RtlSIZETAdd - NtDoc

#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
//
// SIZE_T addition
//
#ifdef _WIN64

#define RtlSIZETAdd      RtlULongLongAdd

#endif
#endif
#endif
View code on GitHub

#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
//
// SIZE_T addition
//
#ifdef _WIN64
// ...
#else

_Must_inspect_result_
__inline
NTSTATUS
RtlSIZETAdd(
    _In_ SIZE_T Augend,
    _In_ SIZE_T Addend,
    _Out_ _Deref_out_range_(==, Augend + Addend) SIZE_T* pResult)
{
    NTSTATUS status;

    if ((Augend + Addend) >= Augend)
    {
        *pResult = (Augend + Addend);
        status = STATUS_SUCCESS;
    }
    else
    {
        *pResult = _SIZE_T_ERROR;
        status = STATUS_INTEGER_OVERFLOW;
    }
    
    return status;
}

#endif
#endif
#endif
View code on GitHub