queue - implementations of linked lists and queues
The
<sys/queue.h> header file provides a set of macros that define
and operate on the following data structures:
- SLIST
- singly linked lists
- LIST
- doubly linked lists
- STAILQ
- singly linked tail queues
- TAILQ
- doubly linked tail queues
- CIRCLEQ
- doubly linked circular queues
All structures support the following functionality:
- •
- Insertion of a new entry at the head of the list.
- •
- Insertion of a new entry after any element in the
list.
- •
- O(1) removal of an entry from the head of the list.
- •
- Forward traversal through the list.
Code size and execution time depend on the complexity of the data structure
being used, so programmers should take care to choose the appropriate one.
Singly linked lists are the simplest and support only the above functionality.
Singly linked lists are ideal for applications with large datasets and few or
no removals, or for implementing a LIFO queue. Singly linked lists add the
following functionality:
- •
- O(n) removal of any entry in the list.
Singly linked tail queues add the following functionality:
- •
- Entries can be added at the end of a list.
- •
- O(n) removal of any entry in the list.
- •
- They may be concatenated.
However:
- •
- All list insertions must specify the head of the list.
- •
- Each head entry requires two pointers rather than one.
Singly linked tail 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) additionally
allow:
- •
- Insertion of a new entry before any element in the
list.
- •
- O(1) removal of any entry in the list.
However:
- •
- Each element requires two pointers rather than one.
Linked lists are the simplest of the doubly linked data structures. They add the
following functionality over the above:
- •
- They may be traversed backwards.
However:
- •
- To traverse backwards, an entry to begin the traversal and
the list in which it is contained must be specified.
Tail queues add the following functionality:
- •
- Entries can be added at the end of a list.
- •
- They may be traversed backwards, from tail to head.
- •
- They may be concatenated.
However:
- •
- All list insertions and removals must specify the head of
the list.
- •
- Each head entry requires two pointers rather than one.
Circular queues add the following functionality over the above:
- •
- The first and last entries are connected.
However:
- •
- The termination condition for traversal is more
complex.
Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008. Present on the BSDs.
<sys/queue.h> macros first appeared in 4.4BSD.
Some BSDs provide SIMPLEQ instead of STAILQ. They are identical, but for
historical reasons they were named differently on different BSDs. STAILQ
originated on FreeBSD, and SIMPLEQ originated on NetBSD. For compatibility
reasons, some systems provide both sets of macros. glibc provides both STAILQ
and SIMPLEQ, which are identical except for a missing SIMPLEQ equivalent to
STAILQ_CONCAT().
circleq(3),
insque(3),
list(3),
slist(3),
stailq(3),
tailq(3)