bsd/man/man2/socket.2

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  34 .\"     @(#)socket.2    8.1 (Berkeley) 6/4/93
  35 .\"
  36 .Dd March 18, 2015
  37 .Dt SOCKET 2
  38 .Os
  39 .Sh NAME
  40 .Nm socket
  41 .Nd create an endpoint for communication
  42 .Sh SYNOPSIS
  43 .Fd #include <sys/socket.h>
  44 .Ft int
  45 .Fo socket
  46 .Fa "int domain"
  47 .Fa "int type"
  48 .Fa "int protocol"
  49 .Fc
  50 .Sh DESCRIPTION
  51 .Fn socket
  52 creates an endpoint for communication and returns a descriptor.
  53 .Pp
  54 The
  55 .Fa domain
  56 parameter specifies a communications domain within which
  57 communication will take place; this selects the protocol family
  58 which should be used.
  59 These families are defined in the include file
  60 .Ao Pa sys/socket.h Ac .
  61 The currently understood formats are
  62 .Pp
  63 .Bd -literal -offset indent -compact
  64 PF_LOCAL        Host-internal protocols, formerly called PF_UNIX,
  65 PF_UNIX         Host-internal protocols, deprecated, use PF_LOCAL,
  66 PF_INET         Internet version 4 protocols,
  67 PF_ROUTE        Internal Routing protocol,
  68 PF_KEY          Internal key-management function,
  69 PF_INET6        Internet version 6 protocols,
  70 PF_SYSTEM       System domain,
  71 PF_NDRV         Raw access to network device,
  72 PF_VSOCK        VM Sockets protocols
  73 .Ed
  74 .Pp
  75 The socket has the indicated
  76 .Fa type ,
  77 which specifies the semantics of communication.  Currently
  78 defined types are:
  79 .Pp
  80 .Bd -literal -offset indent -compact
  81 SOCK_STREAM
  82 SOCK_DGRAM
  83 SOCK_RAW
  84 .\"SOCK_SEQPACKET
  85 .\"SOCK_RDM
  86 .Ed
  87 .Pp
  88 A
  89 .Dv SOCK_STREAM
  90 type provides sequenced, reliable,
  91 two-way connection based byte streams.
  92 An out-of-band data transmission mechanism may be supported.
  93 A
  94 .Dv SOCK_DGRAM
  95 socket supports
  96 datagrams (connectionless, unreliable messages of
  97 a fixed (typically small) maximum length).
  98 .\"A
  99 .\".Dv SOCK_SEQPACKET
 100 .\"socket may provide a sequenced, reliable,
 101 .\"two-way connection-based data transmission path for datagrams
 102 .\"of fixed maximum length; a consumer may be required to read
 103 .\"an entire packet with each read system call.
 104 .\"This facility is protocol specific, and presently implemented
 105 .\"only for
 106 .\".Dv PF_NS .
 107 .Dv SOCK_RAW
 108 sockets provide access to internal network protocols and interfaces.
 109 The type
 110 .Dv SOCK_RAW ,
 111 which is available only to the super-user.
 112 .\" , and
 113 .\" .Dv SOCK_RDM ,
 114 .\" which is planned,
 115 .\" but not yet implemented, are not described here.
 116 .Pp
 117 The
 118 .Fa protocol
 119 specifies a particular protocol to be used with the socket.
 120 Normally only a single protocol exists to support a particular
 121 socket type within a given protocol family.  However, it is possible
 122 that many protocols may exist, in which case a particular protocol
 123 must be specified in this manner.  The protocol number to use is
 124 particular to the \*(lqcommunication domain\*(rq in which communication
 125 is to take place; see
 126 .Xr protocols 5 .
 127 .Pp
 128 Sockets of type
 129 .Dv SOCK_STREAM
 130 are full-duplex byte streams, similar
 131 to pipes.  A stream socket must be in a
 132 .Em connected
 133 state before any data may be sent or received
 134 on it.  A connection to another socket is created with a
 135 .Xr connect 2
 136 or
 137 .Xr connectx 2
 138 call.  Once connected, data may be transferred using
 139 .Xr read 2
 140 and
 141 .Xr write 2
 142 calls or some variant of the
 143 .Xr send 2
 144 and
 145 .Xr recv 2
 146 calls.  When a session has been completed a
 147 .Xr close 2
 148 may be performed.
 149 Out-of-band data may also be transmitted as described in
 150 .Xr send 2
 151 and received as described in
 152 .Xr recv 2 .
 153 .Pp
 154 The communications protocols used to implement a
 155 .Dv SOCK_STREAM
 156 insure that data
 157 is not lost or duplicated.  If a piece of data for which the
 158 peer protocol has buffer space cannot be successfully transmitted
 159 within a reasonable length of time, then
 160 the connection is considered broken and calls
 161 will indicate an error with
 162 -1 returns and with
 163 .Dv ETIMEDOUT
 164 as the specific code
 165 in the global variable
 166 .Va errno .
 167 The protocols optionally keep sockets
 168 .Dq warm
 169 by forcing transmissions
 170 roughly every minute in the absence of other activity.
 171 An error is then indicated if no response can be
 172 elicited on an otherwise
 173 idle connection for a extended period (e.g. 5 minutes).
 174 A
 175 .Dv SIGPIPE
 176 signal is raised if a process sends
 177 on a broken stream; this causes naive processes,
 178 which do not handle the signal, to exit.
 179 .\" .Pp
 180 .\" .Dv SOCK_SEQPACKET
 181 .\" sockets employ the same system calls
 182 .\" as
 183 .\" .Dv SOCK_STREAM
 184 .\" sockets.  The only difference
 185 .\" is that
 186 .\" .Xr read 2
 187 .\" calls will return only the amount of data requested,
 188 .\" and any remaining in the arriving packet will be discarded.
 189 .Pp
 190 .Dv SOCK_DGRAM
 191 and
 192 .Dv SOCK_RAW
 193 sockets allow sending of datagrams to correspondents
 194 named in
 195 .Xr send 2
 196 calls.  Datagrams are generally received with
 197 .Xr recvfrom 2 ,
 198 which returns the next datagram with its return address.
 199 .Pp
 200 An
 201 .Xr fcntl 2
 202 call can be used to specify a process group to receive
 203 a
 204 .Dv SIGURG
 205 signal when the out-of-band data arrives.
 206 It may also enable non-blocking I/O
 207 and asynchronous notification of I/O events
 208 via
 209 .Dv SIGIO .
 210 .Pp
 211 The operation of sockets is controlled by socket level
 212 .Em options .
 213 These options are defined in the file
 214 .Ao Pa sys/socket.h Ac .
 215 .Xr Setsockopt 2
 216 and
 217 .Xr getsockopt 2
 218 are used to set and get options, respectively.
 219 .Sh RETURN VALUES
 220 A -1 is returned if an error occurs, otherwise the return
 221 value is a descriptor referencing the socket.
 222 .Sh ERRORS
 223 The
 224 .Fn socket
 225 system call fails if:
 226 .Bl -tag -width Er
 227 .\" ===========
 228 .It Bq Er EACCES
 229 Permission to create a socket of the specified type and/or protocol
 230 is denied.
 231 .\" ===========
 232 .It Bq Er EAFNOSUPPORT
 233 The specified address family is not supported.
 234 .\" ===========
 235 .It Bq Er EMFILE
 236 The per-process descriptor table is full.
 237 .\" ===========
 238 .It Bq Er ENFILE
 239 The system file table is full.
 240 .\" ===========
 241 .It Bq Er ENOBUFS
 242 Insufficient buffer space is available.
 243 The socket cannot be created until sufficient resources are freed.
 244 .\" ===========
 245 .It Bq Er ENOMEM
 246 Insufficient memory was available to fulfill the request.
 247 .\" ===========
 248 .It Bq Er EPROTONOSUPPORT
 249 The protocol type or the specified protocol is not supported
 250 within this domain.
 251 .\" ===========
 252 .It Bq Er EPROTOTYPE
 253 The socket type is not supported by the protocol.
 254 .El
 255 .Pp
 256 If a new protocol family is defined,
 257 the socreate process is free to return any desired error code.
 258 The
 259 .Fn socket
 260 system call will pass this error code along
 261 (even if it is undefined).
 262 .Sh LEGACY SYNOPSIS
 263 .Fd #include <sys/types.h>
 264 .Fd #include <sys/socket.h>
 265 .Pp
 266 The include file
 267 .In sys/types.h
 268 is necessary.
 269 .Sh SEE ALSO
 270 .Xr accept 2 ,
 271 .Xr bind 2 ,
 272 .Xr connect 2 ,
 273 .Xr connectx 2 ,
 274 .Xr disconnectx 2 ,
 275 .Xr getsockname 2 ,
 276 .Xr getsockopt 2 ,
 277 .Xr ioctl 2 ,
 278 .Xr listen 2 ,
 279 .Xr read 2 ,
 280 .Xr recv 2 ,
 281 .Xr select 2 ,
 282 .Xr send 2 ,
 283 .Xr shutdown 2 ,
 284 .Xr socketpair 2 ,
 285 .Xr write 2 ,
 286 .Xr getprotoent 3 ,
 287 .Xr inet 4 ,
 288 .Xr inet6 4 ,
 289 .Xr unix 4 ,
 290 .Xr compat 5
 291 .Rs
 292 .%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
 293 .%O "reprinted in UNIX Programmer's Supplementary Documents Volume 1"
 294 .Re
 295 .Rs
 296 .%T "BSD Interprocess Communication Tutorial"
 297 .%O "reprinted in UNIX Programmer's Supplementary Documents Volume 1"
 298 .Re
 299 .Sh HISTORY
 300 The
 301 .Fn socket
 302 function call appeared in
 303 .Bx 4.2 .