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.\"     @(#)socket.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 28, 2022
.Dt SOCKET 2
.Os
.Sh NAME
.Nm socket
.Nd create an endpoint for communication
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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:
.Bl -column -offset indent ".Dv PF_APPLETALK"
.It Dv PF_LOCAL Ta local domain Po previously Tn UNIX domain Pc protocols
.It Dv PF_INET Ta Tn ARPA Internet protocols
.It Dv PF_INET6 Ta Tn IPv6 (Internet Protocol version 6) protocols
.It Dv PF_NS Ta Xerox Network Systems protocols
.It Dv PF_APPLETALK Ta AppleTalk protocols
.It Dv PF_BLUETOOTH Ta Bluetooth protocols
.It Dv PF_CAN Ta Tn CAN bus protocols
.El
.Pp
The socket has the indicated
.Fa type ,
which specifies the semantics of communication.
Currently defined types are:
.Bl -tag -offset indent -width Dv
.It Dv SOCK_STREAM
Provides sequenced, reliable, two-way connection based byte streams.
An out-of-band data transmission mechanism may be supported.
.It Dv SOCK_DGRAM
Supports datagrams: connectionless, unreliable messages of a
fixed\(emtypically small\(emmaximum length.
.It Dv SOCK_RAW
Provides access to internal network protocols and interfaces.
Available only to the super-user.
Not described here.
.It Dv SOCK_SEQPACKET
Provides 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.
.It Dv SOCK_RDM
Not implemented.
.El
.Pp
The following flags can be or'ed to the socket type to add conditions to
the returned file descriptor:
.Bl -tag -offset indent -width Dv
.It Dv SOCK_CLOEXEC
Set the close on exec property.
.It Dv SOCK_NONBLOCK
Set non-blocking I/O.
.It Dv SOCK_NOSIGPIPE
Return
.Er EPIPE
instead of raising
.Dv SIGPIPE .
.El
.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 .
.Pp
Sockets of type
.Dv SOCK_STREAM
are full-duplex byte streams.
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 an 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 .
The
.Xr setsockopt 2
and
.Xr getsockopt 2
system calls 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 EACCES
Permission to create a socket of the specified type and/or protocol
is denied.
.It Bq Er EAFNOSUPPORT
The address family (domain) is not supported or
the specified domain is not supported by this protocol family.
.It Bq Er EMFILE
The per-process descriptor table is full.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOBUFS
Insufficient buffer space is available.
The socket cannot be created until sufficient resources are freed.
.It Bq Er EPROTONOSUPPORT
The protocol family is not supported or
the specified protocol is not supported within this domain.
.It Bq Er EPROTOTYPE
The socket type is not supported by the protocol.
.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
.Rs
.%T "An Introductory 4.4BSD Interprocess Communication Tutorial"
.%A Stuart Sechrest
.Re
.Pq see Pa /usr/share/doc/reference/ref3/sockets
.Rs
.%T "Advanced 4.4BSD IPC Tutorial"
.%A Samuel J. Leffler
.%A Robert S. Fabry
.%A William N. Joy
.%A Phil Lapsley
.%A Steve Miller
.%A Chris Torek
.Re
.Pq see Pa /usr/share/doc/reference/ref3/sockets-advanced
.Sh HISTORY
The
.Fn socket
function call appeared in
.Bx 4.2 .