Redefine macro to append item(s) to it

Question

I am trying to redefine a macro to append another macro to it.

When APPEND_ITEM is defined, I got the following error at compile time:
warning: "LIST" redefined
error: 'LIST' undeclared here (not a function)

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

#define LIST 1, 2, 3
#define ITEM 4

#ifdef APPEND_ITEM
#define LIST LIST, ITEM
#endif

uint8_t arr[] = {LIST};

int main()
{
    for (uint8_t i = 0; i < sizeof(arr); i++)
    {
        printf("%d ", arr[i]);
    }
    printf("\r\n");

    return 0;
}

Expected output:
1 2 3 4

I am using GCC.

Answer 1

I want to be able to change the size of arr[] variable at compile time

In this case you don't need to redefine the list (which on the other hand is not possible as others have explained since LIST expands to its literal values when used), but you can embed the ITEM directly into the array:

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

#define LIST 1, 2, 3
#define ITEM 4

#define APPEND_ITEM

uint8_t arr[] = {
    LIST
#ifdef APPEND_ITEM
    , ITEM
#endif
};

int main()
{
    for (uint8_t i = 0; i < sizeof(arr); i++)
    {
        printf("%d ", arr[i]);
    }
    printf("\r\n");
    return 0;
}