---
name: "LIST_HEAD"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/list/3"
---

# LIST\_HEAD(3)

implementation of a doubly linked list

Part of [list(3)](/list/3.md).

## 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)](/insque/3.md), [**queue**(7)](/queue/7.md)