--- name: "TAILQ_FIRST" 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/tailq" --- # TAILQ\_FIRST(3) implementation of a doubly linked tail queue Part of [tailq(3)](/tailq/3.md). ## NAME 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 - implementation of a doubly linked tail queue ## LIBRARY Standard C library (*libc*, *-lc*) ## SYNOPSIS ``` #include ``` **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***);** ## DESCRIPTION These macros define and operate on doubly linked tail queues. In the macro definitions, *TYPE* is the name of a user defined structure, that must contain a field of type *TAILQ\_ENTRY*, named *NAME*. The argument *HEADNAME* is the name of a user defined structure that must be declared using the macro **TAILQ\_HEAD**(). ### 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. The elements are doubly linked so that an arbitrary element can be removed without traversing the queue. 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 as follows: ``` 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; ``` (The names *head* and *headp* are user selectable.) **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 **TAILQ\_EMPTY**() evaluates to true if there are no items on the queue. *head*. ### Insertion **TAILQ\_INSERT\_HEAD**() inserts the new element *elm* at the head of the queue. **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. ### 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. ## 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*. ## STANDARDS BSD. ## HISTORY 4\.4BSD. ## CAVEATS **TAILQ\_FOREACH**() and **TAILQ\_FOREACH\_REVERSE**() don't allow *var* to be removed or freed within the loop, as it would interfere with the traversal. **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. ## EXAMPLES ``` #include #include #include #include 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); } ``` ## SEE ALSO [**insque**(3)](/insque/3.md), [**queue**(7)](/queue/7.md)