Exporting Global Data from a DLL

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.

It is strongly recommended to avoid having global data in DLLs due to following reasons:

• EKA2 emulator allows only a DLL with WSD to load into a single process.

• 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.

• Chunks are a finite resource on ARMv5. Every process loading WSD enabled DLLs uses a chunk to hold the data.

• There are ARM architecture 4 and 5 specific costs and limitations that apply only to DLLs that link against “fixed processes”.

• There is a limit on the number of DLLs in a process with WSD.

On having understood the above limitations, the following pattern can be used for exporting global data from a DLL:

1. Do not export global variables. Within DLL, say there is one global variable, for example:

   int globalVal; 

2. Export one method that returns a pointer to that variable.

   extern "C" EXPORT_C int* GlbData ()
       {
       return &globalVal
       }
   

3. Define a macro for the user of the DLL. Within the DLL header (for example, xxx.h), define the following:

   #ifdef __cplusplus
   extern "C" 
   #endif
   IMPORT_C int* GlbData ();
   #define globalVal (*GlbData())

   And the usage is like:

   #include <xxx.h>  // DLL header
   int main()
       {
       int i = 0;
       globalVal = 10;
       globalVal++;
       i = globalVal;
       return 0;
       }