To exit from the while(1) loop

[mhs100]

Hi all ,

I have a simple question that if in a callback function of a command button, i use a While(bLoop)
loop , where bLoop is a boolean variable initially set to TRUE, then how can i break or exit from this while Loop by using a control on the front panel  , which exits the while loop by setting     bLoop = FALSE .

I have tried it but it is not exiting  

[jr_2005 · Accepted solution]

The problem here is that CVI normally only executes one callback at a time. While you are in one control callback, no GUI processing and subsequent calls to other callbacks normally occurs.

You can try the following approach - but it is not really the best method:

    while (b_loop) {              // Inside a callback

        ...                       // Perform your tight program loop

        ProcessSystemEvents ();   // Force CVI to see if any GUI controls need servicing

    }

If you have a different control callback to clear the global b_loop, this quick-and-dirty approach might work for you.

You should only use this technique for very simple programs - it can rapidly spiral out of control and end up in a mess if you're not careful. NI recommends that you do not use ProcessSystemEvents() inside a control callback.

JR

[Al_S]

JR:

NI recommends that you do not use ProcessSystemEvents() inside a control callback?  That's surprising!  For me, that's one of the main uses of ProcessSystemEvents.  You are absolutely right that care needs to be taken.  CVI 9 help tells you to take care when using it, and warns that ProcessSystemEvents won't return from a tracking loop for a pull-down menu until the user dismisses the menu, But I haven't seen any warnings not to use it in a control callback.  It wouldn't be the first time I missed something (or misused something).  Do you have a reference that will put that into perspective?

Thanks

[jr_2005]

Al,

I must confess I can't lay my hands on a definitive statement from NI - perhaps it was a topic in the forum at some stage? Certainly some forum users frown on the idea of calling ProcessSystemEvents() inside a control callback. A programmer needs to be careful with this approach, and novices tend not to be careful, so as far as I'm concerned the general recommendation to beginners stands. (Its a bit like learning to drive: once you have achieved a level of experience you can start to do things your driving instructor would not approve of.)

Having said that, almost all of my programs have a call to ProcessSystemEvents() at the end of a Timer control callback - but here of course there is a significant difference: a Timer callback function is not re-entrant - once entered it cannot be called again until the original callback has completed. This eliminates the possibility of catastrophic recursion which can occur in other (re-entrant) control callbacks, with a carelessly positioned ProcessSystemEvents().

JR