#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
    );
View code on GitHub// ntifs.h
NTSYSAPI NTSTATUS RtlUnicodeToOemN(
  [out]           PCHAR  OemString,
  [in]            ULONG  MaxBytesInOemString,
  [out, optional] PULONG BytesInOemString,
  [in]            PCWCH  UnicodeString,
  [in]            ULONG  BytesInUnicodeString
);
View the official Windows Driver Kit DDI referenceThis function is documented in Windows Driver Kit.
The RtlUnicodeToOemN routine translates a given Unicode string to an OEM string, using the current system OEM code page.
OemString [out]Pointer to a caller-allocated buffer to receive the translated string.
MaxBytesInOemString [in]Maximum number of bytes to be written to OemString.
BytesInOemString [out, optional]Pointer to a caller-allocated variable that receives the number of bytes in the translated string. This parameter can be NULL.
UnicodeString [in]Pointer to the Unicode source string to be translated.
BytesInUnicodeString [in]Size, in bytes, of the string at UnicodeString.
RtlUnicodeToOemN returns STATUS_SUCCESS if the full string at UnicodeString was translated and returned at OemString.
For the return value STATUS_BUFFER_OVERFLOW, the truncated string at OemString was translated without error.
For the return value STATUS_SUCCESS, the value at BytesInOemString, if any, indicates the length of the returned string, rather than the given MaxBytesInOemString.
RtlUnicodeToOemN does not modify the source string. It returns a null-terminated OEM string if the given BytesInUnicodeString included a NULL terminator and if the given MaxBytesInOemString did not cause truncation.
For information about other string-handling routines, see Run-Time Library (RTL) Routines.
RtlUnicodeStringToCountedOemString