Q66455: Problems May Occur When Passing the Same Array Element Twice

Article: Q66455
Product(s): See article
Version(s): 6.00 6.00b 7.00 7.10 | 6.00 6.00b 7.00 7.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# S901018-67 B_QuickBAS | mspl13_basic
Last Modified: 12-NOV-1990

The following program may give unexpected results when the same array
element is passed twice to the subprogram. The problem results from a
form of variable aliasing, where the same memory location is
referenced by two different variables.

To avoid aliasing problems, never pass the same variable twice in a
given parameter list.

Passing the same array element twice in the same parameter list can
give incorrect or unexpected results regardless of array type or
dynamic or static array allocation. The results may also vary between
compiler versions. A customer reported that the program below gave the
results that he wanted in QuickBASIC 4.00, but not in Microsoft BASIC
Professional Development System (PDS) version 7.10; Microsoft has not
confirmed this report.

This behavior results from the fact the BASIC often requires a far
pointer to access arrays, but parameters need to be passed as near
pointers. On a CALL, BASIC sets aside a temporary location holding the
array element and then passes a pointer to the temporary area.

There are two options in this sort of situation: Recode the subprogram
so that it is not necessary to pass the array element twice, or assign
one of the parameters to a temporary variable and then pass the
temporary variable.

References:

For a similar article on variable aliasing when a parameter is both
SHARED and passed as a parameter to a subprogram, query in this
Knowledge Base on the following words:

   DYNAMIC and ARRAY and ALIASES

A variable should not be passed twice in the list of arguments passed
to a procedure; otherwise, variable-aliasing problems will occur. This
restriction is documented under "The Problem of Variable Aliasing" on
Page 64 in the "Microsoft BASIC 7.0: Programmer's Guide" for BASIC PDS
versions 7.00 and 7.10, on Page 68 of the "Microsoft QuickBASIC 4.5:
Programming in BASIC" manual, and on Page 78 of the "Microsoft
QuickBASIC 4.0: Programming in BASIC: Selected Topics" manual for
QuickBASIC versions 4.00 and 4.00b.

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

   DECLARE SUB MakeUpper(instring AS STRING, outstring AS STRING)
   DIM a$(15)
   a$(4)="abcdefg"
   CALL MakeUpper(a$(4), a$(4))
   PRINT a$(4)
   END

   SUB MakeUpper(instring AS STRING, outstring AS STRING)
     outstring = UCASE$(instring)
   END SUB

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