SOCKET(2) | System Calls Manual | SOCKET(2) |
socket
— create an
endpoint for communication
#include
<sys/socket.h>
int
socket
(int
domain, int type,
int protocol);
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:
The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:
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
AF_UNIX
. SOCK_RAW
sockets
provide access to internal network protocols and interfaces, and are
available only to the superuser.
Any combination of the following flags may additionally be used in the type argument:
AF_INET
or
AF_INET6
, only allow
connect(2),
sendto(2), or
sendmsg(2) to the DNS port
(typically 53).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). A value of 0 for protocol will let the system select an appropriate protocol for the requested socket type.
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>
.
setsockopt(2) and
getsockopt(2) are used to
set and get options, respectively.
A -1 is returned if an error occurs, otherwise the return value is a descriptor referencing the socket.
The socket
() call fails if:
EAFNOSUPPORT
]EPROTONOSUPPORT
]EPROTOTYPE
]EMFILE
]ENFILE
]ENOBUFS
]EACCES
]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), inet(4), inet6(4), netintro(4), unix(4)
An Introductory 4.3 BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's Supplementary Documents Volume 1.
BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's Supplementary Documents Volume 1.
The socket
() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”). The
SOCK_CLOEXEC
and
SOCK_NONBLOCK
flags are expected to conform to a
future revision of that standard.
The SOCK_DNS
flag is an
OpenBSD extension.
The socket
() system call first appeared in
4.1cBSD. Support for the
SOCK_CLOEXEC
and
SOCK_NONBLOCK
flags appeared in
OpenBSD 5.7. Support for the
SOCK_DNS
flag appeared in OpenBSD
5.9.
March 19, 2016 | OpenBSD-6.4 |