RtlInitUnicodeStringEx - NtDoc

#ifndef _NTRTL_H
#ifndef PHNT_NO_INLINE_INIT_STRING

FORCEINLINE NTSTATUS RtlInitUnicodeStringEx(
    _Out_ PUNICODE_STRING DestinationString,
    _In_opt_z_ PCWSTR SourceString
    )
{
    size_t stringLength;

    DestinationString->Length = 0;
    DestinationString->Buffer = (PWCH)SourceString;

    if (!SourceString)
        return STATUS_SUCCESS;

    stringLength = wcslen(SourceString);

    if (stringLength <= UNICODE_STRING_MAX_CHARS - 1)
    {
        DestinationString->Length = (USHORT)stringLength * sizeof(WCHAR);
        DestinationString->MaximumLength = DestinationString->Length + sizeof(UNICODE_NULL);
        return STATUS_SUCCESS;
    }

    return STATUS_NAME_TOO_LONG;
}

#endif
#endif
View code on GitHub

#ifndef _NTRTL_H
#ifndef PHNT_NO_INLINE_INIT_STRING
// ...
#else

NTSYSAPI
NTSTATUS
NTAPI
RtlInitUnicodeStringEx(
    _Out_ PUNICODE_STRING DestinationString,
    _In_opt_z_ PCWSTR SourceString
    );

#endif
#endif
View code on GitHub