CUDA copy linked lists from device to host - struct

Question

I am trying to populate a number of linked lists on the device and then return those lists back to the hosts.
From my understanding I need to allocate memory for my struct Element, but I don't know how to go about it since I will have many linked lists, each with an unknown number of elements. I've tried a couple of different things but it still didn't work. So I'm back to the starting point. Here is my code:
//NODE CLASS
class Node{
public:
int x,y;
Node *parent;
__device__ __host__ Node(){}
__device__ __host__ Node(int cX, int cY){x = cX; y = cY;}
__device__ __host__ int get_row() { return x; }
__device__ __host__ int get_col() { return y; }
};
//LINKED LIST
class LinkedList{
public:
__device__ __host__ struct Element{
Node n1;
Element *next;
};
__device__ __host__ LinkedList(){
head = NULL;
}
__device__ __host__ void addNode(Node n){
Element *el = new Element();
el->n1 = n;
el->next = head;
head = el;
}
__device__ __host__ Node popFirstNode(){
Element *cur = head;
Node n;
if(cur != NULL){
n = cur -> n1;
head = head -> next;
}
delete cur;
return n;
}
__device__ __host__ bool isEmpty(){
Element *cur = head;
if(cur == NULL){
return true;
}
return false;
}
Element *head;
};
//LISTS
__global__ void listsKernel(LinkedList* d_Results, int numLists){
int idx = blockIdx.x * blockDim.x + threadIdx.x;
Node n(1,1);
if(idx < numLists){
d_Results[idx].addNode(n);
d_Results[idx].addNode(n);
d_Results[idx].addNode(n);
d_Results[idx].addNode(n);
}
}
int main(){
int numLists = 10;
size_t size = numLists * sizeof(LinkedList);
LinkedList curList;
LinkedList* h_Results = (LinkedList*)malloc(size);
LinkedList* d_Results;
cudaMalloc((void**)&d_Results, size);
listsKernel<<<256,256>>>(d_Results, numLists);
cudaMemcpy(h_Results, d_Results, sizeof(LinkedList)*numLists, cudaMemcpyDeviceToHost);
for(int i = 0; i < numLists; i++){
curList = h_Results[i];
while(curList.isEmpty() == false){
Node n = curList.popFirstNode();
std::cout << "x: " << n.get_row() << " y: " << n.get_col();
}
}
}
As you can see I'm trying to populate 10 linked lists on the device and then return them back to the host, but the code above results in unhandled exception - Access violation reading location. I am assuming it is not coping the pointers from the device.
Any help would be great.

Answer 1

Just eyeballing the code, it seems you have a fundamental misconception: there is host memory which cannot be accessed from the device, and device memory which cannot be accessed from the host. So when you create linked list nodes in device memory and copy the pointers back to the host, the host cannot dereference those pointers, because they are pointing to device memory.
If you truly want to pass linked lists back and forth between host and device, your best bet is probably to copy the entries into an array, do the memcpy, then copy the array back into a linked list. Other things can be done too, depending on just what your use case is.
(it is possible to allocate a region of memory that is accessible both from the host and from the device, but there is some awkwardness with it and I have no experience using it)