RtlAreBitsSet - NtDoc

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

_Check_return_
NTSYSAPI
BOOLEAN
NTAPI
RtlAreBitsSet(
    _In_ PRTL_BITMAP BitMapHeader,
    _In_ ULONG StartingIndex,
    _In_ ULONG Length
    );

#endif

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This function is documented in Windows Driver Kit.