socket(n) Tcl Built-In Commands socket(n)______________________________________________________________________________NAMEsocket - Open a TCP network connection
SYNOPSISsocket ?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 con‐
nection 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 addi‐
tional 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 asyn‐
chronously. This means that the socket will be created immedi‐
ately 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 con‐
nection is in progress, because it may have to do several con‐
nection 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 incom‐
ing 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 proce‐
dure 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 allo‐
cated 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 con‐
figuration 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 num‐
ber 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 ALSOchan(n), flush(n), open(n), read(n)KEYWORDS
asynchronous I/O, bind, channel, connection, domain name, host, network
address, socket, tcp
Tcl 8.6 socket(n)
