Provided by: tcl9.0-doc_9.0.1+dfsg-1_all 
NAME
socket - Open a TCP network connection
SYNOPSIS
socket ?options? host port
socket -server command ?options? port
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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:
-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 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:
-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 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.
-reuseaddr boolean
Tells the kernel whether to reuse the local address if there is no socket actively listening on
it. This is the default on Windows.
-reuseport boolean
Tells the kernel whether to allow the binding of multiple sockets to the same address and 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.
CONFIGURATION OPTIONS
The chan configure command can be used to query several readonly configuration options for socket
channels or in some cases to set alternative properties on 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.
Note that the error status is reset by the read operation; this mimics the underlying
getsockopt(SO_ERROR) call.
-sockname
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.
-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.
-connecting
This option is not supported by server sockets. For client sockets, this option returns 1 if an
asynchronous connect is still in progress, 0 otherwise.
-keepalive
This option sets or queries the TCP keepalive option on the socket as 1 if keepalive is turned on,
0 otherwise.
-nodelay
This option sets or queries the TCP nodelay option on the socket as 1 if nodelay is turned on, 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"
SEE ALSO
chan(3tcl), flush(3tcl), open(3tcl), read(3tcl)
KEYWORDS
asynchronous I/O, bind, channel, connection, domain name, host, network address, socket, tcp
Tcl 8.6 socket(3tcl)