RTL_SEGMENT_HEAP_MEMORY_SOURCE - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H

typedef struct _RTL_SEGMENT_HEAP_MEMORY_SOURCE
{
    ULONG Flags;
    ULONG MemoryTypeMask; // Mask of RTL_MEMORY_TYPE members.
    ULONG NumaNode;
    union
    {
        HANDLE PartitionHandle;
        RTL_SEGMENT_HEAP_VA_CALLBACKS *Callbacks;
    };
    SIZE_T Reserved[2];
} RTL_SEGMENT_HEAP_MEMORY_SOURCE, *PRTL_SEGMENT_HEAP_MEMORY_SOURCE;

#endif

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This structure is documented in Windows Driver Kit.