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//0x20 bytes (sizeof)
struct _HEAP_USERDATA_HEADER
{
    union
    {
        struct _SINGLE_LIST_ENTRY SFreeListEntry;                           //0x0
        struct _HEAP_SUBSEGMENT* SubSegment;                                //0x0
    };
    VOID* Reserved;                                                         //0x8
    ULONGLONG SizeIndex;                                                    //0x10
    ULONGLONG Signature;                                                    //0x18
};