insque(3) — Linux manual page


INSQUE(3)                 Linux Programmer's Manual                INSQUE(3)

NAME         top

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

SYNOPSIS         top

       #include <search.h>

       void insque(void *elem, void *prev);

       void remque(void *elem);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       insque(), remque():
           _XOPEN_SOURCE >= 500
               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
               || /* Glibc versions <= 2.19: */ _SVID_SOURCE

DESCRIPTION         top

       The insque() and remque() functions manipulate doubly-linked lists.
       Each element in the list is a structure of which the first two
       elements are a forward and a backward pointer.  The linked list may
       be linear (i.e., NULL forward pointer at the end of the list and NULL
       backward pointer at the start of the list) or circular.

       The insque() function inserts the element pointed to by elem
       immediately after the element pointed to by prev.

       If the list is linear, then the call insque(elem, NULL) can be used
       to insert the initial list element, and the call sets the forward and
       backward pointers of elem to NULL.

       If the list is circular, the caller should ensure that the forward
       and backward pointers of the first element are initialized to point
       to that element, and the prev argument of the insque() call should
       also point to the element.

       The remque() function removes the element pointed to by elem from the
       doubly-linked list.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface          Attribute     Value   │
       │insque(), remque() │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

       On ancient systems, the arguments of these functions were of type
       struct qelem *, defined as:

           struct qelem {
               struct qelem *q_forw;
               struct qelem *q_back;
               char          q_data[1];

       This is still what you will get if _GNU_SOURCE is defined before
       including <search.h>.

       The location of the prototypes for these functions differs among sev‐
       eral versions of UNIX.  The above is the POSIX version.  Some systems
       place them in <string.h>.

BUGS         top

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

EXAMPLES         top

       The program below demonstrates the use of insque().  Here is an
       example run of the program:

           $ ./a.out -c a b c
           Traversing completed list:
           That was a circular list

   Program source

       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <search.h>

       struct element {
           struct element *forward;
           struct element *backward;
           char *name;

       static struct element *
           struct element *e;

           e = malloc(sizeof(struct element));
           if (e == NULL) {
               fprintf(stderr, "malloc() failed\n");

           return e;

       main(int argc, char *argv[])
           struct element *first, *elem, *prev;
           int circular, opt, errfnd;

           /* The "-c" command-line option can be used to specify that the
              list is circular */

           errfnd = 0;
           circular = 0;
           while ((opt = getopt(argc, argv, "c")) != -1) {
               switch (opt) {
               case 'c':
                   circular = 1;
                   errfnd = 1;

           if (errfnd || optind >= argc) {
               fprintf(stderr,  "Usage: %s [-c] string...\n", argv[0]);

           /* Create first element and place it in the linked list */

           elem = new_element();
           first = elem;

           elem->name = argv[optind];

           if (circular) {
               elem->forward = elem;
               elem->backward = elem;
               insque(elem, elem);
           } else {
               insque(elem, NULL);

           /* Add remaining command-line arguments as list elements */

           while (++optind < argc) {
               prev = elem;

               elem = new_element();
               elem->name = argv[optind];
               insque(elem, prev);

           /* Traverse the list from the start, printing element names */

           printf("Traversing completed list:\n");
           elem = first;
           do {
               printf("    %s\n", elem->name);
               elem = elem->forward;
           } while (elem != NULL && elem != first);

           if (elem == first)
               printf("That was a circular list\n");


SEE ALSO         top


COLOPHON         top

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       latest version of this page, can be found at

                                 2020-06-09                        INSQUE(3)