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Q35143: Using GOSUB, GOTO, ON Event GOSUB, ON ERROR GOTO in Modules

Article: Q35143
Product(s): See article
Version(s): Ox2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 16-DEC-1989

A normal GOSUB Label or GOTO Label statement must be at the same level
of code as the label. For example, if a GOSUB Label statement occurs
inside a SUB...END SUB (or FUNCTION...END FUNCTION) procedure block,
the label must also occur within that block.

This information applies to Microsoft QuickBASIC Versions 4.00 4.00b
and 4.50 for MS-DOS, to Microsoft BASIC Compiler Versions 6.00 and
6.00b for MS-DOS and MS OS/2, and to Microsoft BASIC PDS Version 7.00
for MS-DOS and MS OS/2.

Only the BASIC compiler 7.00 has an enhanced form of local error
trapping in which the label may be local to the current SUB or
FUNCTION, as the following example shows:

SUB SUB1
     ON LOCAL ERROR GOTO errorhandler

EXIT SUB
errorhandler:
     RESUME NEXT
END SUB

However, the following statements must have their labels at the module
level:

   ON ERROR GOTO Label
   ON Event GOSUB Label (where "Event" may be COM, KEY, PEN, PLAY,
                        STRIG, or TIMER)

The "module level" is defined as the area in a source file that is
outside all subprogram or FUNCTION procedure blocks.

Once the ON Event GOSUB Label and Event ON statements are executed,
event trapping continues until an Event OFF is executed (where "Event"
may be COM, KEY, PEN, PLAY, STRIG, or TIMER).

In QuickBASIC Versions 2.00, 2.01, 3.00, and 4.00, the ON ERROR GOTO
Label statement must be established for each module; otherwise, an
error occurring in that module will not be trapped, and the program
will stop.

In contrast to earlier versions, QuickBASIC Versions 4.00b and 4.50
and Microsoft BASIC Compiler Versions 6.00, 6.00b, and 7.00 for MS-DOS
and MS OS/2 offer global handling of ON ERROR (as explained in the
UPDATE.DOC disk file). With global error-handling, an ON ERROR handler
in the main program handles errors occurring in separate modules when
those modules do not have their own ON ERROR handlers.

For related information query on the following words:

   subprogram and module and error and resume and QuickBASIC

The following is a code example:

' MODULE 1
DECLARE SUB mod2sub2 ()
DECLARE SUB mod1sub1 ()
DECLARE SUB mod2sub1 ()
ON ERROR GOTO errhand1  ' Sets first error handler in main module
KEY 15, CHR$(0) + CHR$(1)  ' Sets up trapping of ESCAPE key.
KEY(15) ON                 ' Turns on trapping of ESCAPE key.
ON KEY(15) GOSUB lower     ' "lower" label is at module level
PRINT "in main"
GOSUB lower    ' You may GOSUB or GOTO a label in this module
               ' only at the level of the main module.
CALL mod1sub1
CALL mod2sub1  ' Establishes an error handler for mod2
CALL mod2sub2
ERROR 3
END
lower:
  PRINT "This gosub/label can be reached";
  PRINT " only from main level mod1 or"
  PRINT "by pressing the ESCAPE key the";
  PRINT " first time in mod2 sub1": PRINT
  RETURN
errhand1:
  PRINT "take care of error on main level mod 1": PRINT
  RESUME NEXT
SUB mod1sub1
    GOSUB localLabel  ' localLabel must be local to the subroutine
    GOTO done:
  localLabel:
    PRINT "I am here in mod1 sub1 simulating error in mod1 subprog1"
    ERROR 1    'this error is handled ok
    RETURN
  done:
END SUB

' MODULE 2
mod2: PRINT "this does not get activated/ mod2 main "
mod2main:
  PRINT "simulating error in nextmod mod2 main called from subprog2"
  ERROR 1
  RETURN
errhand2:
  PRINT " took care of the error in mod2 top level":PRINT
  RESUME NEXT
SUB mod2sub1
   ON ERROR GOTO errhand2
'  Must get new direction once per module in
'  QuickBASIC 3 AND 4. However, in QuickBASIC
'  4.00b and BASIC Compiler 6.00, errhand1 will still trap the error
'  but RESUME NEXT resumes at the END SUB.
   PRINT "simulating an error "
   ERROR 1
   PRINT "press escape"    ' ESCAPE here causes jump
   FOR i = 1 TO 10000: NEXT ' to routine defined in main
   ON KEY(15) GOSUB mod2main ' but after this
   PRINT "press escape again" ' ESCAPE now causes jump
   FOR i = 1 TO 10000: NEXT    ' to nextmod
END SUB
SUB mod2sub2
   ERROR 2  'this is ok if mod2sub1 is called first
END SUB

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