Nested linked lists in C - c

Question

I'm trying to implement a nested linked list in C, that will be used for a hierarchical menu. However, the GCC (v4.9.3-1) is complaining to nested structures, and I have no idea how to fix this. Here is the minimum (non)working example.
Is this nesting even possible in C?
main.c
#include "menu.h"
int main(void) {
Init_Menu();
return 0;
}
menu.c
#include "menu.h"
MenuItem_t LVL_0_MainMenu = {
.size = 0,
};
MenuItem_t LVL_1_Measurements = {
.size = 0,
};
void Init_Menu(void) {
Menu_Add_Child(&LVL_0_MainMenu, &LVL_1_Measurements);
}
void Menu_Add_Child(MenuItem_t *parent, MenuItem_t *child) {
parent->children[parent->size] = child;
child->parent = parent;
parent->size++;
}
menu.h
typedef struct {
unsigned char size;
MenuItem_t children[10];
MenuItem_t *parent;
} MenuItem_t;
extern MenuItem_t LVL_0_MainMenu;
extern MenuItem_t LVL_1_Measurements;
void Init_Menu(void);
void Menu_Add_Child(MenuItem_t *parent, MenuItem_t *child);
Based on answers by #bolov and #sps (once again, thanks to both of them), here is the minimum working example:
main.c
#include "menu.h"
int main(void) {
Init_Menu();
return 0;
}
menu.c
#include "menu.h"
MenuItem_t LVL_0_MainMenu = {
.size = 0,
};
MenuItem_t LVL_1_Measurements = {
.size = 0,
};
void Init_Menu(void) {
Menu_Add_Child(&LVL_0_MainMenu, &LVL_1_Measurements);
}
void Menu_Add_Child(MenuItem_t *parent, MenuItem_t *child) {
parent->children[parent->size] = child;
child->parent = parent;
parent->size++;
}
menu.h
struct MenuItem_t {
unsigned char size;
struct MenuItem_t *children[10];
struct MenuItem_t *parent;
};
typedef struct MenuItem_t MenuItem_t;
extern MenuItem_t LVL_0_MainMenu;
extern MenuItem_t LVL_1_Measurements;
void Init_Menu(void);
void Menu_Add_Child(MenuItem_t *parent, MenuItem_t *child);
The difference between this corrected program and the original (non)working program, is that the children array is defined as an array of pointers to variables of the type MenuItem_t instead of the array of variables of the same type. The other difference is that a nested list (inside the structure) should also contain the keyword struct as #bolov explained.

Answer 1

You need to use struct for the type used inside itself, even if you typedef it later on.
E.g. this won't work:
struct X_ {
X* next;
};
typedef struct X_ X;
But this will
struct X_ {
struct X_* next;
};
As a side note, I really don't like this form:
typedef struct {
} X;
I use:
struct X {
};
typedef struct X X;
But maybe this is just me being more fond of C++.
If you want to use that form, it's the same: you need to add struct and it works:
typedef struct {
struct X2* next;
} X2;
regarding:
struct X {
struct X arr[10];
};
You can't have that! The array is just in our way to understand why. So let's simplify:
struct X {
int a;
struct X var;
};
This can't be. What size would X be? sizeof(X) = sizeof(int) + sizeof(X) + padding. Do you see the problem? All you can do is have a pointer to X, but not an object X inside X.
Returning to your array. You need dynamic arrays:
struct X {
struct X* arr;
int arr_size;
};
It gets more complicated as you need to manage the memory (malloc/free fun), but you can't avoid it.

Answer 2

First of all, you cannot do,
typedef struct {
SomeName_t some_var;
} SomeName_t;
You need to do,
typedef struct somename {
struct somename some_var;
} SomeName_t;
Also, a struct cannot have a member which is an array of structure itself. However, a struct can have a member which is an array of pointer to the same structure.
struct foo {
struct foo foo_arr[10]; /* Will give error */
struct foo *foo_ptr_arr[10]; /* Legal */
};
However, I dont see a reason that your children member should be an array of struct anyways. Because, as can be seen in menu.c, you are doing
parent->children[parent->size] = child;
where the type of child is MenuItem_t *. So I think you basically wanted MenuItem_t.children to be an array of MenuItem_t *, and not an array of MenuItem_t.
So making this change should resolve your issue:
menu.h
typedef struct menuitem {
unsigned char size;
/* MenuItem_t children[10]; */ /* Not possible */
struct menuitem *children[10]; /* This is what you want to do */
struct menutem *parent;
} MenuItem_t;