socket - create an endpoint for communication
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. The protocol family generally is the same as the address
family for the addresses supplied in later operations on the socket.
These families are defined in the include file <sys/socket.h>. The
currently understood formats are:
PF_INET (DARPA Internet protocols)
PF_RAW (Link-level protocols)
PF_UNIX (4.3BSD UNIX internal protocols)
The formats PF_NS (Xerox Network Systems protocols) and PF_IMPLINK (IMP
link layer) are not implemented.
The socket has the indicated type, which specifies the semantics of
communication. Currently implemented 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).
SOCK_RAW sockets, which are available only to the super-user, provide
access to internal network protocols and interfaces. The types
SOCK_SEQPACKET and SOCK_RDM are currently unimplemented.
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
Sockets of type SOCK_STREAM are full-duplex byte streams, similar to
pipes. 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
calls. Note that for the read and recv-style calls, the number of bytes
actually read may be less than the number requested. 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 insure 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 a 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
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
An fcntl(2) call can be used to specify a process group to receive a
SIGURG signal when the out-of-band data arrives. The FIONBIO i/o control
(see ioctl(2)) or the FNDELAY fcntl (see fcntl(2)) 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:
[EPROTONOSUPPORT] The protocol type or the specified protocol is
not supported within this domain.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[EACCES] Permission to create a socket of the specified
type and/or protocol is denied.
[ENOBUFS] Insufficient buffer space is available. The
socket cannot be created until sufficient
resources are freed.
accept(2), bind(2), connect(2), fcntl(2), getsockname(2), getsockopt(2),
ioctl(2), listen(2), read(2), recv(2), select(2), send(2), socketpair(2),
write(2), inet(7F), raw(7F), unix(7F)
ABI-compliant versions of the above calls can be obtained from
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