use host function on device - cuda

Question

How can I use a host function in a device one ?
For example in below function ,I want to return a value
__device__ float magnitude2( void ) {
return r * r + i * i;
}
But this function is a device function and I received this error :
calling a host function from a __device__/__global__ function is not allowed
What's the best approach for this problem ?
for extra comment on the code :
I want to define this struct :
struct cuComplex {
float r;
float i;
cuComplex( float a, float b ) : r(a), i(b) {}
__device__ float magnitude2( void ) {
return r * r + i * i;
}
__device__ cuComplex operator*(const cuComplex& a) {
return cuComplex(r*a.r - i*a.i, i*a.r + r*a.i);
}
__device__ cuComplex operator+(const cuComplex& a) {
return cuComplex(r+a.r, i+a.i);
}
};

Answer 1

Now that we know the question involves a C++ structure, the answer is obvious - the constructor of the class must also be available as a __device__ function in order to be able to instantiate the class inside a kernel. In your example, the structure should be defined like this:
struct cuComplex {
float r;
float i;
__device__ __host__
cuComplex( float a, float b ) : r(a), i(b) {}
__device__
float magnitude2( void ) {
return r * r + i * i;
}
__device__
cuComplex operator*(const cuComplex& a) {
return cuComplex(r*a.r - i*a.i, i*a.r + r*a.i);
}
__device__
cuComplex operator+(const cuComplex& a) {
return cuComplex(r+a.r, i+a.i);
}
};
The error you are seeing arises because the constructor needs to be called whenever the class is instantiated. In your original code, the constructor is a declared only as a host function, leading to a compilation error.