PRB: ScrollText & Improper RETURN from ON Event GOSUB Can Bomb (48727)

MORE INFORMATION

Below is an example of an incorrectly designed QuickBasic program that does not return properly from an ON MENU GOSUB routine. The program is condensed from a customer's larger program. The problem occurs from doing an ON MENU GOSUB statement followed by a GOSUB to a routine that turns MENU ON (mistakenly), invokes ScrollText, and is improperly exited by trapping the user-defined QUIT menu option.

The program needs restructuring so that MENU remains OFF while handling a MENU event (between the ON MENU GOSUB and the RETURN). Also, the ScrollText routine should be moved to a loop at the main level code so MENU events can be used in proper conjunction. You should also execute DisposeScroll when finished using the ScrollText statement.

The necessary program revisions are not written in the program below. Please refer to the ToolBoxSampler program in the "QB Demos" folder on the QuickBasic 1.00 release disk for a similar working example. ToolBoxSampler polls the MENU() function instead of trapping MENU events with the ON MENU GOSUB statement.

Example of Incorrect Programming

To encounter the system bomb, choose the QUIT option under the OPTION One menu two times. This produces a system bomb the first time when run as a compiled application, but in the environment you might have to run it a few times to get a system bomb.

DEFINT a-z
DIM HILOW$(31)
QuitFlag=1
GOSUB restoreall
ON MENU GOSUB doMenu: MENU ON
WHILE 1 : WEND

doMenu:
     MenuID = MENU(0)
     MenuItem = MENU(1)  'SELECT CASE MenuID
     CLS
     SELECT CASE MenuID
           CASE 1  'FILE  menu
               SELECT CASE MenuItem
                   CASE 1 : GOSUB Quitsub
               END   SELECT
           CASE 2
               SELECT CASE MenuItem
                   CASE 2 : GOSUB HILOWLIST
               END SELECT
     CASE ELSE
     END SELECT
     MENU
RETURN

HILOWLIST:
   MENU ON   ' Bad program design: should be no MENU ON here,
             ' and you can't expect to trap MENU selection events in
             ' the ScrollText loop below, since you haven't yet
             ' RETURNed from the MENU event which got you here.
   linenum% =0:  top%=0:  S&=0:  in%=0:  SS&=0
   SetRect scr%(1),3,60,275,270
   FRAMERECT VARPTR (scr%(1))
   SetRect Bar%(1),274,60,290,270
   NewScroll S&,Bar%(1),1,1,20,1: 'verticle
   QuitFlag=3
   WHILE MOUSE (0)<>1  'This Loop is only exited by the Menu Option
        ScrollText S&,scr%(1),HILOW$(1),top%,31,linenum%
   WEND
RETURN                 'This RETURN is never executed.

restoreall:
   MENU 1,0,1,"File"
   MENU 1,1,1,"QUIT":cmdkey 1,1,"Q"
   MENU 2,0,1,"OPTION ONE"
   MENU 2,2,1,"OPTION TWO":cmdkey 2,2,"H"
RETURN

hilowend:
     DisposeScroll S&
     WINDOW CLOSE 1
     WINDOW 1
RETURN

Quitsub:
   IF QuitFlag=1 THEN GOSUB restoreall : PRINT "the end" : END
   IF QuitFlag = 3 THEN QuitFlag=1 : GOSUB hilowend : GOSUB restoreall
RETURN