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 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:



  

  
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 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.

  

  
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.
      Synchronous client sockets may be switched (after they have connected) to
      operating in asynchronous mode using:








fconfigure chan -blocking 0




See the fconfigure command for more details.










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
    port:



  

  
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 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
    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:



  

  
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.

  

  
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 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







  

    8.0
    Tcl