Working with a union of structs in C

Question

Say I have the following types:

typedef struct TYPEA
{
    int type;
    char[12] id;
} TYPEA;

typedef struct TYPEB
{
    int type;
    int value;
} TYPEB;

I want to use create a union of these types and 'int', so that I can access the "type" int without needing to know whether TYPEA or TYPEB is stored in the union (the value of int lets me determine which is actually stored there). I can't get the right syntax though.

My union:

typedef union MEMBER
{
    int type;
    struct TYPEA a;
    struct TYPEB b;
} MEMBER;

The union is accessed via:

typedef struct WRAPPER
{
    union MEMBER* member;
    struct WRAPPER* next;
} WRAPPER;

Questions:

1. (With 'w' as a pointer to an allocated WRAPPER struct) Accessing using w->member->a.id gives "request for member 'id' in something not a structure or union.
2. Can I assign a pointer to an already malloc'd TYPEA/B to w->member directly? Or does a union need to be malloced specially?

Thanks.

Solution 1

1. Use w->member->type.
2. You need to allocate the union specifically.

One note that may be a point of misunderstanding is that the union holds EITHER the int, or TYPEA, or TYPEB, so in particular you cannot rely on your int type; in the union to tell you which struct the union holds.

Edit to respond to question in comments:

You probably want something like this:

struct TYPEA {
  char data[30]; // or whatever
};

struct TYPEB {
  double x, y; // or whatever
};

struct some_info {
  int type; // set accordingly
  union {
    struct TYPEA a;
    struct TYPEB b;
  } data; // access with some_info_object.data.a or some_info_object.data.b
};

Solution 2

1. You defined the member field as a pointer, so you should use w->member->type instead of w->member.type.
2. You should malloc the union type. When you allocate a union, you'll get a structure that has a sizeof equal to the largest element in the union. If you try to copy structures into union pointers, you'll mess up the alignment.

Comments

[rikkit]

So even though TYPEA and TYPEB structs have an int in the first positions, there's no way to access it without knowing which type I'm working with?

[Arthur]

Is malloc for TYPEA or TYPEB needed after mallocing struct some_info?

[Fraggle]

@Arthur No. The members of data are not pointers so sizeof(data) is at least the larger of sizeof(struct TYPEA) and sizeof(struct TYPEB).