RtlUnicodeToOemN - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlUnicodeToOemN(
    _Out_writes_bytes_to_(MaxBytesInOemString, *BytesInOemString) PCHAR OemString,
    _In_ ULONG MaxBytesInOemString,
    _Out_opt_ PULONG BytesInOemString,
    _In_reads_bytes_(BytesInUnicodeString) PCWCH UnicodeString,
    _In_ ULONG BytesInUnicodeString
    );

#endif

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This function is documented in Windows Driver Kit.