HP Fortran Programmer's Guide (September 2007)

Controlling data storage
Modules vs. common blocks
Chapter 3 113
Modules vs. common blocks
The common block has been a mainstay of Fortran programs throughout the evolution of the
language, and it continues to be a part of Fortran. The common block provides a convenient
means to share data among program units, especially when the program units sharing data
do not otherwise communicate with each other. The common block can also be used to share
data between simultaneously executing Fortran programs (see “Sharing data among
programs” on page 109) and between Fortran program units and C functions linked together
in the same program (see “Sharing data” on page 195).
One of the problems with the common block, however, is that the programmer must replicate
the COMMON declaration in each of the sharing program units. If any of the common variables
are out of order or have a different type or size, the program units may not access the same
data. The compiler gives no indication of this discrepancy because it assumes that the
programmer is giving one program unit a different view of the shared storage—even when the
discrepancy is owing to oversight.
To deal with this problem, many implementations of FORTRAN 77 have provided the
INCLUDE extension. This extension enables the user to centralize common block definitions in
one file. At compile-time, the compiler reads the file into program units that have the
INCLUDE line. While this approach eliminates the problem of discrepant common blocks, it
introduces another problem: the INCLUDE facility is nonstandard FORTRAN 77, and its use is
nonportable.
To deal with the portability issue, Standard Fortran defines the INCLUDE line. Unfortunately,
the definition in the Standard leaves many of the details up to the implementation, so that
use of the INCLUDE line in Fortran programs still runs the risk of nonportability.
Another problem with the common block—especially when used with equivalencing—is that
it can inhibit optimization. Common block variables are generally ineligible for register
allocation, and aliasing variables in common can prevent the optimization of the program
units that use the aliased variables.
The module program unit is the Fortran answer to the common block. The programmer
declares shareable variables in a module. Any program unit that wants to access them
references the name of the module in a USE statement. The concept of the module eliminates
the need to re-declare the common variables, without requiring the INCLUDE line.
In addition, the module provides the following controls on access to module data:
• The PUBLIC and PRIVATE statements declare which module variables are accessible
outside the module and which are not.