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     7     Nokia Corporation - initial contribution.

     8 Contributors: 

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    10 <!DOCTYPE concept

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

    12 <concept id="GUID-E7C41361-C0B9-5341-A864-B59770FB7C9B" xml:lang="en"><title>Supporting

    13 Writable Static Data in DLLs </title><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <section><title>Introduction</title> <p>Many UNIX programs rely

    15 on Writable Static Data (WSD) for data storage. Native Symbian platform applications

    16 have been developed without any support for WSD on an Emulator for dynamic

    17 link libraries. P.I.P.S. supports WSD and a complete implementation is available

    18 for target phones. The implementation of WSD on an Emulator, however, poses

    19 certain restrictions for dynamic link libraries. </p><p>A Symbian project's

    20 MMP file can specify the <codeph>EPOCALLOWDLLDATA</codeph> directive to enforce

    21 a rule that WSD should not be shared between processes. The P.I.P.S. <codeph>STDEXE</codeph>y

    22 target type sets this directive automatically. The rule imposes the restriction

    23 that only one process can load a DLL with WSD. P.I.P.S. provides a mechanism

    24 to avoid these restrictions. </p> </section>

    25 <section><title>A UNIX DLL with WSD</title><codeblock xml:space="preserve">int refCount = 0;

    26 int IncRefCount()

    27 {

    28   //Increment the reference count

    29   return ++refCount;

    30 }

    31 

    32 int DecRefCount()

    33 {

    34   //Decrement the reference count

    35   return --refCount;

    36 }

    37 </codeblock><p/></section>

    38 <section><title>A Symbian DLL with WSD</title><p>The UNIX code above will

    39 provide indeterminate results on a Symbian emulator. If the DLL had <codeph>EPOCALLOWDLLDATA</codeph> in

    40 its MMP file, then only one process would be able to use the reference counting

    41 functions. Without <codeph>EPOCALLOWDLLDATA</codeph>, the reference count

    42 would be global and not process specific. </p><p>The following code demonstrates

    43 the use of the <xref href="GUID-807C676B-5C46-328C-879C-A4FCB638A5B6.dita"><apiname>Pls()</apiname></xref> function (Process Local Storage),

    44 which is the P.I.P.S. WSD solution to the problem. </p><codeblock xml:space="preserve">#include &lt;pls.h&gt;

    45 

    46 #ifdef __WINSCW__

    47 //The next code will run only on the emulator

    48 

    49 //Put the global count into a structure

    50 struct DLLData

    51 {

    52    int refCount;

    53 };

    54 

    55 //Define a way to access the structure

    56 //On the first call to this function, memory will be allocated with the specified

    57 //UID as an identifier and the Initialization function will be called

    58 //Subsequent calls to this function return the allocated memory

    59 struct DLLData* GetGlobals()

    60 {

    61    return Pls<DLLData>(KDLLUid3, InitializeGlobals);

    62 }

    63 

    64 //Initialization function

    65 TInt InitializeGlobals(DLLData* aData)

    66 {

    67    aData->refCount = 0;

    68    return KErrNone;

    69 }

    70 

    71 //Access the refCount variable

    72 int& get_refCount()

    73 {

    74    return GetClobals()->refCount;

    75 }

    76 

    77 //Macro to replace the variable with our new method

    78 #define refCount get_refCount()

    79 

    80 #else

    81 //Target device code

    82 int refCount;

    83 

    84 #endif

    85 

    86 int IncRefCount()

    87 {

    88   return ++refCount;

    89 }

    90 

    91 int DecRefCount()

    92 {

    93   return --refCount;

    94 }

    95 </codeblock></section>

    96 </conbody></concept>