Internal Procedures

Internal procedures are functions or subroutines that follow a CONTAINS statement in a program unit. The program unit in which the internal procedure appears is called its host.

Internal procedures can appear in the main program, in an external subprogram, or in a module subprogram.

An internal procedure takes the following form:

CONTAINS

internal-subprogram
[internal-subprogram] ...

internal-subprogram
Is a function or subroutine subprogram that defines the procedure. An internal subprogram must not contain any other internal subprograms.

Rules and Behavior

Internal procedures are the same as external procedures, except for the following:

• Only the host program unit can use an internal procedure.
  
  
• An internal procedure has access to host entities by host association; that is, names declared in the host program unit are useable within the internal procedure.
  
  
• In Fortran 95/90, the name of an internal procedure must not be passed as an argument to another procedure. However, Compaq Fortran allows an internal procedure name to be passed as an actual argument to another procedure.
  
  
• An internal procedure must not contain an ENTRY statement.

An internal procedure can reference itself (directly or indirectly); it can be referenced in the execution part of its host and in the execution part of any internal procedure contained in the same host (including itself).

The interface of an internal procedure is always explicit.

Examples

The following example shows an internal procedure:

PROGRAM COLOR_GUIDE
...
CONTAINS
  FUNCTION HUE(BLUE)   ! An internal procedure
  ...
  END FUNCTION HUE
END PROGRAM

The following example program contains an internal subroutine find, which performs calculations that the main program then prints. The variables a, b, and c declared in the host program are also known to the internal subroutine.

 program INTERNAL
 ! shows use of internal subroutine and CONTAINS statement
    real a,b,c
    call find
    print *, c
 contains
    subroutine find
     read *, a,b
     c = sqrt(a**2 + b**2)
    end subroutine find
 end

For More Information:

• See Functions, Subroutines, and Statement Functions.
• See Host association.
• See Procedure Interfaces.
• See CONTAINS.