Linking Error : Calling external function throws undefined refernce error

[assem2002]

Calling a function or subroutine from another file throws error.

func.f90

function f() result(d) 
    implicit none 
    integer :: d 
    d = 9
end function f

prog.f90

program a
    implicit none
    integer :: c
    integer  :: f 
    external :: f
    c = f()
    print *, c
end program a

assem@assem-PC:~/Desktop/fortran/test_05$ ../../../lfortran/src/bin/lfortran --implicit-interface  -c func.f90 
assem@assem-PC:~/Desktop/fortran/test_05$ ../../../lfortran/src/bin/lfortran --implicit-interface  -c prog.f90 
assem@assem-PC:~/Desktop/fortran/test_05$ ../../../lfortran/src/bin/lfortran --implicit-interface  -o mmm func.o prog.o 
/usr/bin/ld: prog.o: in function `main':
LFortran:(.text+0x7): undefined reference to `f'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
The command 'clang -o mmm func.o prog.o  -L"/home/assem/lfortran/src/bin/../runtime" -Wl,-rpath,"/home/assem/lfortran/src/bin/../runtime" -llfortran_runtime -lm' failed.
assem@assem-PC:~/Desktop/fortran/test_05$ 

[assem2002]

same as in #4009

[kmr-srbh]

@assem2002 the above error occurs because there is no function f() defined inside prog.f90. I think this might be the correct way to do what you wished to achieve.

prog.f90

program a
   implicit none
   integer :: c
   interface f1
      integer function f() result(d)
      end function
   end interface f1
   c = f()
   print *, c
end program a

function f() result(d)
   integer :: d
   d = 9
end function f

This gives the required output

(base) saurabh-kumar@Awadh:~/Projects/System/lfortran$ lfortran ./examples/example.f90
9

[kmr-srbh]

For loading the function from an external module, we can do

func.f90

module func
   implicit none
contains
   function f() result(d)
      integer :: d
      d = 9
   end function f
end module func

prog.f90

program a
   use func
   implicit none
   integer :: c
   c = f()
   print *, c
end program a

Which gives the output

(base) saurabh-kumar@Awadh:~/Projects/System/lfortran$ lfortran ./examples/example.f90
9

[assem2002]

@kmr-srbh. Yeah you're right about your suggested code.but we should be able to call an external function that's defined in another file using external keyword.
gfortran compiles these files and the example files mentioned above normally with no issue.
We have to support such a thing because lapack uses this way to build its own library . (Note that lapack uses Fortran 77 style which doesn't support module)

[kmr-srbh]

@assem2002 could you please share the steps you followed to compile this using GFortran? How does the compiler know where f is present?

[assem2002]

@kmr-srbh

assem@assem-PC:~/Desktop/fortran/test_05$ gfortran -c func.f90 
assem@assem-PC:~/Desktop/fortran/test_05$ gfortran -c prog.f90 
assem@assem-PC:~/Desktop/fortran/test_05$ gfortran -o mmm func.o prog.o 
assem@assem-PC:~/Desktop/fortran/test_05$ ./mmm 
           9

I think it just compiles prog.f90 and the object file waits for the undefined variables to be found in linking phase. for sure it doesn't have the advantages that mod file offers so that is why LFortran requires --implicit-interface flag to be used if we call an external function in our code.

[kmr-srbh]

Looks like we are dealing with legacy code here.

[assem2002]

as @Pranavchiku mentioned it would compile normally with --generate-object-code flag.
-c flag in gfortran handles this issue with no problem. should we do the same in LFortran? If not just let's close this issue.