CUDA program giving garbage value - cuda

Question

I really do not understand why the output for the below code is not a and b.
#include<cutil.h>
#include<iostream>
__global__ void p(unsigned char **a){
unsigned char temp[2];
temp[0] = 'a';
temp[1] = 'b';
a[0] = temp;
}
void main(){
unsigned char **a ;
cudaMalloc((void**)&a, sizeof(unsigned char*));
p<<<1,1>>>(a);
unsigned char **c;
unsigned char b[2];
cudaMemcpy(c, a, sizeof(unsigned char *), cudaMemcpyDeviceToHost);
cudaMemcpy(b, c[0], 2*sizeof(unsigned char), cudaMemcpyDeviceToHost);
for( int i=0 ; i < 2; i++){
printf("%c\n", b[i]);
}
getchar();
}
what is wrong with my logic?

Answer 1

Let's leave out CUDA for now. Let's just make a function that writes data to a user-provided array. The user passes the array via a pointer:
void fill_me_up(int * dst)
{
// We sure hope that `dst` points to a large enough area of memory!
dst[0] = 28;
dst[1] = 75;
}
Now, what you're doing with the local variable doesn't make sense, because you want to use the address of a local variable, which becomes invalid after you leave the function scope. The next best thing you could do is memcpy(), or some equivalent C++ algorithm:
void fill_me_up_again(int * dst)
{
int temp[] = { 28, 75 };
memcpy((void *)dst, (const void *)temp, sizeof(temp));
}
OK, now on to calling that function: We first must provide the target memory, and then pass a pointer:
int main()
{
int my_memory[2]; // here's our memory -- automatic local storage
fill_me_up(my_memory); // OK, array decays to pointer-to-beginning
fill_me_up(&my_memory[0]); // A bit more explicit
int * your_memory = malloc(sizeof(int) * 2); // more memory, this time dynamic
fill_me_up_again(your_memory);
/* ... */
free(your_memory);
}
(In C++ you would probably have uses new int[2] and delete your_memory instead, but by using C malloc() the connection to CUDA hopefully becomes clear.)
When you're moving fill_me_up to the CUDA device, you have to give it a device pointer rather than a host pointer, so you have to set that one up first and afterwards copy the results back out, but that's about the only change.