SOCKET(3SOCKET)SOCKET(3SOCKET)NAMEsocket - create an endpoint for communication
cc [ flag ... ] file ... -lsocket-lnsl [ library ... ]
int socket(int domain, int type, int protocol);
The socket() function creates an endpoint for communication and returns
The domain argument specifies the protocol family within which communi‐
cation takes place. The protocol family is generally the same as the
address family for the addresses supplied in later operations on the
socket. These families are defined in <sys/socket.h>.
The currently supported protocol families are:
UNIX system internal protocols
Internet Protocol Version 4 (IPv4)
Internet Protocol Version 6 (IPv6)
Network Cache and Accelerator (NCA) protocols
The socket has the indicated type, which specifies the communication
semantics. Currently defined types are:
The type may be augmented by a bitwise-inclusive-OR of flags from the
following list, defined in <sys/socket.h>.
Creates the socket with the FD_CLOEXEC flag set, causing
the underlying file descriptor to be closed prior to any
future calls to exec(2). This is similar in purpose to the
O_CLOEXEC flag to open(2).
Creates the socket with the O_NDELAY flag set, causing the
socket to provide nonblocking semantics as described for
O_NDELAY in open(2). SOCK_NONBLOCK should normally be used
in preference to SOCK_NDELAY, and takes precedence if both
are set. See open(2) for further details.
Creates the socket with the O_NONBLOCK flag set, causing
the socket to provide nonblocking semantics as described
for O_NONBLOCK in open(2).
There must be an entry in the netconfig(4) file for at least each pro‐
tocol family and type required. If a non-zero protocol has been speci‐
fied but no exact match for the protocol family, type, and protocol is
found, then the first entry containing the specified family and type
with a protocol value of zero will be used.
A SOCK_STREAM type provides sequenced, reliable, two-way connection-
based byte streams. An out-of-band data transmission mechanism may be
supported. A SOCK_DGRAM socket supports datagrams (connectionless,
unreliable messages of a fixed (typically small) maximum length). A
SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way con‐
nection-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 not
implemented for any protocol family. SOCK_RAW sockets provide access to
internal network interfaces. The types SOCK_RAW, which is available
only to a user with the net_rawaccess privilege, and SOCK_RDM, for
which no implementation currently exists, are not described here.
The protocol parameter is a protocol-family-specific value which speci‐
fies a particular protocol to be used with the socket. Normally this
value is zero, as commonly only a single protocol exists to support a
particular socket type within a given protocol family. However, multi‐
ple protocols may exist, in which case a particular protocol may be
specified in this manner.
Sockets of type SOCK_STREAM are full-duplex byte streams, similar to
pipes. A stream socket must be in a connected state before any data may
be sent or received on it. A connection to another socket is created
with a connect(3SOCKET) call. Once connected, data may be transferred
using read(2) and write(2) calls or some variant of the send(3SOCKET)
and recv(3SOCKET) calls. When a session has been completed, a close(2)
may be performed. Out-of-band data may also be transmitted as described
on the send(3SOCKET) manual page and received as described on the
recv(3SOCKET) manual page.
The communications protocols used to implement a SOCK_STREAM insure
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
ETIMEDOUT as the specific code in the global variable errno. The proto‐
cols optionally keep sockets "warm" by forcing transmissions roughly
every minute in the absence of other activity. An error is then indi‐
cated if no response can be elicited on an otherwise idle connection
for a extended period (for instance 5 minutes). A SIGPIPE signal is
raised if a thread sends on a broken stream; this causes naive pro‐
cesses, which do not handle the signal, to exit.
SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM
sockets. The only difference is that read(2) calls will return only
the amount of data requested, and any remaining in the arriving packet
will be discarded.
SOCK_DGRAM and SOCK_RAW sockets allow datagrams to be sent to corre‐
spondents named in sendto(3SOCKET) calls. Datagrams are generally
received with recvfrom(3SOCKET), which returns the next datagram with
its return address.
An fcntl(2) call can be used to specify a process group to receive a
SIGURG signal when the out-of-band data arrives. It can also enable
The operation of sockets is controlled by socket level options. These
options are defined in the file <sys/socket.h>. setsockopt(3SOCKET) and
getsockopt(3SOCKET) are used to set and get options, respectively.
Upon successful completion, a descriptor referencing the socket is
returned. Otherwise, -1 is returned and errno is set to indicate the
The socket() function will fail if:
Permission to create a socket of the specified type
or protocol is denied.
There were insufficient resources available to com‐
plete the operation.
The specified address family is not supported by the
The per-process descriptor table is full.
Insufficient user memory is available.
There were insufficient STREAMS resources available
to complete the operation.
The specified protocol family is not supported.
The protocol type is not supported by the address
The socket type is not supported by the protocol.
One or more of the specified flags is not supported.
See attributes(5) for descriptions of the following attributes:
│ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│MT-Level │ Safe │
SEE ALSOnca(1), close(2), fcntl(2), ioctl(2), read(2), write(2),
accept(3SOCKET), bind(3SOCKET), exec(2), connect(3SOCKET), getsock‐
name(3SOCKET), getsockopt(3SOCKET), in.h(3HEAD),listen(3SOCKET),
recv(3SOCKET), open(2), setsockopt(3SOCKET), send(3SOCKET), shut‐
down(3SOCKET), socket.h(3HEAD), socketpair(3SOCKET), attributes(5)NOTES
Historically, AF_* was commonly used in places where PF_* was meant.
New code should be careful to use PF_* as necessary.
Jan 28, 2009 SOCKET(3SOCKET)