Tcl Built-In Commands socket(1T)
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 com-
mands 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 connec-
tion, 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
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Tcl Built-In Commands socket(1T)
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 con-
nected 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.
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 chan-
nel 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
host:
-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 INADRANY 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;
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Tcl Built-In Commands socket(1T)
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 doesn't
enter the event loop, for example by invoking the vwait com-
mand or calling the C procedure TclDoOneEvent, then no con-
nections 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.
-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
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Tcl Built-In Commands socket(1T)
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(1T), flush(1T), open(1T), read(1T)
KEYWORDS
bind, channel, connection, domain name, host, network
address, socket, tcp
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTcl
Interface Stability Uncommitted
NOTES
Source for Tcl is available on http:/opensolaris.org.
Tcl Last change: 8.0 4
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