You only need one instance of a module. You make it known to any main program or procedure (subroutine or function) that uses it with a use
statement. If you have a subroutine that sets values then, like any other, it has to have a use
statement. If are are setting initial values you can do so in the declaration. If the module is used by the main program then it will always be in scope and the values of the variables will persist throughout the run of the program. If the module is only used by procedure, in principle the module will go out of scope when none of those procedures are in the call chain and the compiler is allowed to forget the values of the module variables. (It is doubtful that any Fortran compiler actually does this.) This can be prevented by declaring each variable with SAVE
. SAVE
is implicit if you declare the variable with an initial value.
Normally you have to compile the modules first before they are used so that the compiler "knows" about them when it encounters the use statement. This is done either by putting them first in the file or compiling their files first. Here is your example reordered:
module Words
character*5 Greet
save
end module
module Vals
double precision x,y
integer i
save
end module
module my_subs
contains
subroutine foo ()
use Words
use Vals
double precision :: z
print *, Greet
z = x + y
print *, z
print *, i
end subroutine
end module my_subs
program main
use Words
use Vals
use my_subs
Greet = 'Hello'
x = 4.1
y = 5.2
i = 3
call foo ()
end program main