Q51597: In .EXE, PAINT Used in a SUB Can Corrupt Passed Variables

Article: Q51597
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom buglist4.00 buglist4.00b buglist4.50 | mspl13_basic
Last Modified: 6-DEC-1989

The PAINT function can corrupt variables that are passed to a SUB when
the program is compiled using BC.EXE 4.00 4.00B or 4.50. The PAINT
function works correctly in the QB.EXE environment, and if compiled
using BC.EXE with the /X switch.

Microsoft has confirmed this to be a problem in QuickBASIC Versions
4.00, 4.00b, and 4.50 and in Microsoft BASIC Compiler Versions 6.00
and 6.00b (buglist6.00 buglist6.00b) for MS-DOS. This problem was
corrected in Microsoft BASIC Compiler Version 7.00 (fixlist7.00).

The following program demonstrates this problem. The values of x and y
will be invalid following the PAINT(x, y) statement; however, the
values of x and y are unaffected at the module level code following
the call to "Sub1":

Compile with either: BC Paint; or BC Paint /O;
Link with:           Link Paint;

Code Example: PAINT.BAS
-----------------------

DECLARE SUB Sub1(x,y)
x = 300                'Initialize x
y = 100                'Initialize y
CALL Sub1(x,y)
PRINT x; y             'These values will be correct
END
SUB Sub1 (x, y)
SCREEN 12              'Any graphics mode
PRINT x; y             'These values will be correct
CIRCLE (x, y), 50
PAINT (x, y)            'Paint the circle
PRINT x; y             '** These values will contain garbage
END SUB

Specifying a definite type for x and y (such as INTEGER, LONG, SINGLE,
or DOUBLE) does not have any effect on the output.

Four possible workarounds for the PAINT problem are as follows:

1. Compile with BC /X.

2. Copy x and y to variables that are local to the SUB.

3. PAINT at the module level instead of at the SUB level.

4. Make x and y SHARED or COMMON SHARED.

This problem was corrected in Microsoft BASIC Compiler 7.00.

Additional reference word: SR# S891128-117

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