RtlUnicodeToCustomCPN - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlUnicodeToCustomCPN(
    _In_ PCPTABLEINFO CustomCP,
    _Out_writes_bytes_to_(MaxBytesInCustomCPString, *BytesInCustomCPString) PCH CustomCPString,
    _In_ ULONG MaxBytesInCustomCPString,
    _Out_opt_ PULONG BytesInCustomCPString,
    _In_reads_bytes_(BytesInUnicodeString) PWCH UnicodeString,
    _In_ ULONG BytesInUnicodeString
    );

#endif

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This function is documented in Windows Driver Kit.