socket(2)
NAME
socket - create an endpoint for communication
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
DESCRIPTION
Socket creates an endpoint for communication and returns a
descriptor.
The domain parameter specifies a communication domain;
this selects the protocol family which will be used for
communication. These families are defined in
<sys/socket.h>. The currently understood formats include:
Name Purpose Man page
PF_UNIX,PF_LOCAL Local communication unix(7)
PF_INET IPv4 Internet protocols ip(7)
PF_INET6 IPv6 Internet protocols
PF_IPX IPX - Novell protocols
PF_NETLINK Kernel user interface device netlink(7)
PF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
PF_AX25 Amateur radio AX.25 protocol
PF_ATMPVC Access to raw ATM PVCs
PF_APPLETALK Appletalk ddp(7)
PF_PACKET Low level packet interface packet(7)
The socket has the indicated type, which specifies the
communication semantics. Currently defined types are:
SOCK_STREAM
Provides sequenced, reliable, two-way, connection-
based byte streams. An out-of-band data transmis
sion mechanism may be supported.
SOCK_DGRAM
Supports datagrams (connectionless, unreliable mes
sages of a fixed maximum length).
SOCK_SEQPACKET
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.
SOCK_RAW
Provides raw network protocol access.
SOCK_RDM
Provides a reliable datagram layer that does not
guarantee ordering.
SOCK_PACKET
Obsolete and should not be used in new programs;
see packet(7).
Some socket types may not be implemented by all protocol
families; for example, SOCK_SEQPACKET is not implemented
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
specific to the "communication domain" in which communica
tion is to take place; see protocols(5). See getprotoent(3)
on how to map protocol name strings to protocol
numbers.
Sockets of type SOCK_STREAM are full-duplex byte streams,
similar to pipes. They do not preserve record boundaries.
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 to be dead. When
SO_KEEPALIVE is enabled on the socket the protocol checks
in a protocol-specific manner if the other end is still
alive. A SIGPIPE signal is raised if a process sends or
receives 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 sock
ets. The only difference is that read(2) calls will
return only the amount of data requested, and any remain
ing in the arriving 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(7)
instead.
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(7).
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
options, respectively.
RETURN VALUE
-1 is returned if an error occurs; otherwise the return
value is a descriptor referencing the socket.
ERRORS
EPROTONOSUPPORT
The protocol type or the specified protocol is not
supported within this domain.
ENFILE Not enough kernel memory to allocate a new socket
structure.
EMFILE Process file table overflow.
EACCES Permission to create a socket of the specified type
and/or protocol is denied.
ENOBUFS or ENOMEM
Insufficient memory is available. The socket can
not be created until sufficient resources are
freed.
EINVAL Unknown protocol, or protocol family not available.
Other errors may be generated by the underlying protocol
modules.
CONFORMING TO
4.4BSD (the socket function call appeared in 4.2BSD). Gen
erally portable to/from non-BSD systems supporting clones
of the BSD socket layer (including System V variants).
NOTE
The manifest constants used under BSD 4.* for protocol
families are PF_UNIX, PF_INET, etc., while AF_UNIX etc.
are used for address families. However, already the BSD
man page promises: "The protocol family generally is the
same as the address family", and subsequent standards use
AF_* everywhere.
BUGS
SOCK_UUCP is not implemented yet.
SEE ALSO
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),
write(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.
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