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socket(n) Tcl Built-In Commands socket(n) 





NAME


socket - 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 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).






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 (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:



  

  
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.

  

  
This 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:






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.








SERVER SOCKETS


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:



  

  
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 wildcard address
      so that it can accept connections from any interface. If addr is a
      domain name that resolves to multiple IP addresses that are available on
      the local machine, the socket will listen on all of them.




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.






CONFIGURATION OPTIONS


The chan configure 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.






Note that the error status is reset by the read operation; this
    mimics the underlying getsockopt(SO_ERROR) call.





  

  
For client sockets (including the channels that get created when a client
      connects to a server socket) 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.






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.





  

  
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.

  

  
This option is not supported by server sockets. For client sockets, this
      option returns 1 if an asyncroneous connect is still in progress, 0
      otherwise.








EXAMPLES


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"








HISTORY


Support for IPv6 was added in Tcl 8.6.






SEE ALSO


chan(n), flush(n), open(n), read(n)






KEYWORDS


asynchronous I/O, bind, channel, connection, domain name, host,
    network address, socket, tcp







  

    8.6
    Tcl