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NAME
socket - create an endpoint for communication
SYNOPSIS
#include <network.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 communications domain
within which communication will take place; this selects
the protocol family which should be used. These families
are defined in <network.h>. The currently understood
formats include:
PF_INET
IPv4 Internet protocols; see ip(4)
The socket has the indicated type, which specifies the
semantics of communication. 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.
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
numbers.
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-
col-specific manner if the other end is still alive.
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.
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
options, respectively.
RETURN VALUES
-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.
EMFILE There are too many open files.
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 avail-
able.
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