Can't free/deallocate space in my pop function?

Question

Here is my code for pop:

int pop (struct_of_ints *head_node){
int val;
if (head_node == NULL){
    fprintf(stderr, "Empty stack.\n");
    return -1;
}
else {
    struct_of_ints *curr;
    struct_of_ints *prev;
    curr = head_node;
    prev = NULL;

        while (curr->next != NULL) {
            prev = curr;
            curr = curr->next;
        }

        val = curr->value;

        if (prev == NULL) 
            head_node = NULL;
    else 
            prev->next = curr->next;
        free(curr)
        return val;


}

}

When I try to free(curr), however, I get a segmentation fault and when I run valgrind I get messages like "Invalid free() / delete / delete[]", "Address 0x51c1f60 is 16 bytes inside a block of size 32 free'd", and "Invalid read of size 8"...I have no idea what's wrong. If anyone could help I'd appreciate it. Thanks!

Answer 1

You are passing the pointer *head_node, in the function, which is getting passed by value. To update *head_node, try passing **head_node, and change the code as::

int pop (struct_of_ints **head_node)
{
    int val;
    if (*head_node == NULL)
    {
        fprintf(stderr, "Empty stack.\n");
        return -1;
    }
    else 
    {
        struct_of_ints *curr;
        struct_of_ints *prev;
        curr = *head_node;
        prev = NULL;
        while (curr->next != NULL) 
        {
            prev = curr;
            curr = curr->next;
        }

        val = curr->value;

        if (prev == NULL) 
            *head_node = NULL;
        else 
            prev->next = curr->next;
        free(curr)
        return val;
    }
}