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<a name="Marshalling-and-Unmarshalling-Operations"></a>
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<p>
Next: <a href="Incorrect-Remote-Dispatching.html#Incorrect-Remote-Dispatching" accesskey="n" rel="next">Incorrect Remote Dispatching</a>, Previous: <a href="Partition-Communication-Subsystem.html#Partition-Communication-Subsystem" accesskey="p" rel="prev">Partition Communication Subsystem</a>, Up: <a href="Partition-Communication-Subsystem.html#Partition-Communication-Subsystem" accesskey="u" rel="up">Partition Communication Subsystem</a> &nbsp; [<a href="About-This-Guide.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Marshalling-and-Unmarshalling-Operations-1"></a>
<h4 class="subsection">8.2.1 Marshalling and Unmarshalling Operations</h4>

<p>The Partition Communication Subsystem (PCS) is the runtime library for
distributed features. It marshals and unmarshals client and server requests
into a data stream suitable for network transmission.
</p>
<p>Parameter streams are normally read and written using four attributes:
</p>
<ul>
<li> Write: write an element into a stream, valid only for constrained
types

</li><li> Read: read a constrained element from a stream

</li><li> Output: same as Write, but write discriminants or array bounds as well
if needed

</li><li> Input: same as Read, but read discriminants or bounds from
  the stream (the Input attribute denotes a function)
</li></ul>

<p>An Ada compiler provides default &rsquo;Read and &rsquo;Write operations. But it is
up to the implementation of the PCS to provide default &rsquo;Read and &rsquo;Write
to ensure proper operation between heterogeneous architectures (see
<a href="Heterogeneous-System.html#Heterogeneous-System">Heterogeneous System</a>).
</p>
<p>The user can override these operations, except for predefined
types. Overriding with a custom version provides the user with a way
to debug its application (even outside of the Distributed Systems
Annex). On the other hand, remaining with the default implementation
allows the user to take advantage of optimized and portable representations
provided by the PCS.
</p>
<div class="smallexample">
<pre class="smallexample">
<b>with</b> Ada.Streams; <b>use</b> Ada.Streams;
<b>package</b> New_Integers <b>is</b>
   <b>pragma</b> Pure;

   <b>type</b> New_Integer <b>is</b> <b>new</b> Integer;

   <b>procedure</b> Read
     (S : <b>access</b> Root_Stream_Type'Class;
      V : <b>out</b> New_Integer);
   <b>procedure</b> Write
     (S : <b>access</b> Root_Stream_Type'Class;
      V : <b>in</b> New_Integer);

   <b>for</b> New_Integer'Read  <b>use</b> Read;
   <b>for</b> New_Integer'Write <b>use</b> Write;
<b>end</b> New_Integers;

</pre></div>
<div class="smallexample">
<pre class="smallexample">
<b>package</b> <b>body</b> New_Integers <b>is</b>
   <b>procedure</b> Read
     (S : <b>access</b> Root_Stream_Type'Class;
      V : <b>out</b> New_Integer)
   <b>is</b>
      B : String := String'Input (S);
   <b>begin</b>
      V := New_Integer'Value (B);
   <b>end</b> Read;

   <b>procedure</b> Write
     (S : <b>access</b> Root_Stream_Type'Class;
      V : <b>in</b> New_Integer)
   <b>is</b>
   <b>begin</b>
      String'Output (S, New_Integer'Image (V));
   <b>end</b> Write;
<b>end</b> New_Integers;

</pre></div>

<p>The language forces the user to provide Read and Write operations for
non-remote access types. Transmitting an access value by dumping its
content into a stream makes no sense when the value is going to be transmitted
to another partition (with a different memory space). To transmit non-remote
access types see <a href="Transmitting-Dynamic-Structure.html#Transmitting-Dynamic-Structure">Transmitting Dynamic Structure</a>.
</p>
<hr>
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Next: <a href="Incorrect-Remote-Dispatching.html#Incorrect-Remote-Dispatching" accesskey="n" rel="next">Incorrect Remote Dispatching</a>, Previous: <a href="Partition-Communication-Subsystem.html#Partition-Communication-Subsystem" accesskey="p" rel="prev">Partition Communication Subsystem</a>, Up: <a href="Partition-Communication-Subsystem.html#Partition-Communication-Subsystem" accesskey="u" rel="up">Partition Communication Subsystem</a> &nbsp; [<a href="About-This-Guide.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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