通过将指针设置为NULL来初始化C中的堆栈

Question

我正试图根据以下标头（stack.h）在C中实现堆栈：

#ifndef STACK_H
#define STACK_H
/* An element from which stack is consisting */
typedef struct stack_node_ss {
  struct stack_node_ss *next;    /* pointer to next element in stack */
  void *value;                  /* value of this element */
} stack_node_s;
/* typedef so that stack user doesn't have to worry about the actual type of
 * parameter stack when using this stack implementation.
 */
typedef stack_node_s* stack_s;
/* Initializes a stack pointed by parameter stack. User calls this after he
 * has created a stack_t variable but before he uses the stack.
 */
void stack_init(stack_s *stack);
/* Pushes item to a stack pointed by parameter stack. Returns 0 if succesful,
 * -1 otherwise.
*/
int stack_push(void *p, stack_s *stack);
/* Pops item from a stack pointed by parameter stack. Returns pointer to
 * element removed from stack if succesful, null if there is an error or
 * the stack is empty.
 */
void *stack_pop(stack_s *stack);
#endif

然而，作为C的新手，我一直停留在stack_init函数上，我已经在stack.C中编写了

#include <stdlib.h>
#include <stdio.h>
#include "stack.h"
void stack_init(stack_s *stack) {
    (*stack)->value = NULL;
    (*stack)->next = NULL;
}

主程序开始于：

  int *tmp;
  stack_s stack;
  stack_init(&stack);

这使我的程序崩溃：

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000008
0x0000000100000abf in stack_init (stack=0x7fff5fbffb30) at stack.c:6
6       (*stack)->value = NULL;

你能告诉我正确的路线吗？非常感谢。

Answer 1

您必须为**stack本身分配内存：

*stack = malloc(sizeof(**stack));

但请不要键入def指针类型。这真的很令人困惑，很难阅读。最好按值传递指针，并将其留给调用者来存储指针，如下所示：

typedef struct stack_node_t
{
    struct stack_node_t * next;
    /* ... */
} stack_node;
stack_node * create_stack()
{
    stack_node * res = calloc(1, sizeof(stack_node));
    return res;
}
void destroy_stack(stack_node * s)
{
    if (!next) return;
    stack_node * next = s->next;
    free(s);
    destroy_stack(next);
}
// etc.

然后你可以说：

stack_node * s = create_stack();
// use s
destroy_stack(s);
s = NULL;  // some people like this

Answer 2

您正在取消引用未初始化的指针，从而导致未定义的行为。

因为这个函数正在创建一个新的堆栈，所以您需要为堆栈分配一些动态内存，然后将指针设置为指向新分配的内存：

void stack_init(stack_s *stack) {
    *stack = malloc(sizeof(**stack)); // create memory for the stack
    (*stack)->value = NULL;
    (*stack)->next = NULL;
}
stack_s stack;
stack_init(&stack);

然后您应该有一个名为stack_destroy的函数，它将free动态内存并将指针设置为NULL:

void stack_destroy(stack_s *stack) {
    free(*stack);
    *stack = NULL;
}

Answer 3

您应该将堆栈初始化为NULL，而不是向其推送NULL值：

void stack_init(stack_s *stack) {
    *stack=NULL;
}