RtlWriteRegistryValue - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlWriteRegistryValue(
    _In_ ULONG RelativeTo,
    _In_ PCWSTR Path,
    _In_ PCWSTR ValueName,
    _In_ ULONG ValueType,
    _In_ PVOID ValueData,
    _In_ ULONG ValueLength
    );

#endif

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This function is documented in Windows Driver Kit.