dynamically casting based on a header in a struct

struct_header.c

#include<stdio.h>
#include<stdlib.h>
#include<stdint.h>
#include<string.h>
#include<jemalloc/jemalloc.h>

typedef struct
{
    uint32_t type;
} Header;

typedef struct
{
    Header header;
    uint32_t data;
} Foo;

typedef struct
{
    Header header;
    char data[15];
} Bar;

int main(int argc, char* argv[])
{
    // for structs that start with a header
    Foo* foo = malloc(sizeof(Foo));
    foo->data = 12345;
    foo->header.type = 42;

    Bar* bar = malloc(sizeof(Bar));
    strncpy(bar->data, "hello!", 7);
    bar->header.type = 1;

    // we can reinterpret them as just the header (though allocating space for the biggest possible thing)
    Header* h = malloc(sizeof(Bar));
    h = (Header*) foo; // reinterpret

    printf("h (%p): type = %i (%lu bytes, %lu usable)\n", h, h->type, sizeof(h), malloc_usable_size(h)); // it works!

    // then cast back based on type 
    if(h->type == 42) {
        Foo* newfoo = realloc(h, sizeof(Foo));
        printf("newfoo (%p): %lu bytes, %lu usable\n", newfoo, sizeof(newfoo), malloc_usable_size(newfoo));
        printf("newfoo (%p): header.type = %i\n", newfoo, newfoo->header.type);
        printf("newfoo (%p): data = %i\n", newfoo, newfoo->data);
    }

    if(h->type == 1) {
        Bar* newbar = realloc(h, sizeof(Bar));
        printf("newbar (%p): %lu bytes, %lu usable\n", newbar, sizeof(newbar), malloc_usable_size(newbar));
        printf("newbar (%p): header.type = %i\n", newbar, newbar->header.type);
        printf("newbar (%p): data = %s\n", newbar, newbar->data);
    }

    return 0;
}

/**************************************

OUTPUT:

CASE 1: foo (l. 37: h = (Header*) foo;)

h (0x7f60bd406058): type = 42 (8 bytes, 8 usable)
newfoo (0x7f60bd406058): 8 bytes, 8 usable
newfoo (0x7f60bd406058): header.type = 42
newfoo (0x7f60bd406058): data = 12345

CASE 2: bar (l. 37: h = (Header*) bar;)

h (0x7fa615407040): type = 1 (8 bytes, 32 usable)
newbar (0x7fa615407040): 8 bytes, 32 usable
newbar (0x7fa615407040): header.type = 1
newbar (0x7fa615407040): data = hello!

****************************************/