User's Guide

Conventions for XL Fortran External Names

To assist you in writing mixed-language programs, XL Fortran follows a consistent set of rules when translating the name of a global entity into an external name that the linker can resolve:

• Both the underscore (_) and the dollar sign ($) are valid characters anywhere in names.

  Because names that begin with an underscore are reserved for the names of library routines, do not use an underscore as the first character of a Fortran external name.

  To avoid conflicts between Fortran and non-Fortran function names, you can compile the Fortran program with the -qextname option. This option adds an underscore to the end of the Fortran names. Then use an underscore as the last character of any non-Fortran procedures that you want to call from Fortran.

• Names can be up to 250 characters long.

• Program and symbolic names are interpreted as all lowercase by default. If you are writing new non-Fortran code, use all-lowercase procedure names to simplify calling the procedures from Fortran.

  You can use the -U option or the @PROCESS MIXED directive if you want the names to use both uppercase and lowercase:

  @process mixed
         external C_Func    ! With MIXED, we can call C_Func, not just c_func.
         integer aBc, ABC   ! With MIXED, these are different variables.
         common /xYz/ aBc   ! The same applies to the common block names.
         common /XYZ/ ABC   ! xYz and XYZ are external names that are
                            ! visible during linking.
         end
  

• Names for module procedures are formed by concatenating _ _ (two underscores), the module name, _MOD_, and the name of the module procedure. For example, module procedure MYPROC in module MYMOD has the external name _ _mymod_MOD_myproc.

• The XL compilers generate code that uses main as an external entry point name. You can only use main as an external name in these contexts:

  • A Fortran program or local-variable name. (This restriction means that you cannot use main for the name of an external function, external subroutine, block data program unit, or common block. References to such an object use the compiler-generated main instead of your own.)

  • The name of the top-level main function in a C program.

  • The name of a Pascal program unit.

• Some other potential naming conflicts may occur when linking a program. For instructions on avoiding them, see Linking New Objects with Existing Ones and Avoiding Naming Conflicts during Linking.

If you are porting your application from another system, and your application does encounter naming conflicts like these, you may need to use the -qextname Option. Or you can use the -brename linker option to rename the symbol if there are not too many names to change:

  xlf90 -brename:old_name,new_name interlanguage_calls.f