typedef struct _HEAP_USERDATA_HEADER
{
    union
    {
        SINGLE_LIST_ENTRY SFreeListEntry;
        HEAP_SUBSEGMENT *SubSegment;
    };
    void *Reserved;
    unsigned __int64 SizeIndex;
    unsigned __int64 Signature;
} HEAP_USERDATA_HEADER, *PHEAP_USERDATA_HEADER;