Q44795: CLEAR Causes "Illegal Function Call" after RETURN <linelabel>

Article: Q44795
Product(s): See article
Version(s): 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890517-108 B_BasicCom | mspl13_basic
Last Modified: 13-DEC-1989

A CLEAR statement causes an "Illegal function call" error message
after the following steps are performed:

1. CALL a SUBprogram.

2. Trap a key with ON KEY(n) GOSUB.

3. RETURN to a label.

4. Execute a CLEAR statement.

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

The reason this condition causes an "Illegal function call" is that
the SUBprogram was not exited with END SUB or EXIT SUB. Even though
execution is not physically in the SUBprogram when the CLEAR statement
is executed, BASIC assumes that control is still within the SUBprogram
because the information pushed onto the stack during the CALL to the
SUBprogram is still on the STACK. The CLEAR statement is not allowed
in SUBprograms or FUNCTIONs; therefore, the "Illegal function call"
error message is generated.

The fact that the CLEAR statement is not allowed in SUBprograms or
FUNCTIONS is documented in the BASIC language reference manuals for
QuickBASIC Version 4.50 and the BASIC compiler Versions 6.00, and 6.00b,
and the Microsoft BASIC PDS Version 7.00, as well as in the on-line
help in QuickBASIC 4.50 and Microsoft Basic PDS 7.00.

Code Example
------------

The following code sample causes an "Illegal function call" error at
the CLEAR statement after you press F1:

   ON KEY(1) GOSUB KeyHand
   KEY(1) ON
   CALL test
   END

   ErrorHere: CLEAR    'This statement causes "Illegal function call"
   END

   KeyHand:
   RETURN ErrorHere

   SUB test
   DO : LOOP          'Wait for F1 to be pressed
   END SUB

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