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<H1><A NAME=SECTION03260000000000000000>5.6 Asynchronous Communication</A></H1>
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<A NAME=7346>&#160;</A>
Recall that the need for asynchronous communication can arise when the
<A NAME=7347>&#160;</A>
tasks involved in a computation must access elements of a shared data
structure in an unstructured manner (Section <A HREF="node17.html#seccommas" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node17.html#seccommas">2.3.4</A>).  This
<A NAME=7349>&#160;</A>
requirement can be satisfied in CC++
  in three different ways:
<OL><LI>
The shared data structure can be encapsulated in a set of specialized
<em> data tasks
 </em> to which read and write requests are directed by
using channel operations.
<P>
<LI>
The shared data structure can be distributed among the computation
tasks.  As discussed in Section <A HREF="node17.html#seccommas" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node17.html#seccommas">2.3.4</A>, each computation
task must then <em> poll
 </em> periodically for pending requests.  For
example, we can extend the <tt> Queue</tt> class of
Program <A HREF="node56.html#progccch0" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node56.html#progccch0">5.6</A> to support a poll operation.  The <tt>
IntQData</tt> structure is augmented with a full/empty field, which is
initialized to empty when a list element is created and set to full
when a data value is placed in the list element.  The poll function
simply checks the status of this field in the list element at the head
of the queue.
<P>
<LI>
A third implementation approach exploits CC++
 's RPC mechanism more
directly.  The shared data structure is distributed among the
computation tasks.  However, rather than sending a message on a
channel, a task accesses data in another processor object by making an
RPC to an appropriate member function.  (The <tt> get_x</tt> function of
Program <A HREF="node52.html#progccex2" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node52.html#progccex2">5.1</A> is a simple example of the sort of RPC we
might write.)
</OL>
The third approach is explored in a case study in
Section <A HREF="node63.html#secccfock" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node63.html#secccfock">5.12</A>.
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<I>&#169 Copyright 1995 by <A href="msgs0.htm#6" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/tppmsgs/msgs0.htm#6">Ian Foster</a></I>
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