Q62813: FRE(-1) Decreases with Repeated Use of DEF FNa$ in QBX.EXE

Article: Q62813
Product(s): See article
Version(s): 7.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900520-2 buglist7.00 fixlist7.10 | mspl13_basic
Last Modified: 2-NOV-1990

Repeated use of a DEF FN function to return a string in the QBX.EXE
environment causes far heap memory to decrease as shown in the
following program. This does not occur with a compiled .EXE program or
if a FUNCTION procedure is used in place of the DEF FN function.

Microsoft has confirmed this to be a problem in the QBX.EXE
environment of Microsoft BASIC Professional Development System (PDS)
version 7.00 for MS-DOS. This problem was corrected in BASIC PDS 7.10.

To work around the above problem, use a BASIC FUNCTION procedure in
place of the DEF FN function.

This problem does not occur with any version of Microsoft QuickBASIC
(QB.EXE) or any version of Microsoft BASIC Compiler (BC.EXE).

Microsoft QuickBASIC versions 4.00 and later and Microsoft BASIC
Compiler versions 6.00 and later have a method of implementing
functions that is much more straightforward than DEF FN. These
products allow true FUNCTION procedures to be created, much like the
functions available in Pascal and C. A BASIC FUNCTION...END FUNCTION
block is a procedure that allows you to return a value to the calling
subprogram, but in all other respects is the same as a SUB...END SUB
subprogram procedure. Using a FUNCTION procedure instead of DEF FN
avoids the DEF FN problem described in this article.

The following code example demonstrates the problem when run in the
QuickBASIC Extended environment (QBX.EXE) that comes with BASIC PDS
7.00.

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

DEF FNa$ = "This is a test string."
FOR i% = 1 TO 100
  PRINT FRE(-1)   ' Show available far heap.
  PRINT FNa$
NEXT

Output
------

 179376
This is a test string.
 179344
This is a test string.
 179328
This is a test string.
 179296
This is a test string.

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