列表的双指针 C

Question

A short time ago I began to learn c. When I tried coding list I wrote function with double pointer because I saw this in other resource, then I by myself finished writing this function and it's didn't work, please help me and explain how this work.

void
push(v_stack_t ** node, int num_args, ...)
{

    va_list ap;
    v_stack_t **current = node;

    va_start(ap, num_args);
    for (int i = 0; i < num_args; i++) {
        v_stack_t *new_node = (v_stack_t *) malloc(sizeof(v_stack_t));
        new_node->value = va_arg(ap, int);

        if (*current == NULL) {
            *current = new_node;
            continue;
        }
        while ((*current)->next != NULL) {
            current = &(*current)->next;
        }
        (*current)->next = new_node;
    }
    va_end(ap);
}

Answer 1

You haven't shown us the definition for v_stack_t , but you do not initialize all the members of that struct after you allocate one. new_node->next will have some unknown value in it (likely not NULL), causing problems when you try to add a second node. You should set

new_node->next = NULL;

right after the malloc statement.

Unrelated, you don't need to cast the return value from malloc .

Answer 2

As 1201ProgramAlarm mentioned, you need to set next to NULL .

But, because you use current , its final value is never propagated back to caller (eg you'd need to set *node at the end).

Your inner while loop could be moved above your outer loop.

And, it's far easier to dereference node at the beginning and use single indirect pointers for the bulk of the function. Side note: head is more descriptive of function than node here.

Here's a rework of your code:

void
push(v_stack_t **head, int num_args, ...)
{
    va_list ap;
    v_stack_t *tail;

    // find last element of list
    tail = NULL;
    for (v_stack_t *cur = *head;  cur != NULL;  cur = cur->next)
        tail = cur;

    va_start(ap, num_args);
    for (int i = 0; i < num_args; i++) {
        v_stack_t *new_node = malloc(sizeof(v_stack_t));

        new_node->value = va_arg(ap, int);
        new_node->next = NULL;

        // append to tail of list
        if (tail != NULL)
            tail->next = new_node;

        // add node at head of list
        else
            *head = new_node;

        // set new element as tail of list
        tail = new_node;
    }
    va_end(ap);
}