fortran90 - Wrong result when using a global variable in Fortran

Question

I'm learning the basics of Fortran. I created a simple subroutine initializing a matrix:

program test
   integer, parameter :: n = 1024
   real :: a(n, n)
   call init(a)
   write (*, *) a(1, 1)
end program

subroutine init(a)
   real :: a(n, n)
   a(:, :) = 3.0
end subroutine

Then the output is 0.0 instead of expected 3.0. Apart from that, valgrind says that:

==7006== Conditional jump or move depends on uninitialised value(s)
==7006==    at 0x400754: init_ (in /home/marcin/proj/mimuw/fortran/test)
==7006==    by 0x4007A4: MAIN__ (in /home/marcin/proj/mimuw/fortran/test)
==7006==    by 0x40083B: main (in /home/marcin/proj/mimuw/fortran/test)

Why? The n parameter is correctly recognized by the compiler and should be a global one.

I compiled the program with gfortran 6.3.1

Answer 1

n is not a global variable, it is a local variable of the main program.

The subroutine is a completely independent compilation unit from the main program and they do not share any information.

A subroutine can "see" other variables of the parent module, if it is a module procedure, or variables of a parent (host) procedure or program if it is an internal procedure.

Be sure to read about the structure of Fortran programs and use modules as much as possible. Prefer modules over internal procedures. You will see how to put the subroutine into a module or how to make it internal to the main program in the link.

I did not mention common blocks, just don't use them, they are obsolete. And rember to use implicit none in every compilation unit.