RtlGetVersion - NtDoc

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

// Version

NTSYSAPI
NTSTATUS
NTAPI
RtlGetVersion(
    _Out_ PRTL_OSVERSIONINFOEXW VersionInformation // PRTL_OSVERSIONINFOW
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit.