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" 
    IMPORT_C int* GlbData ();
    #define globalVal (*GlbData())

    And the usage is like:

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