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SOCKET(2) | System Calls Manual | SOCKET(2) |
NAME¶
socket — create an endpoint for communicationLIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <sys/types.h>#include <sys/socket.h> int
socket(int domain, int type, int protocol);
DESCRIPTION¶
The socket() system call creates an endpoint for communication and returns a descriptor. The domain argument 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:PF_LOCAL Host-internal protocols, formerly called PF_UNIX, PF_UNIX Host-internal protocols, deprecated, use PF_LOCAL, PF_INET Internet version 4 protocols, PF_PUP PUP protocols, like BSP, PF_APPLETALK AppleTalk protocols, PF_ROUTE Internal Routing protocol, PF_LINK Link layer interface, PF_IPX Novell Internet Packet eXchange protocol, PF_RTIP Help Identify RTIP packets, PF_PIP Help Identify PIP packets, PF_ISDN Integrated Services Digital Network, PF_KEY Internal key-management function, PF_INET6 Internet version 6 protocols, PF_NATM Native ATM access, PF_ATM ATM, PF_NETGRAPH Netgraph sockets
SOCK_STREAM Stream socket, SOCK_DGRAM Datagram socket, SOCK_RAW Raw-protocol interface, SOCK_RDM Reliably-delivered packet, SOCK_SEQPACKET Sequenced packet stream
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 messages 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 unimplemented. 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 argument 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. 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).
The protocol argument may be set to zero (0) to request
the default implementation of a socket type for the protocol, if any.
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) system call. Once connected, data may be
transferred using read(2) and write(2)
calls or some variant of the send(2) and
recv(2) functions. (Some protocol families, such as the
Internet family, support the notion of an “implied connect”, which
permits data to be sent piggybacked onto a connect operation by using the
sendto(2) system call.) 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
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 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 an extended period (e.g. 5
minutes). By default, a SIGPIPE
signal is raised if a
process sends on a broken stream, but this behavior may be inhibited via
setsockopt(2).
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) system 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 notification 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>. The
setsockopt(2) and getsockopt(2) system
calls 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¶
The socket() system call fails if:- [
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.
- [
EPERM
] - User has insufficient privileges to carry out the requested operation.
SEE ALSO¶
accept(2), bind(2), connect(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), netgraph(4), protocols(5) An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7. BSD Interprocess Communication Tutorial, PS1, 8.HISTORY¶
The socket() system call appeared in 4.2BSD.January 5, 2009 | Debian |