HOWTO: Access FORTRAN COMMON Block Contents From C++

ID: Q51614

• 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 C/C++ version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51

SUMMARY

An application developed in Microsoft FORTRAN can store data in a COMMON block of memory that has the Microsoft C equivalent of a public structure. An application developed in C can access a FORTRAN COMMON block as an "extern public" structure. The only public symbols in FORTRAN are the names of COMMON blocks and of subprograms. Because FORTRAN does not have a syntax to declare an external COMMON block, FORTRAN code always creates the memory locations for data in a mixed-language programming environment.

The most critical aspect to accessing a COMMON block from C code is using the correct naming convention. In C, all public names must have a leading underscore (_) character; FORTRAN code must declare the underscore explicitly. C and FORTRAN each expect public names to contain only uppercase letters. To enforce the correct naming convention, either use the _fortran attribute on the "extern struct" declaration in the C code or use the [C] attribute on the COMMON block name in the FORTRAN code.

MORE INFORMATION

The following project demonstrates using a FORTRAN COMMON block in an application developed in C. The project uses the _fortran keyword to allow the C code to generate the correct name for the COMMON block. Even though the design of the makefile assumes Microsoft FORTRAN version 5.1 and Microsoft C/C++ version 7.0, the information is generally correct for other versions of these compilers. This project creates a driver program, a FORTRAN subprogram that defines a COMMON block, and a C program that accesses the data in the COMMON block. The driver program is written in FORTRAN, it could have as easily been written in C.

A critical switch used in the C compile line is /Gx-. If this switch is not used, you will not be able to access the common block, unless you use "_far" in the extern declaration, in CFUNC.C as follows:

    extern struct comstruct _fortran _far TEST;

Sample Code #1: FORMAIN.FOR

   C Main driver program
   C Compiler options required: None

         PROGRAM COMMONTEST
         CALL FORSUB
         END

Sample Code #2: FORSUB.FOR

   C A FORTRAN subroutine with a COMMON block
   C Compiler options required: None

         SUBROUTINE FORSUB
         COMMON /TEST/ A, B, C
         REAL A
         INTEGER B
         CHARACTER*1 C

         A = 1.
         B = 2
         C = 'F'
         CALL CFUNC
         PRINT*
         PRINT*, 'IN FORSUB FOLLOWING CALL TO CFUNC'
         PRINT*, 'A IS ', A, '           B IS ', B, '           C IS ', C
         RETURN
         END

Sample Code #3: CFUNC.C

   /*
    * Compiler options needed: None
    */ 

   // A C function that uses data in the FORTRAN COMMON block

   #include <stdio.h>

   struct comstruct
   {
      float A;
      long  B;
      char  C;
   };

   extern struct comstruct _fortran TEST;

   void _fortran cfunc(void)
   {
      printf("In cfunc\n");
      printf("a is %f, b is %ld, c is %c\n", TEST.A, TEST.B, TEST.C);
      TEST.A = TEST.A + 1.;
      TEST.B = TEST.B + 1;
      TEST.C = 'C';
   }

Sample Code #4: MAKEFILE

   all: test.exe

   formain.obj: formain.for
      fl /c /AL /Od /Zi formain.for

   forsub.obj: forsub.for
      fl /c /AL /Od /Zi forsub.for

   cfunc.obj: cfunc.c
      cl /c /AL /Od /Zi /Gx- cfunc.c

   test.exe: formain.obj forsub.obj cfunc.obj
      link formain forsub cfunc, test, nul, /nod /noe /co \ 
         llibc7 llibf7rc oldnames;

Keywords          : kb16bitonly kbFortranPS kbLangFortran 
Version           : MS-DOS:4.0,4.01,4.1,5.0,5.1,1.0,1.00a; OS/2:4.1,5.0,5.1
Issue type        : kbhowto

Last Reviewed: August 29, 1997