[1] ->[2] ->[3] ->[4] ->[5] 
            ^           |
            |           U
           [10]        [6]
            ^           |
            |           U
           [9]<- [8]<- [7]

 
int main(void)
{
struct list 
 {
 int  val;
 struct list *pnext;
 } *phead, *plast, *pptr, *ploop, **pptmp, *p1, *p2;
int    i;
phead = NULL;
pptmp = &phead;
for ( i = 1 ; i <= 10 ; ++i )
  {
  plast = (*pptmp) = malloc(sizeof(*phead)) ;
  (*pptmp)->val = i;
  (*pptmp)->pnext = NULL;
  if ( i == 3 )
      ploop = (*pptmp);
  pptmp = &(*pptmp)->pnext;
  }
 
 
/* Doing the error !!!!!!!!!! */
plast->pnext = ploop;
 
 
/* Trying to find the error. e.g. cell 10 */
/* Not its value of course but its address */
getchar();
return(0);
}

 //declare node struct to create list from
 struct list 
 {
     int  val;
     list *pnext;
 } 
 
int main()
{
    list *phead, *plast, *ploop, *pptmp;  //pointers to use 
    int    i;
 
    for ( i = 1 ; i <= 10 ; ++i )  //create total of 10 list objects
    {
      //declare memory for a new list object
      pptmp  = new list ;        
 
      //initialize each objects member variables
      pptmp->val = i;
      pptmp->pnext = NULL;
 
      //keep track of the desired place to create the loop
      if ( i == 3 )
        ploop = pptmp;
 
      //create the list 
      if(i == 1)
        plast = phead = pptemp;
      else
      {
        plast->next = pptmp;
        plast = plast->pnext;
      }
    }
 
 
  /* Doing the error !!!!!!!!!! */
  /*create the loop*/
  plast->pnext = ploop;
 
 
  /* Trying to find the error. e.g. cell 10 */
  /* Not its value of course but its address */
  getchar();
  return 0;
}

while ( node && !node->seen ) {
  node->seen = true;
  node = node->next;
}
if ( node ) {
  // I'm the start of the loop
}