RtlGetGroupSecurityDescriptor - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlGetGroupSecurityDescriptor(
    _In_ PSECURITY_DESCRIPTOR SecurityDescriptor,
    _Outptr_result_maybenull_ PSID *Group,
    _Out_ PBOOLEAN GroupDefaulted
    );

#endif

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This function is documented in Windows Driver Kit.