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

NAME

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

SYNOPSIS

       #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():
	   _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

DESCRIPTION

       The  insque()  and  remque() functions manipulate doubly-linked lists.  Each element in the list is a structure of which the first two ele-
       ments 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.

CONFORMING TO

       POSIX.1-2001.

NOTES

       Traditionally (e.g., SunOS, Linux libc 4 and libc 5), 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>.  Linux libc4 and libc 5 placed them in <stdlib.h>.

BUGS

       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 suit-
       ably initialized.

EXAMPLE

       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:
	       a
	       b
	       c
	   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 *
       new_element(void)
       {
	   struct element *e;

	   e = malloc(sizeof(struct element));
	   if (e == NULL) {
	       fprintf(stderr, "malloc() failed
");
	       exit(EXIT_FAILURE);
	   }

	   return e;
       }

       int
       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;
		   break;
	       default:
		   errfnd = 1;
		   break;
	       }
	   }

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

	   /* 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:
");
	   elem = first;
	   do {
	       printf("    %s
", elem->name);
	       elem = elem->forward;
	   } while (elem != NULL && elem != first);

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

	   exit(EXIT_SUCCESS);
       }

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

								    2010-09-09								 INSQUE(3)