1INSQUE(3)                  Linux Programmer's Manual                 INSQUE(3)
2
3
4

NAME

6       insque, remque - insert/remove an item from a queue
7

SYNOPSIS

9       #include <search.h>
10
11       void insque(void *elem, void *prev);
12
13       void remque(void *elem);
14
15   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
16
17       insque(), remque():
18           _XOPEN_SOURCE >= 500
19               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
20               || /* Glibc versions <= 2.19: */ _SVID_SOURCE
21

DESCRIPTION

23       The  insque()  and  remque()  functions manipulate doubly linked lists.
24       Each element in the list is a structure of which the first two elements
25       are  a  forward  and a backward pointer.  The linked list may be linear
26       (i.e., NULL forward pointer at the end of the list  and  NULL  backward
27       pointer at the start of the list) or circular.
28
29       The  insque()  function  inserts the element pointed to by elem immedi‐
30       ately after the element pointed to by prev.
31
32       If the list is linear, then the call insque(elem, NULL) can be used  to
33       insert  the  initial  list  element,  and the call sets the forward and
34       backward pointers of elem to NULL.
35
36       If the list is circular, the caller should ensure that the forward  and
37       backward pointers of the first element are initialized to point to that
38       element, and the prev argument of the insque() call should  also  point
39       to the element.
40
41       The  remque()  function removes the element pointed to by elem from the
42       doubly linked list.
43

ATTRIBUTES

45       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
46       tributes(7).
47
48       ┌───────────────────┬───────────────┬─────────┐
49Interface          Attribute     Value   
50       ├───────────────────┼───────────────┼─────────┤
51insque(), remque() │ Thread safety │ MT-Safe │
52       └───────────────────┴───────────────┴─────────┘
53

CONFORMING TO

55       POSIX.1-2001, POSIX.1-2008.
56

NOTES

58       On  ancient  systems,  the  arguments  of  these functions were of type
59       struct qelem *, defined as:
60
61           struct qelem {
62               struct qelem *q_forw;
63               struct qelem *q_back;
64               char          q_data[1];
65           };
66
67       This is still what you will get if _GNU_SOURCE is  defined  before  in‐
68       cluding <search.h>.
69
70       The  location  of the prototypes for these functions differs among sev‐
71       eral versions of UNIX.  The above is the POSIX version.   Some  systems
72       place them in <string.h>.
73

BUGS

75       In  glibc 2.4 and earlier, it was not possible to specify prev as NULL.
76       Consequently, to build a linear list, the caller had to  build  a  list
77       using  an  initial  call  that  contained the first two elements of the
78       list, with the forward and backward pointers in each  element  suitably
79       initialized.
80

EXAMPLES

82       The program below demonstrates the use of insque().  Here is an example
83       run of the program:
84
85           $ ./a.out -c a b c
86           Traversing completed list:
87               a
88               b
89               c
90           That was a circular list
91
92   Program source
93
94       #include <stdio.h>
95       #include <stdlib.h>
96       #include <unistd.h>
97       #include <search.h>
98
99       struct element {
100           struct element *forward;
101           struct element *backward;
102           char *name;
103       };
104
105       static struct element *
106       new_element(void)
107       {
108           struct element *e = malloc(sizeof(*e));
109           if (e == NULL) {
110               fprintf(stderr, "malloc() failed\n");
111               exit(EXIT_FAILURE);
112           }
113
114           return e;
115       }
116
117       int
118       main(int argc, char *argv[])
119       {
120           struct element *first, *elem, *prev;
121           int circular, opt, errfnd;
122
123           /* The "-c" command-line option can be used to specify that the
124              list is circular */
125
126           errfnd = 0;
127           circular = 0;
128           while ((opt = getopt(argc, argv, "c")) != -1) {
129               switch (opt) {
130               case 'c':
131                   circular = 1;
132                   break;
133               default:
134                   errfnd = 1;
135                   break;
136               }
137           }
138
139           if (errfnd || optind >= argc) {
140               fprintf(stderr,  "Usage: %s [-c] string...\n", argv[0]);
141               exit(EXIT_FAILURE);
142           }
143
144           /* Create first element and place it in the linked list */
145
146           elem = new_element();
147           first = elem;
148
149           elem->name = argv[optind];
150
151           if (circular) {
152               elem->forward = elem;
153               elem->backward = elem;
154               insque(elem, elem);
155           } else {
156               insque(elem, NULL);
157           }
158
159           /* Add remaining command-line arguments as list elements */
160
161           while (++optind < argc) {
162               prev = elem;
163
164               elem = new_element();
165               elem->name = argv[optind];
166               insque(elem, prev);
167           }
168
169           /* Traverse the list from the start, printing element names */
170
171           printf("Traversing completed list:\n");
172           elem = first;
173           do {
174               printf("    %s\n", elem->name);
175               elem = elem->forward;
176           } while (elem != NULL && elem != first);
177
178           if (elem == first)
179               printf("That was a circular list\n");
180
181           exit(EXIT_SUCCESS);
182       }
183

SEE ALSO

185       queue(7)
186

COLOPHON

188       This page is part of release 5.10 of the Linux  man-pages  project.   A
189       description  of  the project, information about reporting bugs, and the
190       latest    version    of    this    page,    can     be     found     at
191       https://www.kernel.org/doc/man-pages/.
192
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195                                  2020-11-01                         INSQUE(3)
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