program in c runs into infinite loop when I use free()

Question

I was trying out a linked list program in c where I use malloc() to allocate memory dynamically and then when I tried using free() at the end of the function, the program runs into an infinite loop.

Why is this happening ?

void Insert(int x, int pos)
{
    struct Node *newnode = (struct Node*) malloc(sizeof(struct Node));
    newnode->data = x;
    newnode->next = NULL;
    struct Node* temp, *left, *right;
    int i = 1;
    temp = head;

    if(head == NULL)
    {
        head = newnode;
    }
    else{
        while(i != pos - 1)
        {
            temp = temp->next;
            i++;
        }
        newnode->next = temp->next;
        temp->next = newnode;
    }
    free(newnode);
}

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, May 12 '17 at 14:26
Are you sure it runs into an infinite loop because of free? That is pretty weird... — Silveris, May 12 '17 at 14:26
why do you free there, at all? Don;t you wanna use 'em later? — Sourav Ghosh, May 12 '17 at 14:27
yes it runs into infinite loop, and i m not able to figure out why! — she_be_me, May 12 '17 at 14:27
Sourav Ghosh is right, this is the not the right place to use `free()`, you free the node you just inserted. — Silveris, May 12 '17 at 14:28
ohh.. then where should i be using free() then?? And should i use free() at all?? — she_be_me, May 12 '17 at 14:30
The only was I see a (de-facto) infinite loop here is, when you call the function with pos <= 1 and the linked list is circular. Do not free the node before you remove it from the list. — Ctx, May 12 '17 at 14:32
ok!! Thanks a lot @Silveris . I am using it in the wrong place! — she_be_me, May 12 '17 at 14:39
If you've read on some site that you must free() memory in the same function as it is allocated, please delete the link from your history/favourites and try to forget you ever opened it :) — ThingyWotsit, May 12 '17 at 14:41
Not directly related to your problem: but using the global variable `head` is a very bad idea. — Jabberwocky, May 12 '17 at 14:43

score 1 · Accepted Answer · edited May 23 '17 at 10:31

You use free() in the wrong place, resulting in deleting the newly inserted node in your list.

Should I use free() at all?

Yes, since you are using malloc(). The memory you allocated dynamically should be de-allocated by you, otherwise memory leaks shall happen.

then where should I be using free() then?

In the place of your code that you do not need your list anymore. For example, at the end of your main().

Unrelated, but by looking at your insert(), I can see that head is a global variable, which should be avoided, when able. Passing it as a parameter to your list functions, and making this variable non global is a better approach. If you want, take a look in this list's code, which is fully commented and was the one I used to learn.

Do I cast the result of malloc? No!

program in c runs into infinite loop when I use free()

1 Answers1