# 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.

## Description

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.

- 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

Parent topic: Program Units and Procedures

## See Also

Functions, Subroutines, and Statement Functions
Host association
Procedure Interfaces
CONTAINS statement