Problem Using SETMEM and SHELL in QuickBasic 4.50 (62267)

SYMPTOMS

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.

STATUS

Microsoft has confirmed this to be a bug 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).

MORE INFORMATION

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.