list(3) — Linux manual page


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

NAME         top

       implementation of a doubly linked list

SYNOPSIS         top

       #include <sys/queue.h>

       int LIST_EMPTY(LIST_HEAD *head);


       struct TYPE *LIST_FIRST(LIST_HEAD *head);

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



       void LIST_INIT(LIST_HEAD *head);

       void LIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
                       LIST_ENTRY NAME);

       void LIST_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
                       LIST_ENTRY NAME);

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

       struct TYPE *LIST_NEXT(struct TYPE *elm, LIST_ENTRY NAME);

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

DESCRIPTION         top

       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().

       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.)

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

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

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

       The macro LIST_FIRST() returns the first element in the list or
       NULL if the list is empty.

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

       The macro LIST_INIT() initializes the list referenced by head.

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

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

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

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

       The macro LIST_REMOVE() removes the element elm from the list.

RETURN VALUE         top

       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         top

       Not in POSIX.1, POSIX.1-2001 or POSIX.1-2008.  Present on the
       BSDs (LIST macros first appeared in 4.4BSD).

BUGS         top

       The macro LIST_FOREACH() doesn't allow var to be removed or freed
       within the loop, as it would interfere with the traversal.  The
       macro 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         top

       #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);

           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. */
                                                   /* 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);
               n1 = n2;


SEE ALSO         top

       insque(3), queue(7)

COLOPHON         top

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       and the latest version of this page, can be found at

GNU                            2020-12-21                        LIST(3)

Pages that refer to this page: queue(7)