Modules - Global Data

  COMMON blocks have long been regarded as an unsafe feature of Fortran. In FORTRAN 77 they were the only way to achieve global storage; Fortran 90 implements a new mechanism to allow specific procedures to have access to global data. This can be done by:

• declaring the required global objects within a module.
• giving these objects the SAVE attribute.
• attaching the module (via USE) to each program unit which requires access to the global data. (Wherever the MODULE is used, the global objects are visible.)

For example, to declare pi as a global constant

  MODULE  Pye
   REAL, SAVE :: pi = 3.142
  END MODULE Pye

  PROGRAM Area
   USE Pye
   IMPLICIT NONE
   REAL :: r
    READ*, r
    PRINT*, "Area= ",pi*r*r
  END PROGRAM Area

In the example the module Pye defines global data (the constant Pi). Its value can be accessed by USE ing the module.

Name clashes caused by the above can be avoided by using the object renaming facility, (see Section 5.2.5).

If the use of a COMMON block is absolutely essential then a single instance of it should be placed in a MODULE and then used. This will have the effect of ensuring that each time the COMMON block is accessed it will be laid out in exactly the same way and mimics the techniques of placing COMMON blocks in INCLUDE files (a common FORTRAN 77 technique).

• Module Global Data Example
• Module Global Data Example
• Visualisation of Global Storage