lib/libsys/socket.2

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.Dd September 28, 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),
PF_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
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 .
The
.Fa protocol
argument may be set to zero (0) to request the default
implementation of a socket type for the protocol, if any.
.Sh STREAM SOCKET TYPE
The
.Dv SOCK_STREAM
socket type provides reliable, sequenced, full-duplex octet streams between
the socket and a peer to which the socket is connected.
A socket of type
.Dv SOCK_STREAM
needs to be in a
.Em connected
state before any data can be sent or received.
A connection to another socket is created with a
.Xr connect 2
system call.
(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.)
Once connected, data may be sent using
.Xr send 2 ,
.Xr sendto 2 ,
.Xr sendmsg 2
and
.Xr write 2
system calls.
Data may be received using
.Xr recv 2 ,
.Xr recvfrom 2 ,
.Xr recvmsg 2 ,
and
.Xr read 2
system calls.
Record boundaries are not maintained; data sent on a stream socket using output
operations of one size can be received using input operations of smaller or
larger sizes without loss of data.
Data may be buffered; successful return from an output function does not imply
that the data has been delivered to the peer or even transmitted from the local
system.
For certain protocols out-of-band data may also be transmitted as described in
.Xr send 2
and received as described in
.Xr recv 2 .
.Pp
If data cannot be successfully transmitted within a given time then the
connection is considered broken, and subsequent operations shall fail with
a protocol specific error code.
A
.Dv SIGPIPE
signal is raised if a thread attempts to send data on a broken stream (one that
is no longer connected).
The signal can be suppressed by the
.Dv MSG_NOSIGNAL
flag with distinct
.Xr send 2 ,
.Xr sendto 2 ,
and
.Xr sendmsg 2
system calls or by the
.Dv SO_NOSIGPIPE
socket option set on the socket with
.Xr setsockopt 2 .
.Pp
The
.Dv SOCK_STREAM
socket is supported by the following protocol families:
.Dv PF_INET ,
.Dv PF_INET6 ,
.Dv PF_UNIX ,
.Dv PF_BLUETOOTH ,
.Dv PF_HYPERV ,
and
.Dv PF_INET_SDP .
Out-of-band data transmission mechanism is supported for stream sockets of
.Dv PF_INET
and
.Dv PF_INET6
protocol families.
.Sh DATAGRAM SOCKET TYPE
The
.Dv SOCK_DGRAM
socket type supports connectionless data transfer which is not necessarily
acknowledged or reliable.
Datagrams can be sent to the address specified (possibly multicast or
broadcast) in each output operation, and incoming datagrams can be received
from multiple sources.
The source address of each datagram is available when receiving the datagram
with
.Xr recvfrom 2
or
.Xr recvmsg 2 .
An application can also pre-specify a peer address with
.Xr sendto 2
or
.Xr sendmsg 2 ,
in which case calls to output functions that do not specify a peer address
shall send to the pre-specified peer.
If a peer has been specified, only datagrams from that peer shall be received.
A datagram shall be sent in a single output operation, and needs to be received
in a single input operation.
The maximum size of a datagram is protocol-specific.
Output datagrams may be buffered within the system; thus, a successful return
from an output function does not guarantee that a datagram is actually sent or
received.
.Pp
The
.Dv SOCK_DGRAM
socket is supported by the following protocol families:
.Dv PF_INET ,
.Dv PF_INET6 ,
.Dv PF_UNIX ,
.Dv PF_NETGRAPH ,
and
.Dv PF_NETLINK .
.Sh SEQUENCED PACKET SOCKET TYPE
The
.Dv SOCK_SEQPACKET
socket type is similar to the
.Dv SOCK_STREAM
type, and is also connection-oriented.
The only difference between these types is that record boundaries are
maintained using the
.Dv SOCK_SEQPACKET
type.
A record can be sent using one or more output operations and received using one
or more input operations, but a single operation never transfers parts of more
than one record.
Record boundaries are set by the sender with the
.Dv MSG_EOR
flag of
.Xr send 2
or
.Xr sendmsg 2
functions.
There is no possibility to set a record boundary with
.Xr write 2 .
Record boundaries are visible to the receiver via the
.Dv MSG_EOR
flag in the received message flags returned by the
.Xr recvmsg 2
function.
It is protocol-specific whether a maximum record size is imposed.
.Pp
The
.Dv SOCK_SEQPACKET
socket is supported by the following protocol families:
.Dv PF_INET ,
.Dv PF_INET6 ,
and
.Dv PF_UNIX .
.Pp
.Sh RAW SOCKET TYPE
The
.Dv SOCK_RAW
socket type provides access to internal network protocols and interfaces.
It is a datagram socket in its nature, thus has the same semantics of
read and write operations.
The
.Dv SOCK_RAW
type is available only to the super-user and is described in
.Xr ip 4
and
.Xr ip6 4 .
.Sh NON-BLOCKING MODE
A socket can be created in
.Em non-blocking mode
with the help of
.Dv SOCK_NONBLOCK
flag.
Alternatively, the non-blocking mode on a socket can be turned on and off with
the help of the
.Dv O_NONBLOCK
flag of the
.Xr fcntl 2
system call.
.Pp
When a non-blocking socket has not enough data in its receive buffer to fulfill
the application supplied buffer, then data receiving system calls like
.Xr recv 2 ,
.Xr recvfrom 2 ,
.Xr recvmsg 2
and
.Xr read 2
will not block waiting for the data but immediately return.
Return value will indicate amount of bytes read into the supplied buffer.
The
.Va errno
will be set to
.Dv EAGAIN
.Po
has same value as
.Dv EWOULDBLOCK
.Pc .
.Pp
If application tries to send more data on a non-blocking socket than the socket
send buffer can accomodate with
.Xr send 2 ,
.Xr sendto 2 ,
.Xr sendmsg 2
or
.Xr write 2
system calls partial data will be sent.
Return value will indicate amount of bytes sent.
The
.Va errno
will be set to
.Dv EAGAIN .
Note that sockets of
.Dv SOCK_DGRAM
type are unreliable, thus for these sockets sending operations will never fail
with
.Dv EAGAIN
in non-blocking mode neither will block in blocking mode.
.Sh OTHER OPERATIONS ON SOCKETS
Since socket descriptors are file descriptors, many generic file operations
performed by
.Xr fcntl 2 ,
apply.
Socket descriptors can be used with all event engines, such as
.Xr kevent 2 ,
.Xr select 2
and
.Xr poll 2 .
.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.
.Pp
Connection associated with a socket can be terminated by
.Xr close 2
system call.
One direction of communication can be disabled with
.Xr shutdown 2 .
.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 close 2 ,
.Xr connect 2 ,
.Xr fcntl 2 ,
.Xr getpeername 2 ,
.Xr getsockname 2 ,
.Xr getsockopt 2 ,
.Xr ioctl 2 ,
.Xr kevent 2 ,
.Xr listen 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr sendmsg 2 ,
.Xr sendto 2 ,
.Xr signal 3 ,
.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 .