NAME

       SLIST_EMPTY,  SLIST_ENTRY, SLIST_FIRST, SLIST_FOREACH, SLIST_HEAD, SLIST_HEAD_INITIALIZER,
       SLIST_INIT,    SLIST_INSERT_AFTER,    SLIST_INSERT_HEAD,     SLIST_NEXT,     SLIST_REMOVE,
       SLIST_REMOVE_HEAD - implementation of a singly linked list

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/queue.h>

       SLIST_ENTRY(TYPE);

       SLIST_HEAD(HEADNAME, TYPE);
       SLIST_HEAD SLIST_HEAD_INITIALIZER(SLIST_HEAD head);
       void SLIST_INIT(SLIST_HEAD *head);

       int SLIST_EMPTY(SLIST_HEAD *head);

       void SLIST_INSERT_HEAD(SLIST_HEAD *head,
                               struct TYPE *elm, SLIST_ENTRY NAME);
       void SLIST_INSERT_AFTER(struct TYPE *listelm,
                               struct TYPE *elm, SLIST_ENTRY NAME);

       struct TYPE *SLIST_FIRST(SLIST_HEAD *head);
       struct TYPE *SLIST_NEXT(struct TYPE *elm, SLIST_ENTRY NAME);

       SLIST_FOREACH(struct TYPE *var, SLIST_HEAD *head, SLIST_ENTRY NAME);

       void SLIST_REMOVE(SLIST_HEAD *head, struct TYPE *elm,
                               SLIST_ENTRY NAME);
       void SLIST_REMOVE_HEAD(SLIST_HEAD *head,
                               SLIST_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 SLIST_ENTRY, named NAME.  The argument HEADNAME is the  name  of  a  user-
       defined structure that must be declared using the macro SLIST_HEAD().

   Creation
       A  singly  linked  list  is headed by a structure defined by the SLIST_HEAD() macro.  This
       structure contains a single pointer to the first element on the list.   The  elements  are
       singly  linked  for minimum space and pointer manipulation overhead at the expense of O(n)
       removal for arbitrary elements.  New elements can be added to the list after  an  existing
       element or at the head of the list.  An SLIST_HEAD structure is declared as follows:

           SLIST_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.)

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

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

       SLIST_INIT() initializes the list referenced by head.

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

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

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

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

       SLIST_NEXT() returns the next element in the list.

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

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

       SLIST_REMOVE_HEAD() removes the element elm from  the  head  of  the  list.   For  optimum
       efficiency,  elements  being  removed from the head of the list should explicitly use this
       macro instead of the generic SLIST_REMOVE().

RETURN VALUE

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

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

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

STANDARDS

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

BUGS

       SLIST_FOREACH()  doesn't  allow  var  to  be removed or freed within the loop, as it would
       interfere with the traversal.  SLIST_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;
           SLIST_ENTRY(entry) entries;             /* Singly linked list */
       };

       SLIST_HEAD(slisthead, entry);

       int
       main(void)
       {
           struct entry *n1, *n2, *n3, *np;
           struct slisthead head;                  /* Singly linked list
                                                      head */

           SLIST_INIT(&head);                      /* Initialize the queue */

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

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

           SLIST_REMOVE(&head, n2, entry, entries);/* Deletion */
           free(n2);

           n3 = SLIST_FIRST(&head);
           SLIST_REMOVE_HEAD(&head, entries);      /* Deletion from the head */
           free(n3);

           for (unsigned int i = 0; i < 5; i++) {
               n1 = malloc(sizeof(struct entry));
               SLIST_INSERT_HEAD(&head, n1, entries);
               n1->data = i;
           }

                                                   /* Forward traversal */
           SLIST_FOREACH(np, &head, entries)
               printf("%i\n", np->data);

           while (!SLIST_EMPTY(&head)) {           /* List deletion */
               n1 = SLIST_FIRST(&head);
               SLIST_REMOVE_HEAD(&head, entries);
               free(n1);
           }
           SLIST_INIT(&head);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       insque(3), queue(7)