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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-539C4AA4-FF3F-4D6B-90A5-677092DBE54E" xml:lang="en"><title>Exporting

    13 Global Data from a DLL</title><shortdesc>Exporting global data from a DLL to be accessed by either P.I.P.S.

    14 or Symbian C++ applications is one of the typical problems that developers

    15 encounter.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    16 <p>It is strongly recommended to avoid having global data in DLLs due to following

    17 reasons:</p>

    18 <ul>

    19 <li><p>EKA2 emulator allows only a DLL with WSD to load into a single process.</p></li>

    20 <li><p>RAM usage for WSD data chunk is atleast one 4K RAM page (the smallest

    21 possible RAM allocation), irrespective of how much static data is required.</p></li>

    22 <li><p>Chunks are a finite resource on ARMv5. Every process loading WSD enabled

    23 DLLs uses a chunk to hold the data.</p></li>

    24 <li><p>There are ARM architecture 4 and 5 specific costs and limitations that

    25 apply only to DLLs that link against “fixed processes”.</p></li>

    26 <li><p> There is a limit on the number of DLLs in a process with WSD. </p></li>

    27 </ul>

    28 <p>On having understood the above limitations, the following pattern can be

    29 used for exporting global data from a DLL:</p>

    30 <ol>

    31 <li id="GUID-CB5A6042-22D6-4A55-AB95-238D36B29E4D"><p>Do not export global

    32 variables. Within DLL, say there is one global variable, for example:</p><codeblock xml:space="preserve">int globalVal; </codeblock></li>

    33 <li id="GUID-441655B4-C2E0-4923-AA61-9AD71B9E8E43"><p>Export one method that

    34 returns a pointer to that variable.</p><codeblock xml:space="preserve">extern "C" EXPORT_C int* GlbData ()

    35     {

    36     return &globalVal

    37     }

    38 </codeblock></li>

    39 <li id="GUID-8C06D549-3F94-455B-981D-D0D19B004BE1"><p>Define a macro for the

    40 user of the DLL. Within the DLL header (for example, <filepath>xxx.h</filepath>),

    41 define the following:</p><codeblock xml:space="preserve">#ifdef __cplusplus

    42 extern "C" 

    43 #endif

    44 IMPORT_C int* GlbData ();

    45 #define globalVal (*GlbData())</codeblock><p>And the usage is like: </p><codeblock xml:space="preserve">#include &lt;xxx.h&gt;  // DLL header

    46 int main()

    47     {

    48     int i = 0;

    49     globalVal = 10;

    50     globalVal++;

    51     i = globalVal;

    52     return 0;

    53     }

    54 </codeblock></li>

    55 </ol>

    56 </conbody></concept>