Undefined reference to subroutine from module Fortran90

Question

I'm relatively new to fortran90 that I need for a project.

I have three scripts, 2 modules and the main program with the following structure:

script 1:

program main
   use module1
   implicit none
   ..
   call sub_from_mod1
end program main

script 2:

module module_2
   implicit none
   contains
   ..
end module module_2

script 3:

module module_1
   use module_2
   implicit none
   contains 
   ...
   subroutine sub_from_mod1
   ...
end module module_1

When compiling in CodeBlocks the whole project, I get an error:

undefined reference to sub_from_mod1_

Does anyone know what is the case?

Welcome, please take the [tour] and read [ask] and [mcve]. Please show the **actual code** and **actual error messages**. Also, use tag [tag:fortan] for all Fortran questions. — Vladimir F Героям слава, Jun 26 '21 at 15:37
Exactly what you did to compile and link the code could be useful as well - the actual commands. — Ian Bush, Jun 26 '21 at 16:30

score 1 · Answer 1 · answered Jun 27 '21 at 09:19

There is a typo in the use statement of the main program: correct would be

use module_1 !not use module1

Placing the main file in the file main.f90 and the modules in mod1.f90 and mod2.f90 I can compile the code with:

gfortran -c mod2.f90
gfortran -c mod1.f90
gfortran -c main.f90
gfortran main.o mod1.o mod2.o

The -c flag on the first three lines tells the compiler just compile and not link the code. This is necessary because the .mod files need to exist when the program is linked on the last line.

I doubt this is the real reason, the error message would be different if the module did not exist. I fear the code we were shown is severely oversimplified or even crippled. — Vladimir F Героям слава, Jun 27 '21 at 14:42

Undefined reference to subroutine from module Fortran90

1 Answers1