# INTERFACE

Statement: Defines an explicit interface for an external or dummy procedure. It can also be used to define a generic name for procedures, a new operator for functions, and a new form of assignment for subroutines.

INTERFACE [generic-spec]

[interface-body]...

[[MODULE]PROCEDURE [::]name-list]...

END INTERFACE [generic-spec]

generic-spec | (Optional) Is one of the following: A generic name For information on generic names, see Defining Generic Names for Procedures. OPERATOR ( op) Defines a generic operator ( op). It can be a defined unary, defined binary, or extended intrinsic operator. For information on defined operators, see Defining Generic Operators. ASSIGNMENT (=) Defines generic assignment. For information on defined assignment, see Defining Generic Assignment.
interface-body | Is one or more function or subroutine subprograms or a procedure pointer. A function must end with END FUNCTION and a subroutine must end with END SUBROUTINE. The subprogram must not contain a statement function or a DATA, ENTRY, or FORMAT statement; an entry name can be used as a procedure name. The subprogram can contain a USE statement. It can also contain the prefix MODULE before FUNCTION or SUBROUTINE to indicate a separate module procedure.
name-list | Is the name of one or more nonintrinsic procedures that are accessible in the host. The MODULE keyword is only allowed if the interface block specifies a generic-spec and has a host that is a module, or accesses a module by use association. The characteristics of module procedures or internal procedures are not given in interface blocks, but are assumed from the module subprogram definitions or the USE associated interfaces.

## Description

Interface blocks can appear in the specification part of the program unit that invokes the external or dummy procedure.

A generic-spec can only appear in the END INTERFACE statement if one appears in the INTERFACE statement; they must be identical.

The characteristics specified for the external or dummy procedure must be consistent with those specified in the procedure's definition.

An interface block must not appear in a block data program unit.

An interface block comprises its own scoping unit, and does not inherit anything from its host through host association.

Internal, module, and intrinsic procedures are all considered to have explicit interfaces. External procedures have implicit interfaces by default; when you specify an interface block for them, their interface becomes explicit. A procedure must not have more than one explicit interface in a given scoping unit. This means that you cannot include internal, module, or intrinsic procedures in an interface block, unless you want to define a generic name for them.

The function or subroutine named in the interface-body cannot have the same name as a keyword that specifies an intrinsic type.

A interface block containing generic-spec specifies a generic interface for the following procedures:

- The procedures within the interface block

  Any generic name, defined operator, or equals symbol that appears is a generic identifier for all the procedures in the interface block. For the rules on how any two procedures with the same generic identifier must differ, see Unambiguous Generic Procedure References.

- The module procedures listed in the MODULE PROCEDURE statement

  The module procedures must be accessible by a USE statement.

To make an interface block available to multiple program units (through a USE statement), place the interface block in a module.

The following rules apply to interface blocks containing pure procedures:

- The interface specification of a pure procedure must declare the INTENT of all dummy arguments except pointer and procedure arguments.

- A procedure that is declared pure in its definition can also be declared pure in an interface block. However, if it is not declared pure in its definition, it must not be declared pure in an interface block.

## Example

The following example shows a simple procedure interface block with no generic specification:

    SUBROUTINE SUB_B (B, FB)
      REAL B
      ...
      INTERFACE
        FUNCTION FB (GN)
          REAL FB, GN
        END FUNCTION
      END INTERFACE

The following shows another example:

    !An interface to an external subroutine SUB1 with header:
    !SUBROUTINE SUB1(I1,I2,R1,R2)
    !INTEGER I1,I2
    !REAL R1,R2
    INTERFACE
      SUBROUTINE SUB1(int1,int2,real1,real2)
        INTEGER int1,int2
        REAL real1,real2
      END SUBROUTINE SUB1
    END INTERFACE
    INTEGER int
    ...

Parent topic: H to I

## See Also

ABSTRACT INTERFACE
CALL
PROCEDURE
FUNCTION
MODULE
SUBMODULE
MODULE PROCEDURE
SUBROUTINE
PURE
Procedure Interfaces
Procedures that Require Explicit Interfaces
Use and Host Association