Implement aligned malloc

aligned means: returned memory address starts with an address that are divisible by alignment

eg. alignment = 16, we want the allocated memory address starts at 0x10, 0x20, 0x1010 etc, instead of 0x1A, 0x100B etc.

void* aligned_malloc(size_t size, size_t alignment){
    if (size == 0 || alignment == 0){
        return NULL;
    }

    // if alignment is not power of 2
    if ((alignment & (alignment - 1)) != 0){
        return NULL;
    }

    size_t offset = sizeof(void*) + alignment - 1;
    void* raw = malloc(size + offset);
    if (!raw){
        return NULL;
    }

    // round up to next aligned address
    uintptr_t aligned_addr = ((uintptr_t)raw + offset) & ~ (alignment - 1)
    // (unitptr_t)raw + offset has round up that definitely includes the next algnment
    // we just need to clear the lower bits by & ~ (alignment - 1)

    void aligned = (void*)aligned_addr;
    // cast aligned to an array of pointers, and we stored pointer raw right before it
    ((void**)aligned)[-1] = raw;
    return aligned;
}


void aligned_free(void* aligned){
    if (!aligned){
        return;
    }
    // free the raw pointer returned by malloc, just one pointer before aligned, where we stored.
   free(((void**)aligned)[-1]);
}