Thread: Freeing Linked Lists

Greetings all,

Im getting confused about linked lists and wonder if someone can clear it up for me.

Say i have a simple linked list structured like this :

Code:

typedef struct sockets SOCKETS;

struct sockets {
 int state;
 int sock_id;
 struct socks* next;
 struct socks* prev;
};

Now a small function to populate this linked list with some inital data like :

Code:

struct sockets* populate()
{
 struct sockets* head = NULL; 
 struct sockets* first = NULL;
 head = malloc(sizeof(struct sockets));
 first = malloc(sizeof(struct sockets));

 head->state=1;
 head->sock_id=5;
 head->next=first;
 head->prev=NULL;

 first->state=1;
 first->sock_id=6;
 first->next=NULL;
 first->prev=head;

 return head;
}

so in my main to create this list in memory it looks like :

Code:

int main(void)
{
 SOCKETS *socks;
 socks = populate();
}

Ok so now I have my first node held in socks. Sorry for the simplicity of this btw as I could probably just cut to the chase but i need to make sure im doing everything as I should and make it clear what my question is in relation to how i have my lists.

Ok now if i want to delete a node from that list ive always just looped through the list until i found the key like this :

Code:

void del_node(struct sockets* first_node, int sock_id)
{
 struct sockets* current = first_node;
 struct sockets *tmp_next = NULL;
 struct sockets *tmp_previous = NULL;
 while(current != NULL) {
  if(current->sock_id == sock_id) {
   if(current->next == NULL) {
    tmp_previous = current->previous;
    tmp_previous->next = NULL;
   } else {
    tmp_previous = current->previous;
    tmp_next = current->next;
    tmp_next->previous=tmp_previous;
    tmp_previous->next=tmp_next;
   }
   free(current);
   break;
  }
  current = current->next;
 }
}

Ok i know that is a messy for deleting but its all I could come up with while testing. Anyway heres my question (at last!) as you can see in my delete node function im just free'ing the pointer yes ? and not actually freeing the data within the struct ? as after researching this on the web I found some code by someone like this :

Code:

free(head->data);     /* free mem within the list! */   
free(head);

So do i have to free up all the sections in the struct aswell ?
Also ive read somewhere that after ive free a pointer its still there but has no memory allocated to it. So do i need to NULL it after I use it aswell ?

If anyone can explainwith a little example id really appreciate it.

Thanks.

mrpickle

ps. sorry for the long drawn out post !

Thanks for reply.

OK so technically the only thing im doing wrong is not setting the pointers to NULL after ive free'd them?

Reason I ask is because I keep getting random segfaults with chunk_alloc errors reported by GDB in a MUD ive been writing.

And even though I cant ever recreate the segfaults while running the code through Valgrind as it doesnt ever complain about any bad memory operations. So i thought it may have something to do with my pointers this also may be related to what you said about trying to access a dereferenced pointer.

Ok well ill go through my code and make sure I NULL all the pointers I am finished with and pray for no more random seg faults

Thanks

mrpickle.

quzah,

yep i see my way is going all the way around the houses to do what your code makes very simple. I think when I tried to do it like you have, i had trouble getting my head around the use of this piece of code :

node->prev->next

so my way was just a poor workaround due to lack of knowledge. I "think" i know what it does but im still a little confused about it but ill try and figure it out.

thanks

mrpickle.