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a091d6919c
Create a chapter on every important socket type: stream, datagram, seqpacket. Always list what protocol families do support what kinds of sockets. Improve some statements possessing language from the specification [1]. Reduce some statements that are mostly specific to TCP. Provide more external links and references to various important syscalls that can be used on sockets. Add a paragrph on non-blocking mode. The big factual change is documentation of SOCK_SEQPACKET. In FreeBSD 15 this socket now fully follows the specification and is a stream socket with record boundaries. [1] https://pubs.opengroup.org/onlinepubs/9799919799/functions/V2_chap02.html#tag_16_10_06 Differential Revision: https://reviews.freebsd.org/D52771 (cherry picked from commit 86d17239233ea4f5766bee68ba6355804525cefa) (cherry picked from commit ad13fd5a8fa1607805d11ced3428e86dbd94778e)
484 lines
13 KiB
Groff
484 lines
13 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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.Dd September 28, 2025
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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_DIVERT Firewall packet diversion/re-injection,
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PF_ROUTE Internal routing protocol,
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PF_KEY Internal key-management function,
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PF_NETGRAPH Netgraph sockets,
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PF_NETLINK Netlink protocols,
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PF_BLUETOOTH Bluetooth protocols,
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PF_INET_SDP OFED socket direct protocol (IPv4),
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AF_HYPERV HyperV sockets
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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_SEQPACKET Sequenced packet stream
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.Ed
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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_CLOFORK Set close-on-fork 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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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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.Sh STREAM SOCKET TYPE
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The
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.Dv SOCK_STREAM
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socket type provides reliable, sequenced, full-duplex octet streams between
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the socket and a peer to which the socket is connected.
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A socket of type
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.Dv SOCK_STREAM
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needs to be in a
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.Em connected
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state before any data can be sent or received.
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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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(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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Once connected, data may be sent using
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.Xr send 2 ,
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.Xr sendto 2 ,
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.Xr sendmsg 2
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and
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.Xr write 2
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system calls.
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Data may be received using
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.Xr recv 2 ,
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.Xr recvfrom 2 ,
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.Xr recvmsg 2 ,
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and
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.Xr read 2
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system calls.
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Record boundaries are not maintained; data sent on a stream socket using output
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operations of one size can be received using input operations of smaller or
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larger sizes without loss of data.
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Data may be buffered; successful return from an output function does not imply
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that the data has been delivered to the peer or even transmitted from the local
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system.
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For certain protocols 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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If data cannot be successfully transmitted within a given time then the
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connection is considered broken, and subsequent operations shall fail with
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a protocol specific error code.
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A
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.Dv SIGPIPE
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signal is raised if a thread attempts to send data on a broken stream (one that
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is no longer connected).
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The signal can be suppressed by the
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.Dv MSG_NOSIGNAL
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flag with distinct
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.Xr send 2 ,
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.Xr sendto 2 ,
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and
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.Xr sendmsg 2
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system calls or by the
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.Dv SO_NOSIGPIPE
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socket option set on the socket with
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.Xr setsockopt 2 .
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.Pp
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The
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.Dv SOCK_STREAM
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socket is supported by the following protocol families:
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.Dv PF_INET ,
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.Dv PF_INET6 ,
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.Dv PF_UNIX ,
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.Dv PF_BLUETOOTH ,
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.Dv PF_HYPERV ,
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and
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.Dv PF_INET_SDP .
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Out-of-band data transmission mechanism is supported for stream sockets of
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.Dv PF_INET
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and
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.Dv PF_INET6
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protocol families.
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.Sh DATAGRAM SOCKET TYPE
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The
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.Dv SOCK_DGRAM
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socket type supports connectionless data transfer which is not necessarily
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acknowledged or reliable.
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Datagrams can be sent to the address specified (possibly multicast or
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broadcast) in each output operation, and incoming datagrams can be received
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from multiple sources.
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The source address of each datagram is available when receiving the datagram
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with
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.Xr recvfrom 2
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or
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.Xr recvmsg 2 .
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An application can also pre-specify a peer address with
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.Xr sendto 2
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or
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.Xr sendmsg 2 ,
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in which case calls to output functions that do not specify a peer address
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shall send to the pre-specified peer.
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If a peer has been specified, only datagrams from that peer shall be received.
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A datagram shall be sent in a single output operation, and needs to be received
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in a single input operation.
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The maximum size of a datagram is protocol-specific.
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Output datagrams may be buffered within the system; thus, a successful return
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from an output function does not guarantee that a datagram is actually sent or
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received.
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.Pp
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The
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.Dv SOCK_DGRAM
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socket is supported by the following protocol families:
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.Dv PF_INET ,
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.Dv PF_INET6 ,
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.Dv PF_UNIX ,
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.Dv PF_NETGRAPH ,
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and
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.Dv PF_NETLINK .
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.Sh SEQUENCED PACKET SOCKET TYPE
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The
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.Dv SOCK_SEQPACKET
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socket type is similar to the
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.Dv SOCK_STREAM
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type, and is also connection-oriented.
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The only difference between these types is that record boundaries are
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maintained using the
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.Dv SOCK_SEQPACKET
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type.
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A record can be sent using one or more output operations and received using one
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or more input operations, but a single operation never transfers parts of more
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than one record.
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Record boundaries are set by the sender with the
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.Dv MSG_EOR
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flag of
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.Xr send 2
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or
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.Xr sendmsg 2
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functions.
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There is no possibility to set a record boundary with
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.Xr write 2 .
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Record boundaries are visible to the receiver via the
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.Dv MSG_EOR
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flag in the received message flags returned by the
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.Xr recvmsg 2
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function.
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It is protocol-specific whether a maximum record size is imposed.
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.Pp
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The
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.Dv SOCK_SEQPACKET
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socket is supported by the following protocol families:
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.Dv PF_INET ,
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.Dv PF_INET6 ,
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and
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.Dv PF_UNIX .
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.Pp
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.Sh RAW SOCKET TYPE
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The
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.Dv SOCK_RAW
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socket type provides access to internal network protocols and interfaces.
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It is a datagram socket in its nature, thus has the same semantics of
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read and write operations.
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The
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.Dv SOCK_RAW
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type is available only to the super-user and is described in
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.Xr ip 4
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and
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.Xr ip6 4 .
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.Sh NON-BLOCKING MODE
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A socket can be created in
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.Em non-blocking mode
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with the help of
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.Dv SOCK_NONBLOCK
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flag.
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Alternatively, the non-blocking mode on a socket can be turned on and off with
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the help of the
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.Dv O_NONBLOCK
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flag of the
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.Xr fcntl 2
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system call.
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.Pp
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When a non-blocking socket has not enough data in its receive buffer to fulfill
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the application supplied buffer, then data receiving system calls like
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.Xr recv 2 ,
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.Xr recvfrom 2 ,
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.Xr recvmsg 2
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and
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.Xr read 2
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will not block waiting for the data but immediately return.
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Return value will indicate amount of bytes read into the supplied buffer.
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The
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.Va errno
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will be set to
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.Dv EAGAIN
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.Po
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has same value as
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.Dv EWOULDBLOCK
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.Pc .
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.Pp
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If application tries to send more data on a non-blocking socket than the socket
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send buffer can accomodate with
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.Xr send 2 ,
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.Xr sendto 2 ,
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.Xr sendmsg 2
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or
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.Xr write 2
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system calls partial data will be sent.
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Return value will indicate amount of bytes sent.
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The
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.Va errno
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will be set to
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.Dv EAGAIN .
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Note that sockets of
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.Dv SOCK_DGRAM
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type are unreliable, thus for these sockets sending operations will never fail
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with
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.Dv EAGAIN
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in non-blocking mode neither will block in blocking mode.
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.Sh OTHER OPERATIONS ON SOCKETS
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Since socket descriptors are file descriptors, many generic file operations
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performed by
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.Xr fcntl 2 ,
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apply.
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Socket descriptors can be used with all event engines, such as
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.Xr kevent 2 ,
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.Xr select 2
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and
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.Xr poll 2 .
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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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.Pp
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Connection associated with a socket can be terminated by
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.Xr close 2
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system call.
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One direction of communication can be disabled with
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.Xr shutdown 2 .
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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 close 2 ,
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.Xr connect 2 ,
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.Xr fcntl 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 kevent 2 ,
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.Xr listen 2 ,
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.Xr poll 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 sendmsg 2 ,
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.Xr sendto 2 ,
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.Xr signal 3 ,
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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 CMSG_DATA 3 ,
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.Xr getprotoent 3 ,
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.Xr divert 4 ,
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.Xr ip 4 ,
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.Xr ip6 4 ,
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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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and
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.Dv SOCK_CLOFORK
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flags are expected to conform to
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.St -p1003.1-2024 .
|
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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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.Pp
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The
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.Dv SOCK_CLOFORK
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flag appeared in
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.Fx 15.0 .
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