Extending a Common Block with an EQUIVALENCE Statement

ID: Q67229

The information in this article applies to:

Microsoft FORTRAN for MS-DOS, versions 4.0, 4.01, 4.1, 5.0, 5.1
Microsoft FORTRAN for OS/2, versions 4.1, 5.0, 5.1
Microsoft FORTRAN PowerStation for MS-DOS, versions 1.0, 1.0a
Microsoft Fortran Powerstation 32 for Windows NT, versions 1.0, 4.0

SUMMARY

An EQUIVALENCE statement can be used to extend a common block size by EQUIVALENCing the last element of a common block with the first element of an array that is not in a common block. This is demonstrated in the code below.

This article expands upon the information in the EQUIVALENCE statement section of the "Microsoft FORTRAN Reference" manual, which discusses extending common blocks found.

MORE INFORMATION

The following code fragment shows a common block being extended from 4 bytes to 800 bytes in length.


C ***** FORTRAN program fragment *****

      REAL*4 VAR, A(200)
      COMMON /TEST/ VAR
      EQUIVALENCE (VAR, A(1))

In this fragment, the common block TEST is initially 4 bytes in length. The equivalence statement specifies that VAR and A(1) will share the same location in memory. Because all of A's elements are contiguous in memory, the common block TEST is extended to be 200 x 4 or 800 bytes in length. This is shown graphically below:


   |01|02|03|04|05|06|07|08|09|0A|0B|0C|0D|0E|0F|10|
   |------------------------------------------------
   |....VAR....|
   |------------------------------------------------
   |....A(1)...|....A(2)...|....A(3)...|....A(4)...|
   |------------------------------------------------
   |<-- Beginning address of common block TEST

The guidelines listed below must be followed when using EQUIVALENCE to extend a common block's length:

A common block may only be extended by adding elements to the end of the common block. An EQUIVALENCE operation cannot add elements that precede the common block as in the following example:
```
C ***** FORTRAN program fragment *****

      REAL*4 VAR, A(200)
      COMMON /TEST/ VAR
      EQUIVALENCE (VAR, A(2)) 
```
Here VAR and A(2) will share the same memory location. Because A(1) precedes A(2) in memory, it will also precede VAR, which is the beginning of the common block. This is called extending a common block forward and is shown graphically below:
```
      |??|??|??|??|01|02|03|04|05|06|07|08|09|0A|0B|0C|0D|0E|0F|10
      ------------|------------------------------------------------
      ............|....VAR....|
      ------------|------------------------------------------------
      |....A(1)...|....A(2)...|....A(3)...|....A(4)...|....A(5)...
      ------------|------------------------------------------------
      ............|<-- Beginning address of common block TEST 
```
The following error is generated when the code listed above is compiled:

F2320: A : EQUIVALENCE : extends common block TEST forward

The ANSI FORTRAN 77 Standard states that a NAMED common block must be of the same size in all program units of an executable program in which it appears. For this reason, when a NAMED common block is extended, every subsequent subprogram accessing the common block must declare it according to its new size. For example, in the code below, a NAMED common block is again declared with one REAL*4 element and then EQUIVALENCEd with a 200 element REAL*4 array. In the subroutine, the common block is then declared with its new size of 200 elements:


C ***** FORTRAN program fragment *****

      PROGRAM main
      REAL*4 VAR, A(200)
      COMMON /TEST/ VAR
      EQUIVALENCE (VAR, A(1))
      CALL sub()
      .
      .
      END

      SUBROUTINE sub ()
      REAL*4 B
      COMMON /TEST/ B(200)
      .
      .
      RETURN
      END

However, FORTRAN 5.0 will allow a NAMED common block to have different sizes in subprogram units. In this situation, the compiler generates the following warning:

F4329: TEST : COMMON : size changed

If the /4Ys compile option or $STRICT metacommand is used with FORTRAN 5.0, or if the program is compiled with FORTRAN 4.x, the error

F2323: TEST : COMMON : size changed

is generated when a NAMED common block's size is changed in a subprogram.

Under Fortran PowerStation 4.0 using the /4Ys compile option or the $STRICT metacommnad, a change in a subprogram common block's size generates the following error message:

error FOR3750: byte count on numeric data type detected between * and 4

Additional query words: kbinf 1.00 4.00 5.00 5.10

Keywords : kbcode kbFortranPS kbLangFortran
Version : :1.0,1.0a,4.0,4.01,4.1,5.0,5.1
Platform : MS-DOS NT OS/2 WINDOWS
Issue type :