C++-Fortran mixcompile unresolved externals - c++

Question

I am trying to make a fortran-C++ mix compile project. I use the case to test how the visual studio 2017 compiler link the intel fortran .lib file. Here is the example code:
Fortran code here:
!DEC$ IF DEFINED (_DLL)
!DEC$ ATTRIBUTES DLLEXPORT :: ADD
!DEC$ END IF
REAL*8 FUNCTION ADD (A, B, C, D)
REAL*8 B,D
INTEGER*4 A,C
DIMENSION B(4), D(4)
ADD = B(A) + D(C)
RETURN
END
C++ code here:
#include <iostream>
extern "C" {double add_(int *, double[], int *, double[]); }
double ar1[4] = { 1.0, 2.0, 3.0, 4.0 };
double ar2[4] = { 5.0, 6.0, 7.0, 8.0 };
int main()
{
int x, y;
double z;
x = 3;
y = 3;
z = add_(&x, ar1, &y, ar2); /* Call Fortran add routine */
/* Note: Fortran indexes arrays 1..n */
/* C indexes arrays 0..(n-1) */
std::cout << z << std::endl;
return 0;
}
The project setup procedures are:
Open VS studio, create a c++ empty project, add the c++ code in the source file. Solution will be automatically created as well.
Add a new fortran static library project in the solution. Add the fortran code in the source file.
Fortran project-> properties->Libraries->Runtime Libraries-> Debug multithread dll.
Build the fortran project, find the .lib file directory.
C++ project-> linker -> General -> Additional Library Dependencies -> add the .lib file directory here
C++ project-> linker -> Input -> Additional dependencies -> Add .lib file name here end with ";".
Build C++ project.
And the compiler error message is:
1>------ Build started: Project: Cpp_main, Configuration: Debug Win32 ------
1>Source.obj : error LNK2019: unresolved external symbol _add_ referenced in function _main
1>C:\Users\Documents\TEST_Project\MixCompile_Test\Mix_compile\Debug\Cpp_main.exe : fatal error LNK1120: 1 unresolved externals
1>Done building project "Cpp_main.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 1 up-to-date, 0 skipped ==========
Why the compiler cannot resolve the external function call even if I assign the .lib directory in C++ project?
Thanks a lot!
Update 22-6-24:
Thank you Steve. I followed your instruction and made configuration in VC++ directories to add the ifortran include and lib directory into it. But new problem happened:
1>------ Build started: Project: Cpp_main, Configuration: Debug Win32 ------
1>LINK : fatal error LNK1104: cannot open file 'msvcprtd.lib'
1>Done building project "Cpp_main.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 2 up-to-date, 0 skipped ==========
Update 22-06-24
I solved the issue. I will upload the whole solution tomorrow. It's really special problem for VS2017.
Fortran compiler: Inter ifortran19
Cpp compiler: MSVC 2017
Step:
Create your CPP project and solution.
In the solution, add a fortran static library project with name .
Add your fortran headfile and source code in that project.
For every fortran subroutine that will be called in CPP, do like this:
subroutine_name() BIND(C)
USE, INTRINSIC :: ISO_C_BINDING
end subroutine
For CPP headfile, if the fortran compiler is interl fortran, do not add underscore_ after the function name!! Just declare the function name with extern "C" like this:
extern "C"{void <fortran_subroutine>()}
Remember, all the arguments passed from C code should be reference or pointer. Do not pass value.
In the C compiler, add several dependency directories:
6.1 C project properties-> VC++ Directories -> Include Directries -> add $(IFORT_COMPILER19)\compiler\include;
6.2 C project properties-> VC++ Directories -> Libiary Directries -> add $(IFORT_COMPILER19)\compiler\lib; (+\ia32_win or +\intel64_win depend on win32 or X64 compiler used in VS)
In the C linker, add several dependency directories:
7.1 C project properties-> linker-> General-> Additional Dependency Directories->
add C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\lib
(+\x86 or +\x64 depend on win32 or x64 compiler used in VS)
7.2 C project properties-> linker-> General-> Additional Dependency Directories->
add the fortran static library compiled file directory
7.3 C project properties-> linker->input add .lib
In the source code, include the header file with extern “C” keyword declaration and call the function
<fortran_subroutine>()

Answer 1

The linker (not the compiler) can't resolve the reference because the naming conventions don't match. You named the C routine "add_" in lowercase and with a trailing underscore. Neither of those are part of the default Intel Fortran conventions.
A second problem is that your directives to export the name into the DLL are outside the routine - they will be ignored. (Not that this matters if you are building a static library.)
The simplest and most portable way to fix this is:
Remove the trailing underscore from add_ in the C source, wherever it appears.
Add BIND(C) at the end of the SUBROUTINE line in the Fortran source.
Move the directives inside the routine.
This will make the names match, not just in Intel Fortran but any other combination of Fortran and C you may use.
I would also recommend replacing the non-standard Fortran variable declarations and use this instead:
REAL(C_DOUBLE) FUNCTION ADD (A, B, C, D) BIND(C)
USE, INTRINSIC :: ISO_C_BINDING
!DEC$ IF DEFINED (_DLL)
!DEC$ ATTRIBUTES DLLEXPORT :: ADD
!DEC$ END IF
REAL(C_DOUBLE) :: B,D
INTEGER(C_INT) :: A,C
...