# Program Units and Procedures

A Fortran program consists of one or more program units. There are four types of program units:

- Main program

  The program unit that denotes the beginning of execution. It may or may not have a PROGRAM statement as its first statement.

- External procedures

  Program units that are either user-written functions or subroutines.

- Modules and submodules

  Program units that contain declarations, type definitions, procedures, or interfaces that can be shared by other program units. A module can be extended by one or more program units called submodules. A submodule can in turn be extended by one or more submodules.

- Block data program units

  Program units that provide initial values for variables in named common blocks.

A program unit does not have to contain executable statements; for example, it can be a module containing interface blocks for subroutines.

A procedure can be invoked during program execution to perform a specific task. It specifies the EXTERNAL attribute for all procedure entities in the procedure declaration list. A procedure declaration is denoted by a PROCEDURE statement.

There are several kinds of procedures, as follows:

Kind of Procedure | Description
--- | ---
External Procedure | A procedure that is not part of any other program unit.
Module Procedure | A procedure defined within a module.
Internal Procedure1 | A procedure (other than a statement function) contained within a main program, function, or subroutine.
Intrinsic Procedure | A procedure defined by the Fortran language.
Dummy Procedure | A dummy argument specified as a procedure or appearing in a procedure reference. A dummy procedure with the POINTER attribute is a dummy procedure pointer.
Procedure Pointer | A procedure that has the EXTERNAL and POINTER attributes. It may be pointer associated with an external procedure, a module procedure, an intrinsic procedure, or a dummy procedure that is not a procedure pointer.
Statement function | A computing procedure defined by a single statement.
1 The program unit that contains an internal procedure is called its host.

A function is invoked in an expression using the name of the function or a defined operator. It returns a a single value (function result) that is used to evaluate the expression.

A subroutine is invoked in a CALL statement or by a defined assignment statement. It does not directly return a value, but values can be passed back to the calling program unit through arguments (or variables) known to the calling program.

Recursion (direct or indirect) is permitted for functions and subroutines.

A procedure interface refers to the properties of a procedure that interact with or are of concern to the calling program. A procedure interface can be explicitly defined in interface blocks. All program units, except block data program units, can contain interface blocks.

Parent topic: Language Reference

- Main Program
- Procedure Characteristics
- Modules and Module Procedures
- Intrinsic Modules
- Block Data Program Units Overview
- Functions, Subroutines, and Statement Functions
- External Procedures
- Internal Procedures
- Argument Association in Procedures
- Procedure Interfaces
- Interoperability of Procedures and Procedure Interfaces
- Procedure Pointers

## See Also

Program structure
Intrinsic procedures
Scope
RECURSIVE keyword