Procedure Interfaces

Every procedure has an interface, which consists of the name and characteristics of a procedure, the name and characteristics of each dummy argument, and the generic identifier (if any) by which the procedure can be referenced. The characteristics of a procedure are fixed, but the remainder of the interface can change in different scoping units.

If these properties are all known within the scope of the calling program, the procedure interface is explicit; otherwise it is implicit (deduced from its reference and declaration). The following table shows which procedures have implicit or explicit interfaces:

The interface of a recursive subroutine or function is explicit within the subprogram that defines it.

An explicit interface can appear in a procedure's definition, in an interface block, or both. (Internal procedures must not appear in an interface block.)

An abstract interface lets you give a name to a set of characteristics and argument keyword names that create an explicit interface to a procedure. It does not declare any actual procedure to have those characteristics.

You can use a procedure declaration statement to declare procedure pointers, dummy procedures, and external procedures. It specifies the EXTERNAL attribute for all procedure entities in the procedure declaration list.

You can use the IMPORT statement to make host entities accessible in the interface body of an interface block.

You can specify the ALLOCATABLE, OPTIONAL, or POINTER attributes for a dummy argument in a procedure interface that has the BIND attribute.

 INTERFACE
   SUBROUTINE Ext1 (x, y, z)
   REAL, DIMENSION (100,100) :: x, y, z
   END SUBROUTINE Ext1

   SUBROUTINE Ext2 (x, z)
   REAL x
   COMPLEX (KIND = 4) z (2000)
   END SUBROUTINE Ext2

   FUNCTION Ext3 (p, q)
   LOGICAL Ext3
   INTEGER p (1000)
   LOGICAL q (1000)
   END FUNCTION Ext3
 END INTERFACE