求解！为什么多次连续执行会出现不一样的结果？为什么呀？

#include<stdio.h>
main()
{
   int p1,p2,i;
   while ((p1=fork())==-1);
   if(p1==0)
           for(i=0;i<3;i++)
                 printf("daughter %d\n",i);
   else
       {
          while((p2=fork())==-1);
          if(p2==0)
               for(i=0;i<5;i++)
                     printf("son %d\n",i);
          else
               for(i=0;i<4;i++)
                     printf("parent %d\n",i);
        }
}