The block is
defined where the variables are declared:
COMMON
[/<name>/] <list of variables to share>
Then in the
external subprogram, local variables are mapped to local variables:
COMMON
[/<name>/] <list of local variables of correct types>
Though the names
of the local variables in the external subprogram do not have to match
the original names, the types of the variables do. Arrays may also be
passed in COMMON blocks along with variables for their dimensions. But
this requires that an array of exactly the same dimensions of the COMMON
array to already have been locally declared.
There are a few
occasions where COMMON blocks are necessary, however they should be
avoided whenever parameter passing is an option.
MODULES and Dynamic Array Allocation
FORTRAN 90 has
included MODULE to allow blocks of parameters, variables, and
subprograms to be both shared and re-used. The basic structure is as
follows:
MODULE
<name>
<specifications>
END
MODULE <name>
This
module can then be passed to another program through USE. An optional
command ONLY limits the parts of the module that can be referenced.
USE
<name> [ONLY : <partial list of variables in module>]
Modules
also allow for the passing of arrays whose size is not specified in the
program or subprogram referencing them. The Array itself is defined as
ALLOCATABLE in a module, along with a variable for it's size. The values
in that module can be used to later allocate the array, reference the
array in programs and subprograms, and deallocate the array. See the
following site for an example of a program that does this.