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
46       attributes(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
68       including <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

EXAMPLE

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;
109
110           e = malloc(sizeof(struct element));
111           if (e == NULL) {
112               fprintf(stderr, "malloc() failed\n");
113               exit(EXIT_FAILURE);
114           }
115
116           return e;
117       }
118
119       int
120       main(int argc, char *argv[])
121       {
122           struct element *first, *elem, *prev;
123           int circular, opt, errfnd;
124
125           /* The "-c" command-line option can be used to specify that the
126              list is circular */
127
128           errfnd = 0;
129           circular = 0;
130           while ((opt = getopt(argc, argv, "c")) != -1) {
131               switch (opt) {
132               case 'c':
133                   circular = 1;
134                   break;
135               default:
136                   errfnd = 1;
137                   break;
138               }
139           }
140
141           if (errfnd || optind >= argc) {
142               fprintf(stderr,  "Usage: %s [-c] string...\n", argv[0]);
143               exit(EXIT_FAILURE);
144           }
145
146           /* Create first element and place it in the linked list */
147
148           elem = new_element();
149           first = elem;
150
151           elem->name = argv[optind];
152
153           if (circular) {
154               elem->forward = elem;
155               elem->backward = elem;
156               insque(elem, elem);
157           } else {
158               insque(elem, NULL);
159           }
160
161           /* Add remaining command-line arguments as list elements */
162
163           while (++optind < argc) {
164               prev = elem;
165
166               elem = new_element();
167               elem->name = argv[optind];
168               insque(elem, prev);
169           }
170
171           /* Traverse the list from the start, printing element names */
172
173           printf("Traversing completed list:\n");
174           elem = first;
175           do {
176               printf("    %s\n", elem->name);
177               elem = elem->forward;
178           } while (elem != NULL && elem != first);
179
180           if (elem == first)
181               printf("That was a circular list\n");
182
183           exit(EXIT_SUCCESS);
184       }
185

SEE ALSO

187       queue(3)
188

COLOPHON

190       This page is part of release 5.02 of the Linux  man-pages  project.   A
191       description  of  the project, information about reporting bugs, and the
192       latest    version    of    this    page,    can     be     found     at
193       https://www.kernel.org/doc/man-pages/.
194
195
196
197                                  2019-03-06                         INSQUE(3)
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