array of type void

Question

plain C have nice feature - void type pointers, which can be used as pointer to any data type.
But, assume I have following struct:

struct token {
    int type;
    void *value;
};

where value field may point to char array, or to int, or something else.
So when allocating new instance of this struct, I need:

1) allocate memory for this struct;
2) allocate memory for value and assign it to value field.

My question is - is there ways to declare "array of type void", which can be casted to any another type like void pointer?

All I want is to use "flexible member array" (described in 6.7.2.1 of C99 standard) with ability to casting to any type.

Something like this:

struct token {
    int type;
    void value[];
};

struct token *p = malloc(sizeof(struct token) + value_size);
memcpy(p->value, val, value_size);
...
char *ptr = token->value;

I suppose declaring token->value as char or int array and casting to needed type later will do this work, but can be very confusing for someone who will read this code later.

Answer 1

Well, sort of, but it's probably not something you want:

struct token {
  // your fields
  size_t item_size;
  size_t length
};

struct token *make_token(/* your arguments */, size_t item_size, size_t length)
{
    struct token *t = malloc(sizeof *t + item_size * length);
    if(t == NULL) return NULL;
    t->item_size = item_size;
    t->length    = length;
    // rest of initialization
}

The following macro can be used to index your data (assuming x is a struct token *):

#define idx(x, i, t) *(t *)(i < x->length ? sizeof(t) == x->item_size ?
                       (void *)(((char *)x[1]) + x->item_size * i)
                     : NULL : NULL)

And, if you like, the following macro can wrap your make_token function to make it a little more intuitive (or more hackish, if you think about it that way):

#define make_token(/* args */, t, l) (make_token)(/* args */, sizeof(t), l)

Usage:

struct token *p = make_token(/* args */, int, 5); // allocates space for 5 ints
...
idx(p, 2, int) = 10;