memory management - CUDA allocating array of arrays

Question

I have some trouble with allocate array of arrays in CUDA.

void ** data;
cudaMalloc(&data, sizeof(void**)*N); // allocates without problems
for(int i = 0; i < N; i++) {
    cudaMalloc(data + i, getSize(i) * sizeof(void*)); // seg fault is thrown
}

What did I wrong?

Answer 1

You have to allocate the pointers to a host memory, then allocate device memory for each array and store it's pointer in the host memory. Then allocate the memory for storing the pointers into the device and then copy the host memory to the device memory. One example is worth 1000 words:

__global__ void multi_array_kernel( int N, void** arrays ){
    // stuff
}


int main(){

    const int N_ARRAYS = 20;
    void *h_array = malloc(sizeof(void*) * N_ARRAYS);
    for(int i = 0; i < N_ARRAYS; i++){
        cudaMalloc(&h_array[i], i * sizeof(void*));
        //TODO: check error
    }
    void *d_array = cudaMalloc(sizeof(void*) * N_ARRAYS);

    // Copy to device Memory
    cudaMemcpy(d_array, h_array, sizeof(void*) * N_ARRAYS, cudaMemcpyHostToDevice);

    multi_array_kernel<1,1>(N_ARRAYS, d_array);
    cudaThreadSynchronize();

    for(int i = 0; i < N_ARRAYS; i++){
        cudaFree(h_array[i]); //host not device memory
        //TODO: check error
    }
    cudaFree(d_array);
    free(h_array);
}