Segmentation fault in CUDA - cuda

Question

I receive Segmentation fault (core dumped) when i run this code.
I know the cudaMalloc is the problem, but I have no idea how to solve it. I just started learning some CUDA programming and im not familiar with it. I'm working on wsl if it matters.
#include<stdio.h>
#define SIZE 20
__global__ void VectorAdd(int *a,int *b,int *c,int n){
int i = threadIdx.x;
if(i<n)
c[i]=a[i]+b[i];
}
int main(){
int *a,*b,*c;
cudaMalloc((void**)&a,SIZE *sizeof(int));
cudaMalloc((void**)&b, SIZE *sizeof(int));
cudaMalloc((void**)&c,SIZE *sizeof(int));
for (int i=1;i<SIZE;i++){
a[i]=i;
b[i]=i;
c[i]=0;
}
VectorAdd<<<1, SIZE>>>(a,b,c,SIZE);
cudaDeviceSynchronize();
for(int i=1;i<SIZE;i++){
printf("%d \n",c[i]);
}
cudaFree(a);
cudaFree(b);
cudaFree(c);
return 0;
}

Answer 1

As the comments already suggested, you have to initialize values for arrays a and b on the host, copy them to device array, and once computation is completed you have to copy data from c back to the host.
#include<stdio.h>
#define SIZE 20
__global__ void VectorAdd(int *a,int *b,int *c,int n){
int i = threadIdx.x;
if(i<n)
c[i]=a[i]+b[i];
}
int main(){
int *a,*b,*c;
int *h_a, *h_b, *h_c; /*declare pointers to host arrays*/
cudaMalloc((void**)&a,SIZE *sizeof(int));
cudaMalloc((void**)&b, SIZE *sizeof(int));
cudaMalloc((void**)&c,SIZE *sizeof(int));
/* allocate memory for host arrays */
h_a = new int[SIZE];
h_b = new int[SIZE];
h_c = new int[SIZE];
/* initialize values on host arrays */
for (int i = 0; i < SIZE; i++){
h_a[i]=i;
h_b[i]=i;
}
/*copy data from host to device */
cudaMemcpy(a, h_a, SIZE*sizeof(int), cudaMemcpyHostToDevice);
cudaMemcpy(b, h_b, SIZE*sizeof(int), cudaMemcpyHostToDevice);
VectorAdd<<<1, SIZE>>>(a,b,c,SIZE);
// cudaDeviceSynchronize(); /* this is not needed because cudaMemcpy implies sync. */
/*copy results from device to host*/
cudaMemcpy(h_c, c, SIZE*sizeof(int), cudaMemcpyDeviceToHost);
for(int i = 0; i < SIZE; i++){
printf("%d \n",h_c[i]);
}
cudaFree(a);
cudaFree(b);
cudaFree(c);
/* free host memory */
delete [] h_a;
delete [] h_b;
delete [] h_c;
return 0;
}
Notes
For some reason you start iterating from position 1 instead 0 in you for loops! If this is wrong by accident I fixed it!
cudaMemcpy always performs synchronization between host and device. So, cudaDeviceSynchronize() is not necessary after kernel invocation.
To avoid explicit handling of separated host and device data, you could use cudaMallocManaged instead of cudaMalloc.