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.