BUG: Watch Expression Evaluation Gives Unexpected Value

 Last reviewed: July 13, 1995
Article ID: Q118395
The information in this article applies to:
  • Microsoft FORTRAN PowerStation for MS-DOS, versions 1.0 and 1.0a
  • Microsoft FORTRAN PowerStation 32 for Windows NT, version 1.0

SYMPTOMS

Expressions evaluated in the watch window result in incorrect values, and no overflow warnings are given.

CAUSE

The FORTRAN expression evaluator, which calculates the value of expressions that are typed into the watch window, uses the same data type to store intermediate values as it does to store the terms in the expression.

For example, if all of the terms in an expression were INTEGER*2, then the expression will be evaluated with INTEGER*2 arithmetic. It is very common for an overflow to occur in subexpressions of an expression, even if the final value can be stored in INTEGER*2. The result of a calculation following this convention is incorrect because this is not the way expressions are evaluated in FORTRAN code.

RESOLUTION

Avoid creating expressions that could overflow. The most reliable way to do this is to avoid using INTEGER*2 and INTEGER*1.

You can make the subexpressions of an expression be calculated in INTEGER*4 by forcing one of the terms to be stored as INTEGER*4. To do this, use the function INT4() on a term in the subexpression.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft FORTRAN PowerStation for MS-DOS, versions 1.0 and 1.0a, and the Microsoft FORTRAN PowerStation 32 for Windows NT, version 1.0. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The expression "i1*i2/i3" in the sample code below has a subexpression "i1*i2", which causes an overflow in INTEGER*4. This causes an incorrect evaluation of the expression.

To demonstrate the problem, perform the following steps:

  1. Build the sample code in debug mode on the Visual Workbench.

  2. Step to the print statement.

  3. From the Window menu, choose Watch.

  4. Type "IAN" (without the quotes) in the watch window and press the Enter key.

  5. Type "i1*i2/i3" and press the Enter key.

  6. Type "int4(i1)*i2/i3" and press the Enter key.

All three lines should display the 3157 because they are all (500*120/19). However, the watch window actually displays the following: 

   ian = 3157
   i1*i2/i3 = -291
   int4(i1)*i2/i3 = 3157

Sample Code

C Compile options needed: Debug Mode 

      integer*2 i1,i2,i3,ian
      i1 = 500
      i2 = 120
      i3 = 19
      ian = i1*i2/i3
      print *, ian
      end

Additional reference words: 1.00 1.00a
KBCategory: kbtool kbbuglist
KBSubcategory: WBDebug

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Last reviewed: July 13, 1995
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