RtlOemToUnicodeN - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlOemToUnicodeN(
    _Out_writes_bytes_to_(MaxBytesInUnicodeString, *BytesInUnicodeString) PWSTR UnicodeString,
    _In_ ULONG MaxBytesInUnicodeString,
    _Out_opt_ PULONG BytesInUnicodeString,
    _In_reads_bytes_(BytesInOemString) PCCH OemString,
    _In_ ULONG BytesInOemString
    );

#endif

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This function is documented in Windows Driver Kit.