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udp(3tcl) Tcl UDP extension udp(3tcl)


udp - Create UDP sockets in Tcl


package require Tcl 8.2

package require udp 1.0.11

udp_open ?port? ?reuse? ?ipv6?

udp_conf sock host port

udp_conf sock ?-myport? ?-remote? ?-peer? ?-broadcast bool? ?-ttl count? ?-mcastadd "groupaddr ?netwif?"? ?-mcastdrop "groupaddr ?netwif?"? ?-mcastgroups? ?-mcastloop bool?

udp_peek sock ?buffersize?


This package provides support for using UDP through Tcl. The package provides a new channel type and attempts to permit the use of packet oriented UDP over stream oriented Tcl channels. The package defined three commands but udp_conf should be considered depreciated in favour of the standard Tcl command fconfigure.


udp_open will open a UDP socket. If a port is specified the UDP socket will be opened on that port. Otherwise the system will choose a port and the user can use the udp_conf command to obtain the port number if required.

The following keywords can be used to specify options on the opened socket.

Using this keyword sets the SO_REUSEADDR socket option which permits multiple sockets to be bound to the same address/port combination.
By default a IPv4 socket is created. When keyword ipv6 is specified an IPv6 socket is opened.
Deprecated in favour of the standard Tcl fconfigure command.

udp_conf in this configuration is used to specify the remote destination for packets written to this sock. You must call this command before writing data to the UDP socket.

Deprecated in favour of the standard Tcl fconfigure command.

In addition to being used to configure the remote host, the udp_conf command is used to obtain information about the UDP socket. NOTE all these options are now available using the standard Tcl fconfigure command.

Returns the local port number of the socket.
Returns the remote hostname and port number as set using udp_conf sock host port.
Returns the remote hostname and port number for the packet most recently received by this socket.
UDP packets can listen and send on the broadcast address. For some systems a flag must be set on the socket to use broadcast. With no argument this option will return the broadcast setting. With a boolean argument the setting can be modified. This option is not permitted when using IPv6.
The time-to-live is given as the number of router hops the packet may do. For multicast packets this is important in specifying the distribution of the packet. The system default for multicast is 1 which restricts the packet to the local subnet. To permit packets to pass routers, you must increase the ttl. A value of 31 should keep it within a site, while 255 is global.
tcludp sockets can support IPv4 and IPv6 multicast operations. To receive multicast packets the application has to notify the operating system that it should join a particular multicast group. For IPv4 these are specified as addresses in the range to

When specifying only the groupaddr the system will determine the network interface to use. Specifying the netwif will join a multicast group on a specific network interface. This is useful on a multihomed system with multiple network interfaces. On windows you must specify the network interface index. For other platforms the network interface (e.g. 'eth0') name can be specified.

To view the current set of multicast groups for a channel use -mcastgroups

With multicast udp the system can choose to receive packets that it has sent or it can drop them. This is known as multicast loopback and can be controlled using this option. By default the value is true and your application will receive its own transmissions.
Examine a packet without removing it from the buffer. Option buffersize specifies the maximum buffer size. Value must be between 0 and 16.

This function is not available on windows.


# Send data to a remote UDP socket
proc udp_puts {host port} {

set s [udp_open]
fconfigure $s -remote [list $host $port]
puts $s "Hello, World"
close $f }
# A simple UDP server
package require udp
proc udpEventHandler {sock} {

set pkt [read $sock]
set peer [fconfigure $sock -peer]
puts "$peer: [string length $pkt] {$pkt}"
return } proc udp_listen {port} {
set srv [udp_open $port]
fconfigure $srv -buffering none -translation binary
fileevent $srv readable [list ::udpEventHandler $srv]
puts "Listening on udp port: [fconfigure $srv -myport]"
return $srv } set sock [udp_listen 53530] vwait forever close $sock
# A multicast demo.
proc udpEvent {chan} {

set data [read $chan]
set peer [fconfigure $chan -peer]
puts "$peer [string length $data] '$data'"
if {[string match "QUIT*" $data]} {
close $chan
set ::forever 1
return } set group set port 7771 set s [udp_open $port] fconfigure $s -buffering none -blocking 0 fconfigure $s -mcastadd $group -remote [list $group $port] fileevent $s readable [list udpEvent $s] puts -nonewline $s "hello, world" set ::forever 0 vwait ::forever exit


Some of the code in this extension is copied from Michael Miller's tcludp package. ( Compared with Michael's UDP extension, this extension provides Windows support and provides the ability of using 'gets/puts' to read/write the socket. In addition, it provides more configuration ability.

Enhancements to support binary data and to setup the package for the Tcl Extension Architecture by Pat Thoyts.

Support for IPv6 and allowing a multicast join on a specific network interface is added by Huub Eikens.




networking, socket, udp


Copyright (c) 1999-2000 Columbia University; all rights reserved
1.0.11 udp