Matrix-Vector Multiplication parallel program in CUDA

matVecMul.cu

#include<iostream>
#include<cstdlib>
#include<cmath>
#include<time.h>
using namespace std;


__global__ void matrixVectorMultiplication(int *a, int *b, int *c, int n)
{
    int row=threadIdx.x+blockDim.x*blockIdx.x;
    int sum=0;
   
    if(row<n)
    for(int j=0;j<n;j++)
    {
        sum=sum+a[row*n+j]*b[j];
    }
    
    c[row]=sum;
}
int main()
{
    int *a,*b,*c;
    int *a_dev,*b_dev,*c_dev;
    int n=32;
    
    a=new int[n*n];
    b=new int[n];
    c=new int[n];
    int *d=new int[n];
    int size=n*sizeof(int);
    cudaMalloc(&a_dev,size*size);
    cudaMalloc(&b_dev,size);
    cudaMalloc(&c_dev,size);
    
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<n;j++)
        {
            a[i*n+j]= i*n+j+1; //rand()%n;
            
        }
        
        b[i]= i+1; //rand()%n;
        //cout<<a[i]<<" ";
       // d[i]=a[i]+b[i];
    }
    
    
    cudaEvent_t start,end;
    
    cudaEventCreate(&start);
    cudaEventCreate(&end);
    
    cudaMemcpy(a_dev,a,size*size,cudaMemcpyHostToDevice);
    cudaMemcpy(b_dev,b,size,cudaMemcpyHostToDevice);
    
    
    dim3 threadsPerBlock(n, n);
    dim3 blocksPerGrid(1, 1);
    
    if(n*n>512){
        threadsPerBlock.x=512;
        threadsPerBlock.y=512;
        blocksPerGrid.x=ceil((double)n/(double)threadsPerBlock.x);
        blocksPerGrid.y=ceil((double)n/(double)threadsPerBlock.y);
    }
    
    cudaEventRecord(start);
    matrixVectorMultiplication<<<blocksPerGrid,threadsPerBlock>>>(a_dev,b_dev,c_dev,n);
    
    cudaEventRecord(end);
    cudaEventSynchronize(end);
    
    float time=0.0;
    cudaEventElapsedTime(&time,start,end);
    
    cudaMemcpy(c,c_dev,size,cudaMemcpyDeviceToHost);
    cout<<"\nGPU Time Elapsed:  "<<time;
    
    //CPU matrixVector multiplication
    clock_t t=clock();
    int sum=0;
    for(int row=0;row<n;row++)
    {
        sum=0;
        for(int col=0;col<n;col++)
        {
              sum=sum+a[row*n+col]*b[col];  
            
        }
      d[row]=sum;
    }
    t=clock()-t;
        cout<<"\nCPU Time Elapsed:  "<<((double)t);      //((double)t)/CLOCKS_PER_SEC;

    
    int error=0;
    for(int i=0;i<n;i++){
        error+=d[i]-c[i];
       // cout<<" gpu "<<c[i]<<" CPU "<<d[i]<<endl;
    }
    
    cout<<"Error : "<<error;
    
    
    return 0;
}

I am getting 0 as output from GPU after running exactly the same code for matrix vector multiplication and hence the error is obviously coming very large that is the sum of all. Can you please help.