list

LIST(3)                     Linux Programmer's Manual                    LIST(3)



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

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.

CONFORMING TO
       Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008.  Present on the BSDs (LIST
       macros first appeared in 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)

COLOPHON
       This page is part of release 5.11 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



GNU                                2021-03-22                            LIST(3)