Problem with pointers and linked lists

bsimbeck · 02-21-2003, 10:27 AM

I hope someone can help me. I am in the process of working on a linked list project. When I do everything in main (commented out right now) it works nodes are created etc... However when I do it in a function it doesn't work, I get a core dump. Am I doing something wrong with passing the pointers as parameters??

Thanks in advance,

Branden

Code:

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

typedef struct node{
   int number;
   struct node *next;
   struct node *prev;
} NODE;

void createlist(NODE *head, NODE *current, int number)
{
   head = (NODE*)malloc(sizeof(NODE));
   current = head;
   current->number = 15;
   current->next = current;
   current->prev = current;
}

int main()
{
   NODE *head = NULL;
   NODE *current = NULL;

   //create the first node
   //head = malloc(sizeof(NODE));
   //current = head;
   //current->number = 15;
   //current->next = current;
   //current->prev = current;
   //end first node creation
  
 createlist(head, current, 15);
   printf("%i\n", current->number);
   return 0;
}

Monster · 02-21-2003, 10:42 AM

You need to pass the address of the pointer to make it work. This is because you change the pointer with the malloc function:

Code:

void createlist(NODE **head, NODE **current, int number)
{
   *head = (NODE*)malloc(sizeof(NODE));
   *current = *head;
   (*current)->number = 15;
   (*current)->next = *current;
   (*current)->prev = *current;
}

createlist(&head, &current, 15);

Monster · 02-21-2003, 11:05 AM

Some explanation:

In main head is a pointer to NULL.
When calling createlist a copy of the pointer is placed on the stack (it's also pointing to NULL).
In the createlist function memory is allocated and the copy of head is pointing to the allocated memory.
The original head pointer in main is still pointing to NULL.

If you pass the address of the head pointer, the createlist function has access to the original pointer by dereferencing (place * before the variable), so the allocated memory can be assigned to the original head pointer in the main function.

Same story for the "current" variable

Some more info

02-21-2003, 10:42 AM	#2
Monster Me want cookie! Join Date: Dec 2001 Posts: 680	You need to pass the address of the pointer to make it work. This is because you change the pointer with the malloc function: Code: void createlist(NODE head, NODE current, int number) { head = (NODE)malloc(sizeof(NODE)); current = head; (current)->number = 15; (current)->next = current; (current)->prev = *current; } createlist(&head, &current, 15);
Monster is offline

02-21-2003, 11:05 AM	#3
Monster Me want cookie! Join Date: Dec 2001 Posts: 680	Some explanation: In main head is a pointer to NULL. When calling createlist a copy of the pointer is placed on the stack (it's also pointing to NULL). In the createlist function memory is allocated and the copy of head is pointing to the allocated memory. The original head pointer in main is still pointing to NULL. If you pass the address of the head pointer, the createlist function has access to the original pointer by dereferencing (place * before the variable), so the allocated memory can be assigned to the original head pointer in the main function. Same story for the "current" variable Some more info
Monster is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Pointers to pointers in linked lists	G4B3	C++ Programming	2	07-23-2008 03:54 AM
pointers and linked lists in place of arrays	kordric	C++ Programming	8	05-14-2008 10:59 AM
Double pointers and linked lists.	simo_r	C Programming	2	05-06-2008 04:25 AM
Request for comments	Prelude	A Brief History of Cprogramming.com	15	01-02-2004 10:33 AM
Linked Lists & Pointers	fkheng	C Programming	4	06-10-2003 07:26 AM