RtlInitUTF8String - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H
#if (PHNT_VERSION >= PHNT_20H1)

NTSYSAPI
VOID
NTAPI
RtlInitUTF8String(
    _Out_ PUTF8_STRING DestinationString,
    _In_opt_z_ PCSZ SourceString
    );

#endif
#endif

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This function is documented in Windows Driver Kit.