Insertion of Array

#include 

#define length(x) (sizeof x / sizeof *x)

int insert(int *array, int val, size_t size, size_t at)
{
 size_t i;

 /* In range? */
 if (at >= size)
   return -1;

 /* Shift elements to make a hole */
 for (i = size - 1; i > at; i--)
   array[i] = array[i - 1];
 /* Insertion! */
 array[at] = val;

 return 0;
}

int main(void)
{
 int a[11] = {0,1,2,3,4,5,6,7,8,9};
 int i;

 if (insert(a, 6, length(a), 3) != -1)
{
  
for (i = 0; i < length(a); i++)      
printf("%-3d", a[i]);    
printf("\n");  
}  
else    
fprintf(stderr, "Insertion failed\n");  
return 0;
}