insque(3) — Linux manual page


insque(3)               Library Functions Manual               insque(3)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <search.h>

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

   Feature Test Macro Requirements for glibc (see

       insque(), remque():
           _XOPEN_SOURCE >= 500
               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 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 │

VERSIONS         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
       several versions of UNIX.  The above is the POSIX version.  Some
       systems place them in <string.h>.

STANDARDS         top


HISTORY         top


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

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

       static struct element *
           struct element *e;

           e = malloc(sizeof(*e));
           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


Linux man-pages (unreleased)     (date)                        insque(3)

Pages that refer to this page: circleq(3)list(3)slist(3)stailq(3)tailq(3)queue(7)