NAME
socket —
create an endpoint for
communication
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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:
PF_LOCAL local (previously UNIX) domain protocols
PF_INET ARPA Internet protocols
PF_INET6 IPv6 (Internet Protocol version 6) protocols
PF_NS Xerox Network Systems protocols
PF_APPLETALK AppleTalk protocols
PF_BLUETOOTH Bluetooth protocols
PF_CAN CAN bus protocols
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
The following flags can be or'ed to the type to condition the returned file
descriptor: The following flags are valid:
SOCK_CLOEXEC Set the close on
exec property. |
SOCK_NONBLOCK Sets
non-blocking I/O. |
SOCK_NOSIGPIPE Return
EPIPE instead of raising
SIGPIPE . |
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 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 implemented only for
PF_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 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).
Sockets of type
SOCK_STREAM
are full-duplex byte
streams. 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
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). 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 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() call fails if:
-
-
- [
EACCES
]
- Permission to create a socket of the specified type and/or
protocol is denied.
-
-
- [
EAFNOSUPPORT
]
- The address family (domain) is not supported or the
specified domain is not supported by this protocol family.
-
-
- [
EMFILE
]
- The per-process descriptor table is full.
-
-
- [
ENFILE
]
- The system file table is full.
-
-
- [
ENOBUFS
]
- Insufficient buffer space is available. The socket cannot
be created until sufficient resources are freed.
-
-
- [
EPROTONOSUPPORT
]
- The protocol family is not supported or the specified
protocol is not supported within this domain.
-
-
- [
EPROTOTYPE
]
- The socket type is not supported by the protocol.
SEE ALSO
accept(2),
bind(2),
connect(2),
getsockname(2),
getsockopt(2),
ioctl(2),
listen(2),
poll(2),
read(2),
recv(2),
select(2),
send(2),
setsockopt(2),
shutdown(2),
socketpair(2),
write(2),
getprotoent(3)
Stuart Sechrest, An
Introductory 4.4BSD Interprocess Communication Tutorial. (see
/usr/share/doc/psd/20.ipctut)
Samuel J. Leffler,
Robert S. Fabry, William N.
Joy, Phil Lapsley, Steve
Miller, and Chris Torek,
Advanced 4.4BSD IPC Tutorial. (see
/usr/share/doc/psd/21.ipc)
HISTORY
The
socket() function call appeared in
4.2BSD.