vwait - Process events until a variable is written


vwait varName


This command enters the Tcl event loop to process events, blocking the application if no events are ready. It continues processing events until some event handler sets the value of variable varName. Once varName has been set, the vwait command will return as soon as the event handler that modified varName completes. varName must globally scoped (either with a call to global for the varName, or with the full namespace path specification).

In some cases the vwait command may not return immediately after varName is set. This can happen if the event handler that sets varName does not complete immediately. For example, if an event handler sets varName and then itself calls vwait to wait for a different variable, then it may not return for a long time. During this time the top-level vwait is blocked waiting for the event handler to complete, so it cannot return either.


Run the event-loop continually until some event calls exit. (You can use any variable not mentioned elsewhere, but the name forever reminds you at a glance of the intent.)
vwait forever

Wait five seconds for a connection to a server socket, otherwise close the socket and continue running the script:

# Initialise the state
after 5000 set state timeout
set server [socket -server accept 12345]
proc accept {args} {
   global state connectionInfo
   set state accepted
   set connectionInfo $args

# Wait for something to happen
vwait state

# Clean up events that could have happened
close $server
after cancel set state timeout

# Do something based on how the vwait finished...
switch $state {
   timeout {
      puts "no connection on port 12345"
   accepted {
      puts "connection: $connectionInfo"
      puts [lindex $connectionInfo 0] "Hello there!"


global, update


event, variable, wait
Copyright © 1995-1996 Sun Microsystems, Inc.
Copyright © 1995-1997 Roger E. Critchlow Jr.