RtlLengthRequiredSid - NtDoc

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

NTSYSAPI
ULONG
NTAPI
RtlLengthRequiredSid(
    _In_ ULONG SubAuthorityCount
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit.