Shared library in Fortran, minimal example does not work

Question

I am trying to understand how to dynamically create and link a shared library in Fortran under Linux.

I have two files: The first one, liblol.f90, looks like this:

subroutine func()
    print*, 'lol!'
end subroutine func

I compile it with gfortran -shared -fPIC -o liblol.so liblol.f90

The second file, main.f90, looks like this:

program main
    call func()
end program main

When I now try to compile that with the command gfortran -L. -llol main.f90 -o main, I get the following error:

/tmp/ccIUIhcE.o: In function `MAIN__':
main.f90:(.text+0xa): undefined reference to `func_'
collect2: ld returned 1 exit status

I do not understand why it says "undefined reference", since the output of nm -D liblol.so gives me this:

                 w _Jv_RegisterClasses
0000000000201028 A __bss_start
                 w __cxa_finalize
                 w __gmon_start__
0000000000201028 A _edata
0000000000201038 A _end
0000000000000778 T _fini
                 U _gfortran_st_write
                 U _gfortran_st_write_done
                 U _gfortran_transfer_character_write
0000000000000598 T _init
00000000000006cc T func_

Is there any other parameter needed?

Answer 1

The only thing that has to be changed is the order of the arguments, as in

gfortran -L. main.f90 -llol -o main

Yes, only main.f90 and -llol are reversed. I hope this saves someone the year of his life I just lost on this. On a related note, if you are trying to compile a program which uses LAPACK or BLAS (which did not work for me and is why in the first place I tried to create a shared library myself), the same also applies. Write the name of the source file first:

gfortran mylapack.f90 -llapack -lblas -o mylapack

The reason for this can be found in the manual pages, see the top of http://gcc.gnu.org/onlinedocs/gcc/Link-Options.html for the option -l:

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in `z', those functions may not be loaded.