RtlInitEmptyUnicodeString - NtDoc

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

FORCEINLINE
VOID
NTAPI
RtlInitEmptyUnicodeString(
    _Out_ PUNICODE_STRING DestinationString,
    _Writable_bytes_(MaximumLength) _When_(MaximumLength != 0, _Notnull_) PWCHAR Buffer,
    _In_ USHORT MaximumLength
    )
{
    memset(DestinationString, 0, sizeof(UNICODE_STRING));
    DestinationString->MaximumLength = MaximumLength;
    DestinationString->Buffer = Buffer;
}

#endif

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This function is documented in Windows Driver Kit.