RTL_SEGMENT_HEAP_VA_CALLBACKS - NtDoc

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

typedef struct _RTL_SEGMENT_HEAP_VA_CALLBACKS
{
    HANDLE CallbackContext;
    PALLOCATE_VIRTUAL_MEMORY_EX_CALLBACK AllocateVirtualMemory;
    PFREE_VIRTUAL_MEMORY_EX_CALLBACK FreeVirtualMemory;
    PQUERY_VIRTUAL_MEMORY_CALLBACK QueryVirtualMemory;
} RTL_SEGMENT_HEAP_VA_CALLBACKS, *PRTL_SEGMENT_HEAP_VA_CALLBACKS;

#endif

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This structure is documented in Windows Driver Kit.