351 lines
9.3 KiB
Groff
351 lines
9.3 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" From: @(#)socket.2 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd June 10, 2017
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.Dt SOCKET 2
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.Os
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.Sh NAME
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.Nm socket
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.Nd create an endpoint for communication
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/socket.h
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.Ft int
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.Fn socket "int domain" "int type" "int protocol"
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.Sh DESCRIPTION
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The
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.Fn socket
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system call
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creates an endpoint for communication and returns a descriptor.
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.Pp
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The
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.Fa domain
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argument specifies a communications domain within which
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communication will take place; this selects the protocol family
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which should be used.
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These families are defined in the include file
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.In sys/socket.h .
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The currently understood formats are:
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.Pp
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.Bd -literal -offset indent -compact
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PF_LOCAL Host-internal protocols (alias for PF_UNIX),
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PF_UNIX Host-internal protocols,
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PF_INET Internet version 4 protocols,
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PF_INET6 Internet version 6 protocols,
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PF_ROUTE Internal routing protocol,
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PF_LINK Link layer interface,
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PF_KEY Internal key-management function,
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PF_NATM Asynchronous transfer mode protocols,
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PF_NETGRAPH Netgraph sockets,
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PF_IEEE80211 IEEE 802.11 wireless link-layer protocols (WiFi),
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PF_BLUETOOTH Bluetooth protocols,
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PF_INET_SDP OFED socket direct protocol (IPv4),
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PF_INET6_SDP OFED socket direct protocol (IPv6)
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.Ed
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.Pp
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Each protocol family is connected to an address family, which has the
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same name except that the prefix is
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.Dq Dv AF_
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in place of
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.Dq Dv PF_ .
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Other protocol families may be also defined, beginning with
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.Dq Dv PF_ ,
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with corresponding address families.
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.Pp
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The socket has the indicated
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.Fa type ,
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which specifies the semantics of communication.
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Currently
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defined types are:
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.Pp
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.Bd -literal -offset indent -compact
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SOCK_STREAM Stream socket,
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SOCK_DGRAM Datagram socket,
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SOCK_RAW Raw-protocol interface,
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SOCK_RDM Reliably-delivered packet,
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SOCK_SEQPACKET Sequenced packet stream
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.Ed
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.Pp
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A
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.Dv SOCK_STREAM
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type provides sequenced, reliable,
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two-way connection based byte streams.
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An out-of-band data transmission mechanism may be supported.
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A
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.Dv SOCK_DGRAM
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socket supports
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datagrams (connectionless, unreliable messages of
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a fixed (typically small) maximum length).
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A
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.Dv SOCK_SEQPACKET
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socket may provide a sequenced, reliable,
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two-way connection-based data transmission path for datagrams
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of fixed maximum length; a consumer may be required to read
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an entire packet with each read system call.
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This facility may have protocol-specific properties.
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.Dv SOCK_RAW
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sockets provide access to internal network protocols and interfaces.
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The types
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.Dv SOCK_RAW ,
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which is available only to the super-user, and
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.Dv SOCK_RDM ,
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which is planned,
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but not yet implemented, are not described here.
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.Pp
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Additionally, the following flags are allowed in the
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.Fa type
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argument:
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.Pp
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.Bd -literal -offset indent -compact
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SOCK_CLOEXEC Set close-on-exec on the new descriptor,
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SOCK_NONBLOCK Set non-blocking mode on the new socket
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.Ed
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.Pp
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The
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.Fa protocol
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argument
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specifies a particular protocol to be used with the socket.
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Normally only a single protocol exists to support a particular
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socket type within a given protocol family.
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However, it is possible
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that many protocols may exist, in which case a particular protocol
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must be specified in this manner.
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The protocol number to use is
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particular to the
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.Dq "communication domain"
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in which communication
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is to take place; see
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.Xr protocols 5 .
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.Pp
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The
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.Fa protocol
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argument may be set to zero (0) to request the default
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implementation of a socket type for the protocol, if any.
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.Pp
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Sockets of type
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.Dv SOCK_STREAM
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are full-duplex byte streams, similar
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to pipes.
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A stream socket must be in a
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.Em connected
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state before any data may be sent or received
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on it.
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A connection to another socket is created with a
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.Xr connect 2
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system call.
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Once connected, data may be transferred using
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.Xr read 2
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and
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.Xr write 2
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calls or some variant of the
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.Xr send 2
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and
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.Xr recv 2
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functions.
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(Some protocol families, such as the Internet family,
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support the notion of an
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.Dq implied connect ,
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which permits data to be sent piggybacked onto a connect operation by
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using the
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.Xr sendto 2
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system call.)
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When a session has been completed a
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.Xr close 2
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may be performed.
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Out-of-band data may also be transmitted as described in
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.Xr send 2
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and received as described in
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.Xr recv 2 .
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.Pp
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The communications protocols used to implement a
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.Dv SOCK_STREAM
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ensure that data
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is not lost or duplicated.
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If a piece of data for which the
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peer protocol has buffer space cannot be successfully transmitted
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within a reasonable length of time, then
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the connection is considered broken and calls
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will indicate an error with
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-1 returns and with
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.Er ETIMEDOUT
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as the specific code
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in the global variable
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.Va errno .
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The protocols optionally keep sockets
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.Dq warm
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by forcing transmissions
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roughly every minute in the absence of other activity.
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An error is then indicated if no response can be
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elicited on an otherwise
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idle connection for an extended period (e.g.\& 5 minutes).
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By default, a
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.Dv SIGPIPE
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signal is raised if a process sends
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on a broken stream, but this behavior may be inhibited via
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.Xr setsockopt 2 .
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.Pp
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.Dv SOCK_SEQPACKET
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sockets employ the same system calls
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as
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.Dv SOCK_STREAM
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sockets.
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The only difference
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is that
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.Xr read 2
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calls will return only the amount of data requested,
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and any remaining in the arriving packet will be discarded.
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.Pp
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.Dv SOCK_DGRAM
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and
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.Dv SOCK_RAW
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sockets allow sending of datagrams to correspondents
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named in
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.Xr send 2
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calls.
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Datagrams are generally received with
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.Xr recvfrom 2 ,
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which returns the next datagram with its return address.
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.Pp
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An
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.Xr fcntl 2
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system call can be used to specify a process group to receive
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a
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.Dv SIGURG
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signal when the out-of-band data arrives.
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It may also enable non-blocking I/O
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and asynchronous notification of I/O events
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via
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.Dv SIGIO .
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.Pp
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The operation of sockets is controlled by socket level
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.Em options .
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These options are defined in the file
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.In sys/socket.h .
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The
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.Xr setsockopt 2
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and
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.Xr getsockopt 2
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system calls are used to set and get options, respectively.
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.Sh RETURN VALUES
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A -1 is returned if an error occurs, otherwise the return
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value is a descriptor referencing the socket.
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.Sh ERRORS
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The
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.Fn socket
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system call fails if:
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.Bl -tag -width Er
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.It Bq Er EACCES
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Permission to create a socket of the specified type and/or protocol
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is denied.
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.It Bq Er EAFNOSUPPORT
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The address family (domain) is not supported or the
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specified domain is not supported by this protocol family.
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.It Bq Er EMFILE
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The per-process descriptor table is full.
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.It Bq Er ENFILE
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The system file table is full.
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.It Bq Er ENOBUFS
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Insufficient buffer space is available.
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The socket cannot be created until sufficient resources are freed.
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.It Bq Er EPERM
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User has insufficient privileges to carry out the requested operation.
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.It Bq Er EPROTONOSUPPORT
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The protocol type or the specified protocol is not supported
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within this domain.
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.It Bq Er EPROTOTYPE
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The socket type is not supported by the protocol.
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.El
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.Sh SEE ALSO
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.Xr accept 2 ,
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.Xr bind 2 ,
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.Xr connect 2 ,
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.Xr getpeername 2 ,
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.Xr getsockname 2 ,
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.Xr getsockopt 2 ,
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.Xr ioctl 2 ,
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.Xr listen 2 ,
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.Xr read 2 ,
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.Xr recv 2 ,
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.Xr select 2 ,
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.Xr send 2 ,
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.Xr shutdown 2 ,
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.Xr socketpair 2 ,
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.Xr write 2 ,
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.Xr getprotoent 3 ,
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.Xr netgraph 4 ,
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.Xr protocols 5
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.Rs
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.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
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.%B PS1
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.%N 7
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.Re
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.Rs
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.%T "BSD Interprocess Communication Tutorial"
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.%B PS1
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.%N 8
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.Re
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.Sh STANDARDS
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The
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.Fn socket
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function conforms to
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.St -p1003.1-2008 .
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The
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.Tn POSIX
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standard specifies only the
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.Dv AF_INET ,
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.Dv AF_INET6 ,
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and
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.Dv AF_UNIX
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constants for address families, and requires the use of
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.Dv AF_*
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constants for the
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.Fa domain
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argument of
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.Fn socket .
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The
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.Dv SOCK_CLOEXEC
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flag is expected to conform to the next revision of the
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.Tn POSIX
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standard.
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The
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.Dv SOCK_RDM
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.Fa type ,
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the
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.Dv PF_*
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constants, and other address families are
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.Fx
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extensions.
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.Sh HISTORY
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The
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.Fn socket
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system call appeared in
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.Bx 4.2 .
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