How to interface fortran application with C code without getting compiler errors

[Jeff Whittle]

I have an application written in Intel Fortran that needs to call a C library routine. I do not need to pass three of the arguments and the C documentation says that they can be NULL.

I have set up the INTERFACE structure shown below, and have used this approach extensively for years with other C routines, but have never needed to pass a NULL before.

The first dummy argument is "char *" and the other two are "char **".

I have tried passing NULL() but the compiler complains that "This name has not been declared as an array or a function".

      INTERFACE
        INTEGER FUNCTION GRBloadmodel (
        .................. other arguments ...
     +                                 vtype,
     +                                 varnames,
     +                                 constrnames
     +                                )
          !DEC$ ATTRIBUTES STDCALL, ALIAS:"GRBloadmodel"::GRBloadmodel
          .................. other arguments ...
          CHARACTER*(*)    vtype
          !DEC$ ATTRIBUTES REFERENCE::vtype
          CHARACTER*(*)    varnames(*)
          CHARACTER*(*)    constrnames(*)
        END FUNCTION GRBloadmodel
      END INTERFACE