h>.
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The currently understood formats are
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AF_UNIX (UNIX internal protocols),
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AF_INET (ARPA Internet protocols),
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AF_INET6 (ARPA IPv6 protocols),
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AF_ISO (ISO protocols),
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AF_NS (Xerox Network Systems protocols),
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AF_IPX (Internetwork Packet Exchange), and
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AF_IMPLINK (IMP host at IMP link layer).
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The socket has the indicated type, which specifies the semantics of com-
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munication. Currently defined types are:
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SOCK_STREAM
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SOCK_DGRAM
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SOCK_RAW
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SOCK_SEQPACKET
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SOCK_RDM
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A SOCK_STREAM type provides sequenced, reliable, two-way connection based
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byte streams. An out-of-band data transmission mechanism may be sup-
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ported. A SOCK_DGRAM socket supports datagrams (connectionless, unreli-
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able messages of a fixed (typically small) maximum length). A
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SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connec-
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tion-based data transmission path for datagrams of fixed maximum length;
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a consumer may be required to read an entire packet with each read system
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call. This facility is protocol specific, and presently implemented only
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for PF_NS. SOCK_RAW sockets provide access to internal network protocols
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and interfaces. The types SOCK_RAW, which is available only to the supe-
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ruser, and SOCK_RDM, which is planned, but not yet implemented, are not
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described here.
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The protocol specifies a particular protocol to be used with the socket.
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Normally only a single protocol exists to support a particular socket
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type within a given protocol family. However, it is possible that many
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protocols may exist, in which case a particular protocol must be speci-
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fied in this manner. The protocol number to use is particular to the
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communication domain in which communication is to take place; see
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protocols(5). A value of 0 for protocol will let the system select an
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appropriate protocol for the requested socket type.
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Sockets of type SOCK_STREAM are full-duplex byte streams, similar to
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pipes. A stream socket must be in a connected state before any data may
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be sent or received on it. A connection to another socket is created
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with a connect(2) call. Once connected, data may be transferred using
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read(2) and write(2) calls or some variant of the send(2) and recv(2)
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calls. When a session has been completed a close(2) may be performed.
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Out-of-band data may also be transmitted as described in send(2) and
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received as described in recv(2).
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The communications protocols used to implement a SOCK_STREAM ensure that
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data is not lost or duplicated. If a piece of data for which the peer
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protocol has buffer space cannot be successfully transmitted within a
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reasonable length of time, then the connection is considered broken and
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calls will indicate an error with -1 returns and with ETIMEDOUT as the
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specific code in the global variable errno. The protocols optionally
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keep sockets ``warm'' by forcing transmissions roughly every minute in
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the absence of other activity. An error is then indicated if no response
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can be elicited on an otherwise idle connection for a extended period
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(e.g., 5 minutes). A SIGPIPE signal is raised if a process sends on a
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broken stream; this causes naive processes, which do not handle the sig-
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nal, to exit.
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SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sock-
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ets. The only difference is that read(2) calls will return only the
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amount of data requested, and any remaining in the arriving packet will
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be discarded.
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SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspon-
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dents named in send(2) calls. Datagrams are generally received with
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recvfrom(2), which returns the next datagram with its return address.
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An fcntl(2) call can be used to specify a process group to receive a
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SIGURG signal when the out-of-band data arrives. It may also enable non-
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blocking I/O and asynchronous notification of I/O events via SIGIO.
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The operation of sockets is controlled by socket level options. These
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options are defined in the file <sys/socket.h>. setsockopt(2) and
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getsockopt(2) are used to set and get options, respectively.
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RETURN VALUES
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A -1 is returned if an error occurs, otherwise the return value is a
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descriptor referencing the socket.
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ERRORS
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The socket() call fails if:
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[EPROTONOSUPPORT] The protocol type or the specified protocol is not
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supported within this domain.
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[EMFILE] The per-process descriptor table is full.
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[ENFILE] The system file table is full.
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[EACCES] Permission to create a socket of the specified type
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and/or protocol is denied.
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[ENOBUFS] Insufficient buffer space is available. The socket
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cannot be created until sufficient resources are
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freed.
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SEE ALSO
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accept(2), bind(2), connect(2), getsockname(2), getsockopt(2), ioctl(2),
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listen(2), poll(2), read(2), recv(2), select(2), send(2), setsockopt(2),
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shutdown(2), socketpair(2), write(2), getprotoent(3), netintro(4)
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An Introductory 4.3 BSD Interprocess Communication Tutorial, reprinted in
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UNIX Programmer's Supplementary Documents Volume 1.
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BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's
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Supplementary Documents Volume 1.
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HISTORY
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The socket() function call appeared in 4.2BSD.
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BSD June 4, 1993 BSD
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</PRE
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