C++Builder 4.0 is the first C++Builder compiler that supports shared memory segments. This document explains how to use this feature in windows DLL.

To change the data segment and the class name, you need to add #pragma option -zR[SEGMENT NAME] and #pragma option -zT[CLASS NAME] to the file you want the data shared from. Below is the source file I am going to export the integer named 'data':

//---------------------------------------------------------------------------
// Borland C++Builder
// Copyright (c) 1987, 1999 Inprise Corporation. All Rights Reserved.
//---------------------------------------------------------------------------

#pragma option -zRSHSEG      // change default data segment name
#pragma option -zTSHCLASS    // change default data class name

// Here is the initialized data that will be shared.
int data = 0;

Notice that the segment name for this file is: SHSEGSHCLASS. A .def file is required for the linker to create the shared segement. Below is what the .def file looks like:

LIBRARY SHAREDDLL

SEGMENTS
  SHSEG CLASS 'SHCLASS' SHARED

Below is the header and source file for the DLL which contains the entry point into the DLL, and the setters and getters for the 'data' variable in the shared memory space:

//---------------------------------------------------------------------------
// Borland C++Builder
// Copyright (c) 1987, 1999 Inprise Corporation. All Rights Reserved.
//---------------------------------------------------------------------------

#ifdef __DLL__
#define DLL_EXPORT __declspec(dllexport)
#else
#define DLL_EXPORT __declspec(dllimport)
#endif

extern "C"
{
void DLL_EXPORT SetData(int x);
int DLL_EXPORT GetData(void);
}

//---------------------------------------------------------------------------
// Borland C++Builder
// Copyright (c) 1987, 1999 Inprise Corporation. All Rights Reserved.
//---------------------------------------------------------------------------

//---------------------------------------------------------------------------
#include 
#pragma hdrstop

#include "Shared.h"
//---------------------------------------------------------------------------
//   Important note about DLL memory management when your DLL uses the
//   static version of the RunTime Library:
//
//   If your DLL exports any functions that pass String objects (or structs/
//   classes containing nested Strings) as parameter or function results,
//   you will need to add the library MEMMGR.LIB to both the DLL project and
//   any other projects that use the DLL.  You will also need to use MEMMGR.LIB
//   if any other projects which use the DLL will be perfomring new or delete
//   operations on any non-TObject-derived classes which are exported from the
//   DLL. Adding MEMMGR.LIB to your project will change the DLL and its calling
//   EXE's to use the BORLNDMM.DLL as their memory manager.  In these cases,
//   the file BORLNDMM.DLL should be deployed along with your DLL.
//
//   To avoid using BORLNDMM.DLL, pass string information using "char *" or
//   ShortString parameters.
//
//   If your DLL uses the dynamic version of the RTL, you do not need to
//   explicitly add MEMMGR.LIB as this will be done implicitly for you
//---------------------------------------------------------------------------
USEUNIT("SharedData.cpp");
USEDEF("Shared.def");
//---------------------------------------------------------------------------
int WINAPI DllEntryPoint(HINSTANCE hinst, unsigned long reason, void*)
{
   return 1;
}
//---------------------------------------------------------------------------
extern int data;

//---------------------------------------------------------------------------
void DLL_EXPORT SetData(int x)
{
  data = x;
}
//---------------------------------------------------------------------------
int DLL_EXPORT GetData(void)
{
  return data;
}
//---------------------------------------------------------------------------