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)