RtlAddAccessAllowedAceEx - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlAddAccessAllowedAceEx(
    _Inout_ PACL Acl,
    _In_ ULONG AceRevision,
    _In_ ULONG AceFlags,
    _In_ ACCESS_MASK AccessMask,
    _In_ PSID Sid
    );

#endif

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This function is documented in Windows Driver Kit.