Darren's Weekly Nugget 06/07/2010

[Darren]

One of the primary reasons we use error clusters in our diagrams is to enforce data flow.  But another (more obvious) reason is that we want to propogate error information through our code.  If an error occurs upstream, we usually do not want downstream functions to execute.  The vast majority of LabVIEW VIs and functions will not execute if they receive an incoming error.  But there are many that still execute, even if they receive an incoming error.  Here is a list of several that come to mind:

• Close Reference
• Close File
• TCP/Bluetooth/IrDA Close Connection
• VISA Close
• TDMS Close
• Release Queue/Notifier
• Close Config Data.vi
• Close Zip File.vi
• Release/Acquire Semaphore.vi

Do you see a pattern?  🙂  Most of these are functions that Close or Release something...it makes sense that, even if an error occurs, you still want to close references and/or release memory assigned to something.  But what about that last pair?  It would seem that even Acquire Semaphore.vi will execute with an incoming error.  So what is the easiest way to determine if a VI or function will still execute, even with an incoming error?  By reading the LabVIEW Help for the function:

Every LabVIEW VI and function that executes even with an incoming error will have this sentence in the error in parameter description in its online help.  And if you find a VI or function that does not behave as described, then we either need to fix the help or fix the function, so make sure to let us know.

[smercurio_fc]

---

Darren wrote:

By reading the LabVIEW Help for the function:

---

Yeah. Good luck with getting people to do that.

[GregSands]

IMAQ Dispose is a VI that presumably should execute despite an incoming error, but doesn't.

Doing a quick search in the LabVIEW help for "runs normally even if" shows a reasonable number of other VIs that also execute with incoming errors, but they're not ones that close or release anything.  They are mostly from the Storage palette (e.g. Create Query, Merge Queries, ID to Refnum ...), the Digital Waveform palette (e.g. Digital Comparison, Analog to Digital, Append Digital Samples ...) or the Waveform Generation palette (e.g. Poisson Noise Waveform, Bernoulli Noise Waveform ...).  Is there any criteria for determining which VIs should behave like this?

[Darren]

---

GregS wrote:

Is there any criteria for determining which VIs should behave like this?

---

The only general criterion that I know of is that close/release functions tend to behave in this manner.  Different APIs are going to implement error handling in different ways.  That's why I recommended reading the help for the functions in the APIs you're using to verify their behavior with error conditions.

[NIquist]

Thanks for that one Darren.  I've always included a 'shutdown2' state in my code that was basically the same thing as 'shutdown' except without the error wires connected.  It ran when 'shutdown' reported any errors just to be sure everything was closed before the main program terminated.  Now I can see why that was superfluous.

I guess I should learn to RTFM a little more carefully. 

[tbob]

---

smercurio_fc wrote:

---

Darren wrote:

By reading the LabVIEW Help for the function:

---

Yeah. Good luck with getting people to do that.

---

Good one Darrin.  Even better one smercurio !!!!!

[hecmar.arreola]

---

NIquist wrote:

Thanks for that one Darren.  I've always included a 'shutdown2' state in my code that was basically the same thing as 'shutdown' except without the error wires connected.  It ran when 'shutdown' reported any errors just to be sure everything was closed before the main program terminated.  Now I can see why that was superfluous.

I guess I should learn to RTFM a little more carefully. 

---

Its a pretty big manual.... /sigh.  Even when I do read about a function thoroughly, I still miss stuff. 

[InfiniteNothing]

---

GregS wrote:

IMAQ Dispose is a VI that presumably should execute despite an incoming error, but doesn't.

Doing a quick search in the LabVIEW help for "runs normally even if" shows a reasonable number of other VIs that also execute with incoming errors, but they're not ones that close or release anything.  They are mostly from the Storage palette (e.g. Create Query, Merge Queries, ID to Refnum ...), the Digital Waveform palette (e.g. Digital Comparison, Analog to Digital, Append Digital Samples ...) or the Waveform Generation palette (e.g. Poisson Noise Waveform, Bernoulli Noise Waveform ...).  Is there any criteria for determining which VIs should behave like this?

---

Actually, it does. Try it 🙂 The documentation should probably be updated.

[GregFreeman]

---

smercurio_fc wrote:

---

Darren wrote:

By reading the LabVIEW Help for the function:

---

Yeah. Good luck with getting people to do that.

---

Read LabVIEW help = forum will become obsolete

[Broken_Arrow]

Here's another function that will run when an error is present on the input....

 

Message Edited by Broken Arrow on 06-08-2010 03:25 PM