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.\" Copyright (c) 1983 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)socket.2	6.1 (Berkeley) %G%
.\"
.TH SOCKET 2 ""
.UC 5
.SH NAME
socket \- create an endpoint for communication
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <sys/socket.h>
.PP
.ft B
s = socket(af, type, protocol)
int s, af, type, protocol;
.fi
.SH DESCRIPTION
.I Socket
creates an endpoint for communication and returns a descriptor.
.PP
The
.I af
parameter specifies an address format with which addresses specified
in later operations using the socket should be interpreted.  These
formats are defined in the include file
.IR <sys/socket.h> .
The currently understood formats are
.PP
.RS
.nf
.ta 1.25i 1.75i
AF_UNIX	(UNIX path names),
AF_INET	(ARPA Internet addresses),
AF_PUP	(Xerox PUP-I Internet addresses), and
AF_IMPLINK	(IMP \*(lqhost at IMP\*(rq addresses).
.fi
.RE
.PP
The socket has the indicated
.I type
which specifies the semantics of communication.  Currently
defined types are:
.PP
.RS
.nf
SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
SOCK_RDM
.fi
.RE
.PP
A SOCK_STREAM type provides sequenced, reliable,
two-way connection based byte streams with an out-of-band data
transmission mechanism.
A SOCK_DGRAM socket supports
datagrams (connectionless, unreliable messages of
a fixed (typically small) maximum length).
SOCK_RAW sockets provide access to internal network interfaces.
The types SOCK_RAW,
which is available only to the super-user, and
SOCK_SEQPACKET and SOCK_RDM, which are planned,
but not yet implemented, are not described here.
.PP
The
.I protocol
specifies a particular protocol to be used with the socket.
Normally only a single protocol exists to support a particular
socket type using a given address format.  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 \*(lqcommunication domain\*(rq in which communication
is to take place; see
.IR services (3N)
and
.IR protocols (3N).
.PP
Sockets of type SOCK_STREAM
are full-duplex byte streams, similar
to pipes.  A stream socket must be in a
.I connected
state before any data may be sent or received
on it.  A connection to another socket is created with a
.IR connect (2)
call.  Once connected, data may be transferred using
.IR read (2)
and
.IR write (2)
calls or some variant of the 
.IR send (2)
and
.IR recv (2)
calls.  When a session has been completed a
.IR close (2)
may be performed.
Out-of-band data may also be transmitted as described in
.IR send (2)
and received as described in
.IR recv (2).
.PP
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 \*(lqwarm\*(rq 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.
.PP
SOCK_DGRAM and SOCK_RAW
sockets allow sending of datagrams to correspondents
named in
.IR send (2)
calls.  It is also possible to receive datagrams at
such a socket with
.IR recv (2).
.PP
An 
.IR fcntl (2)
call can be used to specify a process group to receive
a SIGURG signal when the out-of-band data arrives.
.PP
The operation of sockets is controlled by socket level
.IR options .
These options are defined in the file
.RI < sys/socket.h >
and explained below.  
.I Setsockopt
and
.IR getsockopt (2)
are used to set and get options, respectively.
Options other than SO_LINGER take an 
.I int
parameter which should non-zero if the option is to be
enabled, or zero if it is to be disabled; SO_LINGER
uses a
.I struct linger
parameter, defined in
.RI < sys/socket.h >,
which specifies the desired state of the option and the
linger interval (see below).
.PP
.RS
.DT
.nf
SO_DEBUG 		turn on recording of debugging information
SO_REUSEADDR	allow local address reuse
SO_KEEPALIVE	keep connections alive
SO_DONTROUTE	do not route outgoing messages
SO_LINGER 	linger on close if data present
SO_BROADCAST	permit transmission of broadcast messages
.fi
.RE
.PP
SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates the rules used in validating addresses supplied
in a
.IR bind (2)
call should allow reuse of local addresses.  SO_KEEPALIVE enables the
periodic transmission of messages on a connected socket.  Should the
connected party fail to respond to these messages, the connection is
considered broken and processes using the socket are notified via a
SIGPIPE signal.  SO_DONTROUTE indicates that outgoing messages should
bypass the standard routing facilities.  Instead, messages are directed
to the appropriate network interface according to the network portion
of the destination address.  SO_LINGER
controls the action taken when unsent messags
are queued on socket and a 
.IR close (2)
is performed.
If the socket promises reliable delivery of data and SO_LINGER is set,
the system will block the process on the 
.I close
attempt until it is able to transmit the data or until it decides it
is unable to deliver the information (a timeout period, termed the
linger interval, is specified in the
.IR setsockopt 
call when SO_LINGER is requested). 
If SO_LINGER is disabled and a 
.I close
is issued, the system will process the close in a manner which allows
the process to continue as quickly as possible.
.SH "RETURN VALUE
A \-1 is returned if an error occurs, otherwise the return
value is a descriptor referencing the socket.
.SH "ERRORS
The \fIsocket\fP call fails if:
.TP 20
[EAFNOSUPPORT]
The specified address family is not supported in this version
of the system.
.TP 20
[ESOCKTNOSUPPORT]
The specified socket type is not supported in this address family.
.TP 20
[EPROTONOSUPPORT]
The specified protocol is not supported.
.TP 20
[EMFILE]
The per-process descriptor table is full.
.TP 20
[ENOBUFS]
No buffer space is available.  The socket cannot be created.
.SH SEE ALSO
accept(2), bind(2), connect(2), getsockname(2), getsockopt(2),
ioctl(2), listen(2), recv(2),
select(2), send(2), shutdown(2), socketpair(2)
.br
``A 4.2BSD Interprocess Communication Primer''.
.SH BUGS
The use of keepalives is a questionable feature for this layer.