Intel® Fortran Compiler 18.0 Developer Guide and Reference
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. |
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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.