socket(n) Tcl Built-In Commands socket(n)
socket - Open a TCP network connection
socket ?options? host port
socket -server command ?options? port
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.
Note that the default encoding for all sockets is the system encoding, as returned by
encoding system. Most of the time, you will need to use fconfigure to alter this to some-
thing else, such as utf-8 (ideal for communicating with other Tcl processes) or iso8859-1
(useful for many network protocols, especially the older ones).
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 (or service name, where supported and under-
stood by the host operating system) 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:
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.
Port specifies an integer port number (or service name, where supported and under-
stood by the host operating system) 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
-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 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 block-
ing 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.
If the -server option is specified then the new socket will be a server for the port given
by port (either an integer or a service name, where supported and understood by the host
operating system). 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:
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
If port is specified as zero, the operating system will allocate an unused port for use as
a server socket. The port number actually allocated my be retrieved from the created
server socket using the fconfigure command to retrieve the -sockname option as described
The fconfigure command can be used to query several readonly configuration options for
-error This option gets the current error status of the given socket. This is useful when
you need to determine if an asynchronous connect operation succeeded. If there was
an error, the error message is returned. If there was no error, an empty string is
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.
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 can-
not be computed, the second element of the list is identical to the address, its
fconfigure(n), flush(n), open(n), read(n)
bind, channel, connection, domain name, host, network address, socket, tcp
Tcl 8.0 socket(n)