Q62833: No Array Bounds Checking for Arrays in TYPEs in BC 7.00 .EXE

Article: Q62833
Product(s): See article
Version(s): 7.00   | 7.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | docerr SR# S900521-23 | mspl13_basic
Last Modified: 12-JUN-1990

In Microsoft BASIC Professional Development System (PDS) version 7.00,
the run-time error checking code inserted by the BC /D compiler switch
does not perform array bounds checking for arrays embedded inside
user-defined TYPEs. However, this checking is performed inside the
QBX.EXE environment. This is not a problem with the BC.EXE compiler
but a design limitation. The overhead required for the check would
cause speed degradation and a size increase for an .EXE file.

This information applies to Microsoft BASIC Professional Development
System (PDS) version 7.00 for MS-DOS and MS OS/2.

The BC /D compiler switch (Run-time error checking) inserts array
bounds checking in a module. Whenever an access on an array is
attempted with an illegal subscript, the message "Subscript out of
range" is generated. Without /D, this type of error goes undetected
under DOS and will most likely result in the corruption of other data.
It can also cause corruption of code or possibly even parts of the
operating system itself, which could cause unpredictable results,
including hanging the machine. Under OS/2, an error of this type would
cause a protection violation and abort the program. The /D switch
performs array bounds checking on arrays that are NOT embedded in
user-defined TYPEs, but won't for arrays that ARE embedded in types.
However, in the QBX.EXE environment, the checking is done for all
arrays.

For example, the following program generates a "Subscript out of
range" error in the QBX.EXE environment, but does not when compiled
with the /D switch:

   TYPE aType
      Array (1 TO 100) AS INTEGER
   END TYPE

   DIM aVar as aType

   aVar.Array(1000) = 1000   '1000 as a subscript is out of range

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