Q62456: Incorrect Results When Compiling with Near Strings

Article: Q62456
Product(s): See article
Version(s): 7.00   | 7.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# S900517-165 | mspl13_basic
Last Modified: 1-JUN-1990

A program compiled with the Near Strings option (without /Fs) may
produce incorrect results when the program uses a combination of the
following:

1. The LEN function on a variable length string
2. Integer division
3. String concatenation

The same program will run correctly when run from the QBX.EXE
environment or when compiled with Far Strings (with /Fs).

Microsoft has confirmed this to be a problem in the BC.EXE 7.00
compiler provided with Microsoft BASIC Professional Development System
(PDS) version 7.00 for MS-DOS and MS OS/2. We are researching this
problem and will post new information here as it becomes available.
This problem does not occur in earlier versions of Microsoft BASIC
Compiler (BC.EXE).

This problem results from incorrect optimization by the BC.EXE 7.00
compiler. You can disable these optimizations by either compiling with
the /X compiler switch or by including line numbers in the area of the
program where the error occurs.

The following steps reproduce the problem:

1. Use the LEN function on a variable-length string.

2. Take the number returned by the LEN function and perform integer
   division on that number.

3. Reassign a concatenated string to the variable-length string
   mentioned in step 1.

4. Again, use the LEN function on the variable-length string and
   perform integer division on that result.

If the program is compiled with Near Strings, the result of the
calculation in Step 4 will be incorrect.

The program below demonstrates the problem. The second PRINT statement
should display a value of 9; however, if the program is compiled with
Near Strings, a 0 will be displayed instead. The value returned by
LEN(a$) is correct, but when integer division is performed on this
value, an incorrect result is produced.

   PRINT LEN(a$) \ 2
   a$ = "concatenate " + "strings"
   PRINT LEN(a$) \ 2        'the value printed here is incorrect
   END

Listed below are four different methods to work around the problem:

1. Compile the program with /X or include line numbers in the area
   of the program where the problem occurs. Both of these actions
   disable some of the compiler optimizations that are the cause
   of this problem.

2. Do not use integer division. Instead, perform regular division
   and then use the INT function to get the desired result, as in the
   following example:

      PRINT INT(LEN(a$) / 2)
      a$ = "concatenate " + "strings"
      PRINT INT(LEN(a$) / 2)
      END

3. Avoid string concatenation, as follows:

      PRINT LEN(a$) \ 2
      a$ = "concatenate strings"  'assign a$ to 1 string instead of
                                  'using concatenation
      PRINT LEN(a$) \ 2
      END

4. Assign the string that was created by concatenating strings to
   another string variable. Then, perform the LEN function on the
   new string, as in the following example:

      PRINT LEN(a$) \ 2
      a$ = "concatenate " + "strings"
      b$ = a$                'insert this line
      PRINT LEN(b$) \ 2      'and perform the rest of the
                             'calculations on b$
      END

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