Fortran 90 Handbook

Program Units 11There are several kinds of executable and nonexecutable program units inFortran. Each of these program units provides unique functionality. Theexecutable program units are the main program and procedure subprograms;the nonexecutable program units are block data units, which are noweffectively obsolescent, and modules, which provide definitions used by otherprogram units. This chapter describes each of these as well as the closelyrelated concepts of host association and use association. Only one of theseprogram units—the module—is new in Fortran 90, although internalprocedures, which are procedure subprograms, represent a significant additionto the executable program units. A section on the major uses of modules isincluded.A complete Fortran program might be compared to a complete meal. There area number of parts to a meal—the main course, salad, dessert, etc. Each coursehas a specific part in the meal, just like each program unit (main program,subroutine, etc.) plays a specific part in a complete program. The parts worktogether, and in a well-planned program, as with a well-designed meal, the netresult is something analogous to the whole being greater than the sum of itsparts.11.1 OverviewA Fortran program is a collection of program units. One and only one of theseunits must be a main program. In all but the simplest programs, the individualtasks are typically organized into a collection of function and subroutinesubprograms and module program units. The program may be organized so483Copyright © 1992 J. Adams, W. Brainerd, J. Martin, B. Smith, and J. Wagener