Doubly-linked list in C, insert by value

Question

I decided to do a project on doubly-linked list in order to understand it better. I have already made functions that insert nodes at the head and at the tail but now I'm having trouble in inserting nodes by value. Here is the function:

void f_insert_by_value(the_individual **head, char *str, int a) {
    the_individual *current = *head, *temp = f_create(str, a);

    if (*head == NULL) *head = temp;
    else {
        if (temp->age < (*head)->age) {
            temp->next = (*head);
            (*head)->prev = temp;
            (*head) = (*head)->prev;
        }
        else {
            while (temp->age > current->age && current->next != NULL) current = current->next;
            if (current->next = NULL) {
                temp->prev = current;
                current->next = temp;               
                current = current->next;
            }
            else {
                temp->prev = current->prev;
                temp->next = current;
                current->prev->next = temp;
                current->prev = temp;
            } 
        }
    }
    return;
}

Segmentation fault occurs on line "current->prev->next = temp". I tried to print addresses to see why this happens and found out that node which is first in the input always ends up having it's previous element pointing to NULL. Can someone explain why that happens and how can it be fixed? Thank you.

Answer 1

On the first node, current->prev is null because current is the first one, you got it right.

temp->prev = current->prev;
temp->next = current;

This thing is right, you are setting the new node at the right place, but now current doesn't lead to the right thing. You schema is this one at this point :

NULL <= temp => current
NULL <= current <=> ...

And you want

NULL <= temp <=> current <=> ...

So the only thing missing is that the previous element of current is temp . So I guess that just removing the line

current->prev->next = temp

Should do the trick for the first element insertion, because you set current->prev right after.

So I guess your condition block should be like this :

temp->prev = current->prev;
temp->next = current;
if (current->prev != NULL) {
    current->prev->next = temp;
}
current->prev = temp;

As said in the comments, if you want to avoid the null pointer problem with linked list, you can add a dummy at the beginning and at the very end of your list, which are controllers telling that you reach the limit. You can find more information on these types of linked list (with a Sentinel Node) here