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 over IPv4 and IPv6; 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 chan configure to alter this to something 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 understood by the host operating system) and host is either a domain-style name such as www.tcl.tk or a numerical IPv4 or IPv6 address such as 127.0.0.1 or 2001:DB8::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:
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. Synchronous client sockets may be switched (after they have connected) to operating in asynchronous mode using:
chan configure chan -blocking 0
See the chan configure command for more details.
The Tcl event loop should be running while an asynchronous connection is in progress, because it may have to do several connection attempts in the background. Running the event loop also allows you to set up a writable channel event on the socket to get notified when the asynchronous connection has succeeded or failed. See the vwait and the chan commands for more details on the event loop and channel events.
The chan configure option -connecting may be used to check if the connect is still running. To verify a successful connect, the option -error may be checked when -connecting returned 0.
Operation without the event queue requires at the moment calls to chan configure to advance the internal state machine.
If the -server option is specified then the new socket will be a server that listens on the given port (either an integer or a service name, where supported and understood by the host operating system; if port is zero, the operating system will allocate a free port to the server socket which may be discovered by using chan configure to read the -sockname option). If the host supports both, IPv4 and IPv6, the socket will listen on both address families. 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 (properly a command prefix list, see the EXAMPLES below) 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 port:
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 does not enter the event loop, for example by invoking the vwait command or calling the C procedure Tcl_DoOneEvent, then no connections will be accepted.
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 may be retrieved from the created server socket using the chan configure command to retrieve the -sockname option as described below.
The chan configure command can be used to query several readonly configuration options for socket channels:
Note that the error status is reset by the read operation; this mimics the underlying getsockopt(SO_ERROR) call.
For server sockets this option returns a list of a multiple of three elements each group of which have the same meaning as described above. The list contains more than one group when the server socket was created without -myaddr or with the argument to -myaddr being a domain name that resolves multiple IP addresses that are local to the invoking host.
Here is a very simple time server:
proc Server {startTime channel clientaddr clientport} {
puts "Connection from $clientaddr registered"
set now [clock seconds]
puts $channel [clock format $now]
puts $channel "[expr {$now - $startTime}] since start"
close $channel
}
socket -server [list Server [clock seconds]] 9900
vwait forever
And here is the corresponding client to talk to the server and extract some information:
set server localhost set sockChan [socket $server 9900] gets $sockChan line1 gets $sockChan line2 close $sockChan puts "The time on $server is $line1" puts "That is [lindex $line2 0]s since the server started"