SLIST_INIT(3) BSD Programmer's Manual SLIST_INIT(3)
SLIST_ENTRY, SLIST_HEAD, SLIST_HEAD_INITIALIZER, SLIST_FIRST, SLIST_NEXT,
SLIST_EMPTY, SLIST_FOREACH, SLIST_FOREACH_SAFE, SLIST_INIT,
SLIST_INSERT_AFTER, SLIST_INSERT_HEAD, SLIST_REMOVE_AFTER,
SLIST_REMOVE_HEAD, SLIST_REMOVE, LIST_ENTRY, LIST_HEAD,
LIST_HEAD_INITIALIZER, LIST_FIRST, LIST_NEXT, LIST_EMPTY, LIST_FOREACH,
LIST_FOREACH_SAFE, LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_BEFORE,
LIST_INSERT_HEAD, LIST_REMOVE, LIST_REPLACE, SIMPLEQ_ENTRY, SIMPLEQ_HEAD,
SIMPLEQ_HEAD_INITIALIZER, SIMPLEQ_FIRST, SIMPLEQ_NEXT, SIMPLEQ_EMPTY,
SIMPLEQ_FOREACH, SIMPLEQ_FOREACH_SAFE, SIMPLEQ_INIT,
SIMPLEQ_INSERT_AFTER, SIMPLEQ_INSERT_HEAD, SIMPLEQ_INSERT_TAIL,
SIMPLEQ_REMOVE_AFTER, SIMPLEQ_REMOVE_HEAD, SIMPLEQ_CONCAT, TAILQ_ENTRY,
TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_FIRST, TAILQ_NEXT, TAILQ_LAST,
TAILQ_PREV, TAILQ_EMPTY, TAILQ_FOREACH, TAILQ_FOREACH_SAFE,
TAILQ_FOREACH_REVERSE, TAILQ_FOREACH_REVERSE_SAFE, TAILQ_INIT,
TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD,
TAILQ_INSERT_TAIL, TAILQ_REMOVE, TAILQ_REPLACE, TAILQ_CONCAT - intrusive
singly-linked and doubly-linked lists, simple queues, and tail
#include <sys/queue.h>
SLIST_ENTRY(TYPE);
SLIST_HEAD(HEADNAME, TYPE);
SLIST_HEAD_INITIALIZER(SLIST_HEAD head);
struct TYPE *
SLIST_FIRST(SLIST_HEAD *head);
struct TYPE *
SLIST_NEXT(struct TYPE *listelm, FIELDNAME);
int
SLIST_EMPTY(SLIST_HEAD *head);
SLIST_FOREACH(VARNAME, SLIST_HEAD *head, FIELDNAME);
SLIST_FOREACH_SAFE(VARNAME, SLIST_HEAD *head, FIELDNAME, TEMP_VARNAME);
void
SLIST_INIT(SLIST_HEAD *head);
void
SLIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
SLIST_INSERT_HEAD(SLIST_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SLIST_REMOVE_AFTER(struct TYPE *elm, FIELDNAME);
void
SLIST_REMOVE_HEAD(SLIST_HEAD *head, FIELDNAME);
void
SLIST_REMOVE(SLIST_HEAD *head, struct TYPE *elm, TYPE, FIELDNAME);
LIST_ENTRY(TYPE);
LIST_HEAD(HEADNAME, TYPE);
LIST_HEAD_INITIALIZER(LIST_HEAD head);
struct TYPE *
LIST_FIRST(LIST_HEAD *head);
struct TYPE *
LIST_NEXT(struct TYPE *listelm, FIELDNAME);
int
LIST_EMPTY(LIST_HEAD *head);
LIST_FOREACH(VARNAME, LIST_HEAD *head, FIELDNAME);
LIST_FOREACH_SAFE(VARNAME, LIST_HEAD *head, FIELDNAME, TEMP_VARNAME);
void
LIST_INIT(LIST_HEAD *head);
void
LIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
LIST_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
LIST_INSERT_HEAD(LIST_HEAD *head, struct TYPE *elm, FIELDNAME);
void
LIST_REMOVE(struct TYPE *elm, FIELDNAME);
void
LIST_REPLACE(struct TYPE *elm, struct TYPE *elm2, FIELDNAME);
SIMPLEQ_ENTRY(TYPE);
SIMPLEQ_HEAD(HEADNAME, TYPE);
SIMPLEQ_HEAD_INITIALIZER(SIMPLEQ_HEAD head);
struct TYPE *
SIMPLEQ_FIRST(SIMPLEQ_HEAD *head);
struct TYPE *
SIMPLEQ_NEXT(struct TYPE *listelm, FIELDNAME);
int
SIMPLEQ_EMPTY(SIMPLEQ_HEAD *head);
SIMPLEQ_FOREACH(VARNAME, SIMPLEQ_HEAD *head, FIELDNAME);
SIMPLEQ_FOREACH_SAFE(VARNAME, SIMPLEQ_HEAD *head, FIELDNAME,
TEMP_VARNAME);
void
SIMPLEQ_INIT(SIMPLEQ_HEAD *head);
void
SIMPLEQ_INSERT_AFTER(SIMPLEQ_HEAD *head, struct TYPE *listelm,
struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_INSERT_HEAD(SIMPLEQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_INSERT_TAIL(SIMPLEQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_REMOVE_AFTER(SIMPLEQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_REMOVE_HEAD(SIMPLEQ_HEAD *head, FIELDNAME);
SIMPLEQ_CONCAT(SIMPLEQ_HEAD *head1, SIMPLEQ_HEAD *head2);
TAILQ_ENTRY(TYPE);
TAILQ_HEAD(HEADNAME, TYPE);
TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);
struct TYPE *
TAILQ_FIRST(TAILQ_HEAD *head);
struct TYPE *
TAILQ_NEXT(struct TYPE *listelm, FIELDNAME);
struct TYPE *
TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);
struct TYPE *
TAILQ_PREV(struct TYPE *listelm, HEADNAME, FIELDNAME);
int
TAILQ_EMPTY(TAILQ_HEAD *head);
TAILQ_FOREACH(VARNAME, TAILQ_HEAD *head, FIELDNAME);
TAILQ_FOREACH_SAFE(VARNAME, TAILQ_HEAD *head, FIELDNAME, TEMP_VARNAME);
TAILQ_FOREACH_REVERSE(VARNAME, TAILQ_HEAD *head, HEADNAME, FIELDNAME);
TAILQ_FOREACH_REVERSE_SAFE(VARNAME, TAILQ_HEAD *head, HEADNAME, FIELD-
NAME, TEMP_VARNAME);
void
TAILQ_INIT(TAILQ_HEAD *head);
void
TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm,
struct TYPE *elm, FIELDNAME);
void
TAILQ_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
TAILQ_INSERT_HEAD(TAILQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
TAILQ_INSERT_TAIL(TAILQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
TAILQ_REPLACE(TAILQ_HEAD *head, struct TYPE *elm, struct TYPE *elm2,
FIELDNAME);
TAILQ_CONCAT(TAILQ_HEAD *head1, TAILQ_HEAD *head2, FIELDNAME);
These macros define and operate on four types of data structures: singly-
linked lists, simple queues, lists, and tail queues. All four structures
support the following functionality:
1. Insertion of a new entry at the head of the list.
2. Insertion of a new entry after any element in the list.
3. Removal of an entry from the head of the list.
4. Forward traversal through the list.
The following table provides a quick overview of which types support
which additional macros:
LAST, PREV, FOREACH_REVERSE - - -
INSERT_BEFORE, REPLACE - LIST -
INSERT_TAIL, CONCAT - - SIMPLEQ
REMOVE_AFTER, REMOVE_HEAD SLIST - SIMPLEQ
REMOVE SLIST LIST - TAILQ
Singly-linked lists are the simplest of the four data structures and sup-
port only the above functionality. Singly-linked lists are ideal for ap-
plications with large datasets and few or no removals, or for implement-
ing a LIFO queue.
Simple queues add the following functionality:
1. Entries can be added at the end of a list.
However:
1. All list insertions must specify the head of the list.
2. Each head entry requires two pointers rather than one.
3. Code size is about 15% greater and operations run about 20%
slower than singly-linked lists.
Simple queues are ideal for applications with large datasets and few or
no removals, or for implementing a FIFO queue.
All doubly linked types of data structures (lists and tail queues) addi-
tionally allow:
1. Insertion of a new entry before any element in the list.
2. Removal of any entry in the list.
However:
1. Each element requires two pointers rather than one.
2. Code size and execution time of operations (except for remo-
val) is about twice that of the singly-linked data-structures.
Lists are the simplest of the doubly linked data structures and support
only the above functionality over singly-linked lists.
Tail queues add the following functionality:
1. Entries can be added at the end of a list.
2. They may be traversed backwards, at a cost.
However:
1. All list insertions and removals must specify the head of the
list.
2. Each head entry requires two pointers rather than one.
3. Code size is about 15% greater and operations run about 20%
slower than singly-linked lists.
An additional type of data structure, circular queues, violated the C
language aliasing rules and were miscompiled as a result. All code using
them should be converted to another structure; tail queues are usually
the easiest to convert to.
All these lists and queues are intrusive: they link together user defined
structures containing a field of type SLIST_ENTRY, LIST_ENTRY,
SIMPLEQ_ENTRY, or TAILQ_ENTRY. In the macro definitions, TYPE is the name
tag of the user defined structure and FIELDNAME is the name of the
*_ENTRY field. If an instance of the user defined structure needs to be a
member of multiple lists at the same time, the structure requires multi-
ple *_ENTRY fields, one for each list.
The argument HEADNAME is the name tag of a user defined structure that
must be declared using the macros SLIST_HEAD(), LIST_HEAD(),
SIMPLEQ_HEAD(), or TAILQ_HEAD(). See the examples below for further ex-
planation of how these macros are used.
SINGLY-LINKED LISTS
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 ele-
ments. New elements can be added to the list after an existing element or
at the head of the list. A SLIST_HEAD structure is declared as follows:
SLIST_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of 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 HEADNAME facility is often not used, leading to the following bizarre
code:
SLIST_HEAD(, TYPE) head, *headp;
The SLIST_ENTRY() macro declares a structure that connects the elements
in the list.
The SLIST_INIT() macro initializes the list referenced by head.
The list can also be initialized statically by using the
SLIST_HEAD_INITIALIZER() macro like this:
SLIST_HEAD(HEADNAME, TYPE) head = SLIST_HEAD_INITIALIZER(head);
The SLIST_INSERT_HEAD() macro inserts the new element elm at the head of
the list.
The SLIST_INSERT_AFTER() macro inserts the new element elm after the ele-
ment listelm.
The SLIST_REMOVE_HEAD() macro removes the first element of the list
pointed by head.
The SLIST_REMOVE_AFTER() macro removes the list element immediately fol-
lowing elm.
The SLIST_REMOVE() macro removes the element elm of the list pointed by
head.
The SLIST_FIRST() and SLIST_NEXT() macros can be used to traverse the
list:
for (np = SLIST_FIRST(&head); np != NULL; np = SLIST_NEXT(np, FIELDNAME))
Or, for simplicity, one can use the SLIST_FOREACH() macro:
SLIST_FOREACH(np, head, FIELDNAME)
The macro SLIST_FOREACH_SAFE() traverses the list referenced by head in a
forward direction, assigning each element in turn to var. However, unlike
SLIST_FOREACH() it is permitted to remove var as well as free it from
within the loop safely without interfering with the traversal.
The SLIST_EMPTY() macro should be used to check whether a simple list is
empty.
SINGLY-LINKED LIST EXAMPLE
SLIST_HEAD(listhead, entry) head;
struct entry {
...
SLIST_ENTRY(entry) entries; /* Simple list. */
...
} *n1, *n2, *np;
SLIST_INIT(&head); /* Initialize simple list. */
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_FOREACH(np, &head, entries) /* Forward traversal. */
np-> ...
while (!SLIST_EMPTY(&head)) { /* Delete. */
n1 = SLIST_FIRST(&head);
SLIST_REMOVE_HEAD(&head, entries);
free(n1);
}
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 HEADNAME is the name of 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 HEADNAME facility is often not used, leading to the following bizarre
code:
LIST_HEAD(, TYPE) head, *headp;
The LIST_ENTRY() macro declares a structure that connects the elements in
the list.
The LIST_INIT() macro initializes the list referenced by head.
The list can also be initialized statically by using the
LIST_HEAD_INITIALIZER() macro like this:
LIST_HEAD(HEADNAME, TYPE) head = LIST_HEAD_INITIALIZER(head);
The LIST_INSERT_HEAD() macro inserts the new element elm at the head of
the list.
The LIST_INSERT_AFTER() macro inserts the new element elm after the ele-
ment listelm.
The LIST_INSERT_BEFORE() macro inserts the new element elm before the
element listelm.
The LIST_REMOVE() macro removes the element elm from the list.
The LIST_REPLACE() macro replaces the list element elm with the new ele-
ment elm2.
The LIST_FIRST() and LIST_NEXT() macros can be used to traverse the list:
for (np = LIST_FIRST(&head); np != NULL; np = LIST_NEXT(np, FIELDNAME))
Or, for simplicity, one can use the LIST_FOREACH() macro:
LIST_FOREACH(np, head, FIELDNAME)
The macro LIST_FOREACH_SAFE() traverses the list referenced by head in a
forward direction, assigning each element in turn to var. However, unlike
LIST_FOREACH() it is permitted to remove var as well as free it from
within the loop safely without interfering with the traversal.
The LIST_EMPTY() macro should be used to check whether a list is empty.
LIST_HEAD(listhead, entry) head;
struct entry {
...
LIST_ENTRY(entry) entries; /* List. */
...
} *n1, *n2, *np;
LIST_INIT(&head); /* Initialize 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);
n2 = malloc(sizeof(struct entry)); /* Insert before. */
LIST_INSERT_BEFORE(n1, n2, entries);
/* Forward traversal. */
LIST_FOREACH(np, &head, entries)
np-> ...
while (!LIST_EMPTY(&head)) { /* Delete. */
n1 = LIST_FIRST(&head);
LIST_REMOVE(n1, entries);
free(n1);
}
A simple queue is headed by a structure defined by the SIMPLEQ_HEAD()
macro. This structure contains a pair of pointers, one to the first ele-
ment in the simple queue and the other to the last element in the simple
queue. The elements are singly linked. New elements can be added to the
queue after an existing element, at the head of the queue or at the tail
of the queue. A SIMPLEQ_HEAD structure is declared as follows:
SIMPLEQ_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of 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.)
The SIMPLEQ_ENTRY() macro declares a structure that connects the elements
in the queue.
The SIMPLEQ_INIT() macro initializes the queue referenced by head.
The queue can also be initialized statically by using the
SIMPLEQ_HEAD_INITIALIZER() macro like this:
SIMPLEQ_HEAD(HEADNAME, TYPE) head = SIMPLEQ_HEAD_INITIALIZER(head);
The SIMPLEQ_INSERT_AFTER() macro inserts the new element elm after the
element listelm.
The SIMPLEQ_INSERT_HEAD() macro inserts the new element elm at the head
of the queue.
The SIMPLEQ_INSERT_TAIL() macro inserts the new element elm at the end of
the queue.
The SIMPLEQ_REMOVE_AFTER() macro removes the queue element immediately
following elm.
The SIMPLEQ_REMOVE_HEAD() macro removes the first element from the queue.
The SIMPLEQ_CONCAT() macro concatenates all the elements of the queue
referenced by head2 to the end of the queue referenced by head1, emptying
head2 in the process. This is more efficient than removing and inserting
the individual elements as it does not actually traverse head2.
The SIMPLEQ_FIRST() and SIMPLEQ_NEXT() macros can be used to traverse the
queue. The SIMPLEQ_FOREACH() is used for queue traversal:
SIMPLEQ_FOREACH(np, head, FIELDNAME)
The macro SIMPLEQ_FOREACH_SAFE() traverses the queue referenced by head
in a forward direction, assigning each element in turn to var. However,
unlike SIMPLEQ_FOREACH() it is permitted to remove var as well as free it
from within the loop safely without interfering with the traversal.
The SIMPLEQ_EMPTY() macro should be used to check whether a list is emp-
ty.
SIMPLEQ_HEAD(listhead, entry) head = SIMPLEQ_HEAD_INITIALIZER(head);
struct entry {
...
SIMPLEQ_ENTRY(entry) entries; /* Simple queue. */
...
} *n1, *n2, *np;
n1 = malloc(sizeof(struct entry)); /* Insert at the head. */
SIMPLEQ_INSERT_HEAD(&head, n1, entries);
n2 = malloc(sizeof(struct entry)); /* Insert after. */
SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);
n2 = malloc(sizeof(struct entry)); /* Insert at the tail. */
SIMPLEQ_INSERT_TAIL(&head, n2, entries);
/* Forward traversal. */
SIMPLEQ_FOREACH(np, &head, entries)
np-> ...
/* Delete. */
while (!SIMPLEQ_EMPTY(&head)) {
n1 = SIMPLEQ_FIRST(&head);
SIMPLEQ_REMOVE_HEAD(&head, entries);
free(n1);
}
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 tail queue and the other to the last element in the tail queue. The
elements are doubly linked so that an arbitrary element can be removed
without traversing the tail 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 HEADNAME is the name of the structure to be defined, and struct
TYPE is the type of the elements to be linked into the tail queue. A
pointer to the head of the tail queue can later be declared as:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The TAILQ_ENTRY() macro declares a structure that connects the elements
in the tail queue.
The TAILQ_INIT() macro initializes the tail queue referenced by head.
The tail queue can also be initialized statically by using the
TAILQ_HEAD_INITIALIZER() macro.
The TAILQ_INSERT_HEAD() macro inserts the new element elm at the head of
the tail queue.
The TAILQ_INSERT_TAIL() macro inserts the new element elm at the end of
the tail queue.
The TAILQ_INSERT_AFTER() macro inserts the new element elm after the ele-
ment listelm.
The TAILQ_INSERT_BEFORE() macro inserts the new element elm before the
element listelm.
The TAILQ_REMOVE() macro removes the element elm from the tail queue.
The TAILQ_REPLACE() macro replaces the list element elm with the new ele-
ment elm2.
The TAILQ_CONCAT() macro concatenates all the elements of the tail queue
referenced by head2 to the end of the tail queue referenced by head1,
emptying head2 in the process. This is more efficient than removing and
inserting the individual elements as it does not actually traverse head2.
TAILQ_FOREACH() and TAILQ_FOREACH_REVERSE() are used for traversing a
tail queue. TAILQ_FOREACH() starts at the first element and proceeds to-
wards the last. TAILQ_FOREACH_REVERSE() starts at the last element and
proceeds towards the first.
TAILQ_FOREACH(np, &head, FIELDNAME)
TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, FIELDNAME)
The macros TAILQ_FOREACH_SAFE() and TAILQ_FOREACH_REVERSE_SAFE() traverse
the list referenced by head in a forward or reverse direction respective-
ly, assigning each element in turn to var. However, unlike their unsafe
counterparts, they permit both the removal of var as well as freeing it
from within the loop safely without interfering with the traversal.
The TAILQ_FIRST(), TAILQ_NEXT(), TAILQ_LAST() and TAILQ_PREV() macros can
be used to manually traverse a tail queue or an arbitrary part of one.
The TAILQ_EMPTY() macro should be used to check whether a tail queue is
empty.
TAILQ_HEAD(tailhead, entry) head;
struct entry {
...
TAILQ_ENTRY(entry) entries; /* Tail queue. */
...
} *n1, *n2, *np;
TAILQ_INIT(&head); /* Initialize 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);
n2 = malloc(sizeof(struct entry)); /* Insert before. */
TAILQ_INSERT_BEFORE(n1, n2, entries);
/* Forward traversal. */
TAILQ_FOREACH(np, &head, entries)
np-> ...
/* Manual forward traversal. */
for (np = n2; np != NULL; np = TAILQ_NEXT(np, entries))
np-> ...
/* Delete. */
while ((np = TAILQ_FIRST(&head))) {
TAILQ_REMOVE(&head, np, entries);
free(np);
}
tree(3)
It is an error to assume the next and previous fields are preserved after
an element has been removed from a list or queue. Using any macro (except
the various forms of insertion) on an element removed from a list or
queue is incorrect. An example of erroneous usage is removing the same
element twice.
The SLIST_END(), LIST_END(), SIMPLEQ_END() and TAILQ_END() macros are
deprecated; they provided symmetry with the historical CIRCLEQ_END() and
just expand to NULL.
Trying to free a list in the following way is a common error:
LIST_FOREACH(var, head, entry)
free(var);
free(head);
Since var is free'd, the FOREACH macros refer to a pointer that may have
been reallocated already. A similar situation occurs when the current
element is deleted from the list. In cases like these the data
structure's FOREACH_SAFE macros should be used instead.
The queue functions first appeared in 4.4BSD. The historical circle queue
macros were deprecated in OpenBSD 5.5.
MirBSD #10-current November 7, 2020 10