Delete all nodes in a linked list (in c)

Question

I'm solving exercise 17.7 on page 454 in the book C Programming: A Modern Approach (http://knking.com/books/c/). The question is below:

The following loop is supposed to delete all nodes from a linked list and release the memory they occupy. Unfortunately, the loop is incorrect. Explain what's wrong with it and show how to fix the bug.

for (p = first; p != NULL; p = p->next)
    free(p);

My solution is this:

struct node *next_node;
p = first;

while (p != NULL) {
    next_node = p->next;
    free(p);
    p = next_node;
}

Is my answer correct?

Answer 1

Your approach is fine, but if all you provide is a corrected version, your answer is incomplete: you should first explain why p = p->next has undefined behavior once p has been freed with free(p).

You might also define next_node with a tighter scope, inside the loop:

p = first;
while (p != NULL) {
    struct node *next_node = p->next;
    free(p);
    p = next_node;
}

Here is an alternate correction as a single statement for consistency with the code fragment in the question.

for (p = first; p != NULL; ) {
    struct node *next_node = p->next;
    free(p);
    p = next_node;
}