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



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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 invo-cations invocations
       cations of commands like read, puts and flush.  At present only the  TCP  network  protocol  is  sup-ported; supported;
       ported;   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 sys-tem. system.
       tem.  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 proto-cols, protocols,
       cols, especially the older ones).

CLIENT SOCKETS
       If the -server option is not specified, then the client side of a connection is opened and  the  com-mand command
       mand returns a channel identifier that can be used for both reading and writing.  Port and host spec-ify specify
       ify 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 net-work network
              work 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. 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.

SERVER SOCKETS
       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  dis-covered discovered
       covered 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 chan-nel, channel,
       nel, 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:

       -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 net-work network
              work 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 connec-tions. connections.
       tions. 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;  how-ever, however,
       ever, 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.

CONFIGURATION OPTIONS
       The fconfigure command can be used to query several readonly configuration options for  socket  chan-nels: channels:
       nels:

       -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.


EXAMPLES
       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"


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


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



Tcl                                                  8.0                                           socket(n)