c - 释放结构体变量数组的内存

Question

 typedef struct _TRANSITION TRANSITION;

typedef struct {
    char name[4];
    bool is_initial;
    bool is_final;
    TRANSITION *transitions;
    int number_transitions;
} STATE;

struct _TRANSITION {
    char c;
    STATE *dest;
};

 void destroy_state(STATE **pp_state) {
        if (pp_state != NULL && *pp_state != NULL) {
            free((*pp_state)->transitions);
            (*pp_state)->transitions = NULL;
        }
        free(*pp_state);
        *pp_state = NULL;
  }

我想要做的是删除一个STATE。所以我需要释放我所在州拥有的所有 TRANSITION 。在这里您可以看到创建这些结构的函数:

STATE *create_state(char *name, bool is_initial, bool is_final) {
    STATE *state = (STATE *) malloc(sizeof(STATE));
    if (state != NULL) {
        strcpy(state->name, name);
        state->is_initial = is_initial;
        state->is_final = is_final;
        state->transitions = NULL;
        state->number_transitions = 0;
    }
    return state;
}

void add_transition(STATE *source, char sign, STATE *dest) {
    if (source != NULL && dest != NULL) {
        TRANSITION *transitions = source->transitions;
        TRANSITION *trans = source->transitions;
        int i;
        if (transitions != NULL) {
            transitions = (TRANSITION *) realloc(transitions, sizeof(TRANSITION) * ((source->number_transitions) + 1));           
        } else {
            transitions = (TRANSITION *) malloc(sizeof(TRANSITION));
        }
        trans = (transitions + source->number_transitions);
        trans->c = sign;
        trans->dest = dest;
        (source->number_transitions)++;
        source->transitions = transitions;
    }
}

但是，当我用以下方法在 main 方法中测试我的函数时:

STATE *first = create_state("S'0", true, false);
STATE *second = create_state("S'1", false, false);
printf("%s: Initial: %d, Final: %d\n", first->name, first->is_initial, first->is_final);
add_transition(first, 'A', second);
add_transition(first, 'B', second);
add_transition(first, 'C', second);
TRANSITION *transition = first->transitions;
printf("%c", transition[0].c); // Output: A
printf("%c", transition[1].c); // Output: B
printf("%c\n", transition[2].c); // Output: C

destroy_state(&first);

printf("%c", transition[0].c); // Output: 
printf("%c", transition[1].c); // Output: B
printf("%c", transition[2].c); // Output: C

看起来我刚刚删除了第一个 TRANSITION (输出对 printf() 函数进行了注释)。

显然我做错了什么。我对这个问题的第一个解决方案是我必须迭代转换。但由于我只是为一组 TRANSITION 分配空间，所以我看不到它起作用(正如我所期望的那样，它不起作用)。

我的错误是什么？

Answer 1

当你这样做时

TRANSITION *transition = first->transitions;

您将指针保存到内存中。然后释放内存，但这不会更改您在上面定义和初始化的指针变量 transition。因此，当您取消引用transition时，您会访问未分配的内存，并且会出现未定义的行为。

您必须记住，就像分配内存不会初始化已分配内存的内容一样，释放内存也不会对内存的内容执行任何操作。因此，在释放它后访问它时，它的部分内容可能仍然保留其旧内容。