指向结构和函数指针的指针->段故障

Question

I always get a segmentation fault during execution of this test program. I can't figure out why. Maybe someone could explain it to me please, i'm sure i mixed up the pointer stuff.

#include <stdio.h>

struct xy {
    int         (*read)();
    void        (*write)(int);
};

struct z {
    struct xy    *st_xy;
};


static void write_val(int val)
{
    printf("write %d\n", val);
}

static int read_val()
{
    /* return something just for testing */
    return 100;
}

int init(struct xy *cfg)
{
    cfg->read = read_val;
    cfg->write = write_val;
    return 0;
}

int reset(struct z *st_z)
{
    /* write something just for testing */
    st_z->st_xy->write(111);

    return 55;
}

int main(int argc, char **argv)
{
    static struct z test;
    int ret;
    int ret2;

    ret = init(test.st_xy);
    printf("init returned with %d\n", ret);

    ret2 = reset(&test);
    printf("reset returned with %d\n", ret2);

    return 0;
}

Answer 1

You never allocate the actual xy object. Your test.st_xy is just a garbage pointer that you're not allowed to dereference.

Instead, do something like this:

 static struct z test;
 static struct xy inner_test;
 test.st_xy = &inner_test;

 // ...

 ret = init(test.st_xy);

Answer 2

You pass an uninitialized pointer to xy to the init function.

init(test.st_xy);

st_xy has not been initialized. I think there is no need for st_xy to be a pointer.

struct z {
   struct xy st_xy;
};

int main(int argc, char **argv)
{
  static struct z test;
  init(&test.st_xy);
}