Calling Fortran code from C++ on Visual Studio 2015

Author Note: This post was originally hosted on my previous webpage. Some information are updated in December 2020

In case you have to use some Fortran libraries on Windows, there are a few free options:

1. Bash on Ubuntu on Windows (a.k.a. WSL Windows Subsystem for Linux)
2. MING-W64 + Visual Studio
3. Cygwin environment
4. PGI Fortran Compiler in their free community version (now Nvidia)
5. As of Dec 2020, it seems that Intel’s compilers are now free (?).

This post is about option 2. However, option 1 is obviously the most portable way..

Anyway, here I describe how to call Fortran code from C++ using Visual Studio. I will be focusing on MinGW gfortran as that is a popular free Fortran compiler on Windows platform.

System Requirements:

• Visual Studio 2015 (C++) (e.g., free community version)
• MinGW64 gfortran (64bit, using the one from Msys2 here. Make sure you are using the one from mingw64 not msys)

My Fortran code is just a simple function in a module (testfor.f90):

module testfor
    use, intrinsic :: iso_c_binding
    implicit none
contains
    subroutine testingfor(x) bind(c, name="testingfor")
        implicit none
        double precision, intent(in) :: x
 
        print *, "# calling testfor in Fortran!!"
        print *, "x = ", x 
    end subroutine
end module testfor

For simplicity I put this file in the same folder as my C++ codes. Note that I am using Fortran 2003 standard iso_c_binding, which is a standardized way for C/Fortran interface. The bind syntax allows you to customize the function name to be called in C++. I used the same name testingfor in the C++ function below. If you are using the old (pre-2003) style, the naming convention for a function in a module may be a little complicated. The new standard helps clean up the code a little.

Next, my C++ main function is very simple (main.cpp):

extern "C" {
    void testingfor(double* x);
}
 
int main(int argc, char* argv[]) {
    double y = 1.0;
 
    testingfor(&y);
}

The extern C syntax acts as an interface. Note that the input argument for the Fortran function must be a pointer as Fortran is pass-by-reference only.

The main dish: In Windows, we need an “import” library (.lib) and a dynamic library (.dll). The former is used in the compile-time and the latter is for run-time. To generate these files, we need to the following steps (in the Windows’ command prompt):

gfortran -c testfor.f90
 
dllwrap.exe --export-all-symbols testfor.o -lgfortran --output-def testfor.def -o testfor.dll

lib.exe /machine:x64 /def:testfor.def

Above commands work in a command prompt if you have set the path correctly. In short, gfortran and dllwrap are the ones in msys2, and lib.exe is a Windows command. The first command is simply to compile our Fortran module into an object file (testfor.o). Second command creates a definition file (.def) and a dll file (.dll). The dllwrap.exe comes with MinGW as for as I know. We may ignore the following error:

dllwrap.exe: no export definition file provided.
Creating one, but that may not be what you want

The last line with lib.exe in the previous code block is to create the import lib file with the def file. Now we have what we want: testfor.lib and testfor.dll.

The final step is to set up the Visual Studio to include the Fortran library files. I don’t go over the details here. In short, we need to add testfor.lib to the resource folder and let VS know you need it to compile. This can be done by adding testfor.lib into Project-> Properties-> Configuration Properties-> Linker-> Input-> Additional Dependencies. Note that the resultant exe file does require the dynamic library testfor.dll in the PATH in order to run. Here is the result:

Trouble-shooting

• In some cases (pointed by some readers), there are also some DLL files from msys2 that need to be included. They can be found by running ldd against some executable generated by gfortran in msys2 shell. For examples,
  • libgfortran-5.dll, libquadmath-0.dll, libgcc_s_seh-1.dll, libwinpthread-1.dll

Hopefully this is useful. Reference: http://www.cs.cmu.edu/~barbic/arpack.html

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