How can I link a fortran program with a module dependency for use in c++?

Question

fortranModule.f90

module fortranModule
    use iso_c_binding
    implicit none
    contains

    subroutine print_f90()
        implicit none
        write(*,*) "This is a message from Fortran 90."
    end subroutine print_f90
end module fortranModule

fortranProgram.f90

program fortranProgram
    use iso_c_binding
    use fortranModule
    implicit none

    !Do some work as input for print_f90()
    call print_f90()
end program fortranProgram

main.cpp

extern "C" {
    extern void fortranProgram(void);
}

int main(void) {
    fortranProgram();
    return 0;
}

Attempting to compile with the following steps:

gfortran -c fortranModule.f90 fortranProgram.f90

g++ -c main.cpp

g++ main.o fortranModule.o fortraProgram.o -o main -lgfortran

gives

duplicate symbol _main in:
    main.o
    fortranProgram.o
ld: 1 duplicate symbol for architecture x86_64

How can I link a fortran program with module dependencies to a c++ program? Is this possible? Do I need to rewrite fortranProgram.f90 as a module or subroutine? Maybe there is a naming scheme similar to __modulename_MOD_subroutinename ?

Answer 1

Changing fortranProgram.f90 to

subroutine call_print_f90(num) bind(c, name='call_print_f90')
    use iso_c_binding
    use fortranModule
    implicit none

    call print_f90(num)
end subroutine call_print_f90

and changing main.cpp to

extern "C" {
    extern void call_print_f90(void);
}

int main(void) {
    call_print_f90();
    return 0;
}

resolves the problem of having two conflicting "mains".

Below is a solution in which fortranProgram.f90 is refactored as a module.

fortranProgram.f90

module fortranProgram
    use iso_c_binding
    use fortranModule
    implicit none
    contains

    subroutine call_print_f90() bind(c, name='call_print_f90')
        call print_f90(num)
    end subroutine call_print_f90
end module fortranProgram

main.cpp

extern "C" {
    extern void call_print_f90(void);
}

int main(void) {
    call_print_f90();
    return 0;
}