How to free a linked list with pointers - c

Question

I was trying to free memory for a linked list iteratively. The list has a struct which looks like this, and in this linked list, I don't add a url if it's in this list.
struct Node {
char *url;
struct Node *next;
};
Once done working with this linked list, I tried to free it, but got segmentation fault, I'm still in the way of learning c, I haven't got much clue of how to debugging such error other than directly searching for related topics. Referenced a few SOs this one, this one and this one, still cannot figure out where it got crashed.
Here's my code. Free feel to add comments if you think anything I missed in this implementation.
void url_push(struct Node *head, const char *url, size_t url_size) {
struct Node *new_node = (struct Node *) malloc(sizeof(struct Node));
new_node->url = malloc(url_size);
new_node->next = NULL;
for (int i = 0; i < url_size; i++)
*(new_node->url + i) = *(url + i);
struct Node *current = head;
while(1) {
if (strcmp(current->url, new_node->url) == 0) {
printf("Seen page %s!!!!!\n", new_node->url);
free(new_node);
break;
} else if (current->next == NULL) {
current->next = new_node;
break;
} else {
current = current->next;
}
}
}
int main() {
struct Node *head = (struct Node*)malloc(sizeof(struct Node));
head->url = "/";
head->next = NULL;
char *url = "www.google.com";
url_push(head, url, strlen(url));
url = "www.yahoo.com";
url_push(head, url, strlen(url));
url = "www.google.com";
url_push(head, url, strlen(url));
url = "www.wsj.com";
url_push(head, url, strlen(url));
struct Node *current = NULL;
while ((current = head) != NULL) {
printf("url: %s\n", head->url);
head = head->next;
free(current->url);
free(current);
}
}
Edited:
To reduce the confusions, I revised the struct. The purpose of using strcmp is to avoid adding a url that already seen.

Answer 1

head->url = "/";
That is not malloced data so you can't free it!
Your other problem is in url_push() with new_node->url = malloc(url_size); which does not allocate enough space for the terminating 0 in the string (nor does it copy the terminating 0 so you end up not "stomping on memory" but do have unterminated strings...). Try new_node->url = strdup(url); instead.
Style wise: calculate url_size in url_push() instead of making each caller call strlen() do it once inside the function being called (note if you use strdup() then you don't need the url_size at all.
Final note: A tool like valgrind would find both of these problems easily.

Answer 2

There are multiple problems in your code:
you do not allocate space for the null terminator for the new_node->url strings in url_push, causing strcmp() too have undefined behavior.
the first node is not properly constructed: its url pointer is not allocated.
you so not check for memory allocation failure
You should make url_push() more generic: it should handle empty lists by returning the new head pointer. You do not need to pass the length of the url string, just use strdup(), and you should avoid allocating a new node before checking for duplicates.
Here is a modified version:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Node {
char *url;
struct Node *next;
};
struct Node *url_push(struct Node *head, const char *url) {
struct Node *current = head;
if (current != NULL) {
for (;;) {
if (strcmp(current->url, url) == 0) {
printf("Seen page %s before!!!!!\n", url);
return head;
} else if (current->next == NULL) {
break;
} else {
current = current->next;
}
}
}
struct Node *new_node = malloc(sizeof(struct Node));
if (new_node == NULL || (new_node->url = strdup(url)) == NULL) {
fprintf(stderr, "memory allocation failure\n");
exit(1);
}
new_node->next = NULL;
if (current == NULL) {
head = new_node;
} else {
current->next = new_node;
}
return head;
}
int main() {
struct Node *head = NULL;
head = url_push(head, "/");
head = url_push(head, "www.google.com");
head = url_push(head, "www.yahoo.com");
head = url_push(head, "www.google.com");
head = url_push(head, "www.wsj.com");
while (head != NULL) {
printf("url: %s\n", head->url);
struct Node *current = head;
head = head->next;
free(current->url);
free(current);
}
return 0;
}