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 ⟨sys/socket.h⟩. The currently understood formats are:
- UNIX internal protocols
- ARPA Internet protocols
- IPv6 (Internet Protocol version 6) protocols
- IMP host at IMP link layer
- Bluetooth protocols
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
sockets provide access to internal network protocols and interfaces, and are
available only to the superuser.
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
write(2) calls or some variant of the
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
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
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
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.
socket() call fails if:
- The protocol type or the specified protocol is not supported within this domain.
- The per-process descriptor table is full.
- The system file table is full.
- Permission to create a socket of the specified type and/or protocol is denied.
- Insufficient buffer space is available. The socket cannot be created until sufficient resources are freed.
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 call appeared in