\chapter{Optional Operating System Services}The modules described in this chapter provide interfaces to operatingsystem features that are available on selected operating systems only.The interfaces are generally modelled after the \UNIX{} or Cinterfaces but they are available on some other systems as well(e.g. Windows or NT). Here's an overview:\begin{description}\item[signal]--- Set handlers for asynchronous events.\item[socket]--- Low-level networking interface.\item[select]--- Wait for I/O completion on multiple streams.\item[thread]--- Create multiple threads of control within one namespace.\end{description}