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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022

vwait (1t)


vwait - Process events until a variable is written


vwait varName


vwait(1t)                    Tcl Built-In Commands                   vwait(1t)


       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 the global variable varName.
       Once varName has been set, the vwait command will return as soon as the
       event handler that modified varName completes.  The varName argument is
       always interpreted as a variable name with respect to the global names-
       pace, but can refer to any namespace's variables if the fully-qualified
       name is given.

       In some cases the vwait command may not return immediately  after  var-
       Name  is set.  This happens if the event handler that sets varName does
       not complete immediately.  For example, if an event handler  sets  var-
       Name 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 can-
       not return either. (See the NESTED VWAITS BY EXAMPLE below.)

       To be clear, multiple vwait calls will nest and will not happen in par-
       allel.  The outermost call to vwait will not return until all the inner
       ones do.  It is recommended that code should never nest vwait calls (by
       avoiding  putting  them in event callbacks) but when that is not possi-
       ble, care should be taken to add interlock variables  to  the  code  to
       prevent  all  reentrant calls to vwait that are not strictly necessary.
       Be aware that the synchronous modes of operation of some  Tcl  packages
       (e.g., http)  use vwait internally; if using the event loop, it is best
       to use the asynchronous callback-based  modes  of  operation  of  those
       packages where available.

       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!"

       A command that will wait for some time delay by waiting for a namespace
       variable to be set.  Includes an interlock to prevent nested waits.

              namespace eval example {
                  variable v done
                  proc wait {delay} {
                      variable v
                      if {$v ne "waiting"} {
                          set v waiting
                          after $delay [namespace code {set v done}]
                          vwait [namespace which -variable v]
                      return $v

       When  running  inside  a coroutine, an alternative to using vwait is to
       yield to an outer event loop and to get recommenced when  the  variable
       is set, or at an idle moment after that.

              coroutine task apply {{} {
                  # simulate [after 1000]
                  after 1000 [info coroutine]

                  # schedule the setting of a global variable, as normal
                  after 2000 {set var 1}

                  # simulate [vwait var]
                  proc updatedVar {task args} {
                      after idle $task
                      trace remove variable ::var write "updatedVar $task"
                  trace add variable ::var write "updatedVar [info coroutine]"

       This  example  demonstrates  what  can happen when the vwait command is
       nested. The script will never finish because  the  waiting  for  the  a
       variable  never  finishes;  that  vwait  command is still waiting for a
       script scheduled with after to complete, which just happens to be  run-
       ning  an inner vwait (for b) even though the event that the outer vwait
       was waiting for (the setting of a) has occurred.

              after 500 {
                  puts "waiting for b"
                  vwait b
                  puts "b was set"
              after 1000 {
                  puts "setting a"
                  set a 10
              puts "waiting for a"
              vwait a
              puts "a was set"
              puts "setting b"
              set b 42

       If you run the above code, you get this output:

              waiting for a
              waiting for b
              setting a

       The script will never print "a was set" until after it has  printed  "b
       was  set"  because of the nesting of vwait commands, and yet b will not
       be set until after the outer vwait returns, so  the  script  has  dead-
       locked.   The only ways to avoid this are to either structure the over-
       all program in continuation-passing style or to use coroutine  to  make
       the continuations implicit. The first of these options would be written

              after 500 {
                  puts "waiting for b"
                  trace add variable b write {apply {args {
                      global a b
                      trace remove variable ::b write \
                              [lrange [info level 0] 0 1]
                      puts "b was set"
                      set ::done ok
              after 1000 {
                  puts "setting a"
                  set a 10
              puts "waiting for a"
              trace add variable a write {apply {args {
                  global a b
                  trace remove variable a write [lrange [info level 0] 0 1]
                  puts "a was set"
                  puts "setting b"
                  set b 42
              vwait done

       The second option, with coroutine and some helper procedures,  is  done
       like this:

              # A coroutine-based wait-for-variable command
              proc waitvar globalVar {
                  trace add variable ::$globalVar write \
                          [list apply {{v c args} {
                      trace remove variable $v write \
                              [lrange [info level 0] 0 3]
                      after 0 $c
                  }} ::$globalVar [info coroutine]]
              # A coroutine-based wait-for-some-time command
              proc waittime ms {
                  after $ms [info coroutine]

              coroutine task-1 eval {
                  puts "waiting for a"
                  waitvar a
                  puts "a was set"
                  puts "setting b"
                  set b 42
              coroutine task-2 eval {
                  waittime 500
                  puts "waiting for b"
                  waitvar b
                  puts "b was set"
                  set done ok
              coroutine task-3 eval {
                  waittime 1000
                  puts "setting a"
                  set a 10
              vwait done

       See attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       global(n), update(n)

       asynchronous I/O, event, variable, wait

       Source  code  for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   8.0                            vwait(1t)