RtlDowncaseUnicodeString - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlDowncaseUnicodeString(
    _Inout_ PUNICODE_STRING DestinationString,
    _In_ PUNICODE_STRING SourceString,
    _In_ BOOLEAN AllocateDestinationString
    );

#endif

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This function is documented in Windows Driver Kit.