.\"	$OpenBSD: socket.2,v 1.43 2019/05/26 09:47:28 krw 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 $Mdocdate: May 26 2019 $
.Dt SOCKET 2
.Os
.Sh NAME
.Nm socket
.Nd create an endpoint for communication
.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
.In sys/socket.h .
The currently understood formats are:
.Pp
.Bl -tag -width "AF_INET6XXX" -offset indent -compact
.It AF_UNIX
UNIX internal protocols
.It AF_INET
Internet Protocol version 4 (IPv4) protocol family
.It AF_INET6
Internet Protocol version 6 (IPv6) protocol family
.El
.Pp
The socket has the indicated
.Fa type ,
which specifies the semantics of communication.
Currently defined types are:
.Pp
.Bl -tag -width "SOCK_SEQPACKETXXX" -offset indent -compact
.It SOCK_STREAM
.It SOCK_DGRAM
.It SOCK_RAW
.It SOCK_SEQPACKET
.El
.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 AF_UNIX .
.Dv SOCK_RAW
sockets provide access to internal network protocols and interfaces,
and are available only to the superuser.
.Pp
Any combination of the following flags may additionally be used in the
.Fa type
argument:
.Pp
.Bl -tag -width "SOCK_NONBLOCKX" -offset indent -compact
.It SOCK_CLOEXEC
Set close-on-exec flag on the new descriptor.
.It SOCK_NONBLOCK
Set non-blocking I/O mode on the new socket.
.It SOCK_DNS
For domains
.Dv AF_INET
or
.Dv AF_INET6 ,
only allow
.Xr connect 2 ,
.Xr sendto 2 ,
or
.Xr sendmsg 2
to the DNS port (typically 53).
.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
.Dq communication domain
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.
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
.In sys/socket.h .
.Xr setsockopt 2
and
.Xr getsockopt 2
are used to set and get options, respectively.
.Sh RETURN VALUES
If successful,
.Fn socket
returns a non-negative integer, the socket file descriptor.
Otherwise, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn socket
call fails if:
.Bl -tag -width Er
.It Bq Er EAFNOSUPPORT
The specified address family is not supported on this machine.
.It Bq Er EPROTONOSUPPORT
The protocol type or the specified protocol is not supported
within this domain.
.It Bq Er EPROTOTYPE
The combination of the specified protocol and type is not supported.
.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 resources were available in the system
to perform the operation.
.It Bq Er EACCES
Permission to create a socket of the specified type and/or protocol
is denied.
.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 inet 4 ,
.Xr inet6 4 ,
.Xr netintro 4 ,
.Xr unix 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 STANDARDS
The
.Fn socket
function conforms to
.St -p1003.1-2008 .
The
.Dv SOCK_CLOEXEC
and
.Dv SOCK_NONBLOCK
flags are expected to conform to a future revision of that standard.
.Pp
The
.Dv SOCK_DNS
flag is an
.Ox
extension.
.Sh HISTORY
The
.Fn socket
system call first appeared in
.Bx 4.1c .
Support for the
.Dv SOCK_CLOEXEC
and
.Dv SOCK_NONBLOCK
flags appeared in
.Ox 5.7 .
Support for the
.Dv SOCK_DNS
flag appeared in
.Ox 5.9 .