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Curiously effective debug technique for finding what VI generated particular value

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Giant VI hierarchy. Runs for some long amount of time. Somewhere in the mess, a value is generated that is causing the program to go awry. You only know about it at the point where the program goes wrong, which appears to be a long way from where the value was generated.
Your challenge: Find the VI that is generating that value and make it generate some other value.

I've had to debug things like this in LabVIEW before... I just tried something that worked surprisingly well. It works if the value to be generated is somewhat unique in your application (i.e. a particular string or a particular double value... doesn't work so well for common values like "1" that may occur a lot in your application).


  1. Run a quick bit of scripting to change all of your VIs to enable "Show front panel when called" and disable "Close afterwards if originally closed". See picture.
  2. Run your top-level VI. Set a breakpoint in the subVI where things are going wrong -- which you know has to be after the value is generated.
  3. (optional) Abort the VI hierarchy. This just cuts down on other stuff happening in the system.
  4. Now you can use ctrl+F and do a text find for the value. Because all the panels were open, the value they last emitted is on their panels. Even if it is nested inside an array of cluster of array of cluster of... etc. Even if it eventually got packaged into a DVR or passed through some other data-passing mechanism, the original VI that generated the value will still have the value on its panel unless that VI has been called again for some other purpose.

This does slow down your whole application -- having the panel open makes LV spend a lot of time updating the controls and indicators, which is a lot more data copies. But at least in my most recent case, this strategy was effective. I figured I'd pass it along. By using the "Show front panel when called" option, you get all the reentrant clone VIs, which you won't get if you just open all of the source VIs in your project.


Edited by Aristos Queue
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