Changes

# include <cmath> // This library enable the use of sqrt.  # include <iostream>  # include <ctime>  # include <cuda_runtime.h>    using namespace std;  __global__ void primegen(bool prime, int number2,int x,int *primes_d)  {  int c = 0;    for (int i = 1; i <= x; i++)  {  for ( int j = 2; j <= number2; j++)  {  if ( i!=j && i % j == 0 )  {  prime = false;  break;  }  }  if (prime)  {  primes_d[c]=i;  c += 1;  }  prime = true;  }    }     void primenum(long double); // Prototype...        int main()  {  long double x = 0;  cout<<"\n This program will generate all prime numbers up to the"<<"\n number you have entered below...\n";  cout<<"\n Please enter a number: ";  cin>> x;  cout<<"\n Here are all the prime numbers up to "<<x<<".\n";  primenum(x); //function invocation...  //cout<<endl<<"\nThere are "<<c  //<<" prime numbers less than or equal to "<<x<<".\n\n";  return 0;  }    // This function will determine the primenumbers up to num.  void primenum(long double x)  {  bool prime = true;    int number2;  number2 =(int) floor (sqrt (x));    //Array to hold generated primes on host  int *primes_h = new int[(int)x];  //Device array to hold the primes on the device  int *primes_d = new int[(int)x];  //allocate device memory and initialize device memory  cudaMalloc((void**)&primes_d, (int)x * sizeof(int));  // cudaMalloc((void**)&c_d, sizeof(int));  cudaMemset(&primes_d,0,x * sizeof(int));  //error checking  cudaError_t error ;  int start_s=clock();  //Kernal goes here  primegen<<<1,1>>>(prime,number2,(int)x,primes_d);  int stop_s = clock();  // extract error code from the kernel's execution  error = cudaGetLastError();  if (error != cudaSuccess) {  cout << cudaGetErrorString(error) << endl;  }    //copy the array holding primes from device to host  error =cudaMemcpy(primes_h, primes_d, ((int)x) * sizeof(int), cudaMemcpyDeviceToHost);    if (error != cudaSuccess) {  cout << cudaGetErrorString(error) << endl;  }  // cudaMemcpy(c_h, c_d, sizeof(int), cudaMemcpyDeviceToHost);  //display the primes  for(int i=0; i<(int)x ; i++){  if(primes_h[i]>=2&& primes_h[i]<=(int)x){  cout<<primes_h[i]<<endl;  }  }  cout<< "time: "<< (stop_s-start_s)/double(CLOCKS_PER_SEC)<<endl;  //free allocated memory  delete [] primes_h;  cudaFree(primes_d);    getchar();  }