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.\"     $OpenBSD: socket.2,v 1.18 2000/10/18 05:12:12 aaron Exp $

.\"     $NetBSD: socket.2,v 1.5 1995/02/27 12:37:53 cgd Exp $

.\"

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.\"     @(#)socket.2    8.1 (Berkeley) 6/4/93

.\"

.Dd June 4, 1993

.Dt SOCKET 2

.Os

.Sh NAME

.Nm socket

.Nd create an endpoint for communication

.Sh SYNOPSIS

.Fd #include 

.Fd #include 

.Ft int

.Fn socket "int domain" "int type" "int protocol"

.Sh DESCRIPTION

.Fn socket

creates an endpoint for communication and returns a descriptor.

.Pp

The

.Fa domain

parameter specifies a communications domain within which

communication will take place; this selects the protocol family

which should be used.

These families are defined in the include file

.Ao Pa sys/socket.h Ac .

The currently understood formats are

.Pp

.Bd -literal -offset indent -compact

AF_UNIX         (UNIX internal protocols),

AF_INET         (ARPA Internet protocols),

AF_INET6        (ARPA IPv6 protocols),

AF_ISO          (ISO protocols),

AF_NS           (Xerox Network Systems protocols),

AF_IPX          (Internetwork Packet Exchange), and

AF_IMPLINK      (IMP \*(lqhost at IMP\*(rq link layer).

.Ed

.Pp

The socket has the indicated

.Fa type ,

which specifies the semantics of communication.

Currently defined types are:

.Pp

.Bd -literal -offset indent -compact

SOCK_STREAM

SOCK_DGRAM

SOCK_RAW

SOCK_SEQPACKET

SOCK_RDM

.Ed

.Pp

A

.Dv SOCK_STREAM

type provides sequenced, reliable,

two-way connection based byte streams.

An out-of-band data transmission mechanism may be supported.

A

.Dv SOCK_DGRAM

socket supports

datagrams (connectionless, unreliable messages of

a fixed (typically small) maximum length).

A

.Dv SOCK_SEQPACKET

socket may provide a sequenced, reliable,

two-way connection-based data transmission path for datagrams

of fixed maximum length; a consumer may be required to read

an entire packet with each read system call.

This facility is protocol specific, and presently implemented

only for

.Dv PF_NS .

.Dv SOCK_RAW

sockets provide access to internal network protocols and interfaces.

The types

.Dv SOCK_RAW ,

which is available only to the superuser, and

.Dv SOCK_RDM ,

which is planned,

but not yet implemented, are not described here.

.Pp

The

.Fa protocol

specifies a particular protocol to be used with the socket.

Normally only a single protocol exists to support a particular

socket type within a given protocol family.

However, it is possible that many protocols may exist,

in which case a particular protocol must be specified in this manner.

The protocol number to use is particular to the \*(lqcommunication domain\*(rq

in which communication is to take place; see

.Xr protocols 5 .

A value of 0 for

.Fa protocol

will let the system select an appropriate protocol for the requested

socket type.

.Pp

Sockets of type

.Dv SOCK_STREAM

are full-duplex byte streams, similar to pipes.

A stream socket must be in a

.Em connected

state before any data may be sent or received on it.

A connection to another socket is created with a

.Xr connect 2

call.

Once connected, data may be transferred using

.Xr read 2

and

.Xr write 2

calls or some variant of the

.Xr send 2

and

.Xr recv 2

calls.

When a session has been completed a

.Xr close 2

may be performed.

Out-of-band data may also be transmitted as described in

.Xr send 2

and received as described in

.Xr recv 2 .

.Pp

The communications protocols used to implement a

.Dv SOCK_STREAM

ensure that data is not lost or duplicated.

If a piece of data for which the peer protocol has buffer space cannot

be successfully transmitted within a reasonable length of time, then the

connection is considered broken and calls will indicate an error with \-1

returns and with

.Er ETIMEDOUT

as the specific code in the global variable

.Va errno .

The protocols optionally keep sockets

.Dq warm

by forcing transmissions roughly every minute in the absence of other activity.

An error is then indicated if no response can be elicited on an otherwise

idle connection for a extended period (e.g., 5 minutes).

A

.Dv SIGPIPE

signal is raised if a process sends on a broken stream; this causes

naive processes, which do not handle the signal, to exit.

.Pp

.Dv SOCK_SEQPACKET

sockets employ the same system calls

as

.Dv SOCK_STREAM

sockets.

The only difference is that

.Xr read 2

calls will return only the amount of data requested,

and any remaining in the arriving packet will be discarded.

.Pp

.Dv SOCK_DGRAM

and

.Dv SOCK_RAW

sockets allow sending of datagrams to correspondents named in

.Xr send 2

calls.

Datagrams are generally received with

.Xr recvfrom 2 ,

which returns the next datagram with its return address.

.Pp

An

.Xr fcntl 2

call can be used to specify a process group to receive

a

.Dv SIGURG

signal when the out-of-band data arrives.

It may also enable non-blocking I/O and asynchronous notification

of I/O events via

.Dv SIGIO .

.Pp

The operation of sockets is controlled by socket level

.Em options .

These options are defined in the file

.Ao Pa sys/socket.h Ac .

.Xr setsockopt 2

and

.Xr getsockopt 2

are used to set and get options, respectively.

.Sh RETURN VALUES

A \-1 is returned if an error occurs, otherwise the return

value is a descriptor referencing the socket.

.Sh ERRORS

The

.Fn socket

call fails if:

.Bl -tag -width Er

.It Bq Er EPROTONOSUPPORT

The protocol type or the specified protocol is not supported

within this domain.

.It Bq Er EMFILE

The per-process descriptor table is full.

.It Bq Er ENFILE

The system file table is full.

.It Bq Er EACCES

Permission to create a socket of the specified type and/or protocol

is denied.

.It Bq Er ENOBUFS

Insufficient buffer space is available.

The socket cannot be created until sufficient resources are freed.

.El

.Sh SEE ALSO

.Xr accept 2 ,

.Xr bind 2 ,

.Xr connect 2 ,

.Xr getsockname 2 ,

.Xr getsockopt 2 ,

.Xr ioctl 2 ,

.Xr listen 2 ,

.Xr poll 2 ,

.Xr read 2 ,

.Xr recv 2 ,

.Xr select 2 ,

.Xr send 2 ,

.Xr setsockopt 2 ,

.Xr shutdown 2 ,

.Xr socketpair 2 ,

.Xr write 2 ,

.Xr getprotoent 3 ,

.Xr netintro 4

.Rs

.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"

.%O "reprinted in UNIX Programmer's Supplementary Documents Volume 1"

.Re

.Rs

.%T "BSD Interprocess Communication Tutorial"

.%O "reprinted in UNIX Programmer's Supplementary Documents Volume 1"

.Re

.Sh HISTORY

The

.Fn socket

function call appeared in

.Bx 4.2 .