Argument Association in Procedures

Procedure arguments provide a way for different program units to access the same data.

When a procedure is referenced in an executable program, the program unit invoking the procedure can use one or more actual arguments to pass values to the procedure's dummy arguments. The dummy arguments are associated with their corresponding actual arguments when control passes to the subprogram.

In general, when control is returned to the calling program unit, the last value assigned to a dummy argument is assigned to the corresponding actual argument.

An actual argument can be a variable, expression, or procedure name. The type and kind parameters, and rank of the actual argument must match those of its associated dummy argument.

A dummy argument is either a dummy data object, a dummy procedure, or an alternate return specifier (*). Except for alternate return specifiers, dummy arguments can be optional.

If argument keywords are not used, argument association is positional. The first dummy argument becomes associated with the first actual argument, and so on. If argument keywords are used, arguments are associated by the keyword name, so actual arguments can be in a different order than dummy arguments.

A keyword is required for an argument only if a preceding optional argument is omitted or if the argument sequence is changed.

A scalar dummy argument can be associated with only a scalar actual argument.

If a dummy argument is an array, it must be no larger than the array that is the actual argument. You can use adjustable arrays to process arrays of different sizes in a single subprogram.

An actual argument associated with a dummy argument that is allocatable or a pointer must have the same type parameters as the dummy argument.

A dummy argument referenced as a subprogram must be associated with an actual argument that has been declared EXTERNAL or INTRINSIC in the calling routine.

If a scalar dummy argument is of type character, its length must not be greater than the length of its associated actual argument.

If the character dummy argument's length is specified as *(*) (assumed length), it uses the length of the associated actual argument.

Once an actual argument has been associated with a dummy argument, no action can be taken that affects the value or availability of the actual argument, except indirectly through the dummy argument. For example, if the following statement is specified:

  CALL SUB_A (B(2:6), B(4:10))

B(4:6) must not be defined, redefined, or become undefined through either dummy argument, since it is associated with both arguments. However, B(2:3) is definable through the first argument, and B(7:10) is definable through the second argument.

Similarly, if any part of the actual argument is defined through a dummy argument, the actual argument can only be referenced through that dummy argument during execution of the procedure. For example, if the following statements are specified:

  MODULE MOD_A
    REAL :: A, B, C, D
  END MODULE MOD_A

  PROGRAM TEST
    USE MOD_A
    CALL SUB_1 (B)
    ...
  END PROGRAM TEST

  SUBROUTINE SUB_1 (F)
    USE MOD_A
    ...
    WRITE (*,*) F
  END SUBROUTINE SUB_1

Variable B must not be directly referenced during the execution of SUB_1 because it is being defined through dummy argument F. However, B can be indirectly referenced through F (and directly referenced when SUB_1 completes execution).

The ultimate argument is the effective argument if the effective argument is not a dummy argument or a subobject of a dummy argument. If the effective argument is a dummy argument, the ultimate argument is the ultimate argument of that dummy argument. If the effective argument is a subobject of a dummy argument, the ultimate argument is the corresponding subobject of the ultimate argument of that dummy argument.

Consider the following sequence of subroutine calls:

INTEGER :: X(100)
CALL SUBA (X)
...
SUBROUTINE SUBA(A)
INTEGER :: A(:)
CALL SUBB (A(1:5), A(5:1:-1))
...
SUBROUTINE SUBB(B, C)
INTEGER :: B(:), C(:)

The ultimate argument of B is X(1:5). The ultimate argument of C is X(5:1:-1), which is not the same object as the ultimate argument of B.

The following sections provide more details on arguments:

• Optional arguments

• The different kinds of arguments:

  • Array arguments

  • Pointer arguments

  • Passed-Object Dummy Arguments

  • Assumed-length character arguments

  • Character constant and Hollerith arguments

  • Alternate return arguments

  • Dummy procedure arguments

  • Coarray Dummy Arguments

• References to generic procedures

• References to non-Fortran procedures (%REF, %VAL, and %LOC)