#ifndef _NTRTL_H

typedef _Function_class_(ALLOCATE_VIRTUAL_MEMORY_EX_CALLBACK)
NTSTATUS NTAPI ALLOCATE_VIRTUAL_MEMORY_EX_CALLBACK(
    _Inout_ HANDLE CallbackContext,
    _In_ HANDLE ProcessHandle,
    _Inout_ _At_ (*BaseAddress, _Readable_bytes_ (*RegionSize) _Writable_bytes_ (*RegionSize) _Post_readable_byte_size_ (*RegionSize)) PVOID* BaseAddress,
    _Inout_ PSIZE_T RegionSize,
    _In_ ULONG AllocationType,
    _In_ ULONG PageProtection,
    _Inout_updates_opt_(ExtendedParameterCount) PMEM_EXTENDED_PARAMETER ExtendedParameters,
    _In_ ULONG ExtendedParameterCount
    );

#endif
View code on GitHub

// ntifs.h

ALLOCATE_VIRTUAL_MEMORY_EX_CALLBACK AllocateVirtualMemoryExCallback;

NTSTATUS AllocateVirtualMemoryExCallback(
  HANDLE CallbackContext,
  HANDLE ProcessHandle,
  PVOID *BaseAddress,
  PSIZE_T RegionSize,
  ULONG AllocationType,
  ULONG PageProtection,
  PMEM_EXTENDED_PARAMETER ExtendedParameters,
  ULONG ExtendedParameterCount
)
{...}
View the official Windows Driver Kit DDI reference