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.\"	$OpenBSD: socket.2,v 1.31 2008/11/26 22:40:21 mk Exp $
.\"	$NetBSD: socket.2,v 1.5 1995/02/27 12:37:53 cgd Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)socket.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: November 26 2008 $
.Dt SOCKET 2
.Os
.Sh NAME
.Nm socket
.Nd create an endpoint for communication
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.Ft int
.Fn socket "int domain" "int type" "int protocol"
.Sh DESCRIPTION
.Fn socket
creates an endpoint for communication and returns a descriptor.
.Pp
The
.Fa domain
parameter specifies a communications domain within which
communication will take place; this selects the protocol family
which should be used.
These families are defined in the include file
.Ao Pa sys/socket.h Ac .
The currently understood formats are:
.Pp
.Bl -tag -width "AF_IMPLINKXXX" -offset indent -compact
.It AF_UNIX
UNIX internal protocols
.It AF_INET
ARPA Internet protocols
.It AF_INET6
IPv6 (Internet Protocol version 6) protocols
.It AF_IMPLINK
IMP host at IMP link layer
.It AF_BLUETOOTH
Bluetooth protocols
.El
.Pp
The socket has the indicated
.Fa type ,
which specifies the semantics of communication.
Currently defined types are:
.Pp
.Bl -tag -width "SOCK_SEQPACKETXXX" -offset indent -compact
.It SOCK_STREAM
.It SOCK_DGRAM
.It SOCK_RAW
.It SOCK_SEQPACKET
.It SOCK_RDM
.El
.Pp
A
.Dv SOCK_STREAM
type provides sequenced, reliable,
two-way connection based byte streams.
An out-of-band data transmission mechanism may be supported.
A
.Dv SOCK_DGRAM
socket supports
datagrams (connectionless, unreliable messages of
a fixed (typically small) maximum length).
A
.Dv SOCK_SEQPACKET
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 implemented
only for
.Dv PF_NS .
.Dv SOCK_RAW
sockets provide access to internal network protocols and interfaces,
and are available only to the superuser.
.Pp
The
.Fa 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, 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
.Xr protocols 5 .
A value of 0 for
.Fa protocol
will let the system select an appropriate protocol for the requested
socket type.
.Pp
Sockets of type
.Dv SOCK_STREAM
are full-duplex byte streams.
A stream socket must be in a
.Em connected
state before any data may be sent or received on it.
A connection to another socket is created with a
.Xr connect 2
call.
Once connected, data may be transferred using
.Xr read 2
and
.Xr write 2
calls or some variant of the
.Xr send 2
and
.Xr recv 2
calls.
When a session has been completed a
.Xr close 2
may be performed.
Out-of-band data may also be transmitted as described in
.Xr send 2
and received as described in
.Xr recv 2 .
.Pp
The communications protocols used to implement a
.Dv SOCK_STREAM
ensure 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
.Er ETIMEDOUT
as the specific code in the global variable
.Va errno .
The protocols optionally keep sockets
.Dq 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 an extended period (e.g., 5 minutes).
A
.Dv 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
.Dv SOCK_SEQPACKET
sockets employ the same system calls
as
.Dv SOCK_STREAM
sockets.
The only difference is that
.Xr read 2
calls will return only the amount of data requested,
and any remaining in the arriving packet will be discarded.
.Pp
.Dv SOCK_DGRAM
and
.Dv SOCK_RAW
sockets allow sending of datagrams to correspondents named in
.Xr send 2
calls.
Datagrams are generally received with
.Xr recvfrom 2 ,
which returns the next datagram with its return address.
.Pp
An
.Xr fcntl 2
call can be used to specify a process group to receive
a
.Dv SIGURG
signal when the out-of-band data arrives.
It may also enable non-blocking I/O and asynchronous notification
of I/O events via
.Dv SIGIO .
.Pp
The operation of sockets is controlled by socket level
.Em options .
These options are defined in the file
.Ao Pa sys/socket.h Ac .
.Xr setsockopt 2
and
.Xr getsockopt 2
are used to set and get options, respectively.
.Sh RETURN VALUES
A \-1 is returned if an error occurs, otherwise the return
value is a descriptor referencing the socket.
.Sh ERRORS
The
.Fn socket
call fails if:
.Bl -tag -width Er
.It Bq Er EPROTONOSUPPORT
The protocol type or the specified protocol is not supported
within this domain.
.It Bq Er EMFILE
The per-process descriptor table is full.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er EACCES
Permission to create a socket of the specified type and/or protocol
is denied.
.It Bq Er ENOBUFS
Insufficient buffer space is available.
The socket cannot be created until sufficient resources are freed.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr bind 2 ,
.Xr connect 2 ,
.Xr getsockname 2 ,
.Xr getsockopt 2 ,
.Xr ioctl 2 ,
.Xr listen 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr setsockopt 2 ,
.Xr shutdown 2 ,
.Xr socketpair 2 ,
.Xr write 2 ,
.Xr getprotoent 3 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr netintro 4 ,
.Xr unix 4
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%O "reprinted in UNIX Programmer's Supplementary Documents Volume 1"
.Re
.Rs
.%T "BSD Interprocess Communication Tutorial"
.%O "reprinted in UNIX Programmer's Supplementary Documents Volume 1"
.Re
.Sh HISTORY
The
.Fn socket
function call appeared in
.Bx 4.2 .