Question

I have a program in c which receives messages from different clients and servers. When the messages come in it adds the message to that list. After the message is added to the list I print it on the screen and on the other servers. But i want to delete the node that contains the message after it is printed so when the the print function gets called only prints the new messages. How can I delete the node after print?

Here is my struct:

typedef struct trade_list {
    char* trader_msg;
    u_int32_t id_of_sender;
    int sender_timer;
    int local_time;

    struct trade_list *next;
}trade_list;

trade_list *head = NULL;

And here is how I print:

  void print_trades()
    {

        trade_list * newnode = head;
        trade_list *previous = NULL;

            while (newnode) {

             previous = newnode;

              if ((elapsed - newnode->local_time >= 8)) 

             printf ("%s\n", newnode->trader_msg);
             newnode = newnode->next;

                if (previous == NULL)
                    head = newnode->next;
                else
                    {
                    previous->next = newnode->next;

                    free(newnode);
                    }
                }
}

Thus gives me a segmentation fault. I tried changing the newnode->next to just newnode in the else part. previous->next = new node; It didn’t give me an error but it did not erase the node as it kept printing that node every time the print function was called

Answer 1

At the start of your function:

trade_list *prev = NULL;

At each iteration in your loop, before newnode = newnode->next; add prev = newnode.

Then to delete:

if (prev == NULL) /* replace the head */
   head = newnode->next;
else
   prev->next = newnode->next;

/* free newnode? */

Fairly simple.

EDIT: You should really have some functions like list_create, list_add, list_remove associated with your data structure, rather than just chucking like the remove code into a print function. That’s the first thing I do when I create any sort of data structure.

Another option is to have your linked list like:

typedef struct trade_node {
   char* trader_msg;
   u_int32_t id_of_sender;
   int sender_timer;
   int local_time;

   struct trade_node *next;
} trade_node;

typedef struct trade_list {
   trade_node *head;
   /* trade_node *foot; ? */
   size_t length;
} trade_list;

EDIT2: As for your edit, change print_trades to something like:

void print_trades()
{
   trade_list *prev = NULL, *next;
   trade_list *newnode = head;

   while (newnode) {

      if ((elapsed - newnode->local_time >= 8)) {    
         printf ("%s\n", newnode->trader_msg);

         /* temp variable for newnode->next */
         next = newnode->next;

         if (prev == NULL) /* replace the head */
            head = next;
         else
            prev->next = next;

         /* free newnode->trader_msg? */
         free(newnode);

         /* don't update prev */
         newnode = next;
      }
      else {
         prev = newnode;
         newnode = newnode->next;
      }

   }
}

In your code, previous will never be NULL since you set it to newnode at the start of the loop, and newnode also shouldn’t equal newnode->next until after newnode has been processed completely. You’re also using newnode after you’ve freed it.