create an endpoint for
domain, int type,
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
currently understood formats are:
- UNIX internal protocols
- Internet Protocol version 4 (IPv4) protocol family
- Internet Protocol version 6 (IPv6) protocol family
The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:
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
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
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:
- Set close-on-exec flag on the new descriptor.
- Set non-blocking I/O mode on the new socket.
- For domains
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
are full-duplex byte streams. A stream socket must be in a
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
calls or some variant of the send(2) and recv(2) calls. When a session has been completed, a
be performed. Out-of-band data may also be transmitted as described in
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_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
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
The operation of sockets is controlled by socket
These options are defined in the file
getsockopt(2) are used to set and get options, respectively.
socket() returns a
non-negative integer, the socket file descriptor. Otherwise, a value of -1
is returned and errno is set to indicate the
socket() call fails if:
- The specified address family is not supported on this machine.
- The protocol type or the specified protocol is not supported within this domain.
- The combination of the specified protocol and type is not supported.
- The per-process descriptor table is full.
- The system file table is full.
- Insufficient resources were available in the system to perform the operation.
- Permission to create a socket of the specified type and/or protocol is denied.
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.
socket() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”). The
SOCK_NONBLOCK flags are expected to conform to a
future revision of that standard.
SOCK_DNS flag is an
socket() system call first appeared in
4.1cBSD. Support for the
SOCK_NONBLOCK flags appeared in
OpenBSD 5.7. Support for the
SOCK_DNS flag appeared in OpenBSD