RtlFindUnicodePrefix - NtDoc

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

NTSYSAPI
PUNICODE_PREFIX_TABLE_ENTRY
NTAPI
RtlFindUnicodePrefix(
    _In_ PUNICODE_PREFIX_TABLE PrefixTable,
    _In_ PUNICODE_STRING FullName,
    _In_ ULONG CaseInsensitiveIndex
    );

#endif

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This function is documented in Windows Driver Kit.