A Fortran 95/90 program consists of one or more program units. There are four types of program units:
The program unit that denotes the beginning of execution. It may or may not have a PROGRAM statement as its first statement.
Program units that are either user-written functions or subroutines.
Program units that contain declarations, type definitions, procedures, or interfaces that can be shared by other 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. 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 | |
| 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 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.
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