In some cases it is desirable to maintain a stack reserve region, which is a minimum-sized region that is immediately above a thread's guard region. A reserve region may be desirable to ensure that exceptions or asynchronous signals have stack space to execute on a thread's stack or to ensure that the exception dispatcher and any exception handler that it might call have stack space to execute after an invalid attempt to extend the stack has been detected.
This standard does not require a reserve region.