RtlCreateUnicodeString - NtDoc

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

_Success_(return != 0)
_Must_inspect_result_
NTSYSAPI
BOOLEAN
NTAPI
RtlCreateUnicodeString(
    _Out_ PUNICODE_STRING DestinationString,
    _In_z_ PCWSTR SourceString
    );

#endif

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This function is documented in Windows Driver Kit.