How can i remove and clear memory, from sepecific node of a linked list?

Question

So, i have a linked list with 47 thousand nods. each node has year month and day. I want to remove nodes that dont fit into [month initial, month final]. So if i chose the month initial as 3 and the final as 8, for exemple, i want to remove the ones that dont fit wichever is the year and day.

LISTAPAISES *Filtra_month(LISTAPAISES * head, int month_init, int month_final) {
    LISTAPAISES *aux=NULL, *elim=NULL, *aux2=NULL; 
    aux=head;
    while(aux !=NULL){
       if(aux->country.month>month_final || aux->country.month<month_init){
          elim=aux;
          aux=aux->next;
          free(elim);
       }
       else{
          if(aux2==NULL){
             aux2=aux;
          }
          aux=aux->next;
      }
   }
   return aux2;
}

This seems no get the nods that i want, but instead of clearing them it just puts random numbers in. Any sugestions? Thanks in advance.

Answer 1

Say you have a linked-list A->B->C . When you're freeing B node, the previous node A , by your code, it still points to the old memory location B and not the new node C . The random numbers, if not segmentation faults, is just garbage memory where B used to be being read.

Fix it by having two pointers, aux , and ahead . The aux pointer stays behind ahead one node, and if ahead passes the failing constrain, free it, assign ahead to ahead->next , and update aux accordingly.

LISTAPAISES* ahead = NULL;
aux = head;
// Special case if head passes the failing constain
if(head != NULL && (head->country.month>month_final || head->country.month<month_init)){
    aux = aux->next;
    free(head);
    head = aux;
}
if(aux != NULL){
    ahead = aux->next;
}
while(ahead != NULL){
    if(ahead->country.month>month_final || ahead->country.month<month_init){
        // Create a tmp pointer to hold the node after 'ahead'
        LISTAPAISES* tmp = ahead->next;
        free(ahead);
        // Reassign the previous pointer to now point to `tmp`
        aux->next = tmp;
        // Update 'ahead' to be 'tmp' for the next iteration
        ahead = tmp;
    }
    else{
        ahead = ahead->next;
        aux = aux->next;
    }
}

return head;