| \chapter{Optional Operating System Services} | 
 |  | 
 | The modules described in this chapter provide interfaces to operating | 
 | system features that are available on selected operating systems only. | 
 | The interfaces are generally modelled after the \UNIX{} or C | 
 | interfaces 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. | 
 |  | 
 | \item[Queue] | 
 | --- A stynchronized queue class. | 
 |  | 
 | \item[anydbm] | 
 | --- Generic interface to DBM-style database modules. | 
 |  | 
 | \item[whichdb] | 
 | --- Guess which DBM-style module created a given database. | 
 |  | 
 | \item[zlib] | 
 | \item[gzip] | 
 | --- Compression and decompression compatible with the | 
 | \code{gzip} program (zlib is the low-level interface, gzip the | 
 | high-level one). | 
 |  | 
 | \end{description} |