Struct inside struct

Either do the forward declaration of struct FRIDGE;

Or,

give the definition of FRIDGE before using it in struct PERSON

You have to use members of FRIDGE, after removing all warnings and errors. Declare FRIDGE before PERSON

me.fridge.number = 1

EDITED: I found the bug. You are using anonymous structure, so you should not use the struct keyword but use the typedefed name.

Change struct FRIDGE fridge to FRIDGE fridge

Using typedefs with your structs will get you into this kind of tangle. The struct keyword in front of a struct tag identifier is how structs are supposed to be used, this is also more explicit and easier to read.

There is a long and good blog post with all the details here https://www.microforum.cc/blogs/entry/21-how-to-struct-lessons-on-structures-in-c/

But in short what you really want to do is leave out the typedef's like this

struct FRIDGE;  // This is a forward declaration, now an incomplete type

struct PERSON{
 int age;
 struct FRIDGE fridge;
};

struct FRIDGE{
 int number;
};

struct FRIDGE fr;
fr.number=1;

struct PERSON me;
me.name=1;
me.fridge = fr;

Linus Torvalds also went on about this once, very solid reasoning why using typedefs on all your structs is confusing and bad.

https://yarchive.net/comp/linux/typedefs.html

struct FRIDGE is something different than FRIDGE.

You need to either use type FRIDGE in your other structure.

typedef struct {
 int age;
 FRIDGE fridge;
} PERSON;

or define your fridge as struct FRIDGE

struct FRIDGE {
 int number;
};

Also, the structure may have to be defined before you use it (e.g. above the person).