Linking Fortran and C++ binaries using gcc

Question

I can use gcc to make calls between C and C++ or between C and Fortran by using g++ or gfortran, respectively. But if I try to make procedure calls between C++ and Fortran I get errors when compiling with either g++ or gfortran because neither knows about the other's required libraries.

How can I link a project that uses source code written in both C++ and Fortran?

$ cat print_hi.f90
subroutine print_hi() bind(C)
  implicit none
  write(*,*) "Hello from Fortran."
end subroutine print_hi

$ cat main.cpp
#include <iostream>

extern "C" void print_hi(void);

using namespace std;

int main() {
  print_hi();
  cout << "Hello from C++" << endl;
  return 0;
}
$ gfortran -c print_hi.f90 -o print_hi.o
$ g++ -c main.cpp -o main.o

I try linking with g++:

$ g++ main.o print_hi.o -o main
print_hi.o: In function `print_hi':
print_hi.f90:(.text+0x3f): undefined reference to `_gfortran_st_write'

and further errors regarding undefined references.

And with gfortran:

$ gfortran main.o print_hi.o -o main
main.o: In function `main':
main.cpp:(.text+0xf): undefined reference to `std::cout'

...and so forth.

How can I link binaries using the gfortran and g++ libraries together?

Answer 1

You're looking for g++ main.o print_hi.o -o main -lgfortran to link in the standard Fortran libraries.

You can also use gfortran by passing -lstdc++.