XL Fortran Enterprise Edition for AIX : User's Guide - IBM

Explicit Interface for %VAL and %REF
You can specify an explicit interface for non-Fortran procedures to avoid coding
calls to %VAL and %REF in each argument list, as follows:
INTERFACE
FUNCTION C_FUNC(%VAL(A),%VAL(B)) ! Now you can code "c_func(a,b)"
INTEGER A,B
! instead of
END FUNCTION C_FUNC
! "c_func(%val(a),%val(b))".
END INTERFACE
Returning Values from Fortran Functions
XL Fortran does not support calling certain types of Fortran functions from
non-Fortran procedures. If a Fortran function returns a pointer, array, or character
of nonconstant length, do not call it from outside Fortran.
You can call such a function indirectly:
SUBROUTINE MAT2(A,B,C) ! You can call this subroutine from C, and the
! result is stored in C.
INTEGER, DIMENSION(10,10) :: A,B,C
C = ARRAY_FUNC(A,B) ! But you could not call ARRAY_FUNC directly.
END
Arguments with the OPTIONAL Attribute
When you pass an optional argument by reference, the address in the argument list
is zero if the argument is not present.
When you pass an optional argument by value, the value is zero if the argument is
not present. The compiler uses an extra register argument to differentiate that
value from a regular zero value. If the register has the value 1, the optional
argument is present; if it has the value 0, the optional argument is not present.
Related Information: See “Order of Arguments in Argument List” on page 365.
Arguments with the INTENT Attribute
Currently, declaring arguments with the INTENT attribute does not change the
linkage convention for a procedure. However, because this part of the convention
is subject to change in the future, we recommend not calling from non-Fortran
procedures into Fortran procedures that have INTENT(IN) arguments.
Type Encoding and Checking
Run-time errors are hard to find, and many of them are caused by mismatched
procedure interfaces or conflicting data definitions. Therefore, it is a good idea to
find as many of these problems as possible at compile or link time. To store type
information in the object file so that the linker can detect mismatches, use the
-qextchk compiler option.
Assembler-Level Subroutine Linkage Conventions
The subroutine linkage convention specifies the machine state at subroutine entry
and exit, allowing routines that are compiled separately in the same or different
languages to be linked. The information on subroutine linkage and system calls in
the AIX Commands Reference is the base reference on this topic. You should consult
it for full details. This section summarizes the information needed to write
mixed-language Fortran and assembler programs or to debug at the assembler
level, where you need to be concerned with these kinds of low-level details.
Interlanguage Calls 355