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SOCKET(2) Linux Programmer’s Manual 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 transmission mechanism may be supported.
SOCK_DGRAM
Supports datagrams (connectionless, unreliable messages 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
protocol family, in which a case protocol can be specified as 0. 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 communication 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 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. 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 sig-
nal 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
FIOSETOWN or SIOCSPGRP argument.
When the network signals an error condition to the protocol module (e.g. using a
ICMP message for IP) the pending error flag is set for the socket. The next opera-
tion on this socket will return the error code of the pending error. For some pro-
tocols 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>. The functions 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 ref-
erencing the socket.
ERRORS
EPROTONOSUPPORT
The protocol type or the specified protocol is not supported within this
domain.
EAFNOSUPPORT
The implementation does not support the specified address family.
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 cannot be created until suffi-
cient 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, SUSv2, POSIX 1003.1-2001. The socket function call appeared in 4.2BSD. It
is generally 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), fcntl(2), getpeername(2), getsockname(2), getsock-
opt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2), shutdown(2),
socketpair(2), write(2), getprotoent(3), ip(7), socket(7), tcp(7), udp(7), unix(7)
“An Introductory 4.3 BSD Interprocess Communication Tutorial” is reprinted in UNIX
Programmers Supplementary Documents Volume 1.
“BSD Interprocess Communication Tutorial” is reprinted in UNIX Programmers Supple-
mentary Documents Volume 1.
Linux Man Page 1999-04-24 SOCKET(2)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
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