NAME

       LIST_EMPTY,   LIST_ENTRY,   LIST_FIRST,  LIST_FOREACH,  LIST_HEAD,  LIST_HEAD_INITIALIZER,
       LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_BEFORE, LIST_INSERT_HEAD, LIST_NEXT, LIST_REMOVE
       - implementation of a doubly linked list

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/queue.h>

       LIST_ENTRY(TYPE);

       LIST_HEAD(HEADNAME, TYPE);
       LIST_HEAD LIST_HEAD_INITIALIZER(LIST_HEAD head);
       void LIST_INIT(LIST_HEAD *head);

       int LIST_EMPTY(LIST_HEAD *head);

       void LIST_INSERT_HEAD(LIST_HEAD *head,
                               struct TYPE *elm, LIST_ENTRY NAME);
       void LIST_INSERT_BEFORE(struct TYPE *listelm,
                               struct TYPE *elm, LIST_ENTRY NAME);
       void LIST_INSERT_AFTER(struct TYPE *listelm,
                               struct TYPE *elm, LIST_ENTRY NAME);

       struct TYPE *LIST_FIRST(LIST_HEAD *head);
       struct TYPE *LIST_NEXT(struct TYPE *elm, LIST_ENTRY NAME);

       LIST_FOREACH(struct TYPE *var, LIST_HEAD *head, LIST_ENTRY NAME);

       void LIST_REMOVE(struct TYPE *elm, LIST_ENTRY NAME);

DESCRIPTION

       These macros define and operate on doubly linked lists.

       In  the macro definitions, TYPE is the name of a user-defined structure, that must contain
       a field of type LIST_ENTRY, named NAME.  The argument HEADNAME is  the  name  of  a  user-
       defined structure that must be declared using the macro LIST_HEAD().

   Creation
       A list is headed by a structure defined by the LIST_HEAD() macro.  This structure contains
       a single pointer to the first element on the list.  The elements are doubly linked so that
       an  arbitrary  element  can  be  removed without traversing the list.  New elements can be
       added to the list after an existing element, before an existing element, or at the head of
       the list.  A LIST_HEAD structure is declared as follows:

           LIST_HEAD(HEADNAME, TYPE) head;

       where  struct  HEADNAME is the structure to be defined, and struct TYPE is the type of the
       elements to be linked into the list.  A pointer to the head  of  the  list  can  later  be
       declared as:

           struct HEADNAME *headp;

       (The names head and headp are user selectable.)

       LIST_ENTRY() declares a structure that connects the elements in the list.

       LIST_HEAD_INITIALIZER() evaluates to an initializer for the list head.

       LIST_INIT() initializes the list referenced by head.

       LIST_EMPTY() evaluates to true if there are no elements in the list.

   Insertion
       LIST_INSERT_HEAD() inserts the new element elm at the head of the list.

       LIST_INSERT_BEFORE() inserts the new element elm before the element listelm.

       LIST_INSERT_AFTER() inserts the new element elm after the element listelm.

   Traversal
       LIST_FIRST() returns the first element in the list, or NULL if the list is empty.

       LIST_NEXT() returns the next element in the list, or NULL if this is the last.

       LIST_FOREACH()  traverses  the list referenced by head in the forward direction, assigning
       each element in turn to var.

   Removal
       LIST_REMOVE() removes the element elm from the list.

RETURN VALUE

       LIST_EMPTY() returns nonzero if the list is empty, and zero if the list contains at  least
       one entry.

       LIST_FIRST(),  and  LIST_NEXT()  return  a  pointer  to  the first or next TYPE structure,
       respectively.

       LIST_HEAD_INITIALIZER() returns an initializer that can be assigned to the list head.

STANDARDS

       BSD.

HISTORY

       4.4BSD.

BUGS

       LIST_FOREACH() doesn't allow var to be removed or freed  within  the  loop,  as  it  would
       interfere  with  the  traversal.  LIST_FOREACH_SAFE(), which is present on the BSDs but is
       not present in glibc, fixes this limitation by allowing var to safely be removed from  the
       list and freed from within the loop without interfering with the traversal.

EXAMPLES

       #include <stddef.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/queue.h>

       struct entry {
           int data;
           LIST_ENTRY(entry) entries;              /* List */
       };

       LIST_HEAD(listhead, entry);

       int
       main(void)
       {
           struct entry *n1, *n2, *n3, *np;
           struct listhead head;                   /* List head */
           int i;

           LIST_INIT(&head);                       /* Initialize the list */

           n1 = malloc(sizeof(struct entry));      /* Insert at the head */
           LIST_INSERT_HEAD(&head, n1, entries);

           n2 = malloc(sizeof(struct entry));      /* Insert after */
           LIST_INSERT_AFTER(n1, n2, entries);

           n3 = malloc(sizeof(struct entry));      /* Insert before */
           LIST_INSERT_BEFORE(n2, n3, entries);

           i = 0;                                  /* Forward traversal */
           LIST_FOREACH(np, &head, entries)
               np->data = i++;

           LIST_REMOVE(n2, entries);               /* Deletion */
           free(n2);
                                                   /* Forward traversal */
           LIST_FOREACH(np, &head, entries)
               printf("%i\n", np->data);
                                                   /* List deletion */
           n1 = LIST_FIRST(&head);
           while (n1 != NULL) {
               n2 = LIST_NEXT(n1, entries);
               free(n1);
               n1 = n2;
           }
           LIST_INIT(&head);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       insque(3), queue(7)