SOCKET(2) System Calls SOCKET(2)
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. These families are defined in the include file
<sys/socket.h> The currently understood formats are
PF_LOCAL (Host-internal protocols, formerly called PF_UNIX),
PF_INET (ARPA Internet protocols),
PF_ISO (ISO protocols),
PF_CCITT (ITU-T protocols, like X.25), and
PF_NS (Xerox Network Systems protocols).
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 con-
nection-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 PF_NS. SOCK_RAW sockets provide access to inter-
nal network protocols and interfaces. The types SOCK_RAW, which is
available only to the super-user, and SOCK_RDM, which is planned, but
not yet implemented, are not described here.
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;
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 cre-
ated 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. (Some protocol families, such as the Internet family,
support the notion of an which permits data to be sent piggybacked onto
a connect operation by using the sendto(2) call.) 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 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 pro-
tocols optionally keep sockets 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 pro-
cess 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 corre-
spondents 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 via SIGIO.
The operation of sockets is controlled by socket level options. These
options are defined in the file 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:
The protocol type or the specified protocol is not sup-
ported within this domain.
EMFILE The per-process descriptor table is full.
ENFILE 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 can-
not be created until sufficient resources are freed.
accept(2), bind(2), connect(2), getprotoent(3), getsockname(2), get-
sockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2),
shutdown(2), socketpair(2), write(2)
An Introductory 4.3 BSD Interprocess Communication Tutorial PS1
BSD Interprocess Communication Tutorial PS1 volume 8
The socket function call appeared in 4.2BSD.
GNO 15 February 1995 SOCKET(2)
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