Q41536: PRINT TAB Can Change Value of Parameter in Subprogram

Article: Q41536
Product(s): See article
Version(s): 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist4.50 SR# S890216-214 | mspl13_basic
Last Modified: 14-FEB-1991

The code below demonstrates that a variable that has been passed to a
subprogram procedure may incorrectly be changed by a PRINT TAB(n)
statement. However, the same statement in the main module does not
cause the error.

The problem can be avoided by not using the PRINT TAB(n) statement, or
by assigning a local variable to the passed parameter in the SUB and
using the local variable in the PRINT TAB(n) statement, or by
compiling with the BC /X option. (The /X compiler switch is normally
used only to flag an ON ERROR statement with RESUME, RESUME NEXT, or
RESUME 0.)

Microsoft has confirmed this to be a problem in an .EXE program
compiled with BC.EXE in QuickBASIC version 4.50. No problem occurs in
the QB.EXE environment. We are researching this problem and will post
new information here as it becomes available.

The program example below works correctly with all versions of
QuickBASIC earlier than version 4.50.

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

DECLARE SUB testsub (row%)
row% = 9
' this line is ok -- it is in the main module
LOCATE row%, 1: PRINT "main line row%= "; row%; TAB(38); row%;
row% = 10
CALL testsub(row%)
END

SUB testsub (row%)
  LOCATE row%, 1: PRINT "Sub row%= "; row%;
  ' The value of row% prints incorrectly:
  PRINT TAB(38); "row now = "; row%
END SUB

To work around the problem in QuickBASIC 4.50, print a local variable
instead of the passed parameter, for example:

SUB testsub (row%)
  x%=row%
  LOCATE row%, 1: PRINT "Sub row%= "; row%;
  PRINT TAB(38); "row now = "; x%      ' x% prints ok.
END SUB

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