simple linked list in C

main.c

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

struct List {
    enum  { NODE, END } tag;
    union {
        struct { void* val; struct List* next; } node;
        struct {                               } end;
    } data;
};

struct List End = { .tag = END, .data = {} };

#define ARENA_MAX 4000
struct Arena { 
    uint8_t    data[ARENA_MAX];
    uintptr_t  top;
} ARENA = { .data = {0}, .top = 0 };


// returns a null pointer if the allocation could fail
void* /* aligned top */ arena_allocate(uint8_t* val, size_t size, uintptr_t align) {
    uintptr_t addr = (uintptr_t)(&ARENA.data[0] + ARENA.top) ;
    uintptr_t m = addr % align;
    if (m != 0) ARENA.top += align - m;
    
    if (ARENA.top+size >= ARENA_MAX) return (void*)0;

    uint8_t* new_pos = ((&ARENA.data[0]) + ARENA.top);
    uint8_t* out_pos = new_pos;

    for (size_t i = 0; i < size; ++i) {
        *new_pos = *val;
        ++new_pos;
        ++val;
    }
    ARENA.top += size;

    return (void*)out_pos;
}
#define ARENA_ALLOC(val_ptr, type) (arena_allocate((uint8_t*)val_ptr, sizeof(type), alignof(type)))


void print_float_list(struct List* float_list) {
    printf("\n[");

    for (struct List* float_list_ = float_list ; 
        float_list_ != &End; 
        float_list_ = float_list_->data.node.next )
    {   
        printf("%f", *(float*)float_list_->data.node.val);
        if ( float_list_->data.node.next->tag == NODE) printf(", ");
    }
    
    
    printf("]\n");
}


int main() {
    float a[6] = {1.0, 2.0, 3.0, 4.0, 5.0, 6.0};

    struct List* acc_list = &End;

    for (int i = 6; i!=0;  ) {
        --i;

        void* ptr = ARENA_ALLOC(&a[i], float);
        struct List l = {
            .tag = NODE, 
            .data = {.node = { .val = ptr, .next = acc_list }}
        };
        acc_list = ARENA_ALLOC(&l, struct List);
    }

    print_float_list(acc_list);

}