SOCKET(2) Linux Programmer's Manual SOCKET(2)
NAME
socket - create an endpoint for communication
SYNOPSIS
#include (lt)sys/types.h(gt)
#include (lt)sys/socket.h(gt)
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 cur-
rently 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 trans-
mission mechanism may be supported. A SOCK_DGRAM socket
supports datagrams (connectionless, unreliable messages of
a fixed (typically small) maximum length). A SOCK_SEQ-
PACKET 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 facil-
ity 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
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described here.
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).
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 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 reason-
able 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 connec-
tion 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 sig-
nal, 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 noti-
fication of I/O events via SIGIO.
The operation of sockets is controlled by socket level
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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), getprotoent(3), getsock-
name(2), getsockopt(2), ioctl(2), listen(2), read(2),
recv(2), select(2), send(2), shutdown(2), socketpair(2),
write(2)
"An Introductory 4.3 BSD Interprocess Communication Tuto-
rial" is reprinted in UNIX Programmer's Supplementary Doc-
uments Volume 1
"BSD Interprocess Communication Tutorial" is reprinted in
UNIX Programmer's Supplementary Documents Volume 1
BSD Man Page 24 July 1993 3
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