socket - create an endpoint for communication
int 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 which should be used. These families
are defined in <sys/socket.h>. The currently understood
UNIX protocols for local communication
IPv4 Internet protocols; see ip(4)
IPv6 Internet protocols
PF_IPX IPX - Novell protocols
Kernel user interface device
PF_X25 ITU-T X.25 / ISO-8208 protocol
Amateur radio AX.25 protocol
Access to raw ATM PVCs
Appletalk; see ddp(4)
Low level packet interface; see packet(4)
The socket has the indicated type, which specifies the
semantics of communication. Currently defined types are:
Provides sequenced, reliable, two-way connection-
based byte streams. An out-of-band data transmis
Supports datagrams (connectionless, unreliable mes
sages of a fixed maximum length).
Provides a sequenced, reliable, two-way connection-
based data transmission path for datagrams of fixed
maximum length; a consumer is required to read an
entire packet with each read system call.
Provides raw network protocol access.
Provides a reliable datagram layer that does not
Obsolete and should not be used in new programs;
Some socket types may not be implemented by all protocol
families; for example, SOCK_SEQPACKET is currently not
implemented in Linux for AF_INET.
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 proto
col 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 communi
cation is to take place; see protocols(5). See getpro
toent(3) on how to map protocol name strings to protocol
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(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 which 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 When SO_KEEPALIVE
is enabled on the socket the protocol checks in a proto
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 arriv
ing packet will be discarded. Also all message boundaries
in incoming datagrams are preserved.
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 address.
SOCK_PACKET is an obsolete socket type to receive raw
packets directly from the device driver. Use packet(4)
An fcntl(2) call with the the F_SETOWN argument can be
used to specify a process group to receive a SIGURG signal
when the out-of-band data arrives or SIGPIPE signal when a
SOCK_STREAM connection breaks unexpectedly. It may also
be used to set the process or process group that receives
the I/O and asynchronous notification of I/O events via
SIGIO. Using F_SETOWN is equivalent to an ioctl(2) call
with the SIOSETOWN argument.
When the network signals an error condition to the proto
col module (e.g. using a ICMP message for IP) the pending
error flag is set for the socket. The next operation on
this socket will return the error code of the pending
error. For some protocols it is possible to enable a per-
socket error queue to retrieve detailed information about
the error; see IP_RECVERR in ip(4).
The operation of sockets is controlled by socket level
options. These options are defined in <sys/socket.h>.
Setsockopt(2) and getsockopt(2) are used to set and get
-1 is returned if an error occurs; otherwise the return
value is a descriptor referencing the socket.
The protocol type or the specified protocol is not
supported within this domain.
ENFILE Not enough kernel memory to allocate a new socket
EMFILE Process file table overflow.
type and/or protocol is denied.
ENOBUFS or ENOMEM
Insufficient memory is available. The socket can
not be created until sufficient resources are
EINVAL Unknown protocol, or protocol family not avail
Other errors may be generated by the underlying protocol
4.4BSD (the socket function call appeared in 4.2BSD). Gen
erally portable with non-BSD systems supporting clones of
the BSD socket layer (including System V variants).
accept(2), bind(2), connect(2), getprotoent(3), getsock
name(2), getsockopt(2), ioctl(2), listen(2), read(2),
recv(2), select(2), send(2), shutdown(2), socketpair(2),
"An Introductory 4.3 BSD Interprocess Communication Tuto
rial" is reprinted in UNIX Programmer's Supplementary Doc
uments Volume 1.
"BSD Interprocess Communication Tutorial" is reprinted in
UNIX Programmer's Supplementary Documents Volume 1.