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