Can't compile with mingw linking a library on Linux to create executable for Windows - c++

Question

I'm trying to compile C/C++ code from my Debian partition to generate some executable files for Windows.
Running $ uname -a on the command line gives Linux machine 5.14.0-2-amd64 #1 SMP Debian 5.14.9-2 (2021-10-03) x86_64 GNU/Linux. My processor is an Intel® Core™ i5-1035G4 CPU # 1.10GHz × 8, with a Mesa Intel® Iris(R) Plus Graphics (ICL GT1.5) integrated GPU.
A minimal example to show my current situation includes the following code (called code.cpp):
#include <iostream>
#include <CL/opencl.hpp>
int main()
{
std::vector <cl::Platform> all_platforms; //Get all platforms
cl::Platform::get(&all_platforms);
if (all_platforms.size() == 0)
{
std::cout << "No platforms found. Check OpenCL installation." << std::endl;
exit(1);
}
int pz = all_platforms.size();
std::cout << "Platforms size: " << pz << std::endl;
for (int i = 0; i < pz; i++)
{
cl::Platform default_platform = all_platforms[i];
std::cout << "Using platform: " << default_platform.getInfo<CL_PLATFORM_NAME>() << std::endl;
}
return(0);
}
which uses OpenCL to print all recognized devices. I compile my code writing g++ code.cpp -o code.out -lOpenCL. The executable file code.out works fine, doing what you would expect it to do. I have another program which uses GSL (GNU Scientific Library) written in C which also works well, linking with -lgsl (therefore I think there's not a problem with my code or the regular compilation process). Both OpenCL and GSL were installed from the official repositories (~# apt install ...) with no problem at all. When I execute code.out the output is
Platforms size: 2
Using platform: Intel(R) OpenCL HD Graphics
Using platform: Portable Computing Language
I installed mingw (via ~# apt install mingw-w64) to create executable files to be run on Windows, and for basic programs (i.e. without "external" libraries) it works well (replacing gcc by x86_64-w64-mingw32-gcc or i686-w64-mingw32-gcc). However for the code written above (and for the one using GSL) it doesn't work. Most of the error outputs are very similar for both examples, and I will show the command line outputs for the code using OpenCL.
When I try x86_64-w64-mingw32-g++ code.cpp -o code.out -lOpenCL the output is
code.cpp:2:10: fatal error: CL/opencl.hpp: No such file or directory
2 | #include <CL/opencl.hpp>
| ^~~~~~~~~~~~~~~
compilation terminated.
I thought this meant that I needed to be more specific when linking and including, so I gave the explicit path where the headers are located (found them via dpkg -S opencl.hpp or dpkg -S gsl*.h), and the .so file for OpenCL was found via dpkg -S *OpenCL.so, while the one for GSL was found using dpkg -S *gsl.so. When I try x86_64-w64-mingw32-g++ code.cpp -o code.out -I/usr/include/ -L/usr/lib/x86_64-linux-gnu/libOpenCL.so the output is
In file included from /usr/lib/gcc/x86_64-w64-mingw32/10-win32/include/c++/cwchar:44,
from /usr/lib/gcc/x86_64-w64-mingw32/10-win32/include/c++/bits/postypes.h:40,
from /usr/lib/gcc/x86_64-w64-mingw32/10-win32/include/c++/iosfwd:40,
from /usr/lib/gcc/x86_64-w64-mingw32/10-win32/include/c++/ios:38,
from /usr/lib/gcc/x86_64-w64-mingw32/10-win32/include/c++/ostream:38,
from /usr/lib/gcc/x86_64-w64-mingw32/10-win32/include/c++/iostream:39,
from code.cpp:1:
/usr/include/wchar.h:27:10: fatal error: bits/libc-header-start.h: No such file or directory
27 | #include <bits/libc-header-start.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
Therefore it seems that MinGW needs additional instructions to properly find, include and/or link the libraries. I don't know how to solve this problem. Those are my attempts based on some answers I've found, and the documentation provided by MinGW says nothing about this. The exact same problem occurs no matter if I use x86_64-w64-mingw32-g++ or i686-w64-mingw32-g++, or their gcc counterparts.

Answer 1

When cross-compiling make sure you are only linking things targeting the same platform together. In other words, your dependencies (and their dependencies) must be for the same target platform. You can't link with those libraries for your build platform.
So if you have a Windows 64-bit application that depends on OpenCL, you will need to link it against a Windows 64-bit build of OpenCL.
The OpenCL the sources can be found here:
https://github.com/KhronosGroup/OpenCL-Headers
https://github.com/KhronosGroup/OpenCL-ICD-Loader
so you would need to build those first.