socket() - Unix, Linux System Call
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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 |
| Local communication
| |
| IPv4 Internet protocols | |
| IPv6 Internet protocols | |
| IPX - Novell protocols | |
| Kernel user interface device
| |
| ITU-T X.25 / ISO-8208 protocol | |
| Amateur radio AX.25 protocol
| |
| Access to raw ATM PVCs | |
| Appletalk | |
| Low level packet interface
| |
The socket has the indicated
type, which specifies the communication semantics. Currently defined types
are:
Tag | Description |
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 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 data 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
sendto(2)
calls.
Datagrams are generally received with
recvfrom(2),
which returns the next datagram along with the address of its sender.
SOCK_PACKET is an obsolete socket type to receive raw packets directly from the
device driver. Use
packet(7)
instead.
An
fcntl(2)
F_SETOWN operation can be used to specify a process or process group to receive a
SIGURG signal when the out-of-band data arrives or
SIGPIPE signal when a
SOCK_STREAM connection breaks unexpectedly.
This operation 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 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>. The functions
setsockopt(2)
and
getsockopt(2)
are used to set and get options, respectively.
RETURN VALUE
On success, a file descriptor for the new socket is returned.
On error, -1 is returned, and
errno is set appropriately.
ERRORS
Tag | Description |
EACCES |
Permission to create a socket of the specified type and/or protocol
is denied.
|
EAFNOSUPPORT | |
The implementation does not support the specified address family.
|
EINVAL |
Unknown protocol, or protocol family not available.
|
EMFILE |
Process file table overflow.
|
ENFILE |
The system limit on the total number of open files has been reached.
|
ENOBUFS or ENOMEM | |
Insufficient memory is available. The socket cannot be
created until sufficient resources are freed.
|
EPROTONOSUPPORT | |
The protocol type or the specified protocol is not
supported within this domain.
|
Other errors may be generated by the underlying protocol modules.
CONFORMING TO
4.4BSD, POSIX.1-2001.
socket() 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 4.x BSD 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
An Introductory 4.3BSD Interprocess Communication Tutorial\(rq
is reprinted in
UNIX Programmers Supplementary Documents Volume 1.
\(lqBSD Interprocess Communication Tutorial\(rq
is reprinted in
UNIX Programmers Supplementary Documents Volume 1.
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