linker - How to call a function in Fortran that is defined in a separate file?

Question

I am trying to compile some really old code (1986 and before). This code references an external function. Today's compilers ask for far more code to make this work. And I keep failing. I now created a small hello world program, which demonstrates the problem.

hello.for

  PROGRAM hello
    USE func        
    PRINT *, "Hello World!"
    PRINT *, f ()    
  END PROGRAM hello

func.for

  MODULE func
    PUBLIC f
  CONTAINS
    FUNCTION f () 
        f='Hello Func'
    END FUNCTION
  END MODULE

This has not only one, but two problems:

• How do I define the return type? Docs tell <type> FUNCTION <function> or FUNCTION <function> () <type>::<something> , but neither works.
• How do I make the linker find the function?

gfortran -c func.for works (if I use the default return type real) and creates a mod file but linking does not work

$ gfortran  hello.for 
/tmp/ccHNzcXA.o: In function `MAIN__':
hello.for:(.text+0xa4): undefined reference to `__func_MOD_f'
collect2: error: ld returned 1 exit status

__func_MOD_f is not contained in the mod file, but in the o file there is func.for__func_MOD_f.

Any idea?

thanks

Answer 1

You have two issues, the delcaration of f and properly linking the module.

First, compiling the module yields the error:

% gfortran -c func.f
func.f:5:8:

       f='Hello Func'
        1
Error: Can't convert CHARACTER(1) to REAL(4) at (1)

This error is due to implicit typing of f and an incompatible assignment. Fixing this is simple, declare f explicitly as a character instead of an implicit type. Add:

character(len=30) :: f

to the function and now your module compiles. Here is the modified module:

  MODULE func
  PUBLIC f
  CONTAINS
  FUNCTION f ()
  character(len=30) :: f
  f='Hello Func'
  END FUNCTION
  END MODULE

---

Your second problem is linking. Your command:

gfortran  hello.for  

fails because you did not specify the module object. If you already compiled the module you would specify:

gfortran hello.for func.o

if you were compiling them both at the same time you would do:

gfortran -o hworld func.for hello.for

if you are compiling everything individually:

gfortran -c func.for
gfortran -c hello.for
gfortran -o hworld hello.o func.o

Any of these will compile and run:

% ./hworld 
 Hello World!
 Hello Func   

---

If you are modernizing your code, it would also be worth adding implicit none to avoid any implicit typing and declaring explicit variables for everything. e.g.:

module func
  implicit none
contains
function f 
  implicit none
  character(len=30) :: f
  f='Hello Func'
end function f
end module func

and

program hello
  use func
  implicit none        
  print *, "Hello World!"
  print *, f ()    
end program hello