4

Recently, I was working on a project where some C++ subroutines are called by Fortran scripts (A Fortran solver intends to have some data-post-processing capability which is from a lib developed in C++). The following procedures replay the error producing process. Here I use quite simple Fortran and C++ scripts for a easy and clear demonstration.

A simple Fortran main program calls a CXX subroutine: The CXX subroutine - sub1.cxx:

    #include <stdio.h>
    using namespace :: std;
    extern "C" void func_c_();
    void func_c_()
    {
         printf("%d\n", 100);
    }

The Fortran main program - sub2.f90:

    program func_fortran
          implicit none
          call func_c()
    end program func_fortran

Compile them:

    g++ -c sub1.cxx
    gfortran -o test sub2.f90 sub1.o

We get the executable - test. Up to now, there is no problem.

Then we replace the sub1.cxx by sub1.1.cxx. It looks like:

    #include <iostream>
    using namespace :: std; 
    extern "C" void func_c_();
    void func_c_()
    {
         cout << "I am a CXX." << endl;
    }

The Fortran main program is totally the same with the previous one. We did not touch it.

Now we compile the codes:

    g++ -c sub1.1.cxx
    gfortran -o test sub2.f90 sub1.1.o

We can obtain sub1.1.o. But the error messages are thrown out:

    sub1.1.o: In function `func_c_':
    sub1.1.cxx:(.text+0xa): undefined reference to `std::cout'
    sub1.1.cxx:(.text+0xf): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
    sub1.1.cxx:(.text+0x14): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)'
    sub1.1.cxx:(.text+0x1c): undefined reference to `std::ostream::operator<<(std::ostream& (*)(std::ostream&))'
    sub1.1.o: In function `__static_initialization_and_destruction_0(int, int)':
    sub1.1.cxx:(.text+0x46): undefined reference to `std::ios_base::Init::Init()'
    sub1.1.cxx:(.text+0x55): undefined reference to `std::ios_base::Init::~Init()'
    collect2: error: ld returned 1 exit status

Compared with sub1.cxx, it seems some C++ standards in sub1.1.cxx e.g. std::cout can not be recognized. Any one could figure this problem out? Additionally, here I did not use the iso_c_binding from Fortran 2013. I tried this features in some other cases but it does not work either.

Improve this question
3
  • 2
    The FORTRAN linker doesn't know to link against the C++ runtimes, which is why you see those errors. Your first C++ snippet was essential just C code, which may explain why it managed to link. Commented Dec 16, 2019 at 20:35
  • 2
    Please - Fortran has officially been spelt Fortran (i.e. lower case) for almost 3 decades now. Commented Dec 16, 2019 at 21:12
  • Hi, Mansoor, hi, Lan Bush, thanks for the comments. Please see what Francesco posted. That is the right solution. In order to make a compatible compiling between C and C++, the link flag -lstdc++ should be used. Commented Dec 17, 2019 at 7:56

1 Answer 1

Reset to default
7

You need to link the executable against the c++ standard library:

g++ -c sub1.1.cxx
gfortran -o test sub2.f90 sub1.1.o -lstdc++

With gfortran/g++ you can also use g++ to link against the fortran library:

g++ -c sub1.1.cxx
gfortran -c sub2.f90
g++ -o test sub1.o  sub2.o -lgfortran
Improve this answer
Sign up to request clarification or add additional context in comments.

4 Comments

Hi @Hacunamatata if this or any answer has solved your question please consider accepting it by clicking the check-mark. This indicates to the wider community that you've found a solution and gives some reputation to both the answerer and yourself. There is no obligation to do this.
Hi, Fransesco. Many thanks for the comment. Now it works. I used to add the flag - -lstdc++ right after gfortran instead appending it to the last. Apparently, it is not the right way to define the linking behavior. Again, thanks for your kind comment.
@Hacunamatata Yes, order is important, see, e.g., here
The link you give here is really helpful. Thanks.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.