SOCKET(2) System Calls Manual SOCKET(2)
socket - create an endpoint for communication
socket(int domain, int type, int protocol);
socket() creates an endpoint for communication and returns a descriptor.
The 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
<sys/socket.h>. The currently understood formats are:
AF_UNIX UNIX internal protocols
AF_INET ARPA Internet protocols
AF_INET6 IPv6 (Internet Protocol version 6) protocols
The socket has the indicated type, which specifies the semantics of
communication. Currently defined types are:
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
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 AF_UNIX. SOCK_RAW sockets provide access to
internal network protocols and interfaces, and are available only to the
Any combination of the following flags may additionally be used in the
SOCK_CLOEXEC Set close-on-exec flag on the new descriptor.
SOCK_NONBLOCK Set non-blocking I/O mode on the new socket.
SOCK_DNS For domains AF_INET or AF_INET6, only allow
connect(2), sendto(2), or sendmsg(2) to the DNS
port (typically 53).
The 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 ``communication domain'' in which communication is to take place; see
protocols(5). A value of 0 for protocol will let the system select an
appropriate protocol for the requested socket type.
Sockets of type SOCK_STREAM are full-duplex byte streams. 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(2) call. Once connected, data may be transferred using read(2)
and write(2) calls or some variant of the send(2) and recv(2) calls.
When a session has been completed a close(2) may be performed. Out-of-
band data may also be transmitted as described in send(2) and received as
described in recv(2).
The communications protocols used to implement a 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 ETIMEDOUT as the
specific code in the global variable errno. The protocols optionally
keep sockets ``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 SIGPIPE signal is raised if a process sends on a
broken stream; this causes naive processes, 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
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to
correspondents named in send(2) calls. Datagrams are generally received
with recvfrom(2), which returns the next datagram with its return
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 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>. setsockopt(2) and
getsockopt(2) are used to set and get options, respectively.
A -1 is returned if an error occurs, otherwise the return value is a
descriptor referencing the socket.
The socket() call fails if:
[EAFNOSUPPORT] The specified address family is not supported on this
[EPROTONOSUPPORT] The protocol type or the specified protocol is not
supported within this domain.
[EPROTOTYPE] The combination of the specified protocol and type is
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[ENOBUFS] Insufficient resources were available in the system to
perform the operation.
[EACCES] Permission to create a socket of the specified type
and/or protocol is denied.
accept(2), bind(2), connect(2), getsockname(2), getsockopt(2), ioctl(2),
listen(2), poll(2), read(2), recv(2), select(2), send(2), setsockopt(2),
shutdown(2), socketpair(2), write(2), getprotoent(3), inet(4), inet6(4),
An Introductory 4.3 BSD Interprocess Communication Tutorial, reprinted in
UNIX Programmer's Supplementary Documents Volume 1.
BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's
Supplementary Documents Volume 1.
The socket() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').
The socket() system call first appeared in 4.1cBSD.
OpenBSD 5.9 October 20, 2015 OpenBSD 5.9
[Unix Hosting |
[Engineering & Automation |
Software Development |