NAME

       queue - implementations of linked lists and queues

DESCRIPTION

       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 (SLIST)
       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 (STAILQ)
       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.

   Doubly linked data structures
       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.

   Doubly linked lists (LIST)
       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.

   Doubly linked tail queues (TAILQ)
       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.

   Doubly linked circular queues (CIRCLEQ)
       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.

STANDARDS

       Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008.  Present on the BSDs.  <sys/queue.h> macros
       first appeared in 4.4BSD.

NOTES

       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().

SEE ALSO

       circleq(3), insque(3), list(3), slist(3), stailq(3), tailq(3)