C中嵌套结构的指针类型

Question

Is it possible to create a pointer to the inner struct of a nested struct in C?

#include <stdio.h>
#include <stdint.h>

typedef struct
{
    uint8_t first;
    struct
    {
        uint8_t second;
        uint8_t third[8];
    } inner_struct;
} outer_struct;

int main()
{
    outer_struct foo;
    void * p = &foo.inner_struct;
    printf("%d", sizeof(p->third));
    return 0;
}

Using a void pointer I could point to it, but I get this error

main.c: In function 'main':
main.c:26:26: error: request for member 'third' in something not a structure or union
     printf("%d", sizeof(p->third));
                          ^~

when trying to get the size of the 'third' array. I could also use a pointer to outer_struct but the real example is event more nested and contains long variable names, making it hard to read. Is it possible to create a pointer to the inner struct directly?

Using struct inner_struct * p = &foo.inner_struct; instead of void yields

main.c: In function 'main':
main.c:25:31: warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
     struct inner_struct * p = &foo.inner_struct;
                               ^
main.c:26:26: error: dereferencing pointer to incomplete type 'struct inner_struct'
     printf("%d", sizeof(p->inner_struct.third));
                          ^~

Answer 1

The type void * is a generic pointer that can point to anything. But the compiler doesn't actually know what it points to, so you have to tell it by using casting.

So for p->third to work, you need to cast the pointer p to the correct pointer type.

Unfortunately this is not possible with your current code, as the inner structure is an anonymous structure without a known tag (structure name). You need to create a structure tag that you can use for the casting. For example

typedef struct
{
    uint8_t first;
    struct inner_struct
    {
        uint8_t second;
        uint8_t third[8];
    } inner_struct;
} outer_struct;

Now you can cast the pointer p , like ((struct inner_struct *) p)->third .

Or define p to be the correct type immediately:

struct inner_struct *p = &foo.inner_struct;