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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 <sys/types.h>
.Fd #include <sys/socket.h>
.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 .