Any known issues with lists in Max?

Miscellaneous Forums/MacOS X Discussion/Any known issues with lists in Max?

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I have a strange bug in some code I'm writing - I'm streaming sound using FMOD, which calls a callback function where the user can write their sample data. (See my callback function below). I had it working, then tried to make some improvements and the sound became broken/choppy - weird thing is I can get rid of the problem by removing a for/next that iterates through a list from the function. The loop causes a problem even if it contains no code or there are no elements in the list. Can anyone shed any light on this?

Function FMOD_Stream_Callback(stream:Byte Ptr, buff:Byte Ptr, length:Int, userdata:Byte Ptr)
	'Print "Hello"
	Print "callback!"
	Print length
	
	Local snd_ptr:Short Ptr=Short Ptr(buff)
	'Print "going to loop"
	'1942
	'2183
	
	DebugLog CountList(l_tone_list)
	DebugLog CountList(r_tone_list)
	
	For n=0 To (length/4)-1
	
		l_sound#=0.0
		r_sound#=0.0
		
		rem
		For l_tone:toneinfo=EachIn l_tone_list 
			l_sound:+l_tone.amp*Sin(l_tone.phase)
			l_tone.phase:+l_tone.dphase
			DebugLog l_tone.phase
		Next
		endrem
		
		
		'Comment this for/next out and the jitters disappear!
		For r_tone:toneinfo=EachIn r_tone_list 
			r_sound:+r_tone.amp*Sin(r_tone.phase)
			r_tone.phase:+r_tone.dphase
		Next								
		'......
		
		l_sound=32767.0*Sin(Float(sample))
		
		r_sound=0.0
		sample:+1																											
		snd_ptr[0]=makesignedshort(l_sound)		'Left speaker
		snd_ptr[1]=makesignedshort(r_sound)		'Right speaker
		snd_ptr:+2

	Next
	
	Return 1
End Function