socket - create an endpoint for communication
socket(int domain, int type, int protocol);
socket() creates an endpoint for communication and returns a
The domain parameter specifies a communications domain within which communication
will take place; this selects the protocol family
be used. These families are defined in the include file
The currently understood formats are
AF_UNIX (UNIX internal protocols),
AF_INET (ARPA Internet protocols),
AF_INET6 (ARPA IPv6 protocols),
AF_ISO (ISO protocols),
AF_NS (Xerox Network Systems protocols),
AF_IPX (Internetwork Packet Exchange), and
AF_IMPLINK (IMP host at IMP link layer).
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
byte streams. An out-of-band data transmission mechanism
may be supported.
A SOCK_DGRAM socket supports datagrams (connectionless,
messages of a fixed (typically small) maximum length). A
socket may provide a sequenced, reliable, two-way connection-based data
transmission path for datagrams of fixed maximum length; a
be required to read an entire packet with each read system
facility is protocol specific, and presently implemented only for PF_NS.
SOCK_RAW sockets provide access to internal network protocols and interfaces.
The types SOCK_RAW, which is available only to the
SOCK_RDM, which is planned, but not yet implemented, are not
The protocol specifies a particular protocol to be used with
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
protocols(5). A value of 0 for protocol will let the system
appropriate protocol for the requested socket type.
Sockets of type SOCK_STREAM are full-duplex byte streams. A
must be in a connected state before any data may be sent
on it. A connection to another socket is created with a
Once connected, data may be transferred using read(2) and
or some variant of the send(2) and recv(2) calls. When a
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
data is not lost or duplicated. If a piece of data for
which the peer
protocol has buffer space cannot be successfully transmitted
reasonable length of time, then the connection is considered
calls will indicate an error with -1 returns and with
ETIMEDOUT as the
specific code in the global variable errno. The protocols
keep sockets ``warm'' by forcing transmissions roughly every
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,
SOCK_SEQPACKET sockets employ the same system calls as
The only difference is that read(2) calls will return
amount of data requested, and any remaining in the arriving
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams
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
SIGURG signal when the out-of-band data arrives. It may also enable nonblocking
I/O and asynchronous notification of I/O events via
The operation of sockets is controlled by socket level
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:
The protocol type or the specified protocol is
within this domain.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[EACCES] Permission to create a socket of the specified
protocol is denied.
[ENOBUFS] Insufficient buffer space is available. The
be created until sufficient resources are
accept(2), bind(2), connect(2), getsockname(2), getsockopt(2), ioctl(2),
listen(2), poll(2), read(2), recv(2), select(2), send(2),
shutdown(2), socketpair(2), write(2), getprotoent(3), netintro(4)
An Introductory 4.3 BSD Interprocess Communication Tutorial,
UNIX Programmer's Supplementary Documents Volume 1.
BSD Interprocess Communication Tutorial, reprinted in UNIX
Supplementary Documents Volume 1.
The socket() function call appeared in 4.2BSD.
OpenBSD 3.6 June 4, 1993
[ Back ]