How to export global data from a DLL

Exporting global data from a DLL to be accessed by either Open C or Symbian C++ applications is one of the typical problems that developers encounter.

NOTE! 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:

Do not export global variables.
Export one method that returns a pointer to that variable.
Define a macro for the user of the DLL.

See the example below:

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 (e.g., 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;
    }

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How to export global data from a DLL