Error in a c program to calculate the execution time of program

I'm trying to caculate the execution time of program given below but it is giving me some errors

#include<stdio.h>
#include<time.h>

int main()

{
  clock_t start, end;   
  start = clock();


int a[5] ={1,2,3,4,51};
int b[5] = {2,3,4,5,6};
int c[5] = {3,4,5,6,7};


for (int i =0;i<5;i++)
{
    if(a[i]== NULL )
    printf("%d element null::",a[i]);
    else
    printf("no null values in first array\n");


        if(b[i]== NULL )
    printf("%d element null::",b[i]); 
        else 
        printf("no null values in second array\n");


        if(c[i]== NULL )
    printf("%d element null::",c[i]);
        else
        printf("no null values in third array\n");

}


  end = clock();
  printf("Start time: %i, End time: %i \n" start, end);

return 0;

}

following are the errors and warnings..

        [eshwar@localhost ~]$ gcc -std=c99 ab.c
ab.c: In function ‘main’:
ab.c:18:9: warning: comparison between pointer and integer [enabled by default]
  if(a[i]== NULL )
         ^
ab.c:24:16: warning: comparison between pointer and integer [enabled by default]
         if(b[i]== NULL )
                ^
ab.c:30:16: warning: comparison between pointer and integer [enabled by default]
         if(c[i]== NULL )
                ^
ab.c:39:44: error: expected ‘)’ before ‘start’
   printf("Start time: %i, End time: %i \n" start, end);

and why is this warning given..because I think we can initialise array like this??