PRB: Passing Constants to Subroutines in FORTRAN (35871)

The information in this article applies to:

Microsoft FORTRAN Compiler for MS-DOS 4.0
Microsoft FORTRAN Compiler for MS-DOS 4.1
Microsoft FORTRAN PowerStation for MS-DOS 1.0
Microsoft FORTRAN PowerStation for MS-DOS 1.0a
Microsoft Fortran Powerstation 32 for Windows NT 1.0
Microsoft Fortran Powerstation 32 for Windows NT 4.0

This article was previously published under Q35871

SYMPTOMS

The code below will generate the following output:

1.000000
2.000000

or print 1.000000 and cause a general protection fault.

CAUSE

This behavior is not a problem with Microsoft FORTRAN. The FORTRAN 77 ANSI standard, section 15.9.2, specifies that when passing a constant as an actual argument to a subroutine, the associated dummy argument (in this case x) cannot be modified.

Microsoft FORTRAN does not generate error messages while compiling. The above code does not follow the standard; therefore, it will generate unpredictable results because FORTRAN passes by reference.

RESOLUTION

If the associated dummy argument is going to be modified, pass a variable. For example, use "call sub1(y)" with y=1.

MORE INFORMATION

The following code demonstrates this information:

        write (*,*) 1.0
        call sub1(1.0)
        write (*,*) 1.0
        end

        subroutine sub1(x)
        real x
        x = 2.0
        return
        end

Modification Type:	Major	Last Reviewed:	12/1/2003
Keywords:	kbLangFortran KB35871