How to Link Fortran and C Binaries Using GCC: A Guide to Resolving Undefined Reference Errors?

Linking Fortran and C Binaries Using GCC

Linking projects that use both Fortran and C code can be challenging due to the incompatibilities between the associated libraries. When attempting to compile and link such projects using g or gfortran, undefined reference errors often occur.

For instance, consider the following code:

<code class="fortran">subroutine print_hi() bind(C)
  implicit none
  write(*,*) "Hello from Fortran."
end subroutine print_hi</code>

<code class="cpp">#include <iostream>

extern "C" void print_hi(void);

using namespace std;

int main() {
  print_hi();
  cout <p>Compiling these files with g   or gfortran separately produces object files, but linking them together using g   results in undefined references to Fortran libraries. Similarly, using gfortran results in undefined references to C   libraries.</p>
<p>To resolve this issue, it is necessary to link in the appropriate libraries during the linking stage. For g  , this can be achieved by adding the -lgfortran flag to the link command:</p>
<pre class="brush:php;toolbar:false">g++ main.o print_hi.o -o main -lgfortran

This flag will link in the necessary Fortran libraries. Alternatively, gfortran can be used with the -lstdc flag to link in the required C libraries:

gfortran main.o print_hi.o -o main -lstdc++

By specifying the appropriate linking flags, programmers can successfully link projects that combine Fortran and C code, allowing them to leverage the capabilities of both languages in their applications.

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