NAME
socket - create an endpoint for communication

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>

s = socket(domain, type, protocol)
int s, domain, type, 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. 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_UNIX (UNIX internal protocols),
PF_INET (ARPA Internet protocols), and
PF_APPLETALK (AppleTalk network system 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
SOCK_RDM

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 PF_NS.
SOCK_RAW sockets provide access to internal 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; see protocols(5).

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 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 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 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 VALUE
A -1 is returned if an error occurs, otherwise the return value is
a descriptor referencing the socket.

ERRORS
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.

[EACCESS] 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.

SEE ALSO
accept(2), bind(2), connect(2), getsockname(2), getsockopt(2),
ioctl(2), listen(2), read(2), recv(2), select(2), send(2),
shutdown(2), socketpair(2), write(2)
"An Introductory 4.3BSD Interprocess Communication Tutorial"
(included in the MachTen Programmer’s Guide).
"An Advanced 4.3BSD Interprocess Communication Tutorial"
(included in the MachTen Programmer’s Guide).