NAME
socket
—
create an endpoint for
communication
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:
- AF_UNIX
- UNIX internal protocols
- AF_INET
- ARPA Internet protocols
- AF_INET6
- IPv6 (Internet Protocol version 6) 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
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:
- SOCK_CLOEXEC
- Set close-on-exec flag on the new descriptor.
- SOCK_NONBLOCK
- Set non-blocking I/O mode on the new socket.
- SOCK_DNS
- For domains
AF_INET
orAF_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.
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:
- [
EAFNOSUPPORT
] - The specified address family is not supported on this machine.
- [
EPROTONOSUPPORT
] - The protocol type or the specified protocol is not supported within this domain.
- [
EPROTOTYPE
] - The combination of the specified protocol and type is not supported.
- [
EMFILE
] - The per-process descriptor table is full.
- [
ENFILE
] - The system file table is full.
- [
ENOBUFS
] - Insufficient resources were available in the system to perform the operation.
- [
EACCES
] - Permission to create a socket of the specified type and/or protocol is denied.
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), 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.
STANDARDS
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.
HISTORY
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.