為什么指向結構的指針沒有指向正確的結構？

Question

我正在建立NFA，並希望通過創建指向其他State的State結構來做到這一點。 NFA的構建過程要求我跟蹤哪個State指向NULL ，然后在我知道它們應指向哪個State時對其進行修補。

但是，當我更新鏈表時，它不會更新pointee State 。 我認為我沒有正確引用和更新NULL指針。

這是有問題的代碼的簡化版本：

#include <stdio.h>
#include <stdlib.h>

typedef struct State State;
struct State
{
    char c;
    State *out;
};

typedef struct List List;
struct List
{
    State *s;
    // has a next member that is irrelevant here.
};

State *State_new(char c, State *out)
{
    State *s;
    s = malloc(sizeof(*s));
    s->c = c;
    s->out = out;
    return s;
}

void *List_new(State **outpp)
{
    List *slist = malloc(sizeof(*slist));
    /* 
     * Dereference the pointer to a pointer of a State
     * to get a pointer to a state
     */
    slist->s = *outpp;
    return slist;
}

int main()
{
    State *a = State_new('a', NULL);
    List *l  = List_new(&(a->out));

    /* This printf() will result in a seg fault, since a->out is NULL. */
    //printf("%c\n", a->out->c);

    /* change what State struct is pointed to by l */
    l->s = State_new('b', NULL);

    /* why is this not b? */
    //printf("%c\n", a->out->c);
    return 0;
}

Answer 1

a->out->c不是'b'因為您將指針的副本存儲在List的成員中。 您要提供State**作為參數，但也應這樣存儲它。 您可以簡單地發送State *outp並寫成slist->s = outp; 如果不是這種情況。

#include <stdio.h>
#include <stdlib.h>

typedef struct State State;
struct State
{
    char c;
    State *out;
};

typedef struct List List;
struct List
{
    State **s; //<--- HERE
    // has a next member that is irrelevant here.
};

State *State_new(char c, State *out)
{
    State *s;
    s = malloc(sizeof(*s));
    s->c = c;
    s->out = out;
    return s;
}

void *List_new(State **outpp)
{
    List *slist = malloc(sizeof(*slist));
    /* 
     * Dereference the pointer to a pointer of a State
     * to get a pointer to a state
     */
    slist->s = outpp; //<<--- HERE
    return slist;
}

int main()
{
    State *a = State_new('a', NULL);
    List *l  = List_new(&(a->out));

    /* This printf() will result in a seg fault, since a->out is NULL. */
    //printf("%c\n", a->out->c);

    /* change what State struct is pointed to by l */
    *l->s = State_new('b', NULL);

    printf("%c\n", a->out->c);
    return 0;
}