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 communications domain
within which communication will take place; this selects the
protocol family which should be used. These families are
defined in the include file sys/socket.h. The currently
understood formats are
AF_UNIX (UNIX internal protocols)
AF_INET (ARPA Internet protocols)
AF_ISO (ISO protocols)
AF_NS (Xerox Network Systems protocols)
AF_IMPLINK
(IMP "host at IMP" link layer)
The socket has the indicated type, which specifies the
semantics of communication. Currently defined types are:
SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
SOCK_RDM
A SOCK_STREAM type provides sequenced, reliable, two-way
connection based byte streams. An out-of-band data
transmission mechanism may be supported. A SOCK_DGRAM
socket supports datagrams (connectionless, unreliable mes-
sages of a fixed (typically small) maximum length). A
SOCK_SEQPACKET socket may provide a sequenced, reliable,
two-way connection-based data transmission path for
datagrams of fixed maximum length; a consumer may be
required to read an entire packet with each read system
call. This facility is protocol specific, and presently
implemented only for AF_NS. SOCK_RAW sockets provide access
to internal network protocols and interfaces. The types
SOCK_RAW, which is available only to the super-user, and
SOCK_RDM, which is planned, but not yet implemented, are not
described here.
The protocol specifies a particular protocol to be used with
the socket. Normally only a single protocol exists to sup-
port a particular socket type within a given protocol fam-
ily. 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 communication is to take
place; see protocols(5).
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 con-
nection 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 used to implement a SOCK_STREAM
insure 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 broken and calls will
indicate an error with -1 returns and with ETIMEDOUT as the
specific code in the global variable errno. The protocols
optionally keep sockets warm by forcing transmissions
roughly every minute in the absence of other activity. An
error is then indicated if no response can be elicited on an
otherwise idle connection for a extended period (e.g. 5
minutes). A SIGPIPE signal is raised if a process sends 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.
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.
An fcntl(2) call can be used to specify a process group to
receive a SIGURG signal when the out-of-band data arrives.
It may also enable non-blocking I/O and asynchronous notifi-
cation of I/O events via SIGIO.
The operation of sockets is controlled by socket level
options. These options are defined in the file
sys/socket.h. Setsockopt(2) and getsockopt(2) are used to
set and get options, respectively.
RETURN VALUES
A -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 The per-process descriptor table is full.
ENFILE The system file table is full.
EACCES Permission to create a socket of the specified type
and/or protocol is denied.
ENOBUFS Insufficient buffer space is available. The socket
cannot be created until sufficient resources are
freed.
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).
SEE ALSO
accept(2), bind(2), connect(2), getsockname(2), get-
sockopt(2), ioctl(2), read(2), recv(2), select(2), socket-
pair(2), write(2)
"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