1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 4. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" From: @(#)socket.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD$ 30.\" 31.Dd November 24, 1997 32.Dt SOCKET 2 33.Os 34.Sh NAME 35.Nm socket 36.Nd create an endpoint for communication 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/types.h 41.In sys/socket.h 42.Ft int 43.Fn socket "int domain" "int type" "int protocol" 44.Sh DESCRIPTION 45The 46.Fn socket 47system call 48creates an endpoint for communication and returns a descriptor. 49.Pp 50The 51.Fa domain 52argument specifies a communications domain within which 53communication will take place; this selects the protocol family 54which should be used. 55These families are defined in the include file 56.In sys/socket.h . 57The currently understood formats are: 58.Pp 59.Bd -literal -offset indent -compact 60PF_LOCAL Host-internal protocols, formerly called PF_UNIX, 61PF_UNIX Host-internal protocols, deprecated, use PF_LOCAL, 62PF_INET Internet version 4 protocols, 63PF_PUP PUP protocols, like BSP, 64PF_APPLETALK AppleTalk protocols, 65PF_ROUTE Internal Routing protocol, 66PF_LINK Link layer interface, 67PF_IPX Novell Internet Packet eXchange protocol, 68PF_RTIP Help Identify RTIP packets, 69PF_PIP Help Identify PIP packets, 70PF_ISDN Integrated Services Digital Network, 71PF_KEY Internal key-management function, 72PF_INET6 Internet version 6 protocols, 73PF_NATM Native ATM access, 74PF_ATM ATM, 75PF_NETGRAPH Netgraph sockets 76.Ed 77.Pp 78The socket has the indicated 79.Fa type , 80which specifies the semantics of communication. 81Currently 82defined types are: 83.Pp 84.Bd -literal -offset indent -compact 85SOCK_STREAM Stream socket, 86SOCK_DGRAM Datagram socket, 87SOCK_RAW Raw-protocol interface, 88SOCK_RDM Reliably-delivered packet, 89SOCK_SEQPACKET Sequenced packet stream 90.Ed 91.Pp 92A 93.Dv SOCK_STREAM 94type provides sequenced, reliable, 95two-way connection based byte streams. 96An out-of-band data transmission mechanism may be supported. 97A 98.Dv SOCK_DGRAM 99socket supports 100datagrams (connectionless, unreliable messages of 101a fixed (typically small) maximum length). 102A 103.Dv SOCK_SEQPACKET 104socket may provide a sequenced, reliable, 105two-way connection-based data transmission path for datagrams 106of fixed maximum length; a consumer may be required to read 107an entire packet with each read system call. 108This facility is protocol specific, and presently unimplemented. 109.Dv SOCK_RAW 110sockets provide access to internal network protocols and interfaces. 111The types 112.Dv SOCK_RAW , 113which is available only to the super-user, and 114.Dv SOCK_RDM , 115which is planned, 116but not yet implemented, are not described here. 117.Pp 118The 119.Fa protocol 120argument 121specifies a particular protocol to be used with the socket. 122Normally only a single protocol exists to support a particular 123socket type within a given protocol family. 124However, it is possible 125that many protocols may exist, in which case a particular protocol 126must be specified in this manner. 127The protocol number to use is 128particular to the 129.Dq "communication domain" 130in which communication 131is to take place; see 132.Xr protocols 5 . 133.Pp 134Sockets of type 135.Dv SOCK_STREAM 136are full-duplex byte streams, similar 137to pipes. 138A stream socket must be in a 139.Em connected 140state before any data may be sent or received 141on it. 142A connection to another socket is created with a 143.Xr connect 2 144system call. 145Once connected, data may be transferred using 146.Xr read 2 147and 148.Xr write 2 149calls or some variant of the 150.Xr send 2 151and 152.Xr recv 2 153functions. 154(Some protocol families, such as the Internet family, 155support the notion of an 156.Dq implied connect , 157which permits data to be sent piggybacked onto a connect operation by 158using the 159.Xr sendto 2 160system call.) 161When a session has been completed a 162.Xr close 2 163may be performed. 164Out-of-band data may also be transmitted as described in 165.Xr send 2 166and received as described in 167.Xr recv 2 . 168.Pp 169The communications protocols used to implement a 170.Dv SOCK_STREAM 171insure that data 172is not lost or duplicated. 173If a piece of data for which the 174peer protocol has buffer space cannot be successfully transmitted 175within a reasonable length of time, then 176the connection is considered broken and calls 177will indicate an error with 178-1 returns and with 179.Er ETIMEDOUT 180as the specific code 181in the global variable 182.Va errno . 183The protocols optionally keep sockets 184.Dq warm 185by forcing transmissions 186roughly every minute in the absence of other activity. 187An error is then indicated if no response can be 188elicited on an otherwise 189idle connection for an extended period (e.g.\& 5 minutes). 190A 191.Dv SIGPIPE 192signal is raised if a process sends 193on a broken stream; this causes naive processes, 194which do not handle the signal, to exit. 195.Pp 196.Dv SOCK_SEQPACKET 197sockets employ the same system calls 198as 199.Dv SOCK_STREAM 200sockets. 201The only difference 202is that 203.Xr read 2 204calls will return only the amount of data requested, 205and any remaining in the arriving packet will be discarded. 206.Pp 207.Dv SOCK_DGRAM 208and 209.Dv SOCK_RAW 210sockets allow sending of datagrams to correspondents 211named in 212.Xr send 2 213calls. 214Datagrams are generally received with 215.Xr recvfrom 2 , 216which returns the next datagram with its return address. 217.Pp 218An 219.Xr fcntl 2 220system call can be used to specify a process group to receive 221a 222.Dv SIGURG 223signal when the out-of-band data arrives. 224It may also enable non-blocking I/O 225and asynchronous notification of I/O events 226via 227.Dv SIGIO . 228.Pp 229The operation of sockets is controlled by socket level 230.Em options . 231These options are defined in the file 232.In sys/socket.h . 233The 234.Xr setsockopt 2 235and 236.Xr getsockopt 2 237system calls are used to set and get options, respectively. 238.Sh RETURN VALUES 239A -1 is returned if an error occurs, otherwise the return 240value is a descriptor referencing the socket. 241.Sh ERRORS 242The 243.Fn socket 244system call fails if: 245.Bl -tag -width Er 246.It Bq Er EPROTONOSUPPORT 247The protocol type or the specified protocol is not supported 248within this domain. 249.It Bq Er EMFILE 250The per-process descriptor table is full. 251.It Bq Er ENFILE 252The system file table is full. 253.It Bq Er EACCES 254Permission to create a socket of the specified type and/or protocol 255is denied. 256.It Bq Er ENOBUFS 257Insufficient buffer space is available. 258The socket cannot be created until sufficient resources are freed. 259.El 260.Sh SEE ALSO 261.Xr accept 2 , 262.Xr bind 2 , 263.Xr connect 2 , 264.Xr getpeername 2 , 265.Xr getsockname 2 , 266.Xr getsockopt 2 , 267.Xr ioctl 2 , 268.Xr listen 2 , 269.Xr read 2 , 270.Xr recv 2 , 271.Xr select 2 , 272.Xr send 2 , 273.Xr shutdown 2 , 274.Xr socketpair 2 , 275.Xr write 2 , 276.Xr getprotoent 3 , 277.Xr netgraph 4 , 278.Xr protocols 5 279.Rs 280.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial" 281.%B PS1 282.%N 7 283.Re 284.Rs 285.%T "BSD Interprocess Communication Tutorial" 286.%B PS1 287.%N 8 288.Re 289.Sh HISTORY 290The 291.Fn socket 292system call appeared in 293.Bx 4.2 . 294