Originální popis anglicky:
socket - create an endpoint for communication
Návod, kniha: Linux Programmer's Manual
#include <sys/types.h>
#include <sys/socket.h>
int socket(int domain, int type, int
protocol);
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 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
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.
-1 is returned if an error occurs; otherwise the return value is a descriptor
referencing the socket.
- 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.
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).
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.
SOCK_UUCP is not implemented yet.
accept(2),
bind(2),
connect(2),
fcntl(2),
getpeername(2),
getsockname(2),
getsockopt(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 Programmer's Supplementary Documents Volume 1.
“BSD Interprocess Communication Tutorial” is reprinted in
UNIX
Programmer's Supplementary Documents Volume 1.
