Thread: pointer of current node should be point to first node

I am trying to make circuler linkel list. I have created single linked list. In a code, pointer of current node point to previous node. I want to change pointer so that pointer of current node will point to first node in the list.

Code:

#include<stdio.h>

#include<stdlib.h>
  
struct node 
{
    int data;
    struct node * next;
};

struct node * ADD ( struct node *Head, int value)
{
    struct node * new = malloc( sizeof(*new));
    
    if( new != NULL )
    {
        new -> data = value;
        new -> next = Head;
    }
    
    return new;
}

void show ( struct node* head)
{
    struct node* current;
    
    for ( current = head; current != NULL; current = current -> next )
    {
        printf("%d ", current -> data);
    }
}    
 
int main()
{
    struct node *head = NULL;
    
    head = ADD( head, 1);
    head = ADD( head, 2);
    head = ADD( head, 3);
    
    show(head);
    
    return 0;
}

3 2 1

You must check if head is NULL in your ADD function.

Something like below:

Code:

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

void check_allocation(void *ptr) {
  if (!ptr) {
    fputs("Allocation failed!\n", stderr);
    exit(EXIT_FAILURE);
  }
}

struct node {
  int data;
  struct node * next;
};

/*to make the list circular*/
struct node * ADD ( struct node *Head, int value){
  struct node * new = malloc( sizeof(*new));
  check_allocation(new);
  new->data = value;  
  if (Head) {//check if Head is NULL
    new->next = Head->next;
    Head->next = new;
  }else {
    new->next = new;
  }
  return new;
}

....

> for ( current = head; current != NULL; current = current -> next )
Your end condition is getting back to head, not NULL.

It's a circular list remember.

Simplier:

Code:

// Circular single linked list test.

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

// Macro to initialize a list.
#define CSLIST_INIT(list__) { &(list__) }

// base structure of a single linked circular list.
struct cslist_head {
  struct cslist_head *next_;
};

// Add a node after an element in the list.
static inline void cslist_add_next( struct cslist_head *node, struct cslist_head *prev )
{
  node->next_ = prev->next_;
  prev->next_ = node;
}

// Deletes a node after an element in the list.
static inline struct cslist_head *cslist_del_next( struct cslist_head *prev )
{
  struct cslist_head *node;

  node = prev->next_;
  prev->next_ = node->next_;

  return node;
}

// Macro to iterate to the list.
#define cslist_foreach( iter__, head__ ) \
  for ( iter__ = (head__)->next_; iter__ != head__; iter__ = (iter__)->next_ )

// Simple test.
int main( void )
{
  // Derived from base structure...
  struct mynode {
    struct cslist_head *next_;    // must be here!

    // data.
    int x;
  };

  // auxiliary pointer and the empty list.
  struct cslist_head *t, list = CSLIST_INIT(list);

  struct mynode *node;

  // Add 3 nodes.
  node = malloc( sizeof *node );
  node->x = 1;
  cslist_add_next( (struct cslist_head *)node, &list );
  t = ( struct cslist_head *)node;    // keep pointer to tail.

  node = malloc( sizeof *node );
  node->x = 2;
  cslist_add_next( (struct cslist_head *)node, t );
  t = ( struct cslist_head *)node;    // keep pointer to tail.

  node = malloc( sizeof *node );
  node->x = 3;
  cslist_add_next( (struct cslist_head *)node, t );

  // List all nodes.
  cslist_foreach( t, &list )
  {
    node = ( struct mynode *)t;

    printf( "%d\n", node->x );
  }  

  // while the list still have nodes, free them.
  while ( list.next_ != &list )
  {
    t = cslist_del_next( &list );
    free( t );
  }
}