why is it possible to use undefined struct in c

Question

#include <stdio.h>

int main()
{
  printf("%d", sizeof(struct token *));
}

The above code can be compiled and linked use gcc under Linux. Could anyone of you explain the thing behind the Scenes to me? I know the point take the fix size of memory, so the struct token is irrelevant to sizeof, but even turn on the warning option in gcc, no warnings about the "none exist" struct at all. The context for the question is that I'm reading some source code by others, I'm trying very very hard to find the definition of "struct token", but off course failed.

Answer 1

Because you are trying to get the size of a pointer to struct token. The size of a pointer doesn't depend on how the structure is actually defined.

Generally, you can even declare a variable of type struct token*, but you can't dereference it (e. g. access a member through the pointer).

Answer 2

To paraphrase the C standard, an incomplete type is a type that describes an object but lacks information needed to determine its size.

void is another incomplete type. Unlike other incomplete types, void cannot be completed.

This "incomplete type" is often used for kinds of handle: a library allows you to allocate a "handle" to something, work with it and dispose it again. All this happens encapsulated in the library. You as user have no idea what might happen inside.

Example:

lib.h:

struct data * d_generate(void);
void d_set(struct data *, int);
int d_get(struct data *);
void d_free(struct data*);

lib.c:

#include "lib.h"
#include <stdlib.h>
struct data { int value; };
struct data * d_generate(void) {
    return malloc(sizeof(struct data))
}
void d_set(struct data * d, int v) {
    d -> value = v;
}
int d_get(struct data * d) {
    return d->value;
}
void d_free(struct data* d) {
   free(d);
}

user.c:

#include "lib.h"
[...]
struct data * d = d_generate();
d_set(d, 42);
int v = d_get(d);
// but v = d->value; doesn't work here because of the incomplete definition.
d_free(d);