Provided by: tcl8.6-doc_8.6.1-4ubuntu1_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.
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.
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:
-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.
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(3tcl), flush(3tcl), open(3tcl), read(3tcl)
KEYWORDS
asynchronous I/O, bind, channel, connection, domain name, host, network address, socket,
tcp