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Subject cvs commit: ws-axis/c/docs/arch mem-management.html
Date Wed, 08 Sep 2004 10:28:42 GMT
samisa      2004/09/08 03:28:42

  Added:       c/docs/arch mem-management.html
  Documentation on memory management semantics
  Revision  Changes    Path
  1.1                  ws-axis/c/docs/arch/mem-management.html
  Index: mem-management.html
  <!doctype html public "-//w3c//dtd html 4.0 transitional//en">
     <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
     <title>Axis C++ Memory Management Guide</title>
  <link href="axis.css" rel=stylesheet type=text/css>
  <img SRC="../images/axis.jpg" height=96 width=176></h1></center>
  Axis C++ Memory Management Guide</h1>
  <br><i>1.0 Version</i>
  <br><i>Feedback: <a href=""></a></i>
  <a href="#Introduction">Introduction</a>
  <br><a href="#Semantics">Allocation/De-allocation Semantics</a>
  <br><a href="#IHeaderBlock">Dealing with SOAP Headers</a>
  <br><a href="#Open Issues">Open Issues</a>
  <h2><a NAME="Introduction"></a>Introduction</h2>
  <p> This guide records the memory management semantics and some of the rationale of
the desing decisions on which memory management semantics are based on. Being a C/C++ application,
it is a must that all users as well as developers have a clear understanding on how to deal
with memory allocation and deallocation. 
  <p> The allocation and deallocation mechanisms are centred around the serializer and
deserializer operations in Axis C++. At the moment, because Axis C++ support both C and C++
clients and services, the deserializer uses <i>malloc()</i> for memory allocation.
Hence C++ users may have to live with <i>free()</i> (instead of <i>delete</i>)
for deallocation. Of course one could still use delte from C++ programs, however this may
not gaurentee the cleanest memory deallocation.
  <h2><a NAME="Semantics"></a>Allocation/De-allocation Semantics</h2>
  On the server side all the parameters added to the serializer are de-allocated by the Axis
Engine after serializing. Hence the user would not have to destruct the parameters to a function
call after calling the function.
  On the client side, the user is responsibe for deallocating memory after a function call.
  Please have a look at the base sample ($AXIS_HOME/samples/client/interoptests/base/) for
a client side sample.
  Axis C++ does not provide a special function for dynamic allocation of objects.
  One can select his/her own allocation mechanism. (<i>malloc()</i> in case of
C programs and/or <i>new</i> in case of C++ programs)
  <h3>Other Objects</h3>
  For XML elements and attribute objects that are created by IHandlerSoapSerializer, HeaderBlock
and Attribute the rules for deleting the objects when the request or response is processed
are as follows.
  <li> Any outbound objects created, either by a client application or by a handler,
must be managed by the creator him(her)self. The Axis C++ engine does not delete any objects
(HeaderBlocks, Attributes, BasicNodes etc.).</li> 
  <li> Any inbound objects that are created by the Axis C++ Engine as a result of deserialization
should be deallocated by a target handler or the client application. Axis C++ Engine deallocates
only the headerblocks that will remain in the deserializer (headerblocks with no target handler).</li>
  <h3>Return Values</h3>
  <p> The values returned by the Axis C++ engine must be memory cleaned by the user
written code.
  The C++ code generated by the WSDL2Ws tool does contain destructors. However, at the moment,
it is not gaurenteed that the destructor of a generated class would clean all of the pointer
members (Some memmbers are cleaned while others are not). Hence the users must have a look
at the generated code to understand the semantics of memory cleaning. In case of C code, of
cource the user must take care of memory cleaning.
  <p> Please note that in case of arrays, both for C and C++, the Axis C++ engine returns
a struct. Hence it is a must that the memory is cleaned properly. 
  <br> C++ Example:<br>
          // testing echoIntegerArray
          xsd__int_Array arrint; // Parameter for method call
          arrint.m_Array = new int[ARRAYSIZE];
          arrint.m_Size = ARRAYSIZE;
  	for (x = 0; x < ARRAYSIZE; x++)
              arrint.m_Array[x] = x;
  	printf ("invoking echoIntegerArray...\n");
  	xsd__int_Array arrintResult = ws.echoIntegerArray (arrint);
  	// Deal with the return value
  	if (arrintResult.m_Array != NULL)
              printf ("successful\n");
              // Clean memory of the returned array
              printf ("failed\n");
          // Clean memory allocated for parameter
          delete [] arrint.m_Array;
  <br> Please have a look at the base sample ($AXIS_HOME/samples/client/interoptests/base/)
for more information.
  <h2><a NAME="IHeaderBlock"></a>Dealing with SOAP Headers</h2>
  <h3>From Stubs</h3>
  <p> IHeaderBlock is a virtual class that defines the interface to deal with SOAP headers.
You can create an IHeaderBlock at the client side using the API provided with Stub classs.
  IHeaderBlock* Stub::createSOAPHeaderBlock(AxisChar * pachLocalName, AxisChar * pachUri);
  Here the Stub class will maintain a list of all created Header Blocks, and in the destructor
of the Stub class, it will clean up memory by deleting all those Header Blocks that were created
by the user. 
  <p> <b>Note 1</b>: It is advisable that if a user wants to delete a Hheader
Block, (s)he uses the API provided by the Stub class to do so.<br>
  void deleteCurrentSOAPHeaderBlock();
  Stub class is also equipped with iterator methods to traverse the current list of Header
Blocks created.
  <p> <b>Note 2</b>: IHeaderBlock destructor will take care of the Header
Block member variables and clean them; BasicNodes (i.e its children) and the Attributes. <br>
  <h3>From Handlers</h3>
  <p> If the Header Blocks are created from within a Handler then it is the responsiblity
of the Handler writer to clean those. For that the user can write the cleanup code either
in the fini() method or in the destructor of the Handler, depending on the following situations
  <li> If it is a sessoin handler which needs to maintain its state, then the cleanup
has to be done in the destructor.</li>
  <li> If it is a request type handler the clean up can be done in the fini() mehtod
of the Handler. Here the writer has to explicitly write the clean up code.</li>
  <p> If a target handler access a Header Block created by the deserializer then it
is the responsibility of the Handler to delete it.
  <h2><a NAME="Open Issues"></a>Open Issues</h2>
  <p> As C++ is and object oriented language, one would ideally like to leverage constructors
and destructors for memory management. However, the Axis C++ engine uses structs in some cases
(e.g. Arrays) and use <i>malloc()</i> to allocate memory. Hence the C++ programmer
woul be forced to use <i>free()</i> at times. When using <i>malloc()</i>
and <i>free()</i> constructors and destructors are not called. However, as the
Axis C++ engine currently supports both C and C++, it is not simple to replace all <i>malloc()</i>
with <i>new</i> or <i>free()</i> with <i>delete</i>. At
the same time, there are some places where <i>new</i> and <i>delete</i>
are being used. They too cannot be replaced with <i>malloc()</i> and <i>free()</i>
overnight. This memory management complexity is the price paid for dual support of C and C++.
Efforts are under way to clean up the memroy management mix-ups and still support both C and
C++. Currently the proposed solution is to make the Axis C++ engine pure C++ and use a wrapper
mechanism to support C. 
  <p> When an array is de-serialized it uses C style memory re-allocation mechanism
in the present code. C++ does not support <i>realloc()</i> and if we use <i>new</i>
instead we have to allocate fresh memory blocks each time we need to increase the array size.
This can be more expensive than using <i>realloc()</i>. Again the price paid for
efficiency is that one has to use <i>free()</i> and not <i>delete []</i>
 from C++ code. 

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