socket(n) Tcl Built-In Commands socket(n)_________________________________________________________________NAMEsocket - Open a TCP network connection
SYNOPSISsocket ?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 com-
mands like read, puts and flush. At present only the TCP
network protocol is supported; future releases may
include support for additional protocols. The socket com-
mand 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 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
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socket(n) Tcl Built-In Commands socket(n)
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 read-
only 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
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socket(n) Tcl Built-In Commands socket(n)
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 com-
puted, the second element of the list is identical
to the address, its first element.
SEE ALSOflush(n), open(n), read(n)KEYWORDS
bind, channel, connection, domain name, host, network
address, socket, tcp
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