Delete element of linked list by a certain criterion

Question

I\'ve written this function:

List* delPaintingCode(List* head, char *code)
{
    List *p,*q;

    for(p=head;p!=NULL;q=p,p=p->next)
    {
        if(!strc         


        

Answer 1

The logical problem that leads to a crash is in the if (p==head) branch of your code: when you delete the initial element, you free the head without updating p. This leads to dereferencing a freed pointer on the very next iteration.

You can fix the problem by introducing a fake node with head in its next, and returning the next, like this:

List fake;
fake.next = head;
// This loop always looks ahead by one element, i.e. at p->next.
for(List *p = &fake ; p->next != NULL ; p = p->next) {
    if(strcmp(code, p->next->code)) {
        continue;
    }
    List *q = p->next;
    p->next = q->next;
    free(q);
}
return fake.next;

This approach works for the initial element, too, because we added a fake head to our list, so the first time around p->next is the same as head. This lets us unify treatment of head element and all other elements.