(3) double-linked list: insque remque

1. 

/** * The insque and remque subroutines manipulate queues built from double-linked lists. * Each element in the queue must be in the form of a qelem structure. The next and * prev elements of that structure must point to the elements in the queue immediately * before and after the element to be inserted or deleted. * * The insque subroutine inserts the element pointed to by the Element parameter into * a queue immediately after the element pointed to by the Pred parameter. * The remque subroutine removes the element defined by the Element parameter from a queue. * * #include <search.h> * Inserts or removes an element in a queue. * void insque(void *elem, void *prev); * void remque(void *elem); *struct qelem {struct qelem *q_forw;struct qelem *q_back;};void insque (struct qelem *elem, struct qelem *pred){  elem -> q_forw = pred -> q_forw;  pred -> q_forw -> q_back = elem;  elem -> q_back = pred;  pred -> q_forw = elem;}void remque (struct qelem *elem){  elem -> q_forw -> q_back = elem -> q_back;  elem -> q_back -> q_forw = elem -> q_forw;}*/#ifndef _GNU_SOURCEstruct qelem {struct    qelem *q_forw;struct    qelem *q_back;char      *element;};#endif#include <search.h>#include <stdlib.h>#include <stdio.h>#include <string.h>void *xmalloc(unsigned n) {void *p;p = malloc(n);if (p) return p;fprintf(stderr, "malloc failure, insufficient memory.\n");exit(1);}struct qelem *create_head(){struct qelem *head = (struct qelem *)xmalloc(sizeof(struct qelem));head->q_forw = head;head->q_back = head;return head;}int main(int argc, char **argv){struct qelem *head = create_head();//head's element is blankstruct qelem *prev = head;struct qelem *tmp;//insque, actually insert the first element into ahead of headint i;for(i = 1; i < argc; i++){struct qelem *node = (struct qelem *)xmalloc(sizeof(struct qelem));node->element = argv[i];insque(node, prev);prev = node;}//traversing the listtmp = prev;puts("traversing the list:");while(tmp != head){puts(tmp->element);tmp = tmp->q_back;}//remque the first element after headputs("after remque, go through:");remque(head->q_forw);tmp = head->q_forw;while(tmp != head){puts(tmp->element);tmp = tmp->q_forw;}return 0;}

2

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

  #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\n");               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...\n", 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:\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");           exit(EXIT_SUCCESS);       }