TAILQ(3)

💖 TAILQ(3) 💖 — the tail queue page, but make it a group chat

bestie. sit down. we're talking about TAILQ, and this is genuinely the doubly linked tail queue implementation from the Library Functions Manual, section 3 (yes that means Library Functions Manual, and yes it's the C library). it's giving data structures, it's giving pointer slay, and by the end you're gonna be obsessed. 🦋

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✨ NAME ✨

Okay so here's the whole squad, no one left behind:

TAILQ_CONCAT, TAILQ_EMPTY, TAILQ_ENTRY, TAILQ_FIRST, TAILQ_FOREACH, TAILQ_FOREACH_REVERSE, TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_INIT, TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL, TAILQ_LAST, TAILQ_NEXT, TAILQ_PREV, TAILQ_REMOVE

and what do they all do? they're the implementation of a doubly linked tail queue. periodt. 💅

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📚 LIBRARY 📚

Standard C library, i.e. libc, so you link it with -lc. iconic behavior from libc fr fr.

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🌸 SYNOPSIS 🌸

here's the receipts, exactly as written, don't let anyone tell you otherwise:

#include <sys/queue.h>

TAILQ_ENTRY(TYPE);

TAILQ_HEAD(HEADNAME, TYPE);
TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);
void TAILQ_INIT(TAILQ_HEAD *head);

int TAILQ_EMPTY(TAILQ_HEAD *head);

void TAILQ_INSERT_HEAD(TAILQ_HEAD *head,
                         struct TYPE *elm, TAILQ_ENTRY NAME);
void TAILQ_INSERT_TAIL(TAILQ_HEAD *head,
                         struct TYPE *elm, TAILQ_ENTRY NAME);
void TAILQ_INSERT_BEFORE(struct TYPE *listelm,
                         struct TYPE *elm, TAILQ_ENTRY NAME);
void TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm,
                         struct TYPE *elm, TAILQ_ENTRY NAME);

struct TYPE *TAILQ_FIRST(TAILQ_HEAD *head);
struct TYPE *TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);
struct TYPE *TAILQ_PREV(struct TYPE *elm, HEADNAME, TAILQ_ENTRY NAME);
struct TYPE *TAILQ_NEXT(struct TYPE *elm, TAILQ_ENTRY NAME);

TAILQ_FOREACH(struct TYPE *var, TAILQ_HEAD *head,
                         TAILQ_ENTRY NAME);
TAILQ_FOREACH_REVERSE(struct TYPE *var, TAILQ_HEAD *head, HEADNAME,
                         TAILQ_ENTRY NAME);

void TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm,
                         TAILQ_ENTRY NAME);

void TAILQ_CONCAT(TAILQ_HEAD *head1, TAILQ_HEAD *head2,
                         TAILQ_ENTRY NAME);

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🎀 DESCRIPTION 🎀

These macros define and operate on doubly linked tail queues. that's the whole vibe.

So in alllll of these macro definitions: TYPE is the name of a user defined structure, and that struct has to contain a field of type TAILQ_ENTRY, named NAME. got it? good. and the argument HEADNAME is the name of a user defined structure that has to be declared using the macro TAILQ_HEAD(). no skipping steps bestie, the compiler will simply humble you. 😭

🫶 Creation

A tail queue is headed by a structure defined by the TAILQ_HEAD() macro. This structure contains a pair of pointers — one to the first element in the queue, and the other to the last element in the queue. (a little first-and-last moment, we love a two-pointer head. 👑)

The elements are doubly linked, so that an arbitrary element can be removed without traversing the queue. no walking the whole thing like a peasant, we just unlink and go.

New elements can be added to the queue:

• after an existing element
• before an existing element
• at the head of the queue
• or at the end of the queue

A TAILQ_HEAD structure is declared like this:

TAILQ_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 queue. A pointer to the head of the queue can later be declared as:

struct HEADNAME *headp;

(and hey — the names head and headp are totally user selectable, you do you 💖)

TAILQ_ENTRY() declares a structure that connects the elements in the queue.

TAILQ_HEAD_INITIALIZER() evaluates to an initializer for the queue head.

TAILQ_INIT() initializes the queue referenced by head.

TAILQ_EMPTY() evaluates to true if there are no items on the queue.

⭐ Insertion

• TAILQ_INSERT_HEAD() — inserts the new element elm at the head of the queue. front of the line energy.
• TAILQ_INSERT_TAIL() — inserts the new element elm at the end of the queue.
• TAILQ_INSERT_BEFORE() — inserts the new element elm before the element listelm.
• TAILQ_INSERT_AFTER() — inserts the new element elm after the element listelm.

🦋 Traversal

• TAILQ_FIRST() — returns the first item on the queue, or NULL if the queue is empty.
• TAILQ_LAST() — returns the last item on the queue. If the queue is empty the return value is NULL.
• TAILQ_PREV() — returns the previous item on the queue, or NULL if this item is the first.
• TAILQ_NEXT() — returns the next item on the queue, or NULL if this item is the last.
• TAILQ_FOREACH() — traverses the queue referenced by head in the forward direction, assigning each element in turn to var. var is set to NULL if the loop completes normally, or if there were no elements.
• TAILQ_FOREACH_REVERSE() — traverses the queue referenced by head in the reverse direction, assigning each element in turn to var.

💅 Removal

TAILQ_REMOVE() removes the element elm from the queue. bye boo 👋

🌈 Other features

TAILQ_CONCAT() concatenates the queue headed by head2 onto the end of the one headed by head1, removing all entries from the former. it's a merge, it's a glow-up, it's a whole arc. ✨

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💜 RETURN VALUE 💜

• TAILQ_EMPTY() returns nonzero if the queue is empty, and zero if the queue contains at least one entry.
• TAILQ_FIRST(), TAILQ_LAST(), TAILQ_PREV(), and TAILQ_NEXT() return a pointer to the first, last, previous, or next TYPE structure, respectively.
• TAILQ_HEAD_INITIALIZER() returns an initializer that can be assigned to the queue head.

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🌸 STANDARDS 🌸

BSD. that's it, that's the standard. 😌

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🧚 HISTORY 🧚

4.4BSD. an elder. a legend. we respect our ancestors.

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⚠️ CAVEATS ⚠️

okay real talk, this one matters:

TAILQ_FOREACH() and TAILQ_FOREACH_REVERSE() don't allow var to be removed or freed within the loop, because it would interfere with the traversal. like, do not do that, the traversal will simply fall apart mid-loop. 💀

The fix exists though: TAILQ_FOREACH_SAFE() and TAILQ_FOREACH_REVERSE_SAFE(), which are present on the BSDs but are not present in glibc, fix this limitation by allowing var to safely be removed from the list and freed from within the loop without interfering with the traversal.

so if you're on glibc and you need that: it's not here bestie, plan accordingly. 🙈

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🔥 EXAMPLES 🔥

here's the whole thing, verbatim, go off:

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>

struct entry {
    int data;
    TAILQ_ENTRY(entry) entries;             /* Tail queue */
};

TAILQ_HEAD(tailhead, entry);

int
main(void)
{
    struct entry *n1, *n2, *n3, *np;
    struct tailhead head;                   /* Tail queue head */
    int i;

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

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

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

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

    n3 = malloc(sizeof(struct entry));      /* Insert before */
    TAILQ_INSERT_BEFORE(n2, n3, entries);

    TAILQ_REMOVE(&head, n2, entries);       /* Deletion */
    free(n2);
                                            /* Forward traversal */
    i = 0;
    TAILQ_FOREACH(np, &head, entries)
        np->data = i++;
                                            /* Reverse traversal */
    TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
        printf("%i\n", np->data);
                                            /* TailQ deletion */
    n1 = TAILQ_FIRST(&head);
    while (n1 != NULL) {
        n2 = TAILQ_NEXT(n1, entries);
        free(n1);
        n1 = n2;
    }
    TAILQ_INIT(&head);

    exit(EXIT_SUCCESS);
}

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👀 SEE ALSO 👀

• insque(3)
• queue(7)

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and that's the page!! you now know the tail queue better than most people in your CS class, no cap. go forth and TAILQ_INSERT_TAIL() with confidence 🫶

this lil masterpiece came to you from Linux man-pages 6.20, dated 2025-05-17, filed under TAILQ(3) — mwah, take care of her 💖✨