访问指向联合的指针中的指针

Question

So I have structure containing a union as follows:

struct FILL{
  char *name;
  int id;
};

struct TEST{
  union{
    struct FILL *fill;
    int type;
  } *uni;
};

I don't understand how to access the union members within the structure. I've been trying to do it as follows:

struct TEST *test_struct, *test_int;

test_struct = malloc(sizeof(struct TEST));
test_struct->uni = malloc(sizeof(struct TEST));
test_struct->uni->fill->name = NULL;
test->struct->uni->fill->id = 5;

test_int = malloc(sizeof(int));
test_int->uni->type = 10;

But I get segfaults when I try this. Am I accessing these wrong? How should I do it otherwise?

Edit: Sorry I was focusing on the formatting and I screwed up the declaration for TEST. Its been fixed.

Answer 1

Each of the pointer members of the struct must be initialized, either by allocate dynamic storage by malloc , or assign to other variables. Here are the problems of your code:

struct TEST *test_struct, *test_int;

test_struct = malloc(sizeof(struct TEST));
test_struct->uni = malloc(sizeof(struct TEST)); // uni should be allocated with size of the union, not the struct
test_struct->uni->fill->name = NULL; // uni->fill is a pointer to struct FILL, it should be allocated too before accessing its members
test->struct->uni->fill->id = 5;

test_int = malloc(sizeof(int)); // test_int is of type struct TEST, you are allocating a integer here
test_int->uni->type = 10; // same, uni not allocated

So try the following fix:

struct TEST *test_struct, *test_int;

test_struct = malloc(sizeof(struct TEST));
test_struct->uni = malloc(sizeof(*test_struct->uni));        
test_struct->uni->fill = malloc(sizeof(struct FILL));
test_struct->uni->fill->name = NULL;
test_struct->uni->fill->id = 5;

test_int = malloc(sizeof(struct TEST));
test_int->uni = malloc(sizeof(*test_struct->uni));