#ifndef _NTRTL_H

typedef _Function_class_(FREE_VIRTUAL_MEMORY_EX_CALLBACK)
NTSTATUS NTAPI FREE_VIRTUAL_MEMORY_EX_CALLBACK(
    _Inout_ HANDLE CallbackContext,
    _In_ HANDLE ProcessHandle,
    _Inout_ __drv_freesMem(Mem) PVOID *BaseAddress,
    _Inout_ PSIZE_T RegionSize,
    _In_ ULONG FreeType
    );

#endif
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// ntifs.h

FREE_VIRTUAL_MEMORY_EX_CALLBACK FreeVirtualMemoryExCallback;

NTSTATUS FreeVirtualMemoryExCallback(
  HANDLE CallbackContext,
  HANDLE ProcessHandle,
  PVOID *BaseAddress,
  PSIZE_T RegionSize,
  ULONG FreeType
)
{...}
View the official Windows Driver Kit DDI reference