SOCKET(2) FreeBSD System Calls Manual SOCKET(2)
NAME
socket - create an endpoint for communication
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
int
socket(int domain, int type, int protocol);
DESCRIPTION
The socket() system call creates an endpoint for communication and
returns a descriptor.
The 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
<sys/socket.h>. The currently understood formats are:
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
Each protocol family is connected to an address family, which has the
same name except that the prefix is "AF_" in place of "PF_". Other
protocol families may be also defined, beginning with "PF_", with
corresponding address families.
The socket has the indicated type, which specifies the semantics of
communication. Currently defined types are:
SOCK_STREAM Stream socket,
SOCK_DGRAM Datagram socket,
SOCK_RAW Raw-protocol interface,
SOCK_SEQPACKET Sequenced packet stream
Additionally, the following flags are allowed in the type argument:
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
The 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 "communication domain" in which communication is to take place; see
protocols(5). The protocol argument may be set to zero (0) to request
the default implementation of a socket type for the protocol, if any.
STREAM SOCKET TYPE
The 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 SOCK_STREAM needs to be in a connected state
before any data can be sent or received. A connection to another socket
is created with a connect(2) system call. (Some protocol families, such
as the Internet family, support the notion of an "implied connect", which
permits data to be sent piggybacked onto a connect operation by using the
sendto(2) system call.) Once connected, data may be sent using send(2),
sendto(2), sendmsg(2) and write(2) system calls. Data may be received
using recv(2), recvfrom(2), recvmsg(2), and 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 send(2) and received as described in recv(2).
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 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 MSG_NOSIGNAL flag with
distinct send(2), sendto(2), and sendmsg(2) system calls or by the
SO_NOSIGPIPE socket option set on the socket with setsockopt(2).
The SOCK_STREAM socket is supported by the following protocol families:
PF_INET, PF_INET6, PF_UNIX, PF_BLUETOOTH, PF_HYPERV, and PF_INET_SDP.
Out-of-band data transmission mechanism is supported for stream sockets
of PF_INET and PF_INET6 protocol families.
DATAGRAM SOCKET TYPE
The 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 recvfrom(2) or recvmsg(2). An application can also pre-
specify a peer address with sendto(2) or 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.
The SOCK_DGRAM socket is supported by the following protocol families:
PF_INET, PF_INET6, PF_UNIX, PF_NETGRAPH, and PF_NETLINK.
SEQUENCED PACKET SOCKET TYPE
The SOCK_SEQPACKET socket type is similar to the SOCK_STREAM type, and is
also connection-oriented. The only difference between these types is
that record boundaries are maintained using the 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 MSG_EOR flag of send(2) or sendmsg(2) functions. There is no
possibility to set a record boundary with write(2). Record boundaries
are visible to the receiver via the MSG_EOR flag in the received message
flags returned by the recvmsg(2) function. It is protocol-specific
whether a maximum record size is imposed.
The SOCK_SEQPACKET socket is supported by the following protocol
families: PF_INET, PF_INET6, and PF_UNIX.
RAW SOCKET TYPE
The 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 SOCK_RAW type is available
only to the super-user and is described in ip(4) and ip6(4).
NON-BLOCKING MODE
A socket can be created in non-blocking mode with the help of
SOCK_NONBLOCK flag. Alternatively, the non-blocking mode on a socket can
be turned on and off with the help of the O_NONBLOCK flag of the fcntl(2)
system call.
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 recv(2), recvfrom(2), recvmsg(2) and 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 errno will be set to EAGAIN
(has same value as EWOULDBLOCK).
If application tries to send more data on a non-blocking socket than the
socket send buffer can accomodate with send(2), sendto(2), sendmsg(2) or
write(2) system calls partial data will be sent. Return value will
indicate amount of bytes sent. The errno will be set to EAGAIN. Note
that sockets of SOCK_DGRAM type are unreliable, thus for these sockets
sending operations will never fail with EAGAIN in non-blocking mode
neither will block in blocking mode.
OTHER OPERATIONS ON SOCKETS
Since socket descriptors are file descriptors, many generic file
operations performed by fcntl(2), apply. Socket descriptors can be used
with all event engines, such as kevent(2), select(2) and poll(2).
An fcntl(2) system call can be used to specify a process group to receive
a 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 SIGIO.
The operation of sockets is controlled by socket level options. These
options are defined in the file <sys/socket.h>. The setsockopt(2) and
getsockopt(2) system calls are used to set and get options, respectively.
Connection associated with a socket can be terminated by close(2) system
call. One direction of communication can be disabled with shutdown(2).
RETURN VALUES
A -1 is returned if an error occurs, otherwise the return value is a
descriptor referencing the socket.
ERRORS
The socket() system call fails if:
[EACCES] Permission to create a socket of the specified type
and/or protocol is denied.
[EAFNOSUPPORT] The address family (domain) is not supported or the
specified domain is not supported by this protocol
family.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[ENOBUFS] Insufficient buffer space is available. The socket
cannot be created until sufficient resources are
freed.
[EPERM] User has insufficient privileges to carry out the
requested operation.
[EPROTONOSUPPORT] The protocol type or the specified protocol is not
supported within this domain.
[EPROTOTYPE] The socket type is not supported by the protocol.
SEE ALSO
accept(2), bind(2), close(2), connect(2), fcntl(2), getpeername(2),
getsockname(2), getsockopt(2), ioctl(2), kevent(2), listen(2), poll(2),
read(2), recv(2), select(2), send(2), sendmsg(2), sendto(2), signal(3),
shutdown(2), socketpair(2), write(2), CMSG_DATA(3), getprotoent(3),
divert(4), ip(4), ip6(4), netgraph(4), protocols(5)
"An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.
"BSD Interprocess Communication Tutorial", PS1, 8.
STANDARDS
The socket() function conforms to IEEE Std 1003.1-2008 ("POSIX.1"). The
POSIX standard specifies only the AF_INET, AF_INET6, and AF_UNIX
constants for address families, and requires the use of AF_* constants
for the domain argument of socket(). The SOCK_CLOEXEC and SOCK_CLOFORK
flags are expected to conform to IEEE Std 1003.1-2024 ("POSIX.1"). POSIX
standard. The SOCK_RDM type, the PF_* constants, and other address
families are FreeBSD extensions.
HISTORY
The socket() system call appeared in 4.2BSD.
The SOCK_CLOFORK flag appeared in FreeBSD 15.0.
FreeBSD 15.1-STABLE-HBSD September 28, 2025 SOCKET(2)