How to properly link cuda header file with device functions?

Question

I\'m trying to decouple my code a bit and something fails. Compilation error:

error: calling a __host__ function(\"DecoupledCallGpu\") from a __global__ func         


        

Answer 1

Add the __device__ decorator to the prototype in decoupled_functions.cuh. That should take care of the error message you are seeing.

Then you'll need to use separate compilation and linking amongst your modules. So instead of compiling with -c compile with -dc. And your link command will need to be modified. A basic example is here.

Your question is a bit confusing:

Question: why is it that the DecoupledCallGpu is called from host function and not a kernel as it was supposed to?

I can't tell if you're tripping over english or if there is a misunderstanding here. The actual error message states:

error: calling a __host__ function("DecoupledCallGpu") from a __global__ function("kernel") is not allowed

This is arising due to the fact that within the compilation unit (ie. within the module, within the file that is being compiled, ie. cuda_computations.cu), the only description of the function DecoupledCallGpu() is that which is provided in the prototype in the header:

void DecoupledCallGpu(int *var);

This prototype indicates an undecorated function in CUDA C, and such functions are equivalent to __host__ (only) decorated functions:

__host__ void DecoupledCallGpu(int *var);

That compilation unit has no knowledge of what is actually in decoupled_functions.cu.

Therefore, when you have kernel code like this:

__global__ void kernel(){       //<- __global__ function
...
DecoupledCallGpu(&var_kernel);  //<- appears as a __host__ function to compiler
}

the compiler thinks you are trying to call a __host__ function from a __global__ function, which is illegal.