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

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

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

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

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

reasons:</p>

<ul>

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

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

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

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

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

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

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

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

</ul>

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

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

<ol>

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

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

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

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

    {

    return &globalVal

    }

</codeblock></li>

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

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

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

extern "C" 

#endif

IMPORT_C int* GlbData ();

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

int main()

    {

    int i = 0;

    globalVal = 10;

    globalVal++;

    i = globalVal;

    return 0;

    }

</codeblock></li>

</ol>