RtlFindSetBits - NtDoc

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

_Success_(return != -1)
_Check_return_
NTSYSAPI
ULONG
NTAPI
RtlFindSetBits(
    _In_ PRTL_BITMAP BitMapHeader,
    _In_ ULONG NumberToFind,
    _In_ ULONG HintIndex
    );

#endif

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This function is documented in Windows Driver Kit.