Q62267: Problem Using SETMEM and SHELL in QuickBASIC 4.50

Article: Q62267
Product(s): See article
Version(s): 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom buglist4.00 buglist4.00b buglist4.50 | mspl13_basic
Last Modified: 14-JUN-1990

Using the SETMEM function with a parameter less than 0 (zero) in
Microsoft QuickBASIC versions 4.00, 4.00b, and 4.50 deallocates memory
from BASIC's far heap. Normally, the memory can be regained with
another call to SETMEM with a positive parameter. However, if a SHELL
statement is executed between the two CALLs to SETMEM, the program
will not be able to reclaim the memory. This happens both when
executing from the QB.EXE environment or as an executable (.EXE) file
that uses the BRUNxx.EXE run-time module. The only workaround is to
compile the program as a stand-alone EXE using BC.EXE with the /O
option.

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

The following code sample reproduces this problem:

   before& = FRE(-1)                ' Get original Value.
   temp& = SETMEM(-30000)           ' Deallocate memory.
   SHELL "CLS"
   temp& = SETMEM(30000)            ' Try to Reallocate memory.
   after& = FRE(-1)                 ' Get New value, which should be
                                    ' the same value as the original.
   CLS
   PRINT "Before Call: "; before&   ' If the difference between these
   PRINT "After  Call: "; after&    ' two values is 30000 then the
                                    ' problem is reproduced.
   differ& = before& - after&
   PRINT " Difference  ";differ&    ' This should be 0.

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