RtlUpperString - NtDoc

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

NTSYSAPI
VOID
NTAPI
RtlUpperString(
    _Inout_ PSTRING DestinationString,
    _In_ const STRING* SourceString
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit.