使用带有malloc（）和free（）的指针的可能的内存泄漏，以及与指针符号的混淆

Question

So I'm relatively new to C, and I was trying to write code for a singly linked list.

This is what I wrote for deleting a node from the beginning of the list.

int delete(struct node **head)
{
    int x = -1;
    if(*head != NULL) {
        struct node *old = *head;
        (*head) = (*head)->next;
        x = old->data;
        free(old);
    }
    return x;
}

and I'm confused at the free() function. Am I freeing the space allocated to the pointer old , or am I freeing it to the address at which old points? Maybe the correct way of doing it would be free(*old) ? Will this code cause a memory leak?

Also, if this frees the memory allocated to old, what would be the effect of free(&old) , in that case?

Answer 1

From what you show the memory management seems OK and does not seem to leak memory.

Am I freeing the space allocated to the pointer old

Yes, the code deallocates, frees to memory old points to, which is commonly referred to as the memory being allocated to a pointer .

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free(*old) wouldn't work, as it wouldn't compile, because you tried to pass a struct into where a pointer is expected.

free(&old) wouldn't work, as it would provoke undefined behaviour, because the code tried to free memory being allocated on the stack, that is the memory for the pointer variable old itself.