A Fortran 90 program consists of one or more program units. The principal kinds of program units are: the main program, external subprograms (user-written functions and subroutines), modules, and block data program units.
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 Procedure | 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 |
A function is invoked in an expression and 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, and does not return a particular value.
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.
This chapter contains information on the following topics: