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<H1><A NAME=SECTION032110000000000000000>5.11 Case Study: Channel Library</A></H1>
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<A NAME=secccchan>&#160;</A>
<P>
In the first of two more substantial examples, we present an
implementation of the channel library used in
<A NAME=7689>&#160;</A>
Program <A HREF="node53.html#progccbuild" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node53.html#progccbuild">5.3</A>.  This case study illustrates how
<A NAME=7691>&#160;</A>
CC++
  can be used to develop libraries implementing particular programming
paradigms: in this case, channel communication.
<P>
<P><A NAME=8556>&#160;</A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img864.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img864.gif">
<BR><STRONG>Figure 5.8:</STRONG> <em> Data structures used in the CC++
  channel library.
In addition to a queue, a channel comprises an <tt> OutPort</tt> object
and an <tt> InPort</tt> object used to encapsulate pointers to the queue.
Processes apply send and receive operations to the outport and inport,
respectively.</em><A NAME=figccchan>&#160;</A><BR>
<P>
<P>
The channel library provides functions to create a channel, to extract
pointers to inport and outport structures, and to send and receive
messages.  A channel is constructed from three basic data structures:
the message queue itself and outport and inport objects used to
contain pointers to the queue (Figure <A HREF="node62.html#figccchan" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node62.html#figccchan">5.8</A>).  The outport
is created in the same processor object as the sender, while the
inport and message queue are created in the same processor object as
the receiver.
<P>
The implementation, Program <A HREF="node62.html#progccq" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node62.html#progccq">5.15</A>, defines the processor
object <tt> ChannelUser</tt> and the classes <tt> Channel</tt>, <tt> InPort</tt>,
and <tt> OutPort</tt>.  The processor object provides functions <tt>
create_inport</tt> and <tt> create_outport</tt> that create the inport and
outport associated with a channel; any program wanting to use the
channel library must include this processor object as a base class.
Recall that this was done when defining the processor object <tt>
Construction</tt> in Program <A HREF="node53.html#progccbuild" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node53.html#progccbuild">5.3</A>.
<P>
The <tt> Channel</tt> class provides three public member functions: the
constructor <tt> Channel</tt>, which creates a new channel linking two
specified processor objects; <tt> get_out_port</tt>, which returns a
pointer to the channel's outport; and <tt> get_in_port</tt>, which
returns a pointer to the channel's inport.
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<P><A NAME=progccq>&#160;</A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img865.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img865.gif"><P>
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The <tt> InPort</tt> class is derived from 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>.  It adds to the functions already defined
in that class a new function <tt> receive</tt>, which simply dequeues a
message from the queue.  Finally, the <tt> OutPort</tt> class encapsulates
a global pointer and provides a <tt> send</tt> function that invokes an
<tt> enqueue</tt> operation on the message queue referenced by this
pointer.
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