ALLOCATE_VIRTUAL_MEMORY_EX_CALLBACK - NtDoc

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

typedef NTSTATUS 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

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This callback function is documented in Windows Driver Kit.