socket (n)
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
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
configuration options for socket channels:
-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 returned.
-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
KEYWORDS
bind, channel, connection, domain name, host, network address, socket, tcp
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