<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>Using Poly/ML - Chapter 5: Processes</title> </head> <body> <h1><font SIZE="6"><a name="Processes5">Chapter 5: Processes</a></font></h1> <p>As from Poly/ML version 5.1 parallelism is provided through the <a href="Threads.html">Thread</a> structure. The Process structure is provided for backwards compatibility only.</p> <p><font SIZE="3">This chapter describes Poly/ML's non-standard extensions for handling lightweight processes. The process model used is based on Milner's CCS. The structure </font><b><font FACE="Courier" SIZE="3">Process</font></b><font SIZE="3"> contains the following:</font></p> <p><b><font FACE="Courier" SIZE="3">type 'a channel<br> val channel : unit -> 'a channel<br> val send : 'a channel * 'a -> unit<br> val receive : 'a channel -> 'a</font></b></p> <p><b><font FACE="Courier" SIZE="3">val fork : (unit -> unit) -> unit<br> val console : (unit -> unit) -> unit -> unit<br> val choice : (unit -> unit) * (unit -> unit) -> unit</font></b></p> <p><strong><font face="Courier">val interruptConsoleProcesses : unit -> unit</font></strong></p> <p><b><font FACE="Courier" SIZE="3">channel ()</font></b><font SIZE="3"> creates a new, unbuffered, channel.</font></p> <p><b><font FACE="Courier" SIZE="3">send (c,x)</font></b><font SIZE="3"> writes the value </font><b><font FACE="Courier" SIZE="3">x</font></b><font SIZE="3"> to the channel </font><b><font FACE="Courier" SIZE="3">c</font></b><font SIZE="3">. Since channels are unbuffered, the writing process is suspended until another process attempts to read from the channel </font><b><font FACE="Courier" SIZE="3">c</font></b><font SIZE="3">.</font></p> <p><font FACE="Courier" SIZE="3"><b>read c</b></font><font SIZE="3"> reads a value from the channel </font><b><font FACE="Courier" SIZE="3">c</font></b><font SIZE="3"> and returns it. If no value is currently available on </font><font FACE="Courier" SIZE="3"><b>c</b></font><font SIZE="3">,<b> </b>the reading process is suspended until another process attempts to write to that channel.</font></p> <p><font FACE="Courier" SIZE="3"><b>fork f </b></font><font SIZE="3">creates a new process and executes the function call </font><b><font FACE="Courier" SIZE="3">f ()</font></b><font SIZE="3"> in it. The new process will not receive console interrupts i.e. typing </font><b><font FACE="Courier" SIZE="3"><control-C> f</font></b><font SIZE="3"> will not raise the exception </font><b><font FACE="Courier" SIZE="3">Interrupt</font></b><font SIZE="3"> in that process.</font></p> <p><b><font FACE="Courier" SIZE="3">console f</font></b><font SIZE="3"> is like </font><font FACE="Courier" SIZE="3"><b>fork f </b></font><font SIZE="3">except that the new process will be a console process. Typing </font><b><font FACE="Courier" SIZE="3"><control-C> f </font></b><font SIZE="3">will raise the exception </font><b><font FACE="Courier" SIZE="3">Interrupt</font></b><font SIZE="3"> in all console processes but doesn't raise this exception in non-console processes. In addition, calling </font><b><font FACE="Courier" SIZE="3">console f</font></b><font SIZE="3"> returns a function of type </font><font FACE="Courier" SIZE="3"><b>unit -> unit</b></font><font SIZE="3">. Calling this function will raise the </font><b><font FACE="Courier" SIZE="3">Interrupt</font></b><font SIZE="3"> exception in the new process only. This provides a more selective way to kill aberrant processes.</font></p> <p><font FACE="Courier" SIZE="3"><b>choice (f g) </b></font><font SIZE="3">is a way to implement the choice primitive of CCS. It creates two processes which execute </font><b><font FACE="Courier" SIZE="3">f()</font></b><font SIZE="3"> and </font><b><font FACE="Courier" SIZE="3">g()</font></b><font SIZE="3"> respectively. However, these two processes are not independent: only one of them will be allowed to perform channels actions (sends and receives). As soon as one process successfully performs a channel action, the other process will block forever as soon as it attempts a send or receive. This relationship is also inherited by child processes; if <i>any</i> child process of </font><font FACE="Courier" SIZE="3"><b>f </b></font><font SIZE="3">performs a channel action then all child processes of </font><b><font FACE="Courier" SIZE="3">g</font></b><font SIZE="3"> will block. This behaviour allows calls of the function </font><b><font FACE="Courier" SIZE="3">choice</font></b><font SIZE="3"> to be nested in order to create multiple-choice decision points.</font></p> <p><strong><font face="Courier">interruptConsoleProcesses</font></strong> raises <b><font FACE="Courier" SIZE="3">Interrupt</font></b> in all console processes. It is equivalent to <font SIZE="3">typing </font><b><font face="Courier" size="3"><control-C> f</font></b><font size="3">.</font></p> <p><font size="2"><b>Copyright (c) 2000 CUTS and contributers. </b></font></p> </body> </html>
