Sets a file-reading boundary at n bytes past the current stream location. If there is an attempt to read past this boundary, an error will be flagged. The boundary is removed with a call to RemoveBoundary. Every call to SetBoundary must be balanced by a call to RemoveBoundary.
SetBoundary helps keep a subroutine from reading too much data. However, a malicious subroutine could call RemoveBoundary on its own.