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 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 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 (or service name, where supported and understood 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 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.

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; if port is zero, the operating system will allocate a free port to the server socket which may be discovered by using fconfigure to read the -sockname option). 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.

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 fconfigure command to retrieve the -sockname option as described below.

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.

Here is a very simple time server: proc Server {channel clientaddr clientport} { puts "Connection from $clientaddr registered" puts $channel [clock format [clock seconds]] close $channel }

socket -server Server 9900 vwait forever

And here is the corresponding client to talk to the server: set server localhost set sockChan [socket $server 9900] gets $sockChan line close $sockChan puts "The time on $server is $line"

fconfigure(n), flush(n), open(n), read(n)

bind, channel, connection, domain name, host, network address, socket, tcp