socket(3N) socket(3N)
NAME
socket - create an endpoint for communication
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int socket(domain, type, protocol)
int domain, type, 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
/usr/include/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
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socket(3N) socket(3N)
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
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(3N) call. Once connected, data may be
transferred using read(2) and write(2) calls or some variant of the
send(3N) and recv(3N) calls. When a session has been completed, a
close(2) 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(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 with
SIGIO signals.
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socket(3N) socket(3N)
The operation of sockets is controlled by socket level options.
These options are defined in the file /usr/include/sys/socket.h.
setsockopt(3N) and getsockopt(3N) 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.
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
close(2), fcntl(2), ioctl(2), read(2), write(2), accept(3N),
bind(3N), connect(3N), getsockname(3N), getsockopt(3N), listen(3N),
recv(3N), send(3N), shutdown(3N), socketpair(3N).
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