socket(3N) UNIX System V socket(3N)
NAME
socket - create an endpoint for communication
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
DESCRIPTION
socket creates an endpoint for communication and returns a descriptor.
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(4) 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:
PFUNIX UNIX system internal protocols
PFINET ARPA Internet protocols
The socket has the indicated type, which specifies the communication
semantics. Currently defined types are:
SOCKSTREAM
SOCKDGRAM
SOCKRAW
SOCKSEQPACKET
SOCKRDM
A SOCKSTREAM type provides sequenced, reliable, two-way connection-based
byte streams. An out-of-band data transmission mechanism may be
supported. A SOCKDGRAM socket supports datagrams (connectionless,
unreliable messages of a fixed (typically small) maximum length). A
SOCKSEQPACKET 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 not
implemented for any protocol family. SOCKRAW sockets provide access to
internal network interfaces. The types SOCKRAW, which is available only
to the super-user, and SOCKRDM, for which no implementation currently
exists, are not described here.
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
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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 SOCKSTREAM 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(3N) manual page and
received as described on the recv(3N) manual page.
The communications protocols used to implement a SOCKSTREAM 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.
SOCKSEQPACKET sockets employ the same system calls as SOCKSTREAM
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.
SOCKDGRAM and SOCKRAW 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.
RETURN VALUE
A -1 is returned if an error occurs. Otherwise the return value is a
descriptor referencing the socket.
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socket(3N) UNIX System V socket(3N)
ERRORS
The socket call fails if:
EPROTONOSUPPORT The protocol type or the specified protocol is not
supported within this domain.
EMFILE The per-process descriptor table is full.
EACCESS Permission to create a socket of the specified type
and/or protocol is denied.
ENOMEM Insufficient user memory is available.
ENOSR There were insufficient STREAMS resources available
to complete the operation.
SEE ALSO
accept(3N), bind(3N), connect(3N), getsockname(3N), getsockopt(3N),
listen(3N), recv(3N), send(3N), shutdown(3N), socketpair(3N).
close(2), fcntl(2), ioctl(2), read(2), write(2) in the Programmer's
Reference Manual.
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