}

/* Allocate new listnode.  Internal use only. */
static struct listnode *
listnode_new (void)
{
  return (struct listnode *)calloc(1,sizeof (struct listnode));
}

/* Free listnode. */
static void
listnode_free (struct listnode *node)
{
  free(node);
}

/* Add new data to the list. */
void
listnode_add (struct list *list, void *val)
{
  struct listnode *node;
  
  assert (val != NULL);
  
  node = listnode_new ();

  node->prev = list->tail;
  node->data = val;

  if (list->head == NULL)
    list->head = node;
  else
    list->tail->next = node;
  list->tail = node;

  list->count++;
}

/*
 * Add a node to the list.  If the list was sorted according to the
 * cmp function, insert a new node with the given val such that the
 * list remains sorted.  The new node is always inserted; there is no
 * notion of omitting duplicates.
 */
void
listnode_add_sort (struct list *list, void *val)
{
  struct listnode *n;
  struct listnode *new;
  
  assert (val != NULL);
  
  new = listnode_new ();
  new->data = val;

  if (list->cmp)
    {
      for (n = list->head; n; n = n->next)
	{
	  if ((*list->cmp) (val, n->data) < 0)
	    {	    
	      new->next = n;
	      new->prev = n->prev;

	      if (n->prev)
		n->prev->next = new;
	      else
		list->head = new;
	      n->prev = new;
	      list->count++;
	      return;
	    }
	}
    }

  new->prev = list->tail;

  if (list->tail)
    list->tail->next = new;
  else
    list->head = new;

  list->tail = new;
  list->count++;
}

void
listnode_add_after (struct list *list, struct listnode *pp, void *val)
{
  struct listnode *nn;
  
  assert (val != NULL);
  
  nn = listnode_new ();
  nn->data = val;

  if (pp == NULL)
    {
      if (list->head)
	list->head->prev = nn;
      else
	list->tail = nn;

      nn->next = list->head;
      nn->prev = pp;

      list->head = nn;
    }
  else
    {
      if (pp->next)
	pp->next->prev = nn;
      else
	list->tail = nn;

      nn->next = pp->next;
      nn->prev = pp;

      pp->next = nn;
    }
  list->count++;
}


/* Delete specific date pointer from the list. */
void
listnode_delete (struct list *list, void *val)
{
  struct listnode *node;

  assert(list);
  for (node = list->head; node; node = node->next)
    {
      if (node->data == val)
	{
	  if (node->prev)
	    node->prev->next = node->next;
	  else
	    list->head = node->next;

	  if (node->next)
	    node->next->prev = node->prev;
	  else
	    list->tail = node->prev;

	  list->count--;
	  listnode_free (node);
	  return;
	}
    }
}

/* Return first node's data if it is there.  */
void *
listnode_head (struct list *list)
{
  struct listnode *node;

  assert(list);
  node = list->head;

  if (node)
    return node->data;
  return NULL;
}

/* Delete all listnode from the list. */
void
list_delete_all_node (struct list *list)
{
  struct listnode *node;
  struct listnode *next;

  assert(list);
  for (node = list->head; node; node = next)
    {
      next = node->next;
      if (list->del)
	(*list->del) (node->data);
      listnode_free (node);
    }
  list->head = list->tail = NULL;
  list->count = 0;
}

/* Delete all listnode then free list itself. */
void
list_delete (struct list *list)
{
  assert(list);
  list_delete_all_node (list);
  list_free (list);
}

/* Lookup the node which has given data. */
struct listnode *
listnode_lookup (struct list *list, void *data)
{
  struct listnode *node;

  assert(list);
  for (node = listhead(list); node; node = listnextnode (node))
    if (data == listgetdata (node))
      return node;
  return NULL;
}

/* Delete the node from list.. */
void
list_delete_node (struct list *list, struct listnode *node)
{
  if (node->prev)
    node->prev->next = node->next;
  else
    list->head = node->next;
  if (node->next)
    node->next->prev = node->prev;
  else
    list->tail = node->prev;
  list->count--;
  listnode_free (node);
}


void
list_add_node_prev (struct list *list, struct listnode *current, void *val)
{
  struct listnode *node;
  
  assert (val != NULL);
  
  node = listnode_new ();
  node->next = current;
  node->data = val;

  if (current->prev == NULL)
    list->head = node;
  else
    current->prev->next = node;

  node->prev = current->prev;
  current->prev = node;

  list->count++;
}

void
list_add_node_next (struct list *list, struct listnode *current, void *val)
{
  struct listnode *node;
  
  assert (val != NULL);
  
  node = listnode_new ();
  node->prev = current;
  node->data = val;

  if (current->next == NULL)
    list->tail = node;
  else
    current->next->prev = node;

  node->next = current->next;
  current->next = node;

  list->count++;
}

void
list_add_list (struct list *l, struct list *m)
{
  struct listnode *n;

  for (n = listhead (m); n; n = listnextnode (n))
    listnode_add (l, n->data);
}

2. linklist.h

#ifndef __LINKLIST_H
#define __LINKLIST_H

/* listnodes must always contain data to be valid. Adding an empty node
 * to a list is invalid
 */
struct listnode 
{
  struct listnode *next;
  struct listnode *prev;
  
  /* private member, use getdata() to retrieve, do not access directly */
  void *data;
};

struct list 
{
  struct listnode *head;
  struct listnode *tail;

  /* invariant: count is the number of listnodes in the list */
  unsigned int count;

  /*
   * Returns -1 if val1 < val2, 0 if equal?, 1 if val1 > val2.
   * Used as definition of sorted for listnode_add_sort
   */
  int (*cmp) (void *val1, void *val2);

  /* callback to free user-owned data when listnode is deleted. supplying
   * this callback is very much encouraged!
   */
  void (*del) (void *val);
};

#define listnextnode(X) ((X) ? ((X)->next) : NULL)
#define listhead(X) ((X) ? ((X)->head) : NULL)
#define listtail(X) ((X) ? ((X)->tail) : NULL)
#define listcount(X) ((X)->count)
#define list_isempty(X) ((X)->head == NULL && (X)->tail == NULL)
#define listgetdata(X) (assert((X)->data != NULL), (X)->data)

/* Prototypes. */
extern struct list *list_new(void); /* encouraged: set list.del callback on new lists */
extern void list_free (struct list *);

extern void listnode_add (struct list *, void *);
extern void listnode_add_sort (struct list *, void *);
extern void listnode_add_after (struct list *, struct listnode *, void *);
extern void listnode_delete (struct list *, void *);
extern struct listnode *listnode_lookup (struct list *, void *);
extern void *listnode_head (struct list *);

extern void list_delete (struct list *);
extern void list_delete_all_node (struct list *);

extern void list_delete_node (struct list *, struct listnode *);
extern void list_add_node_prev (struct list *, struct listnode *, void *);
extern void list_add_node_next (struct list *, struct listnode *, void *);
extern void list_add_list (struct list *, struct list *);

/* List iteration macro. 
 * Usage: for (ALL_LIST_ELEMENTS (...) { ... }
 * It is safe to delete the listnode using this macro.
 */
#define ALL_LIST_ELEMENTS(list,node,nextnode,data) \
  (node) = listhead(list), ((data) = NULL); \
  (node) != NULL && \
    ((data) = listgetdata(node),(nextnode) = node->next, 1); \
  (node) = (nextnode), ((data) = NULL)

/* read-only list iteration macro.
 * Usage: as per ALL_LIST_ELEMENTS, but not safe to delete the listnode Only
 * use this macro when it is *immediately obvious* the listnode is not
 * deleted in the body of the loop. Does not have forward-reference overhead
 * of previous macro.
 */
#define ALL_LIST_ELEMENTS_RO(list,node,data) \
  (node) = listhead(list), ((data) = NULL);\
  (node) != NULL && ((data) = listgetdata(node), 1); \
  (node) = listnextnode(node), ((data) = NULL)

/* these *do not* cleanup list nodes and referenced data, as the functions
 * do - these macros simply {de,at}tach a listnode from/to a list.
 */
 
/* List node attach macro.  */
#define LISTNODE_ATTACH(L,N) \
  do { \
    (N)->prev = (L)->tail; \
    if ((L)->head == NULL) \
      (L)->head = (N); \
    else \
      (L)->tail->next = (N); \
    (L)->tail = (N); \
    (L)->count++; \
  } while (0)

/* List node detach macro.  */
#define LISTNODE_DETACH(L,N) \
  do { \
    if ((N)->prev) \
      (N)->prev->next = (N)->next; \
    else \
      (L)->head = (N)->next; \
    if ((N)->next) \
      (N)->next->prev = (N)->prev; \
    else \
      (L)->tail = (N)->prev; \
    (L)->count--; \
  } while (0)


#endif /* __LINKLIST_H */