Use of cudamalloc(). Why the double pointer?

Question

I am currently going through the tutorial examples on http://code.google.com/p/stanford-cs193g-sp2010/ to learn CUDA. The code which demostrates __global__ functions is given below. It simply creates two arrays, one on the CPU and one on the GPU, populates the GPU array with the number 7 and copies the GPU array data into the CPU array.

#include <stdlib.h> #include <stdio.h>  __global__ void kernel(int *array) {   int index = blockIdx.x * blockDim.x + threadIdx.x;    array[index] = 7; }  int main(void) {   int num_elements = 256;    int num_bytes = num_elements * sizeof(int);    // pointers to host & device arrays   int *device_array = 0;   int *host_array = 0;    // malloc a host array   host_array = (int*)malloc(num_bytes);    // cudaMalloc a device array   cudaMalloc((void**)&device_array, num_bytes);    int block_size = 128;   int grid_size = num_elements / block_size;    kernel<<<grid_size,block_size>>>(device_array);    // download and inspect the result on the host:   cudaMemcpy(host_array, device_array, num_bytes, cudaMemcpyDeviceToHost);    // print out the result element by element   for(int i=0; i < num_elements; ++i)   {     printf("%d ", host_array[i]);   }    // deallocate memory   free(host_array);   cudaFree(device_array); }  

My question is why have they worded the cudaMalloc((void**)&device_array, num_bytes); statement with a double pointer? Even here definition of cudamalloc() on says the first argument is a double pointer.

Why not simply return a pointer to the beginning of the allocated memory on the GPU, just like the malloc function does on the CPU?

Answer 1

All CUDA API functions return an error code (or cudaSuccess if no error occured). All other parameters are passed by reference. However, in plain C you cannot have references, that's why you have to pass an address of the variable that you want the return information to be stored. Since you are returning a pointer, you need to pass a double-pointer.

Another well-known function which operates on addresses for the same reason is the scanf function. How many times have you forgotten to write this & before the variable that you want to store the value to? ;)

int i; scanf("%d",&i);