Q69157: Bad Values When QLB Function Used as SUB Parameter

Article: Q69157
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S910118-99 | mspl13_basic
Last Modified: 14-FEB-1991

If a FUNCTION returning a floating-point value is located in a Quick
library and is directly used as the second, third, or later parameter
of a SUB or FUNCTION in a program, then incorrect values may be passed
to the SUB or FUNCTION being called. This problem occurs only in the
QuickBASIC environment.

Microsoft has confirmed this problem with Microsoft QuickBASIC
versions 4.00, 4.00b, and 4.50. This problem does not exist with the
Microsoft BASIC Professional Development System (PDS) versions 7.00
and 7.10.

When compiled and linked (with the FUNCTION in a .LIB library) into an
.EXE file, the same program will run correctly.

A simple workaround for this problem is to assign a temporary variable
to the return value of the Quick library function, then pass that
temporary variable when you invoke the SUB or FUNCTION.

The results may vary depending upon the number and type of parameters
passed to the FUNCTION within the Quick library. If the Quick library
function accepts parameters, bad values may be passed to the calling
SUB or FUNCTION.  However, if no parameters are accepted by the Quick
library function, a machine hang may occur when that function is used
as a parameter to another SUB or FUNCTION.

Using the debug options when creating the Quick library, or using the
ON ERROR GOTO ... RESUME lines in either the main program or in the
Quick library, does not eliminate the problem. The code below
demonstrates the problem.

The following function goes in the Quick library:

DECLARE FUNCTION func1! (x!)
FUNCTION func1! (x!)
    func1! = x! * 2
END FUNCTION

The following program invokes the Quick library's function directly in
the parameter list of the SUB foo, and demonstrates the problem:

DECLARE FUNCTION func1! (x!)
DECLARE SUB foo (a!, b!)
CALL foo (100, func1! (45))
END
SUB foo (a!, b!)
PRINT a!; b!  'This should print 100 90, but incorrectly prints 90 90
END SUB

A simple modification using a temporary variable (y!) in the calling
program works around the problem, as shown below:

DECLARE FUNCTION func1! (x!)
DECLARE SUB foo (a!, b!)
y! = func1! (45)
CALL foo (100, y!)
END
SUB foo (a!, b!)
    PRINT a!; b!
END SUB

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