lib/libsys/socket.2

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.Dd May 17, 2025
.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
The
.Fn socket
system call
creates an endpoint for communication and returns a descriptor.
.Pp
The
.Fa domain
argument 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
.Bd -literal -offset indent -compact
PF_LOCAL	Host-internal protocols (alias for PF_UNIX),
PF_UNIX		Host-internal protocols,
PF_INET		Internet version 4 protocols,
PF_INET6	Internet version 6 protocols,
PF_DIVERT	Firewall packet diversion/re-injection,
PF_ROUTE	Internal routing protocol,
PF_KEY		Internal key-management function,
PF_NETGRAPH	Netgraph sockets,
PF_NETLINK	Netlink protocols,
PF_BLUETOOTH	Bluetooth protocols,
PF_INET_SDP	OFED socket direct protocol (IPv4),
AF_HYPERV	HyperV sockets
.Ed
.Pp
Each protocol family is connected to an address family, which has the
same name except that the prefix is
.Dq Dv AF_
in place of
.Dq Dv PF_ .
Other protocol families may be also defined, beginning with
.Dq Dv PF_ ,
with corresponding address families.
.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	Stream socket,
SOCK_DGRAM	Datagram socket,
SOCK_RAW	Raw-protocol interface,
SOCK_SEQPACKET	Sequenced packet stream
.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 may have protocol-specific properties.
.Dv SOCK_RAW
sockets provide access to internal network protocols and interfaces.
The
.Dv SOCK_RAW
type is available only to the super-user and is described in
.Xr ip 4
and
.Xr ip6 4 .
.Pp
Additionally, the following flags are allowed in the
.Fa type
argument:
.Pp
.Bd -literal -offset indent -compact
SOCK_CLOEXEC	Set close-on-exec on the new descriptor,
SOCK_CLOFORK	Set close-on-fork on the new descriptor,
SOCK_NONBLOCK	Set non-blocking mode on the new socket
.Ed
.Pp
The
.Fa protocol
argument
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 .
.Pp
The
.Fa protocol
argument may be set to zero (0) to request the default
implementation of a socket type for the protocol, if any.
.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
system 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
functions.
(Some protocol families, such as the Internet family,
support the notion of an
.Dq implied connect ,
which permits data to be sent piggybacked onto a connect operation by
using the
.Xr sendto 2
system call.)
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).
By default, a
.Dv SIGPIPE
signal is raised if a process sends
on a broken stream, but this behavior may be inhibited via
.Xr setsockopt 2 .
.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
system 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 .
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
system 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 EPERM
User has insufficient privileges to carry out the requested operation.
.It Bq Er EPROTONOSUPPORT
The protocol type 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 getpeername 2 ,
.Xr getsockname 2 ,
.Xr getsockopt 2 ,
.Xr ioctl 2 ,
.Xr listen 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr shutdown 2 ,
.Xr socketpair 2 ,
.Xr write 2 ,
.Xr CMSG_DATA 3 ,
.Xr getprotoent 3 ,
.Xr divert 4 ,
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr netgraph 4 ,
.Xr protocols 5
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 7
.Re
.Rs
.%T "BSD Interprocess Communication Tutorial"
.%B PS1
.%N 8
.Re
.Sh STANDARDS
The
.Fn socket
function conforms to
.St -p1003.1-2008 .
The
.Tn POSIX
standard specifies only the
.Dv AF_INET ,
.Dv AF_INET6 ,
and
.Dv AF_UNIX
constants for address families, and requires the use of
.Dv AF_*
constants for the
.Fa domain
argument of
.Fn socket .
The
.Dv SOCK_CLOEXEC
and
.Dv SOCK_CLOFORK
flags are expected to conform to
.St -p1003.1-2024 .
.Tn POSIX
standard.
The
.Dv SOCK_RDM
.Fa type ,
the
.Dv PF_*
constants, and other address families are
.Fx
extensions.
.Sh HISTORY
The
.Fn socket
system call appeared in
.Bx 4.2 .
.Pp
The
.Dv SOCK_CLOFORK
flag appeared in
.Fx 15.0 .