socket(3tcl)
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; 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 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 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 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 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 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 discovered 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 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 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 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
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 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
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(3tcl), flush(3tcl), open(3tcl), read(3tcl)
KEYWORDS
bind, channel, connection, domain name, host, network address, socket, tcp
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