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<a name="Lists-of-Pairs-of-Ports"></a>
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<hr size="1">
<a name="Lists-of-Pairs-of-Ports-1"></a>
<h3 class="subsection">3.8.1 Lists of Pairs of Ports</h3>

<a name="index-port"></a>
<p>A <code>port</code> is the name given to a type of pointer used in the library
as the address of a place where a computational result yet to be
evaluated will be sent. Ports are discussed further in <a href="Ports-and-Packets.html#Ports-and-Packets">Ports and Packets</a>,
but are mentioned here because it is sometimes necessary to employ a
list of pairs of them. A pointer to such a list is declared as a
<code>portal</code> type. It refers to a structure of the form
<a name="index-portal"></a>
<a name="index-port_005fpair"></a>
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">struct port_pair
{
  port left;
  port right;
  portal alters;
}
</pre></td></tr></table>

<p>A small selection of functions for <code>portal</code> memory management is
declared as follows in the header file &lsquo;<tt>portals.h</tt>&rsquo;. For reasons of
C-ness, the type declarations themselves are forced to be in
&lsquo;<tt>lists.h</tt>&rsquo;.
</p>
<dl>
<dt><a name="index-avm_005fnew_005fportal"></a><u>Function:</u> portal <b>avm_new_portal</b><i> (portal <var>alters</var>)</i></dt>
<dd><p>This function is used to create storage for a new <code>port_pair</code>
structure, and returns a <code>portal</code> pointer to it if successful. If
the storage can&rsquo;t be allocated, a <code>NULL</code> pointer is returned.
The <code>alters</code> field of the result is initialized as the given
parameter supplied by the caller. All other fields are filled with zeros.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fseal"></a><u>Function:</u> void <b>avm_seal</b><i> (portal <var>fate</var>)</i></dt>
<dd><p>This function performs the reclamation of storage associated with
<code>portal</code> pointers, either by freeing them or by consigning them
temporarily to a local cache for performance reasons. Client programs
should use only this function for disposing of <code>portal</code> storage
rather than using <code>free</code> directly, so as to allow accurate record
keeping.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005finitialize_005fportals"></a><u>Function:</u> void <b>avm_initialize_portals</b><i> ()</i></dt>
<dd><p>This function should be called by a client program prior to calling
either of the above memory management functions in order to initialize
some local variables. Anomalous results are possible otherwise.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fcount_005fportals"></a><u>Function:</u> void <b>avm_count_portals</b><i> ()</i></dt>
<dd><p>This function should be called at the end of a run or after the last
call to any of the other functions in this section as a way of detecting
memory leaks associated with <code>portal</code> pointers. A warning message
will be written to standard error if any remains unreclaimed.
</p></dd></dl>

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