memcpy实现，主要指针传值都是void *

void mymemcpy(void *dest,void *src,size_t count)
{
   if(dest == NULL || src == NULL)
   {
       return;
   }
   char *pdest = (char *)dest;//对void指针进行强制转换
   char *psrc = (char *)src;
   while(count--)
   {
       *pdest++=*psrc++;
   } 
}