socket(n) Tcl (7.5) socket(n)
_________________________________________________________________
NAME
socket - Open a TCP network connection
SYNOPSIS
socket ?options? host port
socket-server command ?options? port
_________________________________________________________________
DESCRIPTION
This command opens a network socket and returns a channel
identifier that may be used in future invocations of
commands like read, puts and flush. At present only the TCP
network protocol is supported; future releases may include
support for additional protocols. The socket command may be
used to open either the client or server side of a
connection, depending on whether the -server switch is
specified.
CLIENT SOCKETS
If the -server option is not specified, then the client side
of a connection is opened and the command returns a channel
identifier that can be used for both reading and writing.
Port and host specify a port to connect to; there must be a
server accepting connections on this port. Port is an
integer port number and host is either a domain-style name
such as www.sunlabs.com or a numerical IP address such as
127.0.0.1. Use localhost to refer to the host on which the
command is invoked.
The following options may also be present before host to
specify additional information about the connection:
-myaddr addr
Addr gives the domain-style name or numerical IP
address of the client-side network interface to use for
the connection. This option may be useful if the
client machine has multiple network interfaces. If the
option is omitted then the client-side interface will
be chosen by the system software.
-myport port
Port specifies an integer port number to use for the
client's side of the connection. If this option is
omitted, the client's port number will be chosen at
random by the system software.
-async
Page 1 (printed 2/19/99)
socket(n) Tcl (7.5) socket(n)
The -async option will cause the client socket to be
connected asynchronously. This means that the socket
will be created immediately but may not yet be
connected to the server, when the call to socket
returns. When a gets or flush is done on the socket
before the connection attempt succeeds or fails, if the
socket is in blocking mode, the operation will wait
until the connection is completed or fails. If the
socket is in nonblocking mode and a gets or flush is
done on the socket before the connection attempt
succeeds or fails, the operation returns immediately
and fblocked on the socket returns 1.
SERVER SOCKETS
If the -server option is specified then the new socket will
be a server for the port given by port. Tcl will
automatically accept connections to the given port. For
each connection Tcl will create a new channel that may be
used to communicate with the client. Tcl then invokes
command with three additional arguments: the name of the new
channel, the address, in network address notation, of the
client's host, and the client's port number.
The following additional option may also be specified before
host:
-myaddr addr
Addr gives the domain-style name or numerical IP
address of the server-side network interface to use for
the connection. This option may be useful if the
server machine has multiple network interfaces. If the
option is omitted then the server socket is bound to
the special address INADDR_ANY so that it can accept
connections from any interface.
Server channels cannot be used for input or output; their
sole use is to accept new client connections. The channels
created for each incoming client connection are opened for
input and output. Closing the server channel shuts down the
server so that no new connections will be accepted;
however, existing connections will be unaffected.
Server sockets depend on the Tcl event mechanism to find out
when new connections are opened. If the application doesn't
enter the event loop, for example by invoking the vwait
command or calling the C procedure Tcl_DoOneEvent, then no
connections will be accepted.
CONFIGURATION OPTIONS
The fconfigure command can be used to query several readonly
Page 2 (printed 2/19/99)
socket(n) Tcl (7.5) socket(n)
configuration options for socket channels:
-sockname
This option returns a list of three elements, the
address, the host name and the port number for the
socket. If the host name cannot be computed, the second
element is identical to the address, the first element
of the list.
-peername
This option is not supported by server sockets. For
client and accepted sockets, this option returns a list
of three elements; these are the address, the host name
and the port to which the peer socket is connected or
bound. If the host name cannot be computed, the second
element of the list is identical to the address, its
first element.
SEE ALSO
flush(n), open(n), read(n)
KEYWORDS
bind, channel, connection, domain name, host, network
address, socket, tcp
Page 3 (printed 2/19/99)
