Return Code	Significance
-1	callback 0 pointer passed
1	callback function already entered
0	callback function added

With the help of the check function you can decide, whether a button is to be entered into the button area. Several check functions can be put into a callback queue and be sequentially executed.

The check function must correspond to the specification int fnc(typ,idb,selnam,selusx,usxpar). The parameters are:

• char typ: 'B' for button area
• DtvLong idb: Menu Id
• char selnam: Selection name
• char selusx: Function name
• char usxpar: Parameter

If the check function passed returns a Return Code = 0, checking will be continued in the queue. In case of Return Code different from 0 the queue will be left.