1queue(7)               Miscellaneous Information Manual               queue(7)
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NAME

6       queue - implementations of linked lists and queues
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DESCRIPTION

9       The  <sys/queue.h> header file provides a set of macros that define and
10       operate on the following data structures:
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12       SLIST  singly linked lists
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14       LIST   doubly linked lists
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16       STAILQ singly linked tail queues
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18       TAILQ  doubly linked tail queues
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20       CIRCLEQ
21              doubly linked circular queues
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23       All structures support the following functionality:
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25       •  Insertion of a new entry at the head of the list.
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27       •  Insertion of a new entry after any element in the list.
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29       •  O(1) removal of an entry from the head of the list.
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31       •  Forward traversal through the list.
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33       Code size and execution time depend  on  the  complexity  of  the  data
34       structure being used, so programmers should take care to choose the ap‐
35       propriate one.
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37   Singly linked lists (SLIST)
38       Singly linked lists are the simplest and support only the  above  func‐
39       tionality.   Singly  linked lists are ideal for applications with large
40       datasets and few or no removals, or  for  implementing  a  LIFO  queue.
41       Singly linked lists add the following functionality:
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43       •  O(n) removal of any entry in the list.
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45   Singly linked tail queues (STAILQ)
46       Singly linked tail queues add the following functionality:
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48       •  Entries can be added at the end of a list.
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50       •  O(n) removal of any entry in the list.
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52       •  They may be concatenated.
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54       However:
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56       •  All list insertions must specify the head of the list.
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58       •  Each head entry requires two pointers rather than one.
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60       Singly  linked  tail  queues  are  ideal  for  applications  with large
61       datasets and few or no removals, or for implementing a FIFO queue.
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63   Doubly linked data structures
64       All doubly linked types of data structures (lists and tail queues)  ad‐
65       ditionally allow:
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67       •  Insertion of a new entry before any element in the list.
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69       •  O(1) removal of any entry in the list.
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71       However:
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73       •  Each element requires two pointers rather than one.
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75   Doubly linked lists (LIST)
76       Linked  lists  are  the  simplest of the doubly linked data structures.
77       They add the following functionality over the above:
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79       •  They may be traversed backwards.
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81       However:
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83       •  To traverse backwards, an entry to begin the traversal and the  list
84          in which it is contained must be specified.
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86   Doubly linked tail queues (TAILQ)
87       Tail queues add the following functionality:
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89       •  Entries can be added at the end of a list.
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91       •  They may be traversed backwards, from tail to head.
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93       •  They may be concatenated.
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95       However:
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97       •  All list insertions and removals must specify the head of the list.
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99       •  Each head entry requires two pointers rather than one.
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101   Doubly linked circular queues (CIRCLEQ)
102       Circular queues add the following functionality over the above:
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104       •  The first and last entries are connected.
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106       However:
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108       •  The termination condition for traversal is more complex.
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STANDARDS

111       BSD.
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HISTORY

114       <sys/queue.h> macros first appeared in 4.4BSD.
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NOTES

117       Some  BSDs  provide SIMPLEQ instead of STAILQ.  They are identical, but
118       for historical reasons they were named differently on  different  BSDs.
119       STAILQ  originated  on  FreeBSD, and SIMPLEQ originated on NetBSD.  For
120       compatibility reasons, some systems provide both sets of macros.  glibc
121       provides  both  STAILQ  and  SIMPLEQ,  which are identical except for a
122       missing SIMPLEQ equivalent to STAILQ_CONCAT().
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SEE ALSO

125       circleq(3), insque(3), list(3), slist(3), stailq(3), tailq(3)
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129Linux man-pages 6.04              2023-03-30                          queue(7)
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