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Threads in HALion

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HALion provides two threads:

  • Parameter changes and the storage of them are handled by the Controller thread.
  • MIDI event processing and sound reproduction happen in the Processor thread.

You can think of these threads as two sections of code that are exectued in concurrency by HALion. Basically, the two threads are needed to separate longer-lasting function calls from timing-critical function calls. The functions that are called in the Controller thread are executed only as fast as required, while the functions that are called in the Processor thread are executed within an ASIO block.

The information whether a function can be called in the Controller thread, the Processor thread, or in both threads, can be found on the Reference pages below the description of each function.

It looks like this:

Available in: Controller, Processor.

Script Error - Wrong Thread

If you call a function in the wrong thread, the script module will output an error message.

Example 1

--[[
  Example for a script error when calling a function in the wrong thread.
  The onNote callback runs in the Processor thread. openURL runs in the Controller thread.
  Therefore, openURL cannot be called in onNote. The script produces a script error when playing a note.
--]]
 
function onNote(event)     
  openURL("http://www.steinberg.net/en/home.html")
end

The output message for the script error of the above example looks like this:

Script error - Wrong Thread

If this ever happens to you, please review your code and try to place the function call elsewhere in your script. Alternatively, you could use runAsync.

Using runAsync

By calling runAsync in the Processor thread you can invoke a function that is executed in the Controller thread. The execution of runAsync takes at least one audio block, or longer, depending on the function that was called. Please note that when using runAsync, the callback that called runAsync is put on hold until the function has completed.

Example 2

-- Using runAsync to call openURL within onNote.
 
function onNote(event)
  runAsync(function() openURL("http://www.steinberg.net/en/home.html") end)
end

Using runSync

By calling runSync in the Controller thread, you can invoke a function that is executed in the Processor thread. For example, by calling runSync in a parameter change callback, you can invoke an event function like playNote, releaseVoice, controlChange, etc. The callback that called runSync is not stopped and continues its execution. The specified function will be exectued in the next audio block.

-- Fade all voices, triggered by a script parameter.

defineSlotLocal("noteIDs")
noteIDs = {}
  
function onNote(event)
  local id = postEvent(event)
  table.insert(noteIDs, id)
end
  
function syncFadeAllVoices()
  for i, id in ipairs(noteIDs) do
      fade(id, nil, 0, 1000, true)
  end
  noteIDs = {}
end
  
function fadeAllVoices()
  if fadeVoices then
    runSync(syncFadeAllVoices, 1)
  end
end
  
defineParameter("fadeVoices", "Fade All Voices", false, fadeAllVoices)