#include <assert.h>
#include <stdio.h>
#include <stddef.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>

struct arena {
    FILE *stream;
    char *buffer;
    size_t offset;
    size_t size;
};

void arena_create(struct arena *arena)
{
    *arena = (struct arena){open_memstream(&arena->buffer, &arena->size)};
}

void arena_destroy(struct arena *arena)
{
    fclose(arena->stream);
    free(arena->buffer);
    *arena = (struct arena){0};
}

size_t arena_add(struct arena *arena, void const *data, size_t const size)
{
    size_t const offset = arena->offset;
    if (data) {
        fwrite(data, 1, size, arena->stream);
    }
    arena->offset += size;
    return offset;
}

static inline void *arena_get(struct arena const *arena, size_t offset)
{
    fflush(arena->stream);
    return arena->buffer + offset;
}

int main(int argc, char * const argv[])
{
    (void)argc;
    (void)argv;
    struct arena arena;
    arena_create(&arena);
    unsigned data[4] = {1u, 2u, 3u, 4u};
    for (size_t i = 0; i < 50000000lu; i++) {
        size_t const offset = arena_add(&arena, data, sizeof(data));
        data[0] = (uintptr_t)arena_get(&arena, offset);
    }
    arena_destroy(&arena);
}